Behavior of Benzoins and Hydroxy ketones in Acid Media: I. Reaction of 1-Aryl-2-(2,5-dimethylthiophen-3-yl)-2-hydroxyethanones with Thiols in Trifluoroacetic Acid

Abstract

Reactions of 1-aryl-2-(2,5-dimethylthiophen-3-yl)-2-hydroxyethanones with thiols in trifluoroacetic acid lead to formation of substituted 1-aryl-2-(2,5-dimethylthiophen-3-yl)-2-sulfanylethanones.Chemistry of functionally substituted 1,2-dithienylethane derivatives attracts considerable interest, taking into account their wide application in the synthesis of photochromes [1,2]. We previously used 2-hydroxy-1,2-bis(2,5-dimethylthiophen-3-yl)ethanone (I) and its analog to obtain a wide series of photochromic 1,2-dihetarylethenes [35]. The present communication reports on the behavior of these hydroxy-containing compounds in trifluoroacetic acid in the presence of various sulfurcontaining nucleophiles, such as aromatic and heteroaromatic thiols and methyl sulfanylacetate (Scheme 1), synthesis of b-keto sulfides, and their subsequent transformation into substituted ethanones.We have found that hydroxy ketone I reacts with thiols III in trifluoroacetic acid even at room temperature through intermediate conjugate acid IIa and dehydrated species IIb. As a result, the corresponding b-keto sulfides IVaIVg are formed in good yields. The reaction of I with methyl sulfanylacetate in trifluoroacetic acid is the most vigorous: the process is complete within a few minutes, and the yield of compound IVg is almost quantitative. However, in order to obtain sulfide IVg in a high yield, it is necessary to use 5 equiv of methyl sulfanylacetate with respect to hydroxy ketone I. The order of mixing of the reactants is also important: hydroxy ketone I should be added to a solution of methyl 6 2+ 2 6 0H 0H 0H 0H &) &22+ , 6 2 2 6 0H 0H 0H 0H 6 2 6 0H 0H 0H 0H 6 6 2 6 0H 0H 0H 0H + + B + 2 +6 5 ,,, ,, ,,E 5 ,9D ,9J B Scheme 1.III, IV, R = 4-FC 6 H 4 (a), Ph (b), 4-ClC 6 H 4 (c), 5-amino-1,3,4-thiadiazol-2-yl (d), 1-methylimidazol-2-yl (e), 5-(4-methoxyphenyl)-1,2,4-triazin-3-yl (f), MeOCOCH 2 (g).