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

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.