A peculiar transition-metal-free cyclodimerization of propargylic alcohols to vinyl bicyclic ketals

Propargylic alcohols, alkylaryl(hetaryl)ethynyl carbinols, under transition-metal-free conditions (KOH/DMSO, 70-80 °C, 10-15 min) unexpectedly undergo stereoselective cyclodimerization to 7-methylene-6,8-dioxabicyclo[3.2.1]octanes having the scaffold of known insect pheromones and marine toxins. Under acetylenic pressure the yields of bicyclic ketals reach 80%. In all the cases, the cyclodimers are formed exclusively as one diastereomer.