Synthesis, structural properties, electrophilic substitution reactions and DFT computational studies of calix[3]benzofurans

Abstract

Calix[3]benzofurans have been synthesized by a modified TosMIC coupling reaction, followed by acid treatment and an intramolecular cyclization reaction with TMSI (trimethylsilyl iodide); X-ray analysis established the structures of two samples, both showing a cone conformation. ¹H NMR spectroscopic analyses of the calix[3]benzofurans reveal that they can adopt drastically different conformations in solution and undergo very fast conformational changes relative to the NMR time scale. Calix[3]benzofuran 4a exists as two conformers, namely the cone and saddle forms, in a ratio of 83 : 17 at −50 °C. A series of calix[3]benzofuran derivatives was synthesized by electrophilic aromatic substitutions, such as bromination, formylation and acylation, to investigate the influence of the substituents on the conformations of the calix[3]benzofurans. ¹H NMR spectral analyses of the acyl derivatives at room temperature indicated that these macrocycles exist as a mixture of two isomers that are slowly interconverted on the ¹H NMR timescale. The conformational isomers of the calix[3]benzofurans and their derivatives obtained from DFT methods (based on the crystal structure analysis results) were used to estimate the total energies of the different conformations.

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Introduction

The design and synthesis of medium and larger sized ring systems is an area of current interest in supramolecular chemistry.¹ In particular, macrocycles containing aromatic groups represent a vital class of synthetic receptors in molecular recognition due to the hydrophobicity and π-stacking interactions of their aromatic groups.² Calixarenes and their analogues are receiving considerable attention because of their design possibilities as host molecules in supramolecular chemistry.³ One of the characteristic properties of calixarenes is their conformational variety.⁴ By using this variety and design possibilities, calixarenes and their analogues can be useful as basic skeletons for molecular device units in molecular nanotechnology.⁵

Calix[3]benzofurans are calixarene derivatives in which the benzene rings have been replaced by benzofuran rings. Among the arenofurans, benzofurans are very interesting O-heterocycles and numerous synthetic methods have been developed for their preparation.^7–10 A major route for the synthesis of various arene ring-fused furan derivatives is the intramolecular formation of a furan moiety starting from properly substituted arene compounds via dehydrative cyclization of a carbonyl group.^6–11 The Prins cyclization reaction which generally involves a reaction between an aldehyde and a homoallylic alcohol promoted by acid is important for the construction of oxygen-containing heterocyclic units.¹² Many reactions have utilized upper/lower rim modification for the synthesis of new functionalized calixarenes.¹³ Biali and co-workers have synthesized an interesting series of molecules, namely “bis(spirodienones)” prepared by the intramolecular oxidative cyclization through the phenolic hydroxyls of calix[4]arene.¹⁴ Black and co-workers reported for the first time that some activated benzofuranylmethanols undergo acid-catalyzed cyclo-oligomerisation via electrophilic reactions to afford a range of calix[3]benzofurans and calix[4]benzofurans having π-electron rich cavities.¹⁵ Although there have been extensive studies of calix[4]arenes over the last few decades,¹⁶ reports on the preparation and characterization of calix[3]arenes have been very limited.¹⁵ However, the synthetic potential of these molecules for the design of new macrostructures based on their structural features viz. calixarene analogues of metacyclophanes containing benzofuran rings linked by methylene bridges and DFT computational studies thereof, remains mostly unexplored. There are very few reports of DFT computational studies of calixarene analogues. Choe et al. have reported DFT calculations on the conformational characteristics and hydrogen bonding of both p-tert-butylcalix[4]arene and p-tert-butylcalix[5]arene using the B3LYP/6-31+G(d,p) method.¹⁷

In our laboratory, we are now focusing on synthesizing calixarene-type metacyclophanes, with particular interest in their conformations and potential application.¹⁸ The first objective of this research is to synthesize the calix[3]benzofurans and their derivatives by electrophilic substitution reactions such as bromination, formylation and acylation with a view to investigating their conformational properties. The second objective is to determine the energies of different conformational isomers of the calix[3]benzofurans and derivatives using DFT computational studies.

Results and discussion

The synthetic routes and yields of [3.3.3]MCP-2,11,20-triones 2a–b are shown in Scheme 1. The direct coupling reaction of (p-tolylsulfonyl)methyl isocyanide (TosMIC) with 1a–b in the presence of NaH, followed by acid treatment, afforded the tri-ketones 2a–b along with the dimers anti/syn-[3.3]MCP-2,11-diones 3a–b, respectively.¹⁹ In general, a mixture of TosMIC and 1a–b in N,N-dimethylformamide (DMF) was added dropwise to a suspension of NaH in DMF at room temperature. 6,15,24-Tri-tert-butyl-9,18,27-trimethoxy[3.3.3]MCP-2,11,20-trione 2a has been reported previously (Scheme 1).^19c The structure of 9,18,27-trimethoxy[3.3.3]MCP-2,11,20-trione 2b was elucidated by elemental analysis and spectral data. For instance, the mass spectral data for 2b (M⁺ = 486.23) strongly supported a cyclic trimeric structure. The IR spectrum of 2b shows the absorption of the carbonyl stretching vibration at around 1716 cm⁻¹. The ¹H NMR spectrum of macrocycle 2b exhibits two single peaks at δ 3.28 and 3.63 ppm for the methoxy protons and the ArCH₂⁻COCH₂Ar methylene protons.

