Efficient one-pot synthesis of functionalised imidazo[1,2-a]pyridines and unexpected synthesis of novel tetracyclic derivatives by nucleophilic aromatic substitution

Abstract

Novel tetracyclic imidazo[1,2-a]pyridine derivatives have been prepared by intramolecular nucleophilic aromatic substitution of 5-fluoroimidazo[1,2-a]pyridines under basic conditions. Use of the non-nucleophilic alcoholic solvent tert-butanol, rather than methanol, increased the yield of the tetracycles by reducing the competing intermolecular reaction observed for methanol. In addition, a modified protocol for the dehydration of formamides to isocyanides has been found to be tolerant of an unprotected hydroxyl functional group and one-pot conversion to imidazo[1,2-a]pyridyl-aminocyclohexanol analogues is reported.

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Introduction

The imidazopyridine skeleton possesses unique electronic and chemical properties that make it an attractive starting point in the preparation of a broad spectrum of therapeutic agents¹ ranging from sedative drugs such as zolpidem 1, antiviral agents 2, anticancer compounds 3, immunomodulators 4 (ref. 2) and antitubercular agents 5 (ref. 3) (Fig. 1), to mention just a few. Other reports indicate activity as gastric proton pump inhibitors⁴ and as antifungal,⁵ antibacterial⁶ and anxiolytic⁷ agents.

Fig. 1 Biologically active imidazopyridines.

Largely as a result of their biological importance, the development of safe synthetic methodologies that efficiently access imidazopyridines and their associated derivatives continues to generate much research interest in synthetic chemistry.⁸

The utility of the multi-component Groebke–Blackburn–Bienaymé reaction⁹ for preparation of imidazo[1,2-a]pyridines by reaction of an aldehyde, 2-aminopyridine and an isocyanide is well documented in synthetic chemistry literature.^1a,10a,b Given that 2-aminopyridines and aldehydes are generally affordable, the versatility and robustness of this protocol is primarily disadvantaged by the limited variety and high procurement cost of commercially available isocyanides. This mandates researchers to prepare most of the isocyanides that are required to fulfil their research requirements.

More than one and a half centuries ago, Gautier and Hofmann¹¹ first described the preparation of isocyanides. Their apt description of isocyanides as possessing ‘almost overpowering, horrible and extremely distressing odours’ typifies the challenges associated with the preparation and handling of isocyanides even to this present day. Almost a hundred years later, the first generally applicable routes for accessing isocyanides were described, via the dehydration of N-formamides using acyl oxides of group IV–VI elements in the presence of bases.¹² Due to the high toxicity and handling difficulties associated with using phosgene,¹³ phosphorus oxychloride (used together with Et₃N base), a method originally described by Ugi and Meyr,¹⁴ has become one of the most commonly employed N-formamide dehydrating agents for the preparation of isocyanides in synthetic chemistry today.

Nevertheless, the increasing enactment of tightened environmental, health and safety management laws continues to drive the search for safer synthetic routes for accessing isocyanides. Thus, more research and development is still needed to develop safer methodologies that provide ease of access to a large variety of these key substrates. In an interesting development, Wang and co-workers¹⁵ reported the identification of triphenylphosphine and iodine as mild and efficient N-formamide dehydrating agents for generating aromatic isocyanides. Guchhait and colleagues¹⁶ reported the development of a one-pot reaction which employed para-toluenesulfonyl chloride (pTsCl) and DABCO for the dehydration of N-formamide substrates to generate isocyanides in situ for subsequent use in multicomponent reactions.

We have previously reported the identification of novel imidazo[1,2-a]pyridine derivatives as non-nucleoside inhibitors of HIV-1 reverse transcriptase.¹⁷ The novel lead compound, 2-(2-chlorophenyl)-3-(cyclohexylamino)imidazo[1,2-a]pyridine-5-carbonitrile 6 (Fig. 2) exhibited good antiviral activity (whole cell anti-HIV IC₅₀ = 0.18 μM) and displayed excellent selectivity (SI = 868) when screened against the wild-type HI virus. Molecular modelling results indicated that introduction of groups capable of hydrogen-bonding to amino acids in the allosteric site would potentially lead to compounds with increased potency.

Fig. 2 Imidazopyridine lead compound 6; modified target products 7.

Therefore, as part of our ongoing efforts to discover compounds with better antiviral activity profiles against both wild-type and mutant viral strains we planned to expand our imidazo[1,2-a]pyridine library using compound 6 as a starting point. Thus compounds of general structure 7 (Fig. 2) were conceived for synthesis as promising targets for subsequent screening against the HI virus. In this paper, we report a highly efficient modified pTsCl/DABCO protocol¹⁶ as a safe and OH functional group tolerant catalyst methodology for accessing novel imidazo[1,2-a]pyridine heterocyclic targets. In addition, we also report the unexpected ring-closure of 5-fluoro-imidazo[1,2-a]pyridine derivatives, giving rise to novel tetracyclic compounds.

Results and discussion

To access our initial small library of target compounds of general structure 7, 2-trans-hydroxyammonium hydrochloride (rac-8) was neutralised and formylated quantitatively to generate formamide rac-9 using a previously reported method¹⁸ (Scheme 1). After acetylation, compound rac-10 was then dehydrated using POCl₃/Et₃N to produce the isocyanide rac-11 for subsequent multi-component Groebke–Blackburn–Bienaymé reaction. Acetylation was used as a means of protecting the OH group during the dehydration step, as free OH groups are incompatible with the POCl₃/Et₃N methodology. Coupling of rac-11 with 2-chlorobenzaldehyde 12a and 6-substituted-2-aminopyridine 13 (Scheme 1) produced the target acetates 14a–d in excellent yields, specifically over the multicomponent coupling step (v, Scheme 1). Nonetheless, it is noteworthy to point out that a respectable 85% yield of the isocyanide (rac-11) was only achieved at small scale (100 mg of rac-10) whilst efforts to scale up the reaction progressively gave poorer conversions. One of the difficulties of this reaction appeared to be a propensity of rac-11 to undergo a rehydration reaction to regenerate the N-formamide rac-10 during the aqueous work up, given the non-stoichiometric imbalance between the acid and base associated with this protocol.

Scheme 1 Reagents: (i) NaOMe, MeOH, RT; (ii) methyl formate, RT, 12 h; (iii) Ac₂O, pyridine, 4 h; (iv) POCl₃/Et₃N, DCM; (v) 2-chlorobenzaldehyde 12a, 6-substituted-2-aminopyridine 13a–d, K-10 clay, MW 150W, 100 °C, 30 min; (vi) KOH, MeOH, RT, 4 h.

The final step in the synthesis was KOH-catalysed hydrolysis of the respective acetates in MeOH to obtain alcohols 7. Target compounds 7b and 7c were obtained in excellent yields from 14b and 14c, respectively, while unexpected hydrolysis of the nitrile group of 14d under the basic conditions of the deprotection reaction gave rise to carboxamide 7e. Attempted deprotection of compound 14a also did not lead to the expected deprotected compound; instead a roughly equal mixture of two compounds was obtained. On initial inspection of the ¹H NMR spectrum of the first product 15 it was immediately evident that the acetyl group had been removed. The first clear indication that the expected product had not been obtained was the appearance of the signal at 6.17 ppm for H-6 as a doublet showing one ortho-coupling (J = 7.2 Hz). In the starting material 14a, this proton appears at 6.38 ppm as a triplet as a result of ortho-coupling to both H-7 and F with very similar coupling constants (J = 7.1 Hz). The disappearance of F was further confirmed in the ¹³C NMR spectrum of 15 where C-6 appeared as a singlet at 96.6 ppm, rather than the doublet (²J_C–F = 18 Hz) observed at 93.1 ppm for 14a. The signal for C-1′ appeared at 88.4 ppm, far more deshielded than for compound 14a, where this signal appeared at 76.4 ppm. Thus, it appeared that ring-closure of the newly-deprotected hydroxyl group onto the carbon atom originally carrying F had taken place, giving rise to 15. The second product was identified as 7f, where the acetate group had been removed, but where F had been replaced by OMe (Scheme 1). Only these two unexpected products were obtained from the basic deprotection reaction of 14a in MeOH, with none of the expected deprotected hydroxyl product being observed at all. Repeating the KOH hydrolysis reaction of 14a in the non-nucleophilic solvent tert-butanol instead of methanol gave 15 as the sole product in 60% yield.

Excited by the unexpected formation of novel ring-closed heterocyclic product 15, we explored the general applicability of this phenomenon using various fluorine-containing imidazopyridine analogues derived from four randomly selected aldehydes. Given the challenges encountered using the POCl₃/Et₃N reagents as highlighted above, we explored the utility of an alternative para-toluenesulfonyl chloride-based protocol reported in literature¹⁶ that employs equimolar quantities of acid and base as dehydrating agents (Scheme 2).

Scheme 2 Reagents: (i) DCM, anhydr. Na₂SO₄ (1 eq.), pTsCl/DABCO, 0 °C–RT, 6 h, (ii) 0–5 °C, aldehyde 12, 2-amino-6-fluoropyridine 13a, anhydr. Na₂SO₄ (1 eq.); then 50 °C, 12 h; (iii) KOH, MeOH or t-BuOH, RT, 4 h. Yields of 16 are quoted over two steps.

