Alkenylation of thiophenes and furans at the 2-position and a synthesis of allenes conjugated with α,β-unsaturated ester with magnesium alkylidene carbenoids

The reaction of 1-chlorovinyl p-tolyl sulfoxides, derived from ketones and chloromethyl p-tolyl sulfoxide, with i-PrMgCl at −78 °C gave magnesium alkylidene carbenoids. Treatment of the magnesium carbenoids with 2-lithiothiophenes and 2-lithiofurans resulted in the formation of 2-alkenylated thiophenes and furans, respectively, in good to high yields. The intermediates of these reactions were found to be alkenylmagnesium, which could be trapped with several electrophiles to afford thiophenes and furans bearing a fully substituted alkene at the 2-position. Treatment of the magnesium alkylidene carbenoids with 2-lithio-5-methoxyfuran afforded allenes conjugated with α,β-unsaturated methyl ester in moderate yields. These procedures offer a new and versatile one-pot synthesis of 2-alkenylthiophenes, 2-alkenylfurans, and allenes conjugated with α,β-unsaturated methyl ester from 1-chlorovinyl p-tolyl sulfoxides.