Studies on gibberellin synthesis. Assembly of an ethanophenanthrenoid lactone and conversion into a gibbane derivative

Abstract

Diazoketone (13), prepared by standard methods from 5-formyl-8-methoxy-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid (7), was cyclized in trifluoroacetic acid to dienone (1) which under-went intramolecular 1,4-conjugate addition to afford the ethanophenanthrene derivative (15). Refunctionalization furnished dibenzoyloxy diacid (19) which cyclized in sulfuric acid specifically to the 8-carboxy-10,12-dibenzoyloxyperhydro-1H-2,10a-ethanophenanthrene-8,4b-carbolactone (2). Further refunctionalization to ketone (22), then formation of diazoketone (23) followed by photolysis in an aqueous medium, finally gave (1RS,4aRS,4bRS,7SR,8SR,9aSR,10SR,10aRS)-8-benzoyloxy-4a-hydroxy-1-methoxymethyloxymethylgibbane-1,10-dicarboxylic acid 1,4a-lactone (3) which may serve both as a model and intermediate for a radically new approach to the total synthesis of gibberellins.