Scheme 1 Synthesis of 9,18,27-trimethoxy[3.3.3]MCP-2,11,20-triones 2.

The cyclization of ketones 2a–b with phenolic hydroxyl groups produced by treatment with TMSI (generated in situ from TMSCl and NaI in CH₃CN) led to the formation of the furan moiety instead of the expected product 4′ (Scheme 2). Thus nucleophilic intramolecular cyclization of intermediate 4′ afforded the calix[3]benzofurans, 4a and 5a (Scheme 3). Sawada and co-workers have reported hemisphere-shaped calixarene analogues via a single step reaction involving a pinacol rearrangement followed by an intramolecular acetalization from tetrahydroxy-tetramethoxy[2.1.2.1]MCP.^{20 1}H NMR spectroscopy demonstrates that calix[3]benzofurans 4a and 5a adopt radically different conformations in solution at room temperature and undergo very fast conformational changes relative to the NMR time scale. To establish the conformation of 4a, we carried out variable temperature (VT) NMR spectroscopy over the range −50 °C to +70 °C (Fig. 1).

Scheme 2 Synthesis of calix[3]benzofurans 4a and 5a.

Scheme 3 Mechanism for formation of furan moiety.

Fig. 1 Partial VT-NMR spectra of 6,14,22-tri-tert-butylcalix[3]-benzofuran 4a in CDCl₃.

The ¹H NMR spectrum at −50 °C suggests that the cone and saddle conformers of calix[3]benzofuran 4a exist in a ratio of 83 : 17. The interconversion of the conformational structures in solution are readily studied by variable-temperature ¹H NMR spectroscopic techniques by simply monitoring the change in the signals of the bridging methylene protons.^2b The cone conformation was assigned by the observation of a set of doublets for the methylene protons at δ 4.1 and 4.7 ppm, whereas for the saddle, the methylene protons appear as a singlet at δ 4.2 at −50 °C (Fig. 1). The bridging methylene protons are in different chemical environments (axial and equatorial), but quickly interconvert on the NMR time scale at room temperature and appear as a singlet or broad peak.

When the temperature is dropped sufficiently, the conformation becomes more rigid and the interconversion is slower than the NMR time scale, which causes the CH₂ signal to resolve into a pair of doublets. Single crystals of 4a (CCDC-1456536†) were grown from a hexane solution, and were investigated by X-ray crystallography to confirm the conformation; the crystal structure was found to belong to the trigonal crystal system with space group R3̄ (ESI Table S1†). Although calix[3]benzofuran 4a forms drastically different conformations in solution at room temperature, it adopts a rigid cone type hemisphere-shaped symmetrical structure in the solid state (Fig. 2).

Fig. 2 Ortep drawing of 4a. Thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms are omitted for clarity.

Thus, 4a freely interconverts between the cone and saddle conformations in solution. Since the conformation of calix[3]benzofuran at room temperature in solution exists in both the cone and saddle conformations, in order to investigate further the detailed conformational properties, a series of electrophilic substitution reactions was carried out, such as bromination, formylation and acetylation, at the furan moieties (Scheme 4).

Scheme 4 Reagents and conditions: (a) BTMA Br₃, CH₂Cl₂, r.t. 24 h, 52%; (b) Cl₂CHOCH₃, TiCl₄, CH₂Cl₂, r.t. 3 h, 49%; (c) NaBH₄, EtOH/CH₂Cl₂, reflux 24 h, 55%; (d) AcCl, TiCl₄, CH₂Cl₂, r.t. 3 h, 48%.

