A Study of the Interconverison Between 3,4-Dihydro-4-formyl-2-hydroxy-2H-benzopyran and 2,3,3a,8a-Tetrahydro-2-hydroxyfurobenzofuran Moieties, and its Application to a Formal Synthesis of (+/-)-Aflatoxin B1

Article abstract of DOI:10.1016/S0040-4020(01)90487-9

The presence of six compounds, i.e., the epimeric hydroxyfuro<2,3-b>benzofurans 4, a pair of diastereