Formation of acetylenes by ring-opening of 1,1,2-trihalocyclopropanes

This review describes studies into the conversion of substituted 1,1,2-trih alocyclopropanes into acetylenic acetals and acetylenic ketals by applicati on of alcohols or alkoxides under basic conditions. The acetal/ketal ratio turned out to depend both on the substituents attached to the ring and on t he reaction conditions prevailing during the reaction. Under the right cond itions, however, completely regioselective ring-opening occurs to give eith er acetylenic acetals or acetylenic ketals, with formation of either no or only minor amounts of other products. The reactions are thought to involve cyclopropene intermediates, which are consumed by nucleophilic attack of al cohol or alkoxide. In some cases, the cyclopropene intermediates rearrange to the corresponding vinyl carbenes, which undergo insertion reactions with protic species present in the reaction mixture and give various 2-propenal derivatives as by-products.