We commenced this study by bromination of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a with benzyltrimethylammonium tribromide (BTMA-Br₃) in CH₂Cl₂ at room temperature, which afforded only the rigid cone-4,12,20-tri-bromo-6,14,22-tri-tert-butylcalix[3]benzofuran 4b. Two broad single peaks at δ 4.24 and 4.60 ppm in the ¹H NMR (CDCl₃, 400 MHz) spectrum were evidence for the formation of the fixed cone type conformer in solution at room temperature (ESI, Fig. S9†). However at 10 °C, the two broad peaks split into two clearly defined doublets which established the formation of the cone conformation (Fig. 3). The structure of the macrocycle 4b was confirmed by single-crystal X-ray analysis (CCDC-1456535†) and an ORTEP drawing is shown in Fig. 4. The analysis shows the well-defined calixarene molecule lying around a threefold symmetry axis with, on one side, a CHCl₃ solvent molecule (also on the symmetry axis and well-defined) and, on the other side, a complex ring structure, presumably of a disordered array of methanol molecules; in this region, 50 atoms have been refined as isotropic carbon atoms mostly with site occupancies of 0.5, about a point of 3̄ symmetry.

Fig. 3 Partial VT-NMR of 4,12,20-tribromo-6,14,22-tri-tert-butyl-calix[3]benzofuran 4b in CDCl₃ (300 MHz).

Fig. 4 Ortep drawing of 4b. Thermal ellipsoids in the top view are drawn at the 50% probability level and those in the side view at 30%. In both views, the cluster of disordered solvent methanol molecules has been omitted.

Formylation of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a with 1,1-dichlorodimethyl ether in the presence of TiCl₄ at room temperature afforded 6,14,22-tri-tert-butyl-4,12,20-triformylcalix[3]-benzofuran 4c. On replacement of bromine by a more electron withdrawing group (–CHO) at the furan moiety, it is seen that the conformation of 4c in solution at room temperature underwent very fast changes relative to the ¹H NMR timescale and gave a broad peak for the bridging methylene protons at δ 4.59 ppm (ESI, Fig. S11†), consistent with an equilibrium existing between the cone and the saddle conformations. However at low temperature (−30 °C), the isomerization between cone and saddle is slow and the saddle is the major conformation in solution (Table 1). Thus, different conformational properties of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a were observed upon changing the group at the furan moiety. We then introduced the medium-sized –CH₂OH group by reduction of 4c with NaBH₄, which afforded 6,14,22-tri-tert-butyl-4,12,20-trihydroxymethylcalix[3]benzofuran 4d. The CH₂OH group has no significant electron-withdrawing ability and it is surprising that the conformation of the 6,14,22-tri-tert-butyl-4,12,20-trihydroxymethylcalix[3]benzofuran 4d dramatically converted to a fixed cone conformation as in compound 4b. The bridging methylene groups appear as broad singlets and gives peaks at δ 4.22 and 4.66 ppm (ESI, Fig. S13†).

Table 1 Influence of substituents on the conformation of benzofurans

Compound	Tc (°C)	ΔG^‡ (kJ mol^−l)	Cone : saddle (−30 °C)
a Solvent: CDCl3.b Solvent: CDBr3 (300 MHz).
4a; X = H	40^a	61.9 (J = 14.8 Hz)	80 : 20
4b; X = Br	45^a	62.8 (J = 14.4 Hz)	100 : 0
4c; X = CHO	28^a	58.6 (J = 13.5 Hz)	40 : 60
4d; X = CH2OH	50^a	68.2 (J = 14.4 Hz)	100 : 0
4e; X = COMe	75^b	69.0 (J = 13.2 Hz)	20 : 80

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Bromine is electron-withdrawing in nature, but the multiple lone-pairs of electrons are able to increase electron density at the furan moiety and fix the conformation of 4b to the cone conformation. Although the –CH₂OH group has less electron-withdrawing ability than the formyl group (–CHO), the alcohol group (–CH₂OH) is slightly larger, and so steric effects come into play and the molecule is generally locked in one conformation i.e. stabilization of the ring occurs. Thus, both the electron-withdrawing ability and the steric hindrance play a significant role on the conformational preferences of calix[3]benzofuran derivatives. With these results in hand, we elaborated our study by introducing the larger acyl-group (–COMe), which has less electron-withdrawing ability than the formyl group, CHO, but is greater than that of –CH₂OH. Treatment of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a with acetyl chloride in the presence of TiCl₄ at room temperature afforded 4,12,20-triacetyl-6,14,22-tri-tert-butylcalix[3]benzofuran 4e.

¹H NMR spectral analyses of the acylation derivatives indicate that these macrocycles exist as a mixture of two conformers that slowly interconvert on the ¹H NMR timescale. This is evident from the two distinct ¹H signals observed for the bridging methylene protons that appear as two sets of doublets at δ 4.62 and 4.49 ppm for the cone conformation and a single peak at δ 4.55 ppm for the saddle conformation (ESI, Fig. S15†). In this case, the acyl group (–COMe) makes the furan ring electron deficient as in the case of the formyl group (–CHO), but due to the larger size of the acyl group (–COMe), it is possible that steric hindrance prevents complete rapid isomerization at room temperature in solution. At −30 °C, the isomerization between cone and saddle is very slow and the major conformation is saddle (Table 1); a similar result was observed for the electron-withdrawing formyl-group containing derivative of calixbenzofuran 4c. So, both electronic and steric effects of the substituents can play significant roles on the conformations of calix[3]benzofuran derivatives and can lead to interesting conformational changes in solution. In view of the encouraging results in conformational changes obtained by electrophilic substitution reactions of 4a, we extended our studies towards the calix[3]benzofuran 5a.

