Diradical-Promoted (n + 2 − 1) Ring Expansion:  An Efficient Reaction for the Synthesis of Macrocyclic Ketones

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 °C affords the one-carbon ring-expanded cycloalkanones in good yields. Methyl groups on the vinyl moiety are transformed regiospecifically as corresponding α- and β-substituents, respectively. 2-Ethynyl precursor analogues react in a manner similar to give α,β-unsaturated cyclic ketones.