Steric Effect in Regioselective Cyclization of 3,4-Epoxy Alcohols to Oxetanes

A number of 3,4-epoxy alcohols, involving critical structural factors for oxetane formation by intramolecular cyclization, were prepared and treated with base under aqueous (KOH in 75% aq DMSO) and anhydrous conditions (NaH in THF). The result of the reactions of cis- and trans-4,5-epoxy-2-rnethyl-2-undecanols, coupled with that of 1-(2,3-epoxybutyl)- and 1-(2,3-epoxy-3-methylbutyl)-1-cyclohexanols (1a and 1b), indicated that the regioselective oxetane formations of 3,4-epoxy alcohols with alkoxide anions are much insensitive to steric hindrance at the attacked oxirane rings, compared with the corresponding reactions with carbanions, while that of 1-(2,3-epoxypropyl)-1-cycloalkanols revealed that the relevant reactions depend on bulkiness of attacking alkoxide anions. Moreover, the result of the reactions of 1-(1,2-epoxycycloalkyl)-2-methyl-2-propanols under the aqueous conditions demonstrated that the oxetane formations are affected seriously by steric hindrance at the attacked oxirane rings, although the result was in striking contrast with the corresponding result with carbanions. On the other hand, the reactions of 3,4-epoxy alcohols under the anhydrous conditions proceeded in different manner, depending on the structures of the epoxides; namely, fragmentation took place as major reactions with some epoxides, while intramolecular cyclization occurred predominantly with other epoxides.