The Thermal Isomerization of the Sesquiterpenes Isovelleral and Merulidial. A Reversible Ring Opening of the cis-Methylcyclopropanecarbaldehyde Group via an Intramolecular Ene Reaction

Article abstract of DOI:10.1021/jo00040a020

Fungal sesquiterpene dialdehydes of marasmane and isolactarane types, such as isovelleral (1) and merulidial (7), undergo a reversible thermal rearrangement to produce products (e.g., 2 and 8) with inverted orientations of the cyclopropane rings.The process is shown to involve an intramolecular ene reaction with a bicyclic enol intermediate 13 which was trapped as an E-silyl ether 17.In the presence of D2O, deuterium is incorporated quantitatively into the C-12 methyl groups of 1 and 2.A high kinetic isotope effect is observed for the rearrangement of 1 and its deuterated analogue 24, and the reaction parameters are comparable to those reported for the thermal ring-opening reactions of cis-alkylvinylcyclopropanes and cis-alkyl cyclopropyl ketones.In the presence of weak acid or base, an equilibrium is established between 1, 2, and the hydroazulenic dialdehydes 14 and 15.Dialdehyde 7 reacts less cleanly and incorporates deuterium not only at C-13 but also at C-1 (26).The latter process presumably involves enolization via a <1,5> sigmatropic hydrogen shift.