Ring opening of cyclobutene-1,2-dione and 3-trimethylsilylcyclobutene-1,2-dione. Dimerization/decarbonylation of a bisketene to a ketenylbutenolide

Hydrolysis of 3-trimethylsilyl-4,4-dichlorocyclobut-2-ene-1-one (4) in H2SO 4 gave small amounts of cyclobutene-1,2-dione (2a) and 3-trimethylsilylcycl obutene-1,2-dione (2d). Photolysis of 2a gave evidence for the formation of the bisketene (CH=C=O-2) (1a) in solution for the first time, as identifie d by the IR band at 2120 cm(-1) and the H-1 NMR signal at delta 3.20. Therm olysis of 2d gave evidence for the intermediacy of O=C=CHC(SiMe3)=C=O (Id), with an H-1 NMR signal at delta 3.43, and the isolation of the unusual ket enyl lactone 6, which could arise by formation of an unsymmetrical [2+2] ke tene dimer from 1d, followed by rearrangement and decarbonylation.