In our case, we observed that the efficiency of the dehydration protocol was highly dependent on the purity of the pTsCl. Thus, the literature reported purification procedure developed by Whitaker¹⁹ in 2001 was utilized to purify the pTsCl, which was subsequently stored sealed to reduce moisture ingress. The pTsCl/DABCO protocol proved to be highly convenient and efficient with its main attractive feature being the in situ generation of the desired isocyanide rac-11 which obviated the often tedious aqueous workups encountered during isocyanide purification which are commonplace when employing the POCl₃/Et₃N protocol. In addition, it removed the unpleasant odour usually associated with isocyanide isolation. The in situ generated isocyanide rac-11 was then coupled with the requisite aldehydes 12b–e and 2-amino-6-fluoropyridine 13a under sealed conditions at moderate temperatures (50 °C) to obtain the respective fluorine-containing imidazopyridine acetates 16a–d, with the pTsCl/DABCO adducts formed during the dehydration reaction subsequently catalysing the multicomponent coupling reaction (Scheme 2). As a slight deviation from the Guchhait protocol,¹⁶ we observed that the addition of anhydrous Na₂SO₄ during the dehydration stage as well as the multicomponent coupling step, not only obviated the need for N₂ purging, but further simplified the overall preparation of the desired target compounds. These transformations translated to a minimum 70% yield across both the isocyanide generation as well as the multicomponent coupling phases. The pTsCl/DABCO-catalysed reactions were easily scalable without any discernible drop in target product yields, unlike the problems experienced using POCl₃-mediated dehydration. Secondly, the non-stoichiometric addition of dehydrating agents that oftentimes characterises most POCl₃/Et₃N protocols makes it mandatory for preliminary aqueous workups to remove salts and excess reagents and purify the isocyanide. Such reactive salts and excess reagents could also be responsible for catalysing the hydration of the isocyanide to regenerate the formamide, thereby lowering yields.

Subsequent base-catalysed hydrolysis of the acetate 16a in MeOH did indeed give rise to a ring-closed tetracycle 17a in 53% yield (Scheme 2), together with compound rac-18 (Fig. 3), in 47% yield. Close examination of the ¹H NMR spectrum for 17a showed clearly that the fluorine atom had been displaced, as the proton at position 6 appeared as a dd, with one ortho and one meta coupling.

Fig. 3 Compound 18 formed from 16a.

However, the signal for the proton on the cyclohexyl ring carbon atom carrying nitrogen had disappeared, together with the NH proton signal, showing that in fact 17a was an imine, representing the oxidised form of compound 15. Similarly, base-catalysed hydrolysis of acetates 16b–c, this time in t-BuOH, gave rise to oxidised tetracycles 17b–c in good yield (step iii, Scheme 2). Hydrolysis of the bromine containing acetate 16d gave rise to an irresolvable mixture. The identities of the ring-closed products 17a and 17c were confirmed by single crystal X-ray crystallographic analysis (Fig. 4). The formation of the oxidised imine-containing products may possibly be attributed to the stronger electron withdrawing effect on the imidazopyridine skeleton by the nitro and the cyano groups as compared to that exerted by the chlorine atom in the case of unoxidised ring-closed product 15.

Fig. 4 ORTEP diagrams (50% probability level) of 17a (left) and 17c (right).

Given our failure to obtain the originally intended fluorine-containing imidazopyridine targets of general structure 7 via base-catalysed hydrolysis of their respective acetates 14 as explained above, an exploratory attempt was made to directly dehydrate the unprotected 2-trans-hydroxyformamide rac-9 using the modified pTsCl/DABCO protocol and generate the isocyanide rac-19 in situ for the subsequent multicomponent reaction with 2-amino-6-fluoropyridine 13a and selected aldehydes 12 (Scheme 3). To our delight, the expected novel fluorine-containing targets 20a–g were obtained in excellent overall yield, which also demonstrated the excellent OH functional group tolerance of the pTsCl/DABCO protocol. To the best of our knowledge, no similar successful attempts have previously been reported. Guchhait et al.¹⁶ did not test their method on functionalised isocyanides. Although rac-19 has not been prepared previously by dehydration of rac-9, it has been prepared by ring-opening of cyclohexene epoxide using TMSCN and ZnI₂, to give the TMS-protected alcohol that was subsequently deprotected.²⁰ One example of dehydration of a formamide containing a hydroxyl group to the corresponding isocyanide was reported by McCarthy et al. using Burgess reagent, but the reaction was low-yielding (<50%) and took 2 days.²¹ Thus, the method reported here is superior in terms of ease of reaction and yield. The small library of novel imidazo[1,2-a]pyridines produced during the course of this research will be screened for activity against the HI virus and the findings will be reported in due course.

Scheme 3 Reagents: (i) NaOMe, methyl formate, MeOH, RT, 4 h; (ii) DCM, anhydr. Na₂SO₄ (1 eq.), pTsCl/DABCO, 0 °C–RT, 6 h, (iii) 0–5 °C, aldehyde 12, 2-amino-6-fluoropyridine 13a, anhydr. Na₂SO₄ (1 eq.), then 50 °C, 12 h. Yields for 20 are quoted over all three steps.

Experimental

General

All solvents were freshly distilled prior to use. Other reagents were used as purchased from Sigma-Aldrich. All infrared spectra were recorded neat using a Bruker TENSOR 27 single channel infrared spectrometer. All melting points are uncorrected and were performed using open capillary tubes on a Stuart SMP 10 melting point apparatus. ¹H and ¹³C NMR spectra were recorded using either a Bruker AVANCE 111 300, 400 or 500 MHz spectrometer in deuterated chloroform (CDCl₃) with trimethylsilane (TMS) as internal standard (δ = 0) for ¹H NMR, and CDCl₃ (δ = 77.0 ppm) for ¹³C NMR. The chemical shift (δ) is reported in ppm and the coupling constants (J) in Hz. High resolution mass spectral data was collected on a Waters Synapt G2 using an ESI positive source and a cone voltage of 15 V. TLC was performed on aluminium-backed Merck silica gel 60 F₂₅₄ plates. The purification of compounds by column chromatography was performed using gravity (particle size 0.063–0.200 mm) or flash (particle size 0.040–0.063 mm) silica gel 60 purchased from Merck.

Synthetic procedures

Synthesis of N-(2-trans-hydroxycyclohexyl)formamide (rac-9). 2-trans-Hydroxycyclohexylammonium chloride rac-8 (1 eq., 5.00 g, 33.0 mmol) was dissolved in methanol (40 ml) and treated with NaOMe (1.8 eq., 3.298 g, 10.0 ml, 61.0 mmol), to the resulting mixture was added methyl formate (4 eq., 8.00 ml, 130.6 mmol), and the reaction was allowed to stir for 24 h. A white solid precipitated out of the reaction mixture and was removed by filtration. An excess of hexane (relative to the volume of methanol) was added to the collected reaction mixture and was allowed to stand overnight, the resulting precipitate was again removed by filtration and the solvent removed in vacuo to obtain the desired product rac-9 as a white-light grey solid (4.53 g, 96%). From NMR spectroscopy, it was evident that in solution this product occurs as a mixture of rotamers. Mp: 136–139 °C; IR (ν_max/cm⁻¹): 3341 (N–H), 3283 (O–H), 2858–2962 (C–H alkyl), 1635 (C=O); ¹H NMR (300 MHz, CDCl₃) δ: 8.26 (s, 0.7H), 8.06 (d, J = 11.2 Hz, 0.3H), 6.61 (br s, 0.3H), 6.28 (br s, 0.7H), 3.74–3.63 (m, 1H), 3.42–3.20 (m, 1.5H), 3.07–2.98 (m, 0.5H), 2.06–1.97 (m, 2H), 1.90–1.74 (m, 2H), 1.31–1.25 (m, 4H); ¹³C NMR (75 MHz, CDCl₃) δ: 165.14, 162.54, 74.41, 73.23, 58.69, 54.60, 34.38, 33.69, 32.31, 31.57, 24.79, 24.16, 24.44, 24.04; HRMS (ES)⁺: calculated for C₇H₁₄NO₂ [M + H]⁺: 144.1019, found: 144.1019.

Synthesis of 2-trans-formamidocyclohexyl acetate (rac-10). Compound rac-9 (1 eq., 1.20 g, 8.38 mmol) was dissolved in a solution of acetic anhydride (9 eq., 7.00 ml, 74.1 mmol) and pyridine (6 eq., 4.20 ml, 52.1 mmol, 6 eq.) for 4 h at room temperature. The reaction vessel was placed in an ice bath, and excess methanol was added to the resulting solution to quench the excess acetic anhydride. The excess pyridine was removed in vacuo as an azeotropic mixture with toluene to give the desired product rac-10 as a yellow solid (1.50 g, 97%). Mp: 85–88 °C; IR (cm⁻¹): 3271 (NH str.), 2868 and 2937 (CH str.), 1726 (C=O ester), 1658 (C=O aldehyde); ¹H NMR (300 MHz, CDCl₃) δ: 8.15–8.00 (m, 1H), 6.56–6.25 (m, N–H), 4.75–4.52 (m, 1H), 4.03–3.88 (m, 0.8H), 3.35–3.20 (m, 0.2H), 2.12–1.92 (m, 5H), 1.83–1.69 (m, 2H), 1.54–1.19 (m, 4H); ¹³C NMR (75 MHz, CDCl₃) δ: 171.5, 170.3, 164.2, 160.9, 74.8, 74.3, 54.8, 51.1, 32.0, 31.7, 30.8, 30.6, 24.0, 23.9, 23.8, 23.5, 21.0, 20.9; HRMS (ES)⁺: calculated for C₉H₁₆NO₃ [M + H]⁺: 186.1125, found: 186.1127.

