Diradical-promoted (N + 2 - 1) ring expansion: an efficient reaction for the synthesis of macrocyclic ketones.

[reaction: see text] A diradical-promoted (n + 2 - 1) ring expansion reaction based on vinyl side chain insertion (+2C) and decarbonylation (-1C) has been developed. Flash vacuum pyrolysis (FVP) of medium- and large-ring 2-trimethylsilyloxy-2-vinyl-cycloalkanones at 500-600 degrees C affords the one-carbon ring-expanded cycloalkanones in good yields. Methyl groups on the vinyl moiety are transformed regiospecifically as corresponding alpha- and beta-substituents, respectively. 2-Ethynyl precursor analogues react in a manner similar to give alpha,beta-unsaturated cyclic ketones.