Synthesis of the Enantiomeric Furobenzofurans, Late Precursors for the Synthesis of (+)- and (−)-Aflatoxins B₁, B₂, G₁, and G₂

Enantiomeric tetrahydrofuro[2,3-6]benzofurans, representing the ABC tricyclic portion of aflatoxins B₁, B₂, G₁, and G₂, were generated from the oxaza-Cope rearrangement of a suitably functionalized O-aryloxime. The O-aryloxime was, in turn, made from the condensation of an enantiomerically pure aldehyde derived from glutamic acid and a substituted phenoxyamine. High regioselectivity with respect to the A-ring substituents of the ABC tricycle was achieved through the use of electrochemistry. The regioselective electrochemical cleavage of 4,6-bis(tosyloxy)-2-(methoxycarbonyl)-2,3,3a,8a-tetrahydrofuro[2,3-6]benzofuran (22) resulted in a 97/3 mixture of regioisomeric phenols. The regiochemical assignments of the resulting phenols were determined by 2D NOESY NMR. The enantiomeric ratio of the final product was determined to be 96/4 by NMR analysis of diastereomers resulting from the coupling of 31a to (+)- and (±)-phenethylamine.