Synthetic studies directed towards complex diterpenoids. Part 15. Synthesis and stereochemistry of the catalytic reduction of Δ^4b(5)-gibbenes and related compounds

Abstract

The synthesis of, and substituent effects in the stereoselectivity of catalytic hydrogenation of, a series of methyl- or methoxy-substituted (±)-4b,9-cyclogibba-1(10a),2,4-trien-8-ones (1a-g) and (±)-gibba-1(10a), 2,4,4b(5)-tetraen-8-ones (2a-g) are described, which lead to stereocontrolled syntheses of the respective C-4b epimeric gibbanes (3a-g) and (4a-g). The key intermediate tetrahydrofluorene-2-carboxylic acids (10a-g), prepared by cycloaddition reactions, are converted into the corresponding α-diazomethyl ketones (12a-g) which are subjected to intramolecular oxo-carbenoid addition or perchloric acid-trifluoroacetic acid-catalysed cyclisation leading to the cyclopropyl ketones (1a-g) and the Δ^4b(5)-gibbenes (2a-g), respectively.