Exploratory Studies on the Synthesis of the Unusual Diterpenoid Tropone Harringtonolide.

Abstract

Various approaches to the total synthesis of the unusual diterpenoid tropone (2), discovered in the yew species Cephalotaxus harringtonia and C. hainanensis, are described. The rhodium-catalysed intramolecular cyclopropanation reaction of an aryl ring by means of the transition metal catalysed reaction of a diazoacetyl function was used to assemble the 5/7 ring system and to provide a cycloheptatrienyl precursor to the tropone moiety, e.g.(28)→(29) and (38) →(39). In the most promising approach, the carbocyclic system was assembled by means of the aldol reaction (42) →(43) with the newly formed α-hydroxyl being employed subsequently in the formation of the δ-lactone function of (44). The tropone ring may be formed from the methoxycycloheptatriene moiety simply by treatment with mercuric nitrate. Tropone (45) was formed from (44) in this way, but attempts to convert it into harringtonolide by means of transannular oxidation based on the 4-hypoiodite failed. The crystal structure of an intermediate is reported.