Similar phenomena were observed when calix[3]-benzofuran 5a was reacted with acetyl chloride at room temperature in the presence of TiCl₄; the product was 4,12,20-triacetylcalix[3]benzofuran 5b (Scheme 5). ¹H NMR (ESI, Fig. S17†) spectroscopic analysis of 5b indicates that this macrocycle also exists as a mixture of two conformers that are slow to interconvert on the ¹H NMR timescale at room temperature. This is evident by two distinct signals observed for the bridging methylene protons that appear as two doublets at δ 4.68 and 4.99 ppm for the cone conformer and a single peak at δ 4.58 ppm for the saddle conformation (ESI, Fig. S17†). Acylation of macrocycle 5a indicates that the furan ring in benzofuran is electron-rich and electrophilic substitution occurs preferentially in this ring rather than the benzene ring and that the tert-butyl groups have no significant effect on the conformation of this acylated calix[3]benzofuran.

Scheme 5 Acetylation of calix[3]benzofuran 5a.

It is important to emphasize that all the synthesized calix[3]-benzofuran derivatives gave only two sets of distinct proton resonances in their ¹H NMR spectra for the methylene bridges. The ¹H NMR spectra indicated clearly that 4d adopts a shape-persistent cone conformation, whereas 4c derivatives exist as equilibrium mixtures in which various conformers (cone and saddle) undergo rapid interconversion relative to the ¹H NMR time scale evident by a broad peak at δ 4.59 ppm. However, 4e underwent very slow interconversion relative to the ¹H NMR time scale giving two sets of doublets at δ 4.62 and 4.49 ppm for the cone conformation and a single peak at δ 4.55 ppm for the saddle conformation.

The conformations of the present system have also been evaluated by means of dynamic ¹H NMR spectroscopy. By observation of the resonances arising from the ArCH₂Ar methylene protons, the coalescence temperatures (T_c) and free energy barriers (ΔG^‡) have been estimated (Table 1). This behavior indicates that the rate of conformational ring flipping of these macrocycles is faster than the NMR time scale above this temperature (T_c). Density functional theory (DFT) computational studies were carried out to determine the geometry-optimized energies of all the synthesized calix[3]benzofurans 4a–e and 5a–b (ESI Tables S2 and S3†). The starting structures were generated with initial geometries based upon the X-ray structures of 4a and 4b and from the presumed structures of 4c–d (derived from cone-4a and cone-4b) and 5a–b using SpartanPro'10 with the MMFF94 method.²² The individual geometry-optimized structures of these molecules in both CHCl₃ solvent (Fig. 5 and 6) and in the gas phase were then optimized using Gaussian 09 with the B3LYP level of theory and the 6-31G(d) basis set.²³ The computed energies of the two distinct conformers and their energy differences (ΔE, kJ mol⁻¹), are listed in Tables 2 and 3 (and ESI Tables S2 and S3†). From the DFT-optimized B3LYP/6-31G(d) computed energies of all of the synthesized calix[3]benzofurans 4a–e and 5a–b, it is seen that both the cone and saddle conformations have lower ground-state energies in the solvent system than in the gas phase (Table 3). Furthermore, the DFT optimized B3LYP/6-31G(d) energies of tert-butylcalix[3]benzofurans 4a and derivatives 4b–e, suggest that the saddle conformer is energetically more stable than the cone isomer (Table 3). Although 4a and 4b adopt cone conformations in the solid state (Fig. 2 and 3), the DFT optimized B3LYP/6-31G(d) energies of these two conformers imply that the saddle conformers of 4a and 4b are −4 and −35 kJ mol⁻¹, more stable than the cone conformers in the solvent, with similar results in the gas phase (Table 3). On the other hand, in calix[3]benzofuran 5a and its derivative 5b (without tert-butyl groups), the saddle conformers are energetically less stable than the cone conformers by 4 and 10 kJ mol⁻¹ in the gas phase, and by 5 and 7 kJ mol⁻¹ in solvent (Table 3), respectively. Similarly, saddle-4c, 4d, 4e are energetically more stable by 12, 20 and 48 kJ mol⁻¹ than cone-4c, 4d, 4e in the gas phase, respectively (Table 3).

Fig. 5 DFT B3LYP/6-31G(d) optimized molecular structures of the cone (left) and saddle (right) of (i) 4a, (ii) 4b, (iii) 4c, (iv) 4d and (v) 4e in CHCl₃ solvent.

