Chalcogenide‐Lewis Acid Mediated Reactions of Electron‐Deficient Alkynes with Aldehydes

Abstract
Reactions of but-3-yn-2-one (2) with aldehydes 1 in the presence of a Lewis acid and dimethyl sulfide (3 a) predominantly gave (E)-α-(halomethylene)aldols 4–5 in high yields, while reactions of methyl propiolate (6 a) mainly afforded (Z)-3-halogeno-2-(hydroxymethyl)acrylates 7–8 in low to moderate yields. A reaction of dimethyl acetylenedicarboxylate (10) with 1 a in the presence of TiCl4 and 1,1,3,3-tetramethylthiourea (3 c) produced maleate (E)-11 (40 %) and butenolide 12 (40 %). When a reaction of 6 a with 1 a was carried out in the presence of TiBr4 and 3 a (0.2 equiv) at −20 °C for 60 h, 3-(methylthio)-2-(hydroxyalkyl)acrylate 9 a was obtained in an 8 % yield. Experiments were conducted in order to elucidate the formation mechanism of 9 a, and it was made clear that 9 a was formed via the processes of the Michael addition of sulfide 3 a to alkynoate 6 a and an aldol reaction with 1 a and demethylation.

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