Intramolecular cyclization of s-phenyl 3-arylpropynethioates by the action of brønsted and lewis acids

Intramolecular cyclization of N,3-diarylpropynamides or phenyl 3-arylpropynoates in various acidic systems is used for the synthesis of quinoline [1] or coumarin derivatives [2]. The present communication reports on the transformations of S-phenyl 3-arylpropynethioates Ia and Ib by the action of Bronsted (H2SO4, HSO3F, CF3SO3H) or Lewis acids (AlCl3, AlBr3), leading to the corresponding 4-aryl-2H-thiochromen-2-ones IIa and IIb through electrophilic intermediates A (Scheme 1). The reaction conditions and yields are given in table. S-Phenyl 3-phenylpropynethioate (Ia) in sulfuric acid at 20°C was converted in 5 h into 4-phenyl-2H-thiochromen-2-one (IIa) in 96% yield (run no. 1). The transformations of compounds Ia and Ib in stronger acids (HSO3F, CF3SO3H) required a shorter time (1 h at 0–20°C), the yield being fairly high (84–90%; run nos. 3, 4, 8). In HSO3F at –75°C (reaction time 0.25 h) thiocoumarins IIa and IIb were formed in poor yield, and the conversion of initial compounds Ia and Ib was not complete (run nos. 2, 7), in contrast to analogous transformations of phenyl 3-arylpropynoates [2]. Such Lewis acid as AlBr3 also promoted intramolecular cyclization of Ia to thiocoumarin IIa (run no. 5), but the yield of IIa was lower (60%) than in the reactions catalyzed by Bronsted acids. The transformation of propynethioate Ia in the presence of AlCl3 gave not only cyclization product IIa but also isomeric S-phenyl 3-chloro-3-phenylprop2-enethioates E-III and Z-III (run no. 6). The latter are formed via abstraction of chloride ion from associated AlCl3 species by strong electrophile A (Scheme 2). It should be noted that no analogous bromine-containing products were detected in the reaction with AlBr3 (run no. 5). The product mixture obtained in run no. 6 could not be separated by preparative chromatography on silica gel, and it was analyzed by gas chromatography–mass spectrometry and H NMR. Theoretical calculation (ChemDraw Ultra 10.0) of the chemical shifts of vinyl protons in isomers E-III and Z-III gave a larger value for the Z isomer (δcalc 6.80 ppm against 6.42 ppm for the E isomer). Therefore, the isomer characterized by larger experimental chemical shift of the vinyl proton (δexp 6.88 ppm) was assigned Z configuration, and that with δexp 6.68 ppm, E configuration.