Preparation of ent-gibberellane and 16-epi-ent-gibberellane

Abstract

A mixture of ent-gibberellane 1 and 16-epi-ent-gibberellane 2, the parent hydrocarbons of the gibberellin plant hormones, has been prepared from gibberellin A₁₄ dimethyl ester 5 by deoxygenation sequentially at carbons 3, 7 and 19 and then hydrogenation. The most direct route proceeded by dehydration of the 3-ol in 5 to the 2-ene 27, hydride reduction to the 7,19-diol 28, hydride reduction of the corresponding 7,19-dimethanesulfonate 29 to the 19-monomethanesulfonate 30, and then hydrolysis- to the 19-ol 31 which was converted into the phenyl selenide 38. Hydrogenolysis of the selenide 38, with concomitant hydrogenation of the 2- and 16-double bonds, gave a mixture of ent-gibberellane 1 and 16-epi-ent-gibberellane 2 in the ratio of 73:27. This mixture was separated on the analytical scale by capillary GLC and the two epimers were characterised by GLC–MS to provide reference mass spectra and KRI as fossil markers. The structures 1 and 2 were assigned on the basis that the major component was the endo-16-methyl epimer.