Cationic rearrangements and cyclizations of diterpenoid olefins

Abstract

The biogenesis of tetracarbocyclic diterpenes is considered to involve cyclisation via a bicyclic C-13 carbonium ion. This species has been generated in vitro from manool and Δ 13-manool and found to give, in refluxing acetic acid, a 1:1 mixture of Δ 13-manool acetate and olefins. The latter consisted mainly of labdatrienes with smaller amounts of three classes of cyclized products. Ring closure between C-13 and C-17 gave approximately equal amounts of tricyclic α-vinyl isopimaric and β-vinyl pimaric Δ 7, Δ 8 and Δ 8(14) dienes together with products of backbone rearrangement. Each of the dienes was found to be stable under the reaction conditions, indicating backbone rearrangement occurred by transfer of the migrating functions along the backbone of the molecule without participation of the intermediate olefins. The third type of cyclized product was a new cycloocta-1,5-diene which we call 8,13-burnabadiene, generated by cylization between C-15 and C-17. Under refluxing formic acid, formation of labdatrienes was precluded and yields of the initially cyclized pimaradienes and isopimaradienes, the backbone rearranged products and burnabadiene increased. In this reaction the ratio of Δ 13-manool formate to olefins was 1:7. The initial dienes and backbone rearranged products were interconverted by the reaction conditions showing that backbone rearrangement is reversible. A tetracyclic product, hiban-14-α-yl-formate was also isolated and was formed in quantitatively when 8,13-burnabadiene was subjected to the reaction conditions. Deuterium labelling of Δ 13-manool at C-14 showed that hiban-14-α-yl-formate was indeed formed via such a carbon skeleton.