Synthetic studies towards complex diterpenoids. Part 11. Stereochemically defined synthesis of the racemates of 1,2,3,4,4a,9a-hexahydro-7-methoxy-1-methylfluorene-1-carboxylic acid
Two simple synthetic routes to 1,2,3,4-tetrahydro-7-methoxy-1 -methylfluorene-1 -carboxylic acid (1b), a potential intermediate towards gibberellins, and its stereochemically defined transformations to three racemates of 1,2,3,4,4a,9a- hexahydro-7-methoxy-1 -methylfluorene-1 -carboxylic acid (4b)–(6b) are described. Catalytic hydrogenation of (1 b) yields a mixture of (5b) and (6b) in a ratio of ca. 95 : 5, whereas, lithium-liquid ammonia reduction of (1b) gives (4b) and (6b) in a ratio of ca. 23 : 77. A mechanism is proposed to explain the stereochemical results in the lithium–ammonia reduction of (1b) and related systems.