Fig. 6 DFT B3LYP/6-31G(d) optimized molecular structures of the cone (left) and saddle (right) of (i) 5a and (ii) 5b in CHCl₃ solvent.

Table 2 Geometry optimization energies using B3LYP/6-31G(d) (ΔE = E_chlorofom − E_gas-phase)

Compound	Cone	Saddle
	ΔE, kJ mol^−1	ΔE, kJ mol^−1
4a	−17	−17
5a	−18	−17
4b	−13	−15
4c	−23	−28
4d	−35	−31
4e	−26	−28
5b	−25	−28

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Table 3 Geometry optimization energies using B3LYP/6-31G(d) (ΔE = E_saddle − E_cone)

Compound	Gas-phase	Chloroform
	ΔE, kJ mol^−1	ΔE, kJ mol^−1
4a	−4	−4
5a	4	5
4b	−34	−35
4c	−12	−18
4d	−20	−16
4e	−48	−50
5b	10	7

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The results presented in Table S2 and S3† show that among the calix[3]benzofurans, 4b is the energetically most-favored (in both the solvent and gas-phase) and the order is as follows: 4b > 4e > 4d > 4c > 4a > 5b > 5a in both the solvent and gas phase.

So by introducing the different groups at the furan moieties, the derivatives become energetically more favored over the corresponding calix[3]benzofuran according to the increasing size of groups (i.e. COMe > CH₂OH > CHO) except for 4b. In the case of 4b, there may be two factors influencing the stability: bromine is electronegative in nature and it has greater electron-density due to multiple lone-pairs of electrons.

Conclusions

We have described a simple and effective method for the synthesis of flexible calix[3]benzofurans by introducing furan moieties through intramolecular cyclization of [3.3.3]MCP-trione. To explore the rates of conformational interconversion of the described calix[3]benzofurans, a series of electrophilic substitution reactions such as bromination, formylation and acylation reactions of calix[3]benzofurans were studied. The presence of bromine or an alcohol group (–CH₂OH) at the furan ring led to the adoption of the fixed cone conformation, whereas the presence of the larger –COMe forced the adoption of both the cone and saddle conformations in solution and very slow interconversion on the ¹H NMR timescale at room temperature. However, the formyl derivative exhibits rapid conformational transformation as for the calix[3]benzofurans. Conformational flexibility of calix[3]benzofuran derivatives was observed by controlling the steric crowding and electron-withdrawing ability of the groups. The DFT B3LYP/6-31G(d) geometry-optimized computed molecular energies of all synthesized tri-tert-butylcalix[3]benzofurans were determined, and revealed that the saddle conformers were energetically favoured over the corresponding cone conformers; in the calix[3]benzofurans without tert-butyl groups, the cone conformers were the more stable. Further mechanistic details of calix[3]benzofuran derivatives are being explored (by introducing different groups and resolution of their isomers), and will be reported in due course.

Experimental

General

All melting points (Yanagimoto MP-S1) are uncorrected. Proton nuclear magnetic resonance (¹H NMR) spectra and ¹³C NMR spectra were recorded on Nippon Denshi JEOL FT-300 NMR and Varian-400MR-vnmrs400 spectrometers. Chemical shifts are reported as δ values (ppm) relative to internal Me₄Si. Mass spectra were obtained on a Nippon Denshi JMS-01SA-2 mass spectrometer at ionization energy of 70 eV; m/z values reported include the parent ion peak. Infrared (IR) spectra were obtained on a Nippon Denshi JIR-AQ2OM spectrophotometer as KBr disks. Elemental analyses were performed by Yanaco MT-5. G.L.C. analyses were performed by Shimadzu gas chromatograph, GC-14A; Silicone OV-1, 2 m; programmed temperature rise, 12 °C min⁻¹; carrier gas nitrogen, 25 mL min⁻¹. Silica gel columns were prepared by use of Merck silica gel 60 (63–200 μm).

Materials

The preparation of 2,6-bis(bromomethyl)-4-tert-butylanisole 1a and 2,6-bis(bromomethyl)anisole 1b were previously described.²¹ 6,15,24-Tri-tert-butyl-9,18,27-trimethoxy[3.3.3]MCP-2,11,20-trione 2a was prepared according to reported methods.^19c

