Polarized ketene dithioacetals. Part 42. Studies on the reactions of alkyl and aryl Grignard reagents with α-oxoketene dithioacetals

Abstract

The oxoketene dithioacetals (2a–k) derived from a variety of cyclic and acyclic active methylene ketones undergo 1,2-addition with methylmagnesium iodide to give the alcohol acetals (3a–k) which, on subsequent boron trifluoride—ether catalysed methanolysis, yield methyl β-methyl-α,β-unsaturated esters (6a–k) in 51–70% overall yields. Also the alcohols (3a) and (3i–k) underwent partial hydrolysis in the presence of boron trifluoride–ether and water to yield β-methyl-α,β-unsaturated S-methyl esters (7a–d) in 41–60% yield. Under similar conditions however, the oxoketene dithioacetals (21–q) derived from higher homologues of acetophenone yielded the corresponding 2-alkyl-3-methylinden-1-ones (8a–f) in 50–62% overall yields. The reactions of cyclic oxoketene dithioacetals (2g), (2h), and (2j) with higher alkyl Grignard reagents afforded the alcohols (13a–f), formed by sequential 1,4 and 1,2 addition, which on subsequent methanolysis yielded the corresponding 1-(2-alkylcycloalk-1-enyl)alkan-1-ones (14a–f) in 48–58% overall yields. Similarly the phenylmagnesium bromide underwent sequential 1,4 and 1,2 addition to the oxoketene dithioacetals (2a–c), (2h) and (2j) to yield the alcohols (13g–k), which upon subsequent methanolysis yielded either 1,3,3-triarylprop-2-en-1-ones (14g–i) or phenyl (2-phenylcycloalk-1-enyl) ketones (14j–k) in 53–71% yield.