Total synthesis of the macrocyclic diterpene (–)-casbene, the putative biogenetic precursor of lathyrane, tigliane, ingenane, and related terpenoid structures

Abstract

A total synthesis of the [12.1.0]bicyclic diterpene 1S,3R(–)-casbene (19), based on elaboration of 1R,3S(+)-cis-chrystanthemic acid (21) to the bis-allylic bromide (20), followed by closure of the 14-membered ring with tetracarbonylnickel, is described.

Synthetic casbene did not separate from natural casbene, isolated from Ricinus communis, in mixed chromatographic analysis, and their ¹H n.m.r, and mass spectral data were closely similar. ¹³C N.m.r. data support the casbene structure (19), having a 1S,3R-cis-cyclopropane ring, and a 4,8,12-all-E-triene system.