THE ROLE OF DIANION COUPLING IN THE SYNTHESIS OF DIBENZYLBUTANE LIGNANS

Dianion coupling reactions have been used to prepare structurally related lignan natural products in racemic form. A readily available alpha-iodocarboxylic acid 10 has been employed in separate sequences to afford the tetrahydrofuran lignan burseran {6} and the butyrolactone lignan deoxypodorhizon {7}. The key strategy for both burseran and deoxypodorhizon involved reaction of the appropriate dianion with an alpha-iodo carboxylate salt 13. Equilibration under acid or base was employed to create the trans stereochemistry of the substituted benzyl side chains of both burseran and deoxypodorhizon. To achieve proper dianion reactivity and to avoid side reactions in the synthetic sequence to deoxypodorhizon, it was necessary to develop the chemistry of acylsulfonamide dianions. As a further structure proof, our synthetic deoxypodorhizon was also utilized as a substrate in a successful sequence to isostegane {8}, a known relay intermediate to the antineoplastic prototype steganacin {9}.