TOTAL SYNTHESIS AND ABSOLUTE-CONFIGURATION OF PSEUDOSEMIGLABRIN, A PLATELET-AGGREGATION ANTAGONIST, AND ITS DIASTEREOMER SEMIGLABRIN

A general approach to the synthesis of the flavone-furo[2,3-b]furan ring system present in numerous biologically-active secondary metabolites of Tephrosia sp. has been developed and applied in one racemic synthesis and two asymmetric syntheses of four members of the family. It uses 2-diazo-1,3-cyclohexanedione as the keystone of the ring system, uniting it with a dihydrofuran through a rhodium-mediated dipolar cycloaddition. The enolate of this tricyclic intermediate is then utilized to elaborate a salicylate that is subjected to a concise annulation protocol with benzaldehyde to produce the tetracycle. Stereochemical control is accomplished by the use of three strategies. Reduction of a ketone from the more accessible face of a folded bicyclooctane ring system produces the endo stereochemistry. Steric hindrance by a bulky allylic siloxy group directs the cycloaddition to the opposite face of the prochiral alkene to generate the exo stereochemistry. Finally, a novel hydroxyl-directed cycloaddition simultaneously produces the endo stereochemistry and accesses the opposite enantiomeric series.