Cyclisation of acetylenecarboxylic acid. Synthesis of γ-methylenebutyrolactones

Various γ-exo-methylenebutyrolactones have been synthesized in excellent yield by the cyclisation of acetylenecarboxylic acids in the presence of a catalytic amount of mercury(II) oxide. Cyclisation of terminal acetylene compounds[e.g.(1a)–(1e)]proceeded regioselectively to give γ-exo-methylenebutyrolactones as the sole product. Disubstituted acetylenes [(1f)–(1i)] also gave the (Z)-configurational enol lactone, but small amounts of an (E)-isomer of aγ-exo-enol lactone and a δ-lactone (α-pyrone) were also formed. Spectral properties and stereochemistry of the exo-enol lactones are also discussed.