THE THERMAL-ISOMERIZATION OF THE SESQUITERPENES ISOVELLERAL AND MERULIDIAL - A REVERSIBLE RING-OPENING OF THE CIS-METHYLCYCLOPROPANECARBALDEHYDE GROUP VIA AN INTRAMOLECULAR ENE REACTION

Fungal sesquiterpene dialdehydes of marasmane and isolactarane types, such as isovelleral (1) and merulidial (7), undergo a reversible thermal rearrangement to 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.