Synthesis of 2-trans-isocyanocyclohexyl acetate (rac-11). Compound rac-10 (0.100 g, 0.540 mmol) and Et₃N (1.82 g, 18.0 mol) were mixed in dry DCM (10 ml) and the mixture treated with POCl₃ (0.30 ml, 3.21 mmol) at 0 °C. The reaction was warmed to room temperature under N₂, and left to stir for 24 h. The resulting mixture was gradually added to ice-cold water over a period of 30 min to quench the excess POCl₃. The organic layers were separated and the aqueous layer extracted with CH₂Cl₂ (3 × 10 ml). The organic layers were combined and washed with saturated NaHCO₃ solution before being dried over anhydrous Na₂SO₄. The solvent was removed in vacuo and the residue purified by flash column chromatography (elution 4 : 1 EtOAc/Hex) to give the desired product rac-11 as a light yellow oil (0.077 g, 85%). IR (cm⁻¹): 1736 (C=O), 2141 (⁺N≡C⁻); ¹H NMR (300 MHz, CDCl₃) δ: 4.83 (td, J = 9.2, 4.2 Hz, 1H), 3.59–3.47 (m, 1H), 2.25–2.01 (m, 5H), 1.84–1.58 (m, 3H), 1.52–1.20 (m, 3H); ¹³C NMR (75 MHz, CDCl₃) δ: 170.0, 156.5 (t, J_C–N = 9.5 Hz), 73.4, 55.1 (t, J_C–N = 13.5 Hz), 31.3, 29.4, 22.9, 22.7, 20.9 ppm; HRMS (ES)⁺: calculated for C₉H₁₄NO₂ [M + H] ⁺: 168.1019, found: 168.1017.

General procedure for microwave-assisted synthesis of novel imidazo[1,2-a]pyridine derivatives 14a–d

A mixture of a 6-substituted-2-aminopyridine 13a–d (1 mmol), 2-chlorobenzaldehyde 12a (1 mmol), isocyanide rac-11 (170 mg, 1.02 mmol) and montmorillonite K-10 clay (100 mg) in 1,4-dioxane (2.0 ml) was irradiated in a sealed tube for 30 min (150 W, 100 °C). After cooling to room temperature, the K-10 clay was filtered off through Celite which was later washed with EtOAc and the combined organic solvents were removed in vacuo to give a crude residue that was purified using flash silica by eluting with 20–60% EtOAc/Hex.

Synthesis of 2-((2-(2-chlorophenyl)-5-fluoroimidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-14a). MW irradiation of 2-chlorobenzaldehyde 12a (143 mg, 1.0 mmol), 2-amino-6-fluoropyridine 13a (112 mg, 1.0 mmol) and isocyanide rac-11 (167 mg, 1.0 mmol) in the presence of K-10 clay (110 mg) gave rac-14a (220 mg, 60%) as a black oil. IR (cm⁻¹): 3352 (NH str.), 2937 (CH str.), 1730 (C=O str.), 1651 (C=N). ¹H NMR (500 MHz, CDCl₃) δ 7.61–7.58 (m, 1H), 7.50–7.47 (m, 1H), 7.37–7.34 (m, 3H), 7.12–7.08 (m, 1H), 6.38 (t, J = 7.4 Hz, 1H), 4.66–4.55 (m, 1H), 3.55–3.51 (m, 1H), 2.88–2.81 (m, 1H), 1.93–1.87 (m, 4H), 1.71–1.66 (m, 1H), 1.62–1.56 (m, 1H), 1.52–1.47 (m, 1H), 1.24–1.15 (m, 2H), 1.02–0.91 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 170.8, δ 150.6 (d, ¹J_C–F = 268.1 Hz), 143.6 (d, ⁴J_C–F = 3.4 Hz), 136.2, 133.4, 133.2, 132.5, 129.6, 129.5, 126.8, 126.1 (d, ⁴J_C–F = 2.8 Hz), 124.3 (d, ³J_C–F = 6.4 Hz), 113.8 (d, ³J_C–F = 5.0 Hz), 93.1 (d, ²J_C–F = 17.4 Hz), 76.4, 60.9, 30.9, 30.2, 23.7, 23.7, 21.1 ppm; HRMS (ES⁺) calculated for C₂₁H₂₂ClFN₃O₂ [M + H]⁺: 402.1379, found: 402.1400.

Synthesis of 2-((5-chloro-2-(2-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-14b). Reaction of 2-chlorobenzaldehyde 12a (87.5 μL, 0.778 mmol), 2-amino-6-chloropyridine 13b (100 mg, 0.778 mmol) and isocyanide rac-11 (130 mg, 0.778 mmol) under MW irradiation in the presence of K-10 clay (110 mg) gave rac-14b (240 mg, 76%) as a black solid. Mp: 89–91 °C; IR (cm⁻¹): 3355 (NH str.), 2944 (CH str.), 1728 (C=O str.), 1653 (C=N); ¹H NMR (500 MHz, CDCl₃) δ 7.61–7.59 (m, 1H), 7.50–7.48 (m, 2H), 7.37–7.34 (m, 2H), 7.06–7.02 (m, 1H), 6.78–6.76 (m, 1H), 4.57–4.52 (m, 1H), 3.67–3.64 (m, 1H), 2.89–2.84 (m, 1H), 1.91–1.84 (m, 4H), 1.61–1.53 (m, 2H), 1.48–1.43 (m, 1H), 1.22–1.07 (m, 2H), 1.02–0.92 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 170.6, 143.8, 138.3, 133.5, 133.4, 132.5, 129.6, 129.5, 127.6, 126.8, 126.3, 123.7, 116.8, 114.2, 76.9, 60.8, 30.3, 30.0, 23.6, 23.6, 21.1 ppm; HRMS (ES⁺) calculated for C₂₁H₂₂N₃O₂Cl₂ [M + H]⁺: 418.1089, found: 418.1080.

Synthesis of 2-((5-bromo-2-(2-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-14c). 2-Chlorobenzaldehyde 12a (81.2 mg, 0.578 mmol), 2-amino-6-bromopyridine 13c (100 mg, 0.578 mmol) and isocyanide rac-11 (96.5 mg, 0.578 mmol) were reacted under MW irradiation in the presence of K-10 clay (110 mg) to give rac-14c (210 mg, 81%) as a black oil. IR (cm⁻¹): 3352 (NH str.), 2938 (CH str.), 1732 (C=O str.), 1652 (C=N); ¹H NMR (500 MHz, CDCl₃) δ 7.62–7.60 (m, 1H), 7.55–7.52 (m, 1H), 7.49–7.47 (m, 1H), 7.38–7.34 (m, 2H), 7.00–6.94 (m, 2H), 4.58–4.53 (m, 1H), 3.64–3.60 (m, 1H), 2.88–2.82 (m, 1H), 1.92–1.85 (m, 4H), 1.58–1.52 (m, 2H), 1.49–1.43 (m, 1H), 1.22–1.14 (m, 1H), 1.11–1.03 (m, 2H), 0.98–0.90 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 170.5, 143.9, 138.8, 133.6, 133.3, 132.4, 129.6, 129.5, 127.9, 126.8, 124.0, 118.8, 117.3, 112.5, 60.4, 30.1, 29.9, 23.7, 23.6, 21.1 ppm; HRMS (ES⁺) calculated for C₂₁H₂₂BrClN₃O₂ [M + H]⁺: 462.0578, found: 462.0555.

Synthesis of 2-((2-(2-chlorophenyl)-5-cyanoimidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-14d). Reaction of 2-chlorobenzaldehyde 12a (118 mg, 0.84 mmol), 2-amino-6-cyanopyridine 13d (100 mg, 0.84 mmol) and isocyanide rac-11 (140.33 mg, 0.84 mmol) under MW irradiation in the presence of K-10 clay (110 mg) gave rac-14d (150 mg, 44%) as a yellow solid. Mp: 236–238 °C. IR (cm⁻¹): 3348 (NH str.), 2939 (CH str.), 2215 (CN str.), 1733 (C=O str.), 1653 (C=N); ¹H NMR (400 MHz, CDCl₃) δ 7.81 (d, J = 8.9 Hz, 1H), 7.68–7.64 (m, 1H), 7.55–7.51 (m, 1H), 7.45–7.40 (m, 3H), 7.20 (dd, J = 9.0, 7.1 Hz, 1H), 4.59 (td, J = 9.7, 4.4 Hz, 1H), 3.73–3.68 (m, 1H), 2.93–2.86 (m, 1H), 2.00–1.92 (m, 1H), 1.84 (s, 3H), 1.65–1.51 (m, 3H), 1.34–1.21 (m, 3H), 1.12–0.98 (m, 1H); ¹³C NMR (101 MHz, CDCl₃) δ 170.3, 141.3, 139.3, 133.2, 133.0, 132.4, 130.2, 129.9, 127.8, 127.3, 124.3, 123.0, 122.3, 113.8, 108.4, 77.1, 60.0, 30.4, 29.9, 23.8, 23.7, 21.1 ppm; HRMS (ES⁺) calculated for C₂₂H₂₂ClN₄O₂ [M + H]⁺: 409.1426, found: 409.1413.