Direct cyclization of dibromide 1 with TosMIC

To a suspension of NaH (2.1 g, 51 mmol) in DMF (150 mL) a solution of 1a (6.0 g, 17.1 mmol) and TosMIC (3.3 g, 22.0 mmol) in DMF (35.0 mL) was added dropwise over a period of 6 h. After the reaction mixture was stirred for an additional 5 h at room temperature, it was quenched with ice-water (300 mL). The reaction mixture was extracted with CH₂Cl₂ (100 mL × 3), washed with water (100 mL), dried over Na₂SO₄, and concentrated in vacuo to 15 mL. Concentrated HCl (15 mL) was added to the solution and stirring was continued for 15 min. The organic layer was again extracted with CH₂Cl₂ (100 mL × 3), washed with water (100 mL × 2), dried over Na₂SO₄, and concentrated and condensed under reduced pressure. The residue was chromatographed on silica gel using CHCl₃ as eluents to give crude 2a as a pale yellow solid. Recrystallization from hexane afforded 6,15,24-tri-tert-butyl-9,18,27-trimethoxy[3.3.3]MCP-2,11,20-trione 2a (912 mg, 28%) as pale yellow prisms. Mp 227–228 °C (lit.^19c 217–218 °C). IR: ν_max (KBr)/cm⁻¹: 1720 (C=O). ¹H NMR (400 MHz, CDCl₃): δ = 1.19 (27H, s, tBu × 3), 3.33 (9H, s, OMe), 3.61 (12H, s, CH₂) and 6.92 (6H, s, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 31.41, 34.19, 44.10, 60.20, 126.87, 127.01, 146.51, 154.55 and 206.96 ppm. FABMS: m/z: 654.82 [M⁺]. C₄₂H₅₄O₆ (654.90): calcd C 77.03, H 8.31; found: C 76.97, H 8.19.

Compound 2b was similarly prepared.

9,18,27-Trimethoxy[3.3.3]MCP-2,11,20-trione 2b. Recrystallization from hexane afforded 9,18,27-trimethoxy[3.3.3]MCP-2,11,20-trione 2b (640 mg, 22%) as pale yellow prisms. Mp 203–204 °C. IR: ν_max (KBr)/cm⁻¹: 1716 (C=O). ¹H NMR (400 MHz, CDCl₃): δ = 3.28 (9H, s, OMe), 3.63 (12H, s, CH₂), 6.94 (3H, t, J = 7.6, Ar-H) and 7.04 (6H, d, J = 7.6, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 43.29, 60.22, 124.04, 128.12, 130.17, 156.97 and 205.78 ppm. FABMS: m/z: 486.23 [M⁺], 487.23 [M⁺ + 1], C₃₀H₃₀O₆ (486.20): calcd C 74.06, H 6.21; found: C 73.83, H 6.21.

Demethylation of 2a with TMSI

To a solution of 2a (200 mg, 0.3 mmol) in CH₃CN (10.0 mL), NaI (900 mg, 6.0 mmol) was added. After adding trimethyl silyl chloride (0.8 mL, 6.0 mmol), the mixture was stirred at 80–85 °C for 48 h. The reaction mixture was quenched in 20 mL ice water and 10% aqueous sodium thiosulphate solution (40 mL) was added and stirring continued for 1 h at room temperature. Then the mixture was stirred with 10% HCl (20 mL) for 1 h and extracted with CH₂Cl₂ (40 mL × 3). The combined mixture was washed by 10% NaHCO₃ (20 mL) and water (20 mL × 2) and dried over Na₂SO₄, and then concentrated under reduced pressure. The residue was chromatographed on silica gel using hexane : CHCl₃ (5 : 1) as eluents to give crude 4a as a colourless solid. Recrystallisation from hexane afforded 6,14,22-tri-tert-butylcalix[3]benzofuran 4a (138 mg, 82%) as colourless prisms. Mp 190–191 °C. IR: ν_max (KBr)/cm⁻¹: 2960, 2862, 1586, 1480, 1302, 1198, 867. ¹H NMR (400 MHz, CDCl₃): δ = 1.32 (27H, s, tBu × 3), 4.09 (6H, brs, CH₂), 6.43 (3H, s, Ar-H), 7.13 (3H, s, Ar-H) and 7.31 (3H, s, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 30.79, 31.89, 34.59, 102.62, 115.31, 121.43, 129.05, 145.72 and 157.74 ppm. FABMS: m/z: 558.35 [M⁺]. C₃₉H₄₂O₃ (558.31): calcd C 83.83, H 7.58; found: C 83.53, H 7.58.

Compound 5a was similarly prepared.

Calix[3]benzofuran 5a. Recrystallisation from hexane afforded calix[3]benzofuran 5a (92 mg, 79%). Mp 180–181 °C. IR: ν_max (KBr)/cm⁻¹: 2962, 1429, 1194 and 810. ¹H NMR (400 MHz, CDCl₃): δ = 4.15 (6H, s, CH₂), 6.48 (3H, s, Ar-H), 7.08 (3H, t, J = 6.8, Ar-H) and 7.32 (6H, d, J = 6.4, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 30.35, 110.02, 119.20, 123.59, 129.38, 144.19 and 152.75 ppm. HRMS: m/z: 390.1253 [M⁺]. C₂₇H₁₈O₃ (390.1256): calcd C 83.06, H 4.65; found: C 82.78, H 4.72.

