Thiophilic addition of Grignard reagents to methyl dithioates

Thiophilic addition of Grignard reagents to methyl dithioates 

Good yields were generally obtained as reported in Table 4.1. 

Under the same conditions a low yield of dithioacetal was isolated for R = t-Bu (23% with R3 = Et). Only dimerization products of carbene intermediates were obtained for R1 = phenyl however, the corresponding unstable magnesiodithioacetal could be trapped by addition of a good alkylating species before that of the Grignard reagent. An 80% yield of dithioacetal (R1 = Ph, R1 = Et or i-Pr, E = Me) was thus isolated. 

The thiophilic addition ot aliphatic Grignard reagents to dithioesters deserves some comment. Although they are less reactive than organolithium compounds, their reactions are selective. With enethiolizable dithioesters, enethiolization cannot be avoided with the organolithium species, and at low temperatures this can become exclusive [332], Enethiolization does not interfere in the case of the less 

The sequence, addition of one equivalent of MeLi to a solution of dithioester (1) and (2) in THF at -78°C, led. after allowing the temperature to rise slowly to 20°C and the usual work up, to dithioacetal (3) in a 79% yield [333]. Dithioacetalization and a Wittig reaction yielded dehydro-ar-curcumene (4). 

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