Typical procedure for the preparation of compounds 16a–d

A mixture of 2-trans-hydroxycyclohexylammonium chloride rac-8 (2.00 g, 17.12 mmol), sodium methoxide (20 ml, 6.58 mmol) and methyl formate (2 ml) were stirred at room temperature for 2 h. Thereafter, MeOH (20 ml) was added to the reaction and the mixture was stirred at 40 °C for 2 h, and then at room temperature for a further 24 h. After removing the organic solvent in vacuo, acetone (25 ml) was added to the off-white residue and the precipitated salts were removed by filtration. The solvent was evaporated in vacuo to afford an off-white oily product, 2-trans-hydroxycyclohexyl formamide rac-9 (1.83 g. 99% yield). To a mixture of the formamide rac-9 (1.70 g, 16.18 mmol) and a catalytic amount of dimethylaminopyridine (171 mg) in acetonitrile (20 ml) was added acetic anhydride (15 ml) and the reaction mixture stirred at room temperature for 12 h. After evaporating the solvent, the pale-yellow oil was diluted with acetone (20 ml) and after adding sodium bicarbonate (1.2 eq.), the reaction mixture was filtered to afford 2-formamidocyclohexyl acetate rac-10 as a pale yellow oil in quantitative yield.

An appropriate amount of 2-formamidocyclohexyl acetate rac-10 (1.0 mmol (185 mg)–2 mmol (371 mg)), anhydrous Na₂SO₄ (284 mg, 2 mmol) and a magnetic stirrer were added to freshly distilled dichloromethane (10–15 ml) and chilled in an ice bath (10 min). p-Toluenesulfonyl chloride (pTsCl) (1.0 mmol (191 mg)–2.0 mmol (382 mg)) and DABCO (1.0 mmol (112 mg)–2.0 mmol (225 mg)) were added in succession and the closed reaction mixture was stirred under ice-chilled conditions for 1 h. Thereafter, the reaction mixture was allowed to gradually warm to room temperature with stirring for a further 2 h. To this chilled in situ generated isocyanide rac-11 crude mixture was added anhydrous Na₂SO₄ (284 mg, 2 mmol), an appropriate aldehyde 12 (1.0–2.0 mmol) and 2-amino-6-fluoropyridine 13a (1.0–2.0 mmol) and the sealed reaction mixture was heated at 50–60 °C in an oil bath for 10–12 h. Thereafter, the reaction was cooled to room temperature, diluted with DCM (20 ml) and filtered. The filtrate was washed successively with distilled water (2 × 10 ml) and saturated brine solution (10 ml). After drying over Na₂SO₄, the solvent was removed in vacuo and the crude mixture was purified by silica gel flash column chromatography, eluting the title compounds 16a–d with 25–50% EtOAc/hexane.

Synthesis of 2-((5-fluoro-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-16a). 4-Nitrobenzaldehyde (303 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-11 were reacted to generate product rac-16a (454 mg, 74% from rac-10) as a yellow solid. Mp: 196–197 °C; IR (cm⁻¹): 3333 (NH str.), 3081 and 2943 (CH str.), 1727 (C=O str.), 1659 (C=N), 1566 (NH bend), 1451 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.53–8.48 (m, 2H), 8.30–8.26 (m, 2H), 7.36 (d, J = 9.0, Hz, 1H), 7.17–7.12 (m, 1H), 6.41 (t, J = 7.3 Hz, 1H), 4.79 (td, J = 10.0, 4.5 Hz, 1H), 3.63–3.61 (m, 1H), 3.11–3.06 (m, 1H), 2.09–2.05 (m, 1H), 1.94 (s, 3H), 1.78–1.74 (m, 1H), 1.71–1.68 (m, 1H), 1.62–1.58 (m, 1H), 1.33–1.22 (m, 3H), 1.12–1.06 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 170.7, 150.5 (d, ¹J_C–F = 262.5 Hz), 146.7, 144.1 (d, ⁴J_CF = 3.7 Hz), 140.5, 134.7, 127.7, 126.0 (d, ⁴J_C–F = 1.7 Hz), 125.4 (d, ³J_C–F = 6.8 Hz) 123.7, 114.2 (d, ³J_C–F = 4.9 Hz), 93.4, (d, ²J_CF = 17.6 Hz), 77.4, 61.4 (d, ⁵J_C–F = 2.1 Hz), 31.2, 30.7, 24.1, 24.0, 21.1; HRMS (ES⁺) calculated for C₂₁H₂₂FN₄O₄ [M + H]⁺: 413.1620, found: 413.1615.

Synthesis of 2-((5-fluoro-2-(3-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-16b). 3-Nitrobenzaldehyde (303 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-11 were reacted to give product rac-16b (406 mg, 70% from rac-10) as a yellow solid. Mp: 169–171 °C; IR (cm⁻¹): 3341 (NH str.), 3083, 2967 and 2936 (CH str.), 1728 (C=O str.), 1654 (C=N), 1568 (NH bend), 1434 (C=C);.¹H NMR (500 MHz, CDCl₃) δ 9.20 (t, J = 2.0 Hz, 1H), 8.66 (dt, J = 7.9, 1.3 Hz, 1H), 8.17–8.15 (m, 1H), 7.60 (t, J = 8.0 Hz, 1H), 7.37 (d, J = 9.0 Hz, 1H), 7.16–7.12 (m, 1H), 6.40 (t, J = 7.2 Hz, 1H), 4.77 (td, J = 10.1, 4.5 Hz, 1H), 3.63–3.58 (m, 1H), 3.14–3.08 (m, 1H), 2.09–2.03 (m, 1H), 1.85 (s, 3H), 1.70–1.61 (m, 2H), 1.34–1.22 (m, 4H), 1.16–1.08 (m, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 170.1, 150.5 (d, ¹J_C–F = 265.7 Hz), 148.6, 143.9 (d, ⁴J_C–F = 3.7 Hz), 135.8, 134.8, 133.1, 129.2, 125.1 (d, ³J_C–F = 6.3 Hz), 122.1 (d, ³J_C–F = 9.5 Hz), 114.0 (d, ⁴J_C–F = 4.9 Hz), 93.4 (d, ²J_C–F = 17.6 Hz), 77.7, 61.1 (d, ⁵J_C–F = 2.2 Hz), 31.2, 30.7, 24.1, 24.0, 21.0; HRMS (ES⁺) calculated for C₂₁H₂₂FN₄O₄ [M + H]⁺: 413.1620, found: 413.1617.

Synthesis of 2-((2-(3-cyanophenyl)-5-fluoroimidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-16c). Reaction of 3-cyanobenzaldehyde (262 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-11 gave target product rac-16c (448 mg, 76% from rac-10) as a yellow solid. Mp: 185–187 °C; IR (cm⁻¹): 3335 (NH str.), 3055, 2937 and 2855 (CH str.), 2229 (CN str.), 1729 (C=O str.), 1657 (C=N), 1583 (NH bend), 1443 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.62 (d, J = 1.7 Hz, 1H), 8.57 (dt, J = 7.9, 1.5 Hz, 1H), 7.52 (t, J = 7.8 Hz, 1H), 7.34 (d, J = 9.0 Hz, 1H), 7.15–7.11 (m, 1H), 6.40 (t, J = 7.2 Hz, 1H), 4.77 (td, J = 10.0, 4.5 Hz, 1H), 3.58–3.55 (m, 1H), 3.11–3.04 (m, 1H), 2.08–2.05 (m, 1H), 1.94 (s, 3H), 1.77–1.73 (m, 1H), 1.70–1.66 (m, 1H), 1.63–1.57 (m, 1H), 1.33–1.20 (m, 3H), 1.14–1.04 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 170.8, 150.4 (d, ¹J_C–F = 265.9 Hz), 143.9 (d, ⁴J_C–F = 3.8 Hz), 135.2, 134.8, 131.4, 130.8, 130.7, 129.1, 125.1 (d, ³J_C–F = 6.3 Hz), 124.9 (d, ⁴J_C–F = 2.5 Hz), 119.0, 113.9 (d, ³J_C–F = 5.0 Hz), 112.5, 93.3 (d, ²J_C–F = 17.6 Hz), 77.3, 61.1 (d, ⁵J_C–F = 2.2 Hz), 31.2, 30.7, 24.0, 21.0; HRMS (ES⁺) calculated for C₂₂H₂₂FN₄O₂ [M + H]⁺: 393.1721, found: 393.1719.

Synthesis of 2-((2-(3-bromophenyl)-5-fluoroimidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate (rac-16d). 3-Bromobenzaldehyde (370 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ-generated isocyanide rac-11 were reacted to give title compound rac-16d (472 mg, 73% from rac-10) as a yellow solid. Mp: 151–152 °C; IR (cm⁻¹): 3341 (NH str.), 3083, 2957 and 2936 (CH str.), 1728 (C=O str.), 1654 (C=N), 1568 (NH bend), 1434 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.45 (t, J = 1.9 Hz, 1H), 8.24 (dt, J = 7.8, 1.3 Hz, 1H), 7.44–7.42 (m, 1H), 7.34 (d, J = 9.0 Hz, 1H), 7.31–7.27 (m, 1H), 7.11–7.07 (m, 1H), 6.36 (t, J = 7.2, Hz, 1H), 4.75 (td, J = 10.0, 4.5 Hz, 1H), 3.52 (s, 1H), 3.08–3.06 (m, 1H), 2.08–2.04 (m, 1H), 1.89 (s, 3H), 1.79–1.76 (m, 1H), 1.69–1.66 (m, 1H), 1.61–1.58 (m, 1H), 1.32–1.20 (m, 3H), 1.10–1.08 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 170.8, 150.5 (d, ¹J_C–F = 265.7 Hz), 143.9 (d, ⁴J_C–F = 3.7 Hz), 136.0, 135.4, 130.4, 130.1, 129.8, 125.8, 125.7, 124.7 (d, ¹J_C–F = 6.3 Hz), 122.6, 113.8 (d, ⁴J_C–F = 5.0 Hz), 93.1 (d, ²J_C–F = 17.6 Hz), 77.6, 60.9 (d, ⁵J_C–F = 2.1 Hz), 31.1, 30.7, 24.1, 24.0, 21.1; HRMS (ES⁺) calculated for C₂₁H₂₂⁷⁹BrN₃O₂ [M + H]⁺: 446.0874, found: 446.0875.

