[2+2], [4+1], AND [4+2]CYCLOADDITION REACTIONS OF SILYLATED BISKETENES

Cycloaddition reactions of the bisketenes O=C=C(SiMe(3))CR=C=O (15, R = SiMe(3); 16, R = Ph) include BF3-catalyzed [2+2] cycloaddition of 15 with CH3CHO to form an isolable p-lactone Is adduct which undergoes t hermal decarboxylation to the vinylketene 19. Reactions of 15 and 16 w ith CH2N2 proceed by [4+1] cycloaddition to give mixtures of cyclopent ene-1,3-diones 20 and methylenefuranones 21, while Me(3)SiCHN(2) and P hCHN(2) give only 20. The reactions are interpreted in terms of a ster ic preference for nucleophilic attack by the substituted diazomethanes from the side of the ketene bearing the Me(3)SiC=C=O substituent, lea ding to formation of 20. With the less bulky CH2N2, attack occurs from both sides and approach from the side of the R group leads to formati on of lactones 21. Reaction of tetramethoxyethylene with 15 yields bot h a cyclopentene-1,3-dione 24 from net addition of dimethoxycarbene an d a spirocyclopropylbutenolide 25. Free dimethoxycarbene generated by heating an oxadiazoline precursor also reacted with 15 to give dione 2 4. Various electrophilic dienophiles do not react with 15, but nucleop hilic alkynes react with 16 in thermal reactions to give spirocyclopro penylfuranones 33-36, and Me(3)SiC=COEt and 16 react by net [4 + 2] cy cloaddition to give the quinone 37 as the major product.