MEDIUM-RING GAMMA-EPOXY SULFONES - REGIOCHEMISTRY AND STEREOCHEMISTRY OF THE BUTYLLITHIUM-PROMOTED TRANSANNULAR EPOXIDE RING-OPENING

Cyclic 8- and 9-membered E and Z gamma-epoxy sulfones yield bicyclic hydroxy sulfones stereospecifically upon treatment with BuLi in THF. The distribution of regioisomeric products markedly depends on the metalation mode, portionwise or at once. This suggests that the metal/proton exchange between the two positions alpha to the sulfone function is slow, relative to transannular cyclization, under conditions of complete metalation, but becomes fast in the presence of unmetalated sulfone (probably acting as a H+-transfer agent). The product distribution also depends, though to a lesser extent, on temperature, perhaps in relation to changes in the aggregation of the organolithium intermediate. An unexpected phenomenon was observed with the 8-membered Z epoxy sulfone at -70-degrees-C, where the time dependence of the transannular cyclization is consistent with an equilibrium being reached at 2/3 conversion. However, at -40-degrees-C or higher the reaction proceeds to completion without difficulty. A low-temperature metastable equilibrium is suggested involving some kind of aggregate of the lithio sulfone with the lithioalkoxide product.