Bromination of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a

To a solution of 4a (100 mg, 0.18 mmol) in CH₂Cl₂ (7.0 mL), BTMABr₃ (278 mg, 0.71 mmol) was added and the mixture was stirred for 24 h at room temperature. Then the mixture was extracted with CH₂Cl₂ (50 mL × 3) and the combined mixture was washed by water (20 mL × 2) and dried over Na₂SO₄, and then concentrated under reduced pressure. The residue was chromatographed on silica gel using CH₂Cl₂ as eluents to give crude 4b as a yellow powder. Recrystallisation from methanol afforded 4,12,20-tribromo-6,14,22-tri-tert-butylcalix[3]benzofuran 4b (75 mg, 52%) as yellow prisms. Mp 190–191 °C. IR: ν_max (KBr)/cm⁻¹: 2962, 2851, 1522, 1478, 1361, 1196, 867. ¹H NMR (400 MHz, CDCl₃): δ = 1.34 (27H, s, tBu × 3), 4.22 (3H, brs, CH₂), 4.58 (3H, brs, CH₂), 7.21 (3H, s, Ar-H) and 7.47 (3H, s, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 28.52, 31.70, 34.69, 114.12, 119.46, 124.27, 128.04, 146.76 and 152.68 ppm. FABMS: m/z: 793.90 [M⁺]. C₃₉H₃₉Br₃O₃ (792.04): calcd C 58.89, H 4.94; found: C 58.64, H 4.81.

Formylation of 6,14,22-tri-tert-butylcalix[3]benzofuran 4a

To a solution of 4a (100 mg, 0.18 mmol) and Cl₂CHOCH₃ (0.96 mL, 1.08 mmol) in CH₂Cl₂ (2 mL) was added a solution of TiCl₄ (0.12 mL, 1.08 mmol) in CH₂Cl₂ (2 mL) at 0 °C. After stirring the reaction mixture at room temperature for 2 h, it was poured into ice water (20 mL) and then the mixture was extracted with CH₂Cl₂ (50 mL × 3) and the combined mixture was washed by water (20 × 2 mL) and dried over Na₂SO₄, and then concentrated under reduced pressure. The residue was chromatographed on silica gel (Wako C-300, 500 g) using CH₂Cl₂ as eluents to give crude 4c as a yellow powder. Recrystallisation from methanol afforded 6,14,22-tri-tert-butyl-4,12,20-triformylcalix[3]benzofuran 4c (57 mg, 49%) as a yellow powder. Mp 260–261 °C. IR: ν_max (KBr)/cm⁻¹: 2962, 1678 (C=O), 1434. ¹H NMR (400 MHz, CDCl₃): δ = 1.35 (27H, s, tBu × 3), 4.59 (6H, brs, CH₂), 7.48 (3H, s, Ar-H), 8.02 (3H, s, Ar-H) and 10.41 (3H, brs, CHO) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 29.33, 31.77, 34.95, 117.37, 124.20, 153.20, 157.06, 159.94, 166.36 and 196.67 ppm. HRMS: m/z: 642.2994 [M⁺]. C₄₂H₄₂O₆ (642.2981): calcd C 78.48, H 6.59; found: C 78.45, H 6.81.

Preparation of 6,14,22-tri-tert-butyl-4,12,20-trihydroxymethylcalix[3]benzofuran 4d

To a solution of 4c (50 mg, 0.08 mmol) in a mixture of CH₂Cl₂ (2.0 mL), EtOH (2 mL), NaBH₄ (27 mg, 0.7 mmol) was added and the system was reflux for 24 h. Then the mixture was extracted with CH₂Cl₂ (50 mL × 3) and the combined mixture was washed by water (20 mL × 2) and dried over Na₂SO₄, and then concentrated under reduced pressure. The residue was chromatographed on silica gel (Wako C-300, 500 g) using CH₂Cl₂ as eluents to give crude 4d. Recrystallisation from hexane afforded 6,14,22-tri-tert-butyl-4,12,20-trihydroxymethylcalix[3]benzofuran 4d (28 mg, 55%) as a yellow powder. Mp > 280 °C. IR: ν_max (KBr)/cm⁻¹: 3420, 2956, 2871, 1480, 1363, 1001. ¹H NMR (400 MHz, CDCl₃): δ = 0.89 (3H, brs, CH₂OH), 1.33 (27H, s, tBu), 4.22 (3H, brs, CH₂), 4.66 (3H, brs, CH₂), 4.86 (6H, s, CH₂OH), 7.26 (3H, s, Ar-H) and 7.39 (3H, s, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 28.71, 31.85, 34.75, 55.38, 114.20, 121.92, 128.46, 146.59, 150.30 and 155.25 ppm. HRMS: m/z: 648.3472. C₄₂H₄₈O₆ (648.3451): calcd C 77.75, H 7.46; found: 77.52, H 7.28.