General procedure for the preparation of imidazo[1,2-a]pyridines 7b–c, 7e, 18 and ring-closed products 15, 17a–c

An appropriate aminocyclohexyl acetate derivative (14 or 16) and powdered KOH (4 eq.) were stirred in MeOH or tert-butanol (3–5 ml) at room temperature for 4 h. After removing solvent in vacuo, the crude residue was diluted with DCM (15–20 ml) and then washed successively with distilled water (2 × 10 ml) and saturated brine solution (10 ml). After drying over Na₂SO₄, the solvent was removed in vacuo and the crude mixture was purified by silica gel flash column chromatography, eluting the title compound with 25–50% EtOAc/hexane.

Synthesis of 2-((5-chloro-2-(2-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-7b). Compound rac-7b (72 mg, 83%) obtained as a yellow solid from the hydrolysis of acetate rac-14b (100 mg, 0.24 mmol) in methanol. IR (cm⁻¹): 3423 (OH str.), 3340 (NH str.), 2943 (CH str.), 1651 (NH bend), 1442 (C=C); Mp: 183–185 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.60–7.56 (m, 1H), 7.53–7.48 (m, 2H), 7.38–7.34 (m, 2H), 7.05 (dd, J = 8.9, 7.2 Hz, 1H), 6.79 (dd, J = 7.2, 1.1 Hz, 1H), 3.85–3.78 (m, 1H), 3.27–3.21 (m, 1H), 2.70–2.63 (m, 1H), 2.43–2.38 (m, 1H), 1.92–1.86 (m, 1H), 1.61–1.54 (m, 2H), 1.50–1.44 (m, 1H), 1.16–1.07 (m, 2H), 1.11–0.89 (m, 1H), 0.78–0.71 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 143.9, 138.1, 133.6, 133.5, 132.6, 129.8, 129.6, 127.8, 126.8, 126.0, 123.7, 117.0, 114.2, 74.6, 65.3, 33.4, 30.3, 24.4, 24.0; HRMS (ES⁺) calculated for C₁₉H₂₀N₃OCl₂ [M + H]⁺: 376.0983, found: 376.0974.

Synthesis of 2-((5-bromo-2-(2-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-7c). Compound rac-7c (60 mg, 83%) obtained as a yellow solid from the hydrolysis of acetate rac-14c (80 mg, 0.204 mmol) in methanol. Mp: 156–158 °C; IR (cm⁻¹): 3431 (OH str.), 3328 (NH str.), 2940 (CH str.), 1656 (NH bend), 1437 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 7.60–7.54 (m, 2H), 7.51–7.47 (m, 1H), 7.37–7.33 (m, 2H), 7.02–6.95 (m, 2H), 3.90 (s, 1H), 3.27–3.19 (m, 1H), 2.70–2.60 (m, 1H), 2.33 (s, 1H), 1.90–1.80 (m, 1H), 1.57–1.44 (m, 3H), 1.15–1.06 (m, 2H), 0.98–0.90 (m, 1H), 0.85–0.76 (m, 1H); ¹³C NMR (126 MHz, CDCl₃): δ 144.0, 138.5, 133.7, 133.5, 132.5, 129.8, 129.6, 128.0, 126.8, 124.0, 118.8, 117.5, 112.2, 74.7, 64.7, 33.4, 30.1, 24.4, 24.1; HRMS (ES⁺) calculated for C₁₉H₂₀BrClN₃O [M + H]⁺: 420.0473, found: 420.0465.

Synthesis of 2-(2-chlorophenyl)-3-((2-hydroxycyclohexyl)amino)imidazo[1,2-a]pyridine-5-carboxamide (rac-7e). Compound rac-7e (65 mg, 68%) obtained as a yellow solid from the hydrolysis of acetate rac-14d (100 mg, 0.25 mmol) in methanol. Mp: 156–158 °C; IR (cm⁻¹): 3431 (OH str.), 3328 (NH str.), 2940 (CH str.), 1656 (NH bend), 1437 (C=C); ¹H NMR (400 MHz, DMSO-d₆) δ 8.46 (s, 1H), 7.97 (s, 1H), 7.62–7.52 (m, 3H), 7.44–7.39 (m, 2H), 7.23–7.18 (m, 1H), 7.11 (d, J = 6.8 Hz, 1H), 4.36 (d, J = 4.7 Hz, 1H), 4.24 (d, J = 3.2 Hz, 1H), 3.13–3.05 (m, 1H), 1.69–1.60 (m, 1H), 1.48–1.37 (m, 1H), 1.33–1.21 (m, 2H), 1.09–0.92 (m, 2H), 0.79–0.57 (m, 2H); ¹³C NMR (101 MHz, DMSO) δ 165.6, 140.7, 134.4, 133.2, 132.6, 131.8, 129.3, 129.2, 127.9, 126.6, 121.7, 118.8, 113.6, 71.9, 59.3, 32.6, 29.0, 23.0, 22.8.; HRMS (ES⁺) calculated for C₂₀H₂₂N₄O₂Cl [M + H]⁺: 385.1431, found: 385.1418.

Synthesis of 2-((2-(2-chlorophenyl)-5-methoxyimidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-7f). Both compound rac-7f (30 mg, 40%) and rac-15 (50%), which were readily separated by silica gel column chromatography, were obtained from the hydrolysis of rac-14a (80 mg, 0.21 mmol) in methanol. Rac-7f: ¹H NMR (500 MHz, CDCl₃) δ 7.58–7.56 (m, 1H), 7.48–7.46 (m, 1H), 7.35–7.30 (m, 2H), 7.17 (d, J = 8.9 Hz, 1H), 7.05–7.3 (m, 1H), 5.97 (d, J = 7.4 Hz, 1H), 4.04 (s, 3H), 3.85–3.56 (m, 1H), 3.40 (s, 1H), 3.26–3.18 (m, 1H), 2.58–2.51 (m, 1H), 2.00–1.92 (m, 1H), 1.67–1.56 (m, 2H), 1.51–1.44 (m, 1H), 1.18–1.01 (m, 2H), 0.99–0.89 (m, 1H), 0.71–0.62 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 151.8, 143.4, 133.9, 133.8, 133.4, 132.6, 129.7, 129.4, 127.6, 126.8, 124.6, 110.8, 88.4, 74.0, 66.0, 56.3, 33.1, 31.2, 24.8, 24.0; HRMS (ES⁺) calculated for C₂₁H₂₃N₃O₂Cl [M + H]⁺: 372.1479, found: 372.1466.

Synthesis of 1-(2-chlorophenyl)-7,8,9,10,10a,11-hexahydro-6aH-6-oxa-2,2a1,11-triazadibenzo[cd,g]azulene (rac-15). Compound rac-15 (30 mg, 60%) was obtained as an orange oil from the hydrolysis of rac-14a (80 mg, 0.21 mmol) in tert-butanol. IR (cm⁻¹): 3301 (N–H), 3067 (=C–H), 2937 (C–H), 1638 (C=N); ¹H NMR (500 MHz, CDCl₃) δ 7.67 (d, J = 7.6 Hz, 1H), 7.46 (d, J = 8.0 Hz, 1H), 7.35 (t, J = 7.5 Hz, 1H), 7.30–7.26 (m, 1H), 7.17 (d, J = 9.0 Hz, 1H), 6.92 (t, J = 8.1 Hz, 1H), 6.17 (d, J = 7.2 Hz, 1H), 4.13–4.07 (m, 1H), 3.84 (s, 1H), 3.42–3.35 (m, 1H), 2.29–2.23 (m, 1H), 2.06–2.00 (m, 1H), 1.87–1.81 (m, 1H), 1.76–1.62 (m, 2H), 1.50–1.40 (m, 1H), 1.39–1.23 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 147.6, 142.6, 133.4, 133.0, 132.4, 129.8, 128.8, 128.2, 127.1, 124.3, 123.6, 111.7, 96.6, 88.4, 60.4, 32.8, 31.6, 23.9, 23.6 ppm; HRMS (ES⁺) calculated for C₁₉H₁₉N₃OCl [M + H]⁺: 340.1217, found: 340.1205.

