Synthesis of Labdane Diterpenoids

Abstract

<p> The previously known, 8-oxo-(13→17)-pentanorlabda-12,19-dioic acid 19-methyl ester 36, was synthesised via two routes following the extensive investigation of the dehydrobromination of bromo-ketone 48, and also partial ozonolysis of the phenolic ring of podocarpic acid. </p> <p> This keto-acid 36 has been transformed to the corresponding 8-methylene compound in high yield. Application of the latter intermediate to diterpene synthesis has been demonstrated by the synthesis of 12-hydroxylabd-8(17)-en-19-oic acid 37 and methyl 12-ketolambertianate 61. </p> <p> During investigation of other synthetic intermediates a selective reducing agent for carboxylic esters in the presence of the carboxylic acid function - sodium trimethoxyborohydride, has been discovered. </p>