Stereospecific Dicobalt Octacarbonyl Mediated Enyne Cyclization for the Enantiospecific Synthesis of a 6a-Carbocycline Analogue

D-(+)-Ribonolactone 5 was converted into the butenolide 7 by pyrolysis of the derived ortho ester. Treatment of 7 with trisyl bromide gave the corresponding trisylate 9, which was converted into 10 by using Li,(CH,=CH),CuCN. Exposure of 10 to potassium carbonate in methanol gave epoxide 12, which underwent ring opening when treated with lithium (tri- methylsi1yl)acetylide-BF,.OEt, to give lactone 13. Reduction of lactone 13 with LiAlH, gave diol 18, which was converted into its derived acetonide 19. When 19 was treated with CO,(CO)~/CO/P~,PO, bicyclo[3.3.0]octenone 21 was formed in a highly stereoselective process. Conversion of 21 into the carbocycline analogue 28 was achieved by standard methods. The absolute configuration of 21 was established by single-crystal X-ray crystallography on the derived bis(p-bromobenzylidene) derivative 24.