Abstract In the presence of SnCl 4, methoxycarbonyl-, acetyl-, benzoyl-, or cyano-substituted chloromethyl methyl sulfide (1–4) undergoes [2++ 4] polar cycloaddition with conjugated dienes to afford the cycloadducts of type 7, which, on treatment with base, are converted into the 1-acyl-or 1-cyano-1-methylthio-2-vinylcyclopropanes 9–10 via the ylide intermediates of type 8.