Preparation of 4,12,20-triacetyl-6,14,22-tri-tert-butylcalix[3]-benzofuran 4e

To a solution of 4a (100 mg, 0.18 mmol) and AcCl (0.08 mL, 1.08 mmol) in CH₂Cl₂ (2 mL) was added a solution of TiCl₄ (0.12 mL, 1.08 mmol) in CH₂Cl₂ (1 mL) at 0 °C and stirring was continued for 1 h. Then the mixture was stirred at room temperature for 3 h and poured into ice water. Then the mixture was extracted with CH₂Cl₂ (50 mL × 3) and the combined mixture was washed by water (20 mL × 2) and dried over Na₂SO₄, concentrated under reduced pressure. The residue was chromatographed on silica gel (Wako C-300, 500 g) using CH₂Cl₂ as eluents to give crude 4e as a yellow powder. Recrystallisation from hexane afforded 4,12,20-triacetyl-6,14,22-tri-tert-butylcalix[3]benzofuran 4e (59 mg, 48%) as a yellow powder. Mp 164–165 °C. IR: ν_max (KBr)/cm⁻¹: 2960, 2870, 1674, 1464, 1381, 1186. ¹H NMR (400 MHz, CDCl₃): δ = cone – 1.38 (27H, s, tBu × 3), 2.63 (9H, s, –COCH₃), 4.62 (3H, d, J = 13.2 Hz, CH₂), 4.93 (3H, d, J = 14.4 Hz, CH₂), 7.80 (3H, s, Ar-H), 7.88 (3H, s, Ar-H), saddle – 1.36 (27H, s, tBu × 3), 2.63 (9H, s, –COCH₃), 4.55 (6H, s, CH₂), 7.38 (3H, s, Ar-H) and 7.54 (3H, s, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 29.32, 31.32, 31.83, 34.88, 116.49, 117.34, 119.04, 123.96, 125.70, 147.46, 163.25 and 194.24 ppm. HRMS: m/z: 684.3453 [M⁺]. C₄₅H₄₈O₆ (684.3451): calcd C 78.92, H 7.06; found: C 78.83, H 7.28.

Preparation of 4,12,20-triacetylcalix[3]benzofuran 5b

To a solution of 5a (70 mg, 0.18 mmol) and CH₃COCl (0.076 mL, 1.08 mmol) in CH₂Cl₂ (2 mL) was added a solution of TiCl₄ (0.12 mL, 1.08 mmol) in CH₂Cl₂ (2 mL) at 0 °C. After stirring the reaction mixture at room temperature for 2 h, it was poured into ice water (20 mL) and then the mixture was extracted with CH₂Cl₂ (50 mL × 3) and the combined mixture was washed by water (20 mL × 2) and dried over Na₂SO₄, and then concentrated under reduced pressure. The residue was chromatographed on silica gel (Wako C-300, 500 g) using CH₂Cl₂ as eluents to give crude 5b as a yellow powder. Recrystallisation from MeOH/CHCl₃ afforded 4,12,20-triacetylcalix[3]benzofuran 5b (38 mg, 41%) as a yellow powder. Mp 242–243 °C. IR: ν_max (KBr)/cm⁻¹: 1675 (C=O), 1556, 1151. ¹H NMR (400 MHz, CDCl₃): δ = cone – 2.64 (9H, s, –COCH₃), 4.68 (3H, d, J = 13.6 Hz, CH₂), 4.99 (3H, d, J = 13.2 Hz, CH₂), 7.61 (3H, d, J = 8.8 Hz, Ar-H), 7.80 (6H, dd, J = 7.2 Hz, Ar-H), saddle – 2.64 (9H, s, –COCH₃), 4.58 (6H, s, CH₂) and 7.34 (9H, dd, J = 8.0 Hz, Ar-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 31.07, 35.85, 117.45, 124.65, 134.77, 150.48, 159.97, 166.52 and 192.36 ppm. FABMS: m/z: 516.23 [M⁺]. C₃₃H₂₄O₆ (516.16): calcd C 76.73, H 4.68; found: C 77.01, H 4.91.

Acknowledgements

We would like to thank the OTEC at Saga University for financial support. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)”. CR thanks the EPSRC for a travel award. The computational work has been assisted by the use of computing facilities provided by and with the on-going support of Dr G. Shamov of Compute/Calcul Canada via the Westgrid and Acenet facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the single-crystal X-ray crystallographic data and DFT computational data and xyz files. CCDC 1456535 and 1456536. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra06219a

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