Synthesis of 1-(4-nitrophenyl)-7,8,9,10-tetrahydro-6aH-6-oxa-2,2a1,11-triazadibenzo[cd,g]azulene (17a). Compound 17a (46 mg, 53%) was obtained as a dark brown solid and a co-product with 18 (47%) from the hydrolysis of 2-((5-fluoro-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate rac-16a (105 mg, 0.25 mmol) in methanol. Mp: 188–189 °C; IR (cm⁻¹): 3082 and 2946 (CH str.), 1655 (C=N), 1445 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.67–8.65 (m, 2H), 8.31–8.28 (m, 2H), 7.35 (dd, J = 8.9, 1.0 Hz, 1H), 7.25–7.22 (m, 1H), 6.40 (dd, J = 7.3, 1.0 Hz, 1H), 4.72 (dd, J = 9.4, 6.2 Hz, 1H), 2.96–2.90 (m, 1H), 2.69–2.61 (m, 1H), 2.46–2.41 (m, 1H), 2.06–1.94 (m, 3H), 1.85–1.76 (m, 1H), 1.73–1.63 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 163.5, 148.7, 147.0, 145.8, 140.4, 138.7, 129.1, 128.0, 127.1, 123.4, 112.3, 98.7, 83.8, 36.6, 31.6, 23.3, 20.9 ppm; HRMS (ES⁺) calculated for C₁₉H₁₇N₄O₃ [M + H]⁺: 349.1295, found: 349.1287.

Synthesis of 1-(3-nitrophenyl)-7,8,9,10-tetrahydro-6aH-6-oxa-2,2a1,11-triazadibenzo[cd,g]azulene (17b). Compound 17b (45 mg, 65%) was obtained as a yellow solid from the hydrolysis of rac-16b (85 mg, 0.20 mmol) in tert-butanol. Mp: 164–166 °C; IR (cm⁻¹): 3080 and 2952 (CH str.), 1653 (C=N), 1450 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 9.43 (t, J = 2.0 Hz, 1H), 8.75 (d, J = 7.8 Hz, 1H), 8.17 (dd, J = 8.2, 2.4 Hz, 1H), 7.59 (t, J = 8.9, 1H), 7.34 (d, J = 8.9 Hz, 1H), 7.22 (dd, J = 8.9, 7.3 Hz, 1H), 6.39 (d, J = 7.3 Hz, 1H), 4.73–4.69 (m, 1H), 2.96–2.91 (m, 1H), 2.69–2.62 (m, 1H), 2.44–2.41 (m, 1H), 2.05–1.94 (m, 3H), 1.85–1.75 (m, 1H), 1.71–1.64 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 162.9, 148.6, 148.4, 145.6, 138.6, 135.6, 134.1, 129.0, 127.8, 126.4, 123.7, 122.4, 112.1, 98.7, 83.8, 36.6, 31.7, 23.4, 21.0 ppm; HRMS (ES⁺) calculated for C₁₉H₁₇N₄O₃ [M + H]⁺: 349.1295, found: 349.1290.

Synthesis of 3-(7,8,9,10-tetrahydro-6aH-6-oxa-2,2a1,11-triazadibenzo[cd,g]azulen-1-yl)benzonitrile (17c). Hydrolysis of rac-16c (100 mg, 0.25 mmol) with KOH (4 eq.) in tert-butanol gave product 17c (60 mg, 71%) as a brown solid. Mp: 130–132 °C; IR (cm⁻¹): 3078 and 2955 (CH str.), 2225 (CN str.), 1653 (C=N), 1445 (C=C); ¹H NMR (400 MHz, CDCl₃) δ 8.81 (t, J = 1.7 Hz, 1H), 8.66 (dt, J = 8.0, 1.5 Hz, 1H), 7.60 (dt, J = 7.7, 1.5 Hz, 1H), 7.53 (t, J = 7.8 Hz, 1H), 7.32 (d, J = 8.9 Hz, 1H), 7.22 (dd, J = 8.9, 7.3 Hz, 1H), 6.38 (d, J = 7.3 Hz, 1H), 4.71–4.68 (m, 1H), 2.94–2.88 (m, 1H), 2.65–2.61 (m, 1H), 2.42–2.40 (m, 1H), 2.01–1.93 (m, 3H), 1.80–1.78 (m, 1H), 1.71–1.64 (m, 1H); ¹³C NMR (101 MHz, CDCl₃) δ 162.8, 148.6, 145.6, 138.8, 135.1, 132.6, 132.4, 131.0, 128.9, 127.8, 126.3, 119.2, 112.3, 112.1, 98.6, 83.8, 36.5, 31.6, 23.3, 20.9 ppm; HRMS (ES⁺) calculated for C₂₀H₁₇N₄O [M + H]⁺: 329.1397, found: 329.1394.

Synthesis of 2-((5-methoxy-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-18). Compound rac-18 (46 mg, 47%) was obtained as an orange solid co-product with 17a from the hydrolysis of 2-((5-fluoro-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexyl acetate rac-16a (105 mg, 0.25 mmol) in methanol. Mp: 179–180 °C; IR (cm⁻¹): 3401 (OH str.), 3343 (NH str.), 3079 and 2948 (CH str.), 1648 (C=N), 1559 (NH bend), 1448 (C=C); ¹H NMR (400 MHz, CDCl₃) δ 8.48 (d, J = 8.6 Hz, 2H), 8.24 (d, J = 8.6 Hz, 2H), 7.19–7.07 (m, 2H), 5.97 (d, J = 7.2 Hz, 1H), 4.49 (br s, 1H), 4.07 (s, 3H), 3.54–3.49 (m, 1H), 2.79–2.74 (m, 1H), 2.04–1.95 (m, 1H), 1.70–1.61 (m, 1H), 1.56–1.47 (m, 2H), 1.28–1.17 (m, 3H), 1.04–0.93 (m, 2H); ¹³C NMR (101 MHz, CDCl₃) δ 152.0, 146.3, 144.0, 141.5, 133.6, 128.2, 127.8, 125.9, 123.5, 110.7, 88.6, 75.2, 65.5, 56.4, 34.2, 30.1, 24.4, 24.3; HRMS (ES⁺) calculated for C₂₀H₂₃N₄O₄ [M + H]⁺: 383.1714, found: 383.1704.

General procedure for the preparation of imidazo[1,2-a]pyridines 20a–g

To a chilled solution of 2-trans-hydroxycyclohexyl formamide rac-9 (287 mg, 2 mmol) (prepared as described previously) in freshly distilled DCM (10–15 ml) was added a stirrer bar, dried p-TsCl (382 mg, 2 mmol) and DABCO (225 mg, 2 mmol). The sealed reaction mixture was stirred under chilled conditions for 1 h and allowed to warm to room temperature with stirring for a further 3 h. Thereafter, an appropriate aromatic aldehyde 12 (2 mmol), 2-amino-2-fluoropyridine 13a (2 mmol) and anhydrous Na₂SO₄ (282 mg, 2 mmol) were added. The closed reaction mixture was heated at 50–60 °C in an oil bath for 10–12 h and later cooled to room temperature. Thereafter, the mixture was washed consecutively with distilled water (2 × 15 ml) and saturated brine solution (10 ml). After drying with anhydrous Na₂SO₄, the organic was evaporated in vacuo to leave a crude residue which was purified on flash silica gel by eluting with 25–80% EtOAc/Hex to afford title compounds 20a–g.

Synthesis of 4-(5-fluoro-3-((-2-hydroxycyclohexyl)amino)imidazo[1,2-a]pyridin-2-yl)benzonitrile (rac-20a). 4-Cyanobenzaldehyde (303 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 were reacted to give product rac-20a (407 mg, 58% overall yield from rac-8) as a brown solid. Mp: 102–103 °C IR (cm⁻¹): 3305 (NH str.), 3016 and 2933 (CH str.), 2224 (CN str.), 1652 (C=N), 1555 (NH bend), 1434 (C=C); ¹H NMR (300 MHz, CDCl₃) δ 8.49–8.41 (m, 2H), 7.71–7.66 (m, 2H), 7.39 (d, J = 9.0 Hz, 1H), 7.20–7.13 (m, 1H), 6.41 (t, J = 7.2 Hz, 1H), 3.97 (br. s, 1H), 3.57–3.53 (m, 1H), 2.84–2.81 (m, 1H), 2.07–1.93 (m, 2H), 1.69–1.65 (m, 1H), 1.59–1.50 (m, 2H), 1.31–1.20 (m, 2H), 1.07–0.97 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 150.6 (d, ¹J_C–F = 267.1 Hz), 144.1 (d, ⁴J_CF = 2.5 Hz), 138.3, 135.9, 132.1, 128.5, 128.0, 125.6 (d, ³J_CF = 12.0 Hz), 119.2, 113.9 (d, ³J_C–F = 5.0 Hz), 110.8, 93.5 (d, ²J_C–F = 17.6 Hz), 75.3, 65.0 (d, ⁵J_C–F = 2.6 Hz), 34.4, 30.1, 24.4, 24.2 ppm; HRMS (ES⁺) calculated for C₂₀H₂₀FN₄O [M + H]⁺: 351.1616, found: 351.1617.

