Cascade Thiol-Michael–Aldol Reaction Promoted by Tetramethyl Guanidine: Synthesis of 2H-Thiochromene-3-carboxylate Libraries

Abstract

All the reactions were carried out under Nitrogen atmosphere. Unless otherwise noted, all the reagents obtained from commercial sources were used without further purification. All solvents were dried by standard methods. Tetrahydrofuran were dried with sodium and benzophenone and were used immediately after distillation. Dichloromethane was dried with diphosphoruspentoxide (P 2 O 5). Pentane was distilled and then dried with sodium. Analytical thin-layer chromatography (TLC) was performed on 0.25 mm Merck precoated silica gel plates (60-F 254). Column chromatography was carried out with the same kind of silica gel. The TLC plates were visualized with a UV lamp (254 nm and 366 nm) and/or with TLC visualizing solutions activated with heat, including: panisaldehyde solution and potassium permanganate solution. 1 H-NMR and 13 C-NMR were recorded on BRUKER 500 MHz Ascend (Hanoi University of Science, Vietnam National University) instruments using tetramethylsilane (TMS) as the internal standard and CDCl 3 as the solvent (1 H-NMR: TMS at 0.00 ppm, CDCl 3 : at 7.26 ppm; 13 C-NMR: CDCl 3 at 77.0 ppm). Chemical shifts are reported in parts per million (ppm). Data are reported as follows: chemical shift, multiplicity (s= singlet, d= doublet, t= triplet, q= quartet, m= multiplet), coupling constants (Hertz), and integration. X-Ray experiment has been performed with Bruker D8 Quest, Mass spectral analyzes have been performed with a LTQ Orbitrap XL (Hanoi University of Science, Vietnam National University) 3.2 Synthesis of 3-nitro-2-phenyl-2H-thiochromene derivatives General procedure for the synthesis of thiochromene A mixture of 2-mercaptobenzaldehyde (1 ad) (2 equiv), methyl cinnamate derivatives (1 equiv) and TM (1equiv) dissolved in dry toluene was stirred at room temperature for 24h. After completion of the reaction, the solvent was evaporated under vacuum and the crude product was purified by chromatography (n-hexane/ethyl acetate: 80/20) to give a mixture of diastereoisomers 3 as pale yellow liquid. This product was then dissolved in 1ml of toluene with pTSA (1 equiv). The mixture was heated at 90C in one hour. After completion of the reaction, the solvent was evaporated under vacuum and the crude product was purified by chromatography (n-hexane/ethyl acetate: 95/5) to give compound 4 as yellow solid. methyl 2-phenyl-2H-thiochromene-3-carboxylate (4a)