Studies in terpenoid biosynthesis. Part VI. The stereochemistry of some stages in tetracyclic diterpene biosynthesis

Abstract

The stereospecific labelling of the gibberellins and the kaurenolides with 2-(R)-, 2-(S)- and 5-(R)-[³H]mevalonate has been studied. The results lead to the conclusion that the dehydrogenation of ring A in gibberellic acid bio-synthesis is a cis-process and that the hydroxylation of ring B in the formation of the kaurenolides takes place with retention of configuration. One tritium atom from [1-³H₂,2-¹⁴C]geranyl pyrophosphate is retained at position 10 in gibberellic acid. However 5-(R)-mevalonoid hydrogen is lost from ring B during the formation of the gibberellins. Consequently ring contraction takes place with the displacement of an equatorial hydrogen atom from the kauranoid C-6-position.