Synthesis of 2-((5-fluoro-2-(2-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-20b). Reaction of 2-fluorobenzaldehyde (245 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 gave title compound rac-20b (430 mg, 61% overall yield from rac-8) as a brown solid. Mp: 146–147 °C; IR (cm⁻¹): 3389 (NH str.), 3078, 2940 and 2861 (CH str.), 1651 (C=N), 1504 (NH bend), 1430 (C=C); ¹H NMR (300 MHz, CDCl₃) δ 7.79 (td, J = 7.6, 1.8 Hz, 1H), 7.43–7.34 (m, 2H), 7.30–7.25 (m, 1H), 7.20–7.09 (m, 2H), 6.40 (t, J = 7.3 Hz, 1H), 3.58 (br. s, 1H), 3.37–3.31 (m, 1H), 3.18 (br s, 1H), 2.69–2.63 (m, 1H), 2.02–1.98 (m, 1H), 1.63–1.58 (m, 2H), 1.51–1.44 (m, 1H), 1.27–1.12 (m, 2H), 1.03–0.93 (m, 1H), 0.79–0.65 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 159.6 (d, ¹J_C–F = 245.7 Hz), 150.5 (d, ¹J_C–F = 268.4 Hz), 144.2 (d, ⁴J_C–F = 3.8 Hz), 133.2, 131.9 (d, ⁴J_C–F = 3.8 Hz), 129.9 (d, ³J_C–F = 7.6 Hz), 126.5 (d, ⁴J_C–F = 2.5 Hz), 124.73, 124.69 (d, ⁴J_C–F = 3.2 Hz), 121.6 (d, ³J_C–F = 15.1 Hz), 115.9 (d, ¹J_C–F = 22.7 Hz), 113.7 (d, ³J_C–F = 5.0 Hz), 93.4 (d, ²J_C–F = 17.6 Hz), 74.2, 65.5, 33.2, 30.9, 24.6, 24.0 ppm; HRMS (ES⁺) calculated for C₁₉H₂₀F₂N₃O [M + H]⁺: 344.1582, found: 344.1569.

Synthesis of 2-((2-(4-chloro-3-fluorophenyl)-5-fluoroimidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-20c). 4-Chloro-3-fluorobenzaldehyde (318 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 were reacted to give product rac-20c (423 mg, 56% overall yield from rac-8) as a brown solid. Mp: 105–107 °C; IR (cm⁻¹): 3400 (OH str.), 3328 (NH str.), 2932 and 2861 (CH str.), 1653 (C=N), 1512 (NH bend), 1435 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.19 (dd, J = 8.3, 1.9 Hz, 1H), 8.04 (dd, J = 8.3, 2.0 Hz, 1H), 7.41 (t, J = 8.1 Hz, 1H), 7.35 (d, J = 8.9 Hz, 1H), 7.15–7.10 (m, 1H), 6.38 (t, J = 7.3 Hz, 1H), 3.91 (s, 1H), 3.58–3.53 (m, 1H), 2.87–2.81 (m, 1H), 2.07–1.99 (m, 2H), 1.69–1.65 (m, 1H), 1.59–1.52 (m, 2H), 1.29–1.24 (m, 2H), 1.06–1.02 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 157.5 (d, ¹J_C–F = 247 Hz), 150.5 (d, ¹J_C–F = 265.9 Hz), 144.1 (d, ⁴J_C–F = 3.8 Hz), 136.4 (d, ³J_C–F = 5.0 Hz), 134.6 (d, ³J_C–F = 7.6 Hz), 130.3, 125.2 (d, ³J_C–F = 10.1 Hz), 124.5 (d, ⁴J_C–F = 1.3 Hz), 123.9 (d, ⁴J_C–F = 3.8 Hz), 119.4 (d, ²J_C–F = 17.6 Hz), 115.6 (d, ²J_C–F = 22.7 Hz), 113.8 (d, ³J_C–F = 5.0 Hz), 93.2 (d, ²J_C–F = 17.6 Hz), 75.2, 64.9 (d, ⁵J_C–F = 2.7 Hz), 34.4, 29.9, 24.8, 24.4, 24.2 ppm; HRMS (ES⁺) calculated for C₁₉H₁₉ClF₂N₃O [M + H]⁺: 378.1179, found: 378.1167.

Synthesis of 2-((5-fluoro-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-20d). Reaction of 4-nitrobenzaldehyde (303 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 generated product rac-20d (460 mg, 62% overall yield from rac-8) as a brown solid. Mp: 218–220 °C; IR (cm⁻¹): 3361 (OH str.), 3196 (NH str.), 2936 and 2858 (CH str.), 1652 (C=N), 1515 (NH bend), 1439 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 8.55–8.51 (m, 2H), 8.29–8.26 (m, 2H), 7.38 (d, J = 9.0 Hz, 1H), 7.18–7.13 (m, 1H), 6.41 (td, J = 7.3, 0.9 Hz, 1H), 4.01 (s, 1H), 3.60–3.57 (m, 1H), 2.88–2.82 (m, 1H), 2.05–1.99 (m, 1H), 1.80 (s, 1H), 1.70–1.49 (m, 3H), 1.32–1.22 (m, 2H), 1.09–0.98 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 150.6 (d, ¹J_C–F = 265.9 Hz), 146.8, 144.4 (d, ⁴J_C–F = 3.8 Hz), 140.7, 136.0, 128.1, 126.0 (d, ⁴J_C–F = 2.5 Hz), 125.5 (d, ³J_C–F = 7.6 Hz), 123.6, 114.1 (d, ³J_C–F = 5.0 Hz), 93.4 (d, ²J_C–F = 17.6 Hz), 75.3, 65.0 (d, ⁵J_C–F = 2.5 Hz), 34.5, 30.0, 24.4, 24.2 ppm; HRMS (ES⁺) calculated for C₁₉H₂₀FN₄O₃ [M + H]⁺: 371.1514, found: 371.1510.

Synthesis of 3-(5-fluoro-3-((2-hydroxycyclohexyl)amino)imidazo[1,2-a]pyridin-2-yl)benzonitrile (rac-20e). 3-Cyanobenzaldehyde (263 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 were reacted and gave product rac-20e (365 mg, 52% overall yield from rac-8) as a brown solid). Mp: 120–121 °C; IR (cm⁻¹): 3328 (overlapping br. OH and NH str.), 2931 and 2858 (CH str.), 2229 (CN str.), 1652 (C=N), 1516 (NH bend), 1446 (C=C¹H NMR (500 MHz, CDCl₃) δ 8.68–8.66 (m, 1H), 8.55 (dt, J = 8.1, 1.5 Hz, 1H), 7.55 (dt, J = 7.8, 1.5 Hz, 1H), 7.49 (t, J = 7.8, Hz, 1H), 7.39 (d, J = 9.0 Hz, 1H), 7.19–7.15 (m, 1H), 6.42 (t, J = 7.1 Hz, 1H), 4.04 (s, 1H), 3.58–3.52 (m, 1H), 2.85–2.81 (m, 1H), 2.05–1.90 (m, 2H), 1.69–1.63 (m, 1H), 1.57–1.47 (m, 2H), 1.31–1.21 (m, 2H), 1.04–0.97 (m, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 150.6 (d, ¹J_C–F = 267.1 Hz), 143.9 (d, ⁴J_C–F = 3.8 Hz), 135.4, 134.8, 131.8, 131.2, 130.9, 129.1, 126.0 (d, ³J_C–F = 6.3 Hz), 125.1 (d, ³J_C–F = 6.3 Hz), 119.0, 113.5 (d, ⁴J_C–F = 5.0 Hz), 112.3, 93.7 (d, ²J_C–F = 17.6 Hz), 75.0, 64.8 (d, ⁵J_C–F = 2.6 Hz), 34.4, 30.0, 24.4, 24.2 ppm; HRMS (ES⁺) calculated for C₂₀H₂₀FN₄O [M + H]⁺: 351.1616, found: 351.1615.

Synthesis of 2-(5-fluoro-3-((2-hydroxycyclohexyl)amino)imidazo[1,2-a]pyridin-2-yl)benzonitrile (rac-20f). 2-Cyanobenzaldehyde (263 mg, 2.0 mmol), 2-amino-6-fluoropyridine 13a (225 mg, 2.0 mmol) and the in situ generated isocyanide rac-19 were reacted and generated title compound rac-20f (386 mg, 55% overall yield from rac-8) as a brown solid. Mp: 152–153 °C; IR (cm⁻¹): 3328 (OH and NH br. str.), 2931 and 2851 (CH str.), 2225 (CN str.), 1652 (C=N), 1446 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 7.87 (dd, J = 7.9, 1.2 Hz, 1H), 7.77 (dd, J = 7.8, 1.5 Hz, 1H), 7.67 (td, J = 7.7, 1.4 Hz, 1H), 7.47 (td, J = 7.7, 1.3 Hz, 1H), 7.39 (dd, J = 9.0, 0.9 Hz, 1H), 7.18–7.13 (m, 1H), 6.42 (t, J = 7.3 Hz, 1H), 3.83–3.77 (m, 1H), 3.34–3.30 (m, 1H), 2.78 (br s, 1H), 2.70–2.61 (m, 1H), 1.92–1.88 (m, 1H), 1.61–1.55 (m, 1H), 1.54–1.42 (m, 2H), 1.19–1.10 (m, 2H), 1.00–0.93 (m, 1H), 0.77–0.68 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 150.5 (d, ¹J_C–F = 267.1 Hz), 144.3 (d, ⁴J_C–F = 3.8 Hz), 137.7, 136.7, 133.4, 132.6, 131.1, 128.2, 125.7 (d, ⁴J_C–F = 2.5 Hz), 125.3 (d, ³J_C–F = 6.3 Hz), 119.1, 114.1 (d, ³J_C–F = 5.0 Hz), 112.4, 93.5 (d, ²J_C–F = 17.6 Hz), 74.3, 65.0 (d, ⁵J_C–F = 1.9 Hz), 33.5, 30.3, 24.2, 24.1 ppm; HRMS (ES⁺) calculated for C₂₀H₂₀FN₄O [M + H]⁺: 351.1616, found: 351.1615.

