SYNTHETIC STUDIES TOWARD COMPLEX DITERPENOIDS .12. STEREOCONTROLLED TOTAL SYNTHESIS OF SOME GIBBANE SYNTHONS AND DEGRADATION PRODUCTS OF GIBBERELLINS

Stereocontrolled total syntheses of (±)-gibberone (13b) and (±)-4-methyl-9β, 13α-dihydro-16-oxogibba-1, 3, 5-(10)-triene (15a), two degradation products of gibberellins, and a few C-9 epimeric gibbane synthons, 14a, b and 15b, are described. The key intermediates, l, 2, 3, 4-tetrahydro-8-methylfluorene-2-carboxylic acid (6a) and its 2-methyl analogue 6b, prepared by cycloaddition reactions, are converted into the corresponding α'-diazomethyl ketones 11a and lib and are subjected to intramolecular ketocarbenoid addition and boron trifluoride-ethercatalyzed cyclization leading to cyclopropyl ketones 12a, b and Δ9, 11-gibbenes 13a, b, respectively. Catalytic hydrogenation of the cyclopropyl ketones 12a and 12b with palladium-on-charcoal in ethanol proceeded with high stereoselectivity and produced the 9αH epimers 14a and 14b containing minor amount of 9βH epimer 15a in the former case. Under the same conditions reduction of the Δ9, 11-gibbenes 13a and 13b gave a mixture of the epimeric 9αH- and 9βH-gibbanes 14a and 15a (30-20:70-80) and 14b and 15b (~63:37), respectively. © 1979, American Chemical Society. All rights reserved.