[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 BF 3 -catalyzed [2+2] cycloaddition of 15 with CH 3 CHO to form an isolable β-lactone 18 adduct which undergoes thermal decarboxylation to the vinylketene 19 . Reactions of 15 and 16 with CH 2 N 2 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 CH 2 N 2, 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.