Synthesis of 2-((2-(2-chlorophenyl)-5-fluoroimidazo[1,2-a]pyridin-3-yl)amino)cyclohexanol (rac-20g). Reaction of 2-chlorobenzaldehyde (210 mg, 1.0 mmol), 2-amino-6-fluoropyridine 13a (113 mg, 1.0 mmol) and the in situ generated isocyanide rac-19 gave product rac-20g (212 mg, 59% overall yield from rac-8) as a brown solid. Mp: 149–151 °C; IR (cm⁻¹): 3400 (OH br. str.), 3221 (NH str.), 2928 and 2857 (CH str.), 1650 (C=N), 1432 (C=C); ¹H NMR (500 MHz, CDCl₃) δ 7.62–7.57 (m, 1H), 7.52–7.47 (m, 1H), 7.44–7.34 (m, 3H), 7.17–7.14 (m, 1H), 6.43 (t, J = 7.3, 1H), 3.53–3.36 (m, 1H), 3.30–3.25 (m, 1H), 3.08–2.71 (m, 1H), 2.66–2.59 (m, 1H), 2.00–1.94 (m, 1H), 1.66–1.58 (m, 2H), 1.53–1.44 (m, 1H), 1.20–1.10 (m, 2H), 1.02–0.93 (m, 1H), 0.73–0.64 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 150.5 (d, ¹J_C–F = 267.2 Hz), 143.6 (d, ⁴J_C–F = 3.2 Hz), 135.7, 133.2, 132.7, 132.6, 129.8, 129.7, 127.0, 126.2 (d, ⁴J_C–F = 2.9 Hz), 124.8 (d, ³J_C–F = 6.8 Hz), 113.8 (d, ³J_C–F = 5.0 Hz), 93.6 (d, ²J_C–F = 17.7 Hz), 74.1, 65.4, 33.2, 31.0, 24.6, 24.0 ppm; HRMS (ES⁺) calculated for C₁₉H₂₀ClFN₃O [M + H]⁺: 360.1273, found: 360.1272.

Crystallography

Crystal structure and refinement. Intensity data for 17a and 17c were collected on a Bruker Apex-II CCD area detector diffractometer with graphite monochromated Mo Kα radiation (50 kV, 30 mA). The collection method involved ω- and φ-scans of width 0.5° and 1024 × 1024 bit data frames. Using Olex2,²² the crystal structures were solved by with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least Squares minimisation.^23,24 Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix least-squares calculations based on F².

Crystal data for 17a: C₁₉H₁₆N₄O₃ (M =348.36 g mol⁻¹): monoclinic, space group C2/c (no. 15), a = 20.8879(15) Å, b = 7.2791(5) Å, c = 21.5015(15) Å, β = 96.086(5)°, V = 3250.8(4) ų, Z = 8, T = 173.15 K, μ(MoKα) = 0.100 mm⁻¹, D_calc = 1.424 g cm⁻³, 10 492 reflections measured (3.81° ≤ 2θ ≤ 49.998°), 2857 unique (R_int = 0.1096, R_sigma = 0.1327) which were used in all calculations. The final R₁ was 0.0444 (I > 2σ(I)) and wR₂ was 0.0847 (all data). CCDC 1970362.

Crystal data for 17c: C₂₀H₁₆N₄O (M =328.37 g mol⁻¹): monoclinic, space group P2₁/n (no. 14), a = 12.0432(7) Å, b = 7.3016(4) Å, c = 18.3346(12) Å, β = 96.116(4)°, V = 1603.07(17) ų, Z = 4, T = 173.15 K, μ(MoKα) = 0.088 mm⁻¹, D_calc = 1.361 g cm⁻³, 11 067 reflections measured (3.866° ≤ 2θ ≤ 49.98°), 2831 unique (R_int = 0.1479, R_sigma = 0.2114) which were used in all calculations. The final R₁ was 0.0410 (I > 2σ(I)) and wR₂ was 0.0687 (all data). CCDC 1970363.

Conclusions

In summary, we have successfully developed a methodology for the synthesis of novel tetracyclic derivatives through intramolecular nucleophilic aromatic substitution of fluorine at position 5 of the imidazo[1,2-a]pyridine ring. In addition, we have demonstrated the improved utility, convenience and practicality of the pTsCl/DABCO-catalysed isocyanide in situ generation protocol over the conventional POCl₃/Et₃N protocol. Furthermore, we have managed to improve and simplify the existing pTsCl/DABCO N-formamide dehydration methodology to such an extent that it can be employed efficiently and cheaply in the absence of any preliminary purging of the reaction with nitrogen. Of particular importance is that it has also proved to be effective for preparing functionalised isocyanides in situ without the requirement for protection of reactive hydroxyl groups.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank the National Research Foundation (NRF) and the Medical Research Council (MRC) of South Africa and the University of the Witwatersrand through the School of Chemistry and Research Office for their support and financial assistance. The assistance of Mr Thapelo Mbhele with the acquisition of HRMS data is gratefully acknowledged.

Notes and references

1. (a) A. Boltjes and A. Dömling, Eur. J. Org. Chem., 2019, 7007; (b) J. Koubachi, S. El Kazzouli, M. Bousmina and G. Guillaumet, Eur. J. Org. Chem., 2014, 5119.
2. L. Dyminska, Bioorg. Med. Chem., 2015, 23, 6087.
3. T. O'Malley, T. Alling, J. V. Early, H. A. Wescott, A. Kumar, G. C. Moraski, M. J. Miller, T. Masquelin, P. A. Hipskind and T. Parisha, Antimicrob. Agents Chemother., 2018, 62, e02439-17.
4. A. M. Palmer, S. Chrismann, G. Münch, C. Brehm, P. J. Zimmermann, W. Buhr, J. Senn-Bilfinger, M. P. Feth and W. A. Simon, Bioorg. Med. Chem., 2009, 17, 368.
5. F. Göktaş, N. Cesur, D. Şatana and M. Uzun, Turk. J. Chem., 2014, 38, 581.
6. N. M. Shukla, D. B. Salunke, E. Yoo, C. A. Mutz, R. Balakrishna and S. A. David, Bioorg. Med. Chem., 2012, 20, 5850.
7. K. B. Puttaraju and K. Shivashankar, RSC Adv., 2013, 3, 20883.
8. A. K. Bagdi, S. Santra, K. Monir and A. Hajra, Chem. Commun., 2015, 51, 1555.
9. (a) K. Groebke, L. Weber and F. Mehlin, Synlett, 1998, 661; (b) C. Blackburn, B. Guan, P. Fleming, K. Shiosaki and S. Tsai, Tetrahedron Lett., 1998, 39, 3635; (c) H. Bienaymé and K. Bouzid, Angew. Chem., Int. Ed., 1998, 37, 2234.
10. (a) M. L. Bode, D. Gravestock and A. L. Rousseau, Org. Prep. Proced. Int., 2016, 48, 89; (b) A. L. Rousseau, P. Matlaba and C. J. Parkinson, Tetrahedron Lett., 2007, 48, 4079.
11. (a) A. Gautier, Liebigs Ann. Chem., 1867, 142, 289; (b) A. W. Hofmann, Liebigs Ann. Chem., 1867, 144, 114.
12. P. Doz, I. Ugi, U. Fetzer, U. Eholzer, H. Knupfer and K. Offermann, Angew. Chem., Int. Ed., 1965, 4, 472.
13. G. Skorna and I. Ugi, Angew. Chem., Int. Ed., 1977, 16, 259.
14. I. Ugi and R. Meyr, Angew. Chem., 1958, 70, 702.
15. X. Wang, Q.-G. Wang and Q.-L. Luo, Synthesis, 2015, 47, 49.
16. S. K. Guchhait, G. Priyadarshani, V. Chaudhaury, D. S. Seladiya, T. M. Shah and N. P. Bhogayta, RSC Adv., 2013, 3, 10867.
17. M. L. Bode, D. Gravestock, S. S. Moleele, C. W. van der Westhuyzen, S. C. Pelly, P. A. Steenkamp, H. C. Hoppe, T. Khan and L. A. Nkabinde, Bioorg. Med. Chem., 2011, 19, 4227.
18. N. Grenouillat, B. Vauzeilles and J.-M. Beau, Heterocycles, 2007, 73, 891.
19. D. T. Whitaker, K. S. Whitaker and C. R. Johnson, p-Toluenesulfonyl Chloride, in Encyclopedia of Reagents for Organic Synthesis, John Wiley and sons, 2001.
20. (a) P. G. Gassman and T. L. Guggenheim, J. Am. Chem. Soc., 1982, 104, 5849; (b) P. G. Gassman and T. L. Guggenheim, Org. Synth., 1990, 7, 294.
21. S. M. Creedon, H. K. Crowley and D. G. McCarthy, J. Chem. Soc., Perkin Trans. 1, 1998, 1015.
22. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
23. G. M. Sheldrick, Acta Crystallogr., 2015, A71, 3–8.
24. G. M. Sheldrick, Acta Crystallogr., 2015, C71, 3–8.

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Footnote

† Electronic supplementary information (ESI) available: Copies of all NMR spectra and crystallographic data. CCDC 1970362 and 1970363. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ra10447j

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