[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 thermal decarboxylation to the vinylketene 19. Reactions of 15 and 16 with CH2N2 proceed by [4+1] cycloaddition to give mixtures of cyclopentene-1,3-diones 20 and methylenefuranones 21, while Me(3)SiCHN(2) and PhCHN(2) give only 20. The reactions are interpreted in terms of a steric preference for nucleophilic attack by the substituted diazomethanes from the side of the ketene bearing the Me(3)SiC=C=O substituent, leading 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 formation of lactones 21. Reaction of tetramethoxyethylene with 15 yields both a cyclopentene-1,3-dione 24 from net addition of dimethoxycarbene and a spirocyclopropylbutenolide 25. Free dimethoxycarbene generated by heating an oxadiazoline precursor also reacted with 15 to give dione 24. Various electrophilic dienophiles do not react with 15, but nucleophilic alkynes react with 16 in thermal reactions to give spirocyclopropenylfuranones 33-36, and Me(3)SiC=COEt and 16 react by net [4 + 2] cycloaddition to give the quinone 37 as the major product.