A quantitative study on the effect of ring size in the intramolecular ring opening of epoxides by
carbanions is described. Two series of substrates were examined a,a-bis-sulfonyl w-epoxides 1 and
a-cyano-a-sulfonyl w-epoxides 2; in each series the carbanion is tethered to the epoxide by a chain
of variable length from one to four methylene groups. The nucleophile can attack either electrophilic
position of the oxirane ring, or both; exo ring opening of cyano sulfonyl epoxides 2 is followed by
a second cyclization leading eventually to bicyclic, fused y-lactones. Both series of epoxides show
the same trend of reactivity as a function of ring size, in the formation of three- to seven-membered
rings, with reactivity maxima corresponding to the formation of cyclopropane and cyclopentane
derivatives. Unlike 8 N 2 ring closure of w-halogeno carbanions, cyclization to a five-membered ring
is the fastest process in this case. The ratio kdk5 between formation of three- and five-membered
rings drops from over 100, in the SN2 cyclization of w-iodo bis-sulfones, to less than 0.5, in the
cyclization of w-epoxy bis-sulfones 1. The difference is discussed in terms of trajectory of approach
of the carbanion to the nucleophilic center. Cyclization of cyano sulfonyl epoxide 2a, in which the
nucleophilic center and the epoxide are spaced by a single methylene group, is diastereoselective and
leads to a bicyclic product with a cis fusion between the y-lactone and the cyclopropane ring.
