Acid/base-controlled chemodivergent synthesis of two differently functionalized tetrahydroimidazo[1,2-a]pyridines

Abstract

A new approach for the facile chemodivergent synthesis of tetrahydroimidazo[1,2-a]pyridines from β-ketothioamides, aldehydes and heterocyclic ketene aminals is described. In the presence of a base, an unprecedented ring-opening, tautomerization, recyclization cascade is observed and H₂S eliminated tetrahydroimidazo[1,2-a]pyridines were formed in good yields. With catalytic amounts of acid, another type of different substituted tetrahydroimidazo[1,2-a]pyridines were obtained through dehydration.

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Imidazo[1,2-a]pyridine is an important heterocyclic skeleton which can be found in a broad spectrum of pharmaceuticals,¹ such as insecticide against pea aphids (I),² and the inhibitors of TNF-α expression in T cells (II) (Fig. 1).³ Therefore, a variety of methodologies have been developed for the construction of imidazo[1,2-a]pyridines.⁴ Recently, diverse metal-catalyzed methods for the construction of this privileged structure have been reported.⁵ Multicomponent reactions (MCRs) are an important strategy in the synthesis of heterocyclic scaffolds.⁶ The diversity and easy accessibility to a large number of compounds, make MCRs a very important tool in modern organic synthesis.⁷ However, there is only limited number of reports on imidazo[1,2-a]pyridines synthesis through multicomponent reactions under metal-free conditions.⁸

Fig. 1 Selected biologically active imidazo[1,2-a]pyridines.

β-Ketothioamides (KTAs)⁹ and heterocyclic ketene aminals (HKAs)¹⁰ are versatile building blocks for the rapid construction of various heterocyclic compounds.¹¹ The previous reports of KTAs participated reactions mainly involved four types (Fig. 2, modes A–D). In continuation of our interests in the utilization of HKAs and KTAs,¹² and based on our recent works in exploring metal-free MCRs,¹³ herein, we report a metal-free acid/base controlled chemodivergent synthesis of tetrahydroimidazo[1,2-a]pyridines by another reaction mode of KTAs (Fig. 2, mode E).

Fig. 2 Reaction modes of β-ketothioamides.

Initially, a mixture of KTA 1a, aldehyde 2a, and HKA 3a was stirred in refluxing toluene in the presence of Na₂CO₃ for 8 h, a yellow powder 5a was obtained in 28% yield after column chromatography (Table 1, entry 1). Then a series of other bases, including Et₃N, DABCO, DBU, CH₃ONa and K₂CO₃, were screened. Disappointedly, all these bases gave lower yields than Na₂CO₃ (entries 2–7). Subsequently, the model reaction was performed in various solvents. When the solvent was changed to methanol, another compound 4a was obtained besides 5a, albeit in low yield (entry 8). Interestingly, the selectivity of the reaction was changed from 5a to 4a without addition of any base (entry 9). Through carefully reaction conditions screening (entries 10–15), 4a was obtained in high yield in refluxing methanol (entry 10). Furthermore, we found that 4a could be transformed to 5a in high yield using Na₂CO₃ as a base in DMF. Fortunately, the structure of 4a was determined by single crystal X-ray analysis (see ESI†). Thus we reasoned that an interesting ring-opening, tautomerization, recyclization cascade could be involved in the transformation.

Table 1 Optimization of the reaction conditions^a

Entry	Base	Solvent	Temp (°C)	Time (h)	Yield^b (%)
					4a	5a
a The mixture of 1a (0.5 mmol), 2a (0.5 mmol), and 3a (0.5 mmol) was stirred in solvent at indicated temperature in the presence of air.b Isolated yield.
1	Na2CO3	Toluene	110	8	Trace	28
2	Et3N	Toluene	110	8	Trace	20
3	DABCO	Toluene	110	8	Trace	23
4	Pyridine	Toluene	110	8	Trace	18
5	DBU	Toluene	110	8	Trace	16
6	CH3ONa	Toluene	110	8	Trace	13
7	K2CO3	Toluene	110	8	Trace	21
8	Na2CO3	MeOH	65	8	23	15
9		MeOH	65	8	83	Trace
10		Toluene	110	8	30	Trace
11		MeCN	80	8	70	Trace
12		EtOH	80	8	80	Trace
13		Dioxane	100	8	50	Trace
14		MeOH	r.t.	20	Trace	Trace
15		MeOH	50	20	20	Trace

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Under the optimal conditions, the scope of the reaction was examined using a broad range of substituted arylaldehydes (Table 2). Products 5a–i were obtained in moderate to good yields regardless of electron-withdrawing or electron-donating groups on the benzaldehydes. Next, substituted aryl–HKAs were employed to provide the desired products 5j–o in moderate to good yields. Interestingly, tetrahydroimidazo[1,2-a]pyridines 5 were just obtained from ortho substituted aryl–HKAs. No desired products were formed when non-, meta- or para-substituted aryl–HKAs were used as reactants. The reason for this phenomenon is still unclear. The reaction also proceeded smoothly with three other substituted KTAs to afford the tetrahydroimidazo[1,2-a]pyridines 5p–r. The structures of 5 were confirmed by X-ray crystallographic analysis of 5c (see ESI†).

Table 2 Synthesis of tetrahydroimidazo[1,2-a]pyridines 5 under basic conditions^ab

a Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), 3 (0.5 mmol), CH₃OH (2 mL), 70 °C, 8 h. Na₂CO₃ (1.0 mmol), DMF (2 mL), 100 °C, 2 h.b Isolated yield.

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Under the conditions of catalytic sulfuric acid, another different substituted type of tetrahydroimidazo[1,2-a]pyridine 6a could be formed in high yield from intermediate 4a. The substrate scope of this reaction was also evaluated (Table 3). First, a variety of arylaldehydes 2 were reacted with HKA 1 and KTA 3, and products 6a–h were obtained in good yields. Subsequently, tetrahydroimidazo[1,2-a]pyridines 6i–m were formed in moderate to good yields with substituted aryl–HKAs. In addition, products 6n and 6o were also obtained when methyl–HKA and nitro–HKA were used as starting materials. Next, three substituted KTAs were employed with products 6q and 6r formed in good yields, whereas 6p was obtained in relatively low yield. The structures of 6 were confirmed by X-ray crystallographic analysis of 6h (see ESI†). Notably, the ¹H NMR spectra of some compounds 6 (such as 6a–g) are messy due to rotamers when an ortho-chlorine atom was performed on the aromatic ring of HKAs 3. The reason for this phenomenon was explained in detail in our previous report.^12a

Table 3 Synthesis of tetrahydroimidazo[1,2-a]pyridines 6 under acid conditions^ab

a Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), 3 (0.5 mmol), CH₃OH (2 mL), 70 °C, 8 h. Then 0.1 mmol H₂SO₄ 2 h.b Isolated yields.

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Based on the structure of the crucial intermediate 4, a plausible mechanism for the reactions is outlined in Scheme 1. Initially, the Knoevenagel adduct, which is readily formed in situ from aldehyde 2 and β-ketothioamide 3, reacts with HKA 1 via aza-ene process to form intermediate A. After imine–enamine tautomerization and intramolecular cyclization, the intermediate 4 is obtained. The formation of 4 is due to the α-carbon and the carbonyl group of KTAs participated in the reaction under neutral conditions. Notably, in this process, CH₃OH would play an important role in increasing the electrophilicity of the carbonyl group of KTAs by hydrogen bond-driven effect,¹⁴ which is preferentially attacked by nucleophilic reagent giving the ring-closing product 4. Subsequently, in the presence of sodium carbonate, the semi-aminal group would collapse to regenerate the ring-open intermediate B. Then the thiocarbonyl group would tautomerize to form the more stable conjugated thioenol intermediate C under basic conditions. Finally, the tetrahydroimidazo[1,2-a]pyridine 5 is afforded irreversibly through intramolecular cyclization with elimination of H₂S. Otherwise, in the presence of acid, the tetrahydroimidazo[1,2-a]pyridine 6 is obtained directly through dehydration.

Scheme 1 A plausible reaction mechanism.

In conclusion, a chemodivergent metal-free multicomponent synthesis of two differently functionalized tetrahydroimidazo[1,2-a]pyridine derivatives has been developed by adjusting reaction conditions. The ring-opening, tautomerization, recyclization cascade was also first observed in the study of KTAs reactions. The approach features high chemoselectivity, broad substrate scope, and mild reaction conditions, which provides a convenient method for construction of tetrahydroimidazo[1,2-a]pyridine derivatives from readily available starting materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21372137 and 21072110) and the Natural Science Foundation of Shandong Province (ZR2014BM006).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, full spectroscopic data for all new compounds, and crystal data for 4a, 5c and 6h (CIF). CCDC 858868, 878006 and 1003512. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13395e

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