Epoxide of a ketene acetal. The first 2,2-dialkoxyoxirane to be isolated

Dimethoxycarbene, generated at 110°C in benzene by thermolysis of 2,2-dimethoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazoline, reacted with cyclohexanone to afford 2,2-dimethoxyspiro[2.5]-1-oxaoctane. It is the first oxirane of a ketene acetal that could be isolated and characterized by 1H NMR-, 13C NMR-, and IR spectroscopy. The identical oxirane might be expected from conrotatory cyclization of the appropriate carbonyl ylide. That ylide was generated under identical conditions by thermolysis of an oxadiazoline precursor (3,4-diaza-2,2-dimethoxy-1-oxaspiro[4.5]dec-3-ene) (14). The ylide could either cyclize or fragment to dimethoxycarbene and cyclohexanone. Addition of 4-tert-butylcyclohexanone, to trap dimethoxycarbene in competition with the cyclohexanone generated from 14 and, to leave the ylide closure pathway as the only route to the oxirane, showed that the carbonyl ylide does cyclize. However, fragmentation of the carbonyl ylide is relatively fast compared to its cyclization and most of it fragments to dimethoxycarbene and cyclohexanone. Oxirane formation from the carbene and ketone is probably either a concerted cycloaddition or it occurs in two steps, by nucleophilic attack at the carbonyl carbon to form the C—C bond first, prior to ring closure. If the carbene is bonded first to O of the carbonyl group, as it is in the carbonyl ylide, subsequent formation of the C—C bond is too slow, relative to fragmentation of the ylide, to afford the oxirane ring efficiently.Key words: carbonyl ylide, dialkoxyoxirane, dimethoxycarbene, oxadiazoline, oxirane.