FORMAL ASYMMETRIC-SYNTHESIS OF PENTALENOLACTONE-E AND PENTALENOLACTONE-F - 2 - CONSTRUCTION OF THE ANGULAR DIQUINANOID DELTA-LACTONE

A formal asymmetric synthesis of pentalenolactone E (1b) and pentaleno lactone F (1a) has been accomplished. Ozonolysis of the diphenyl-subst ituted triquinane 3 and Kauffmann methylenation of ketone 5 with WOCl3 /2 MeLi yielded the unsubstituted triquinane 9. The crucial rearrangem ent of the linear triquinanoid lactone 11 to the angular triquinanoid lactone 14a was accomplished using orthoformate and acid in methanol. Subjecting triquinanes 14a/b to the selenoxide method gave triquinene 15. Homologation of gamma-lactone 15 to the angular diquinanoid delta- lactone 2 via a Horner-Wadsworth-Emmons or Peterson reaction of hemiac etals 16a/b was, however, not successful. Chemoselective reduction of 14a afforded hemiacetals 21a/b, reaction of which with the phosphonate salt 17a ultimately led to the ketene dithioacetal 22. The angular in termediates 25a/b were obtained from 22 by reduction to give the linea r hemiacetals 24a/b, which rearranged to the dithio ortholactones 25a/ b in the presence of acid. Introduction of the double bond and deprote ction were accomplished via selenation of 25a/b with N,N-diethylbenzen eselenylamide and treatment of selenides 30a/b with silver nitrate. Th e unsaturated aldehydes 28 and 29 thus obtained were converted to 2 an d 31, respectively, by oxidation with manganese dioxide in the presenc e of sodium cyanide, methanol and acetic acid. Alkene 2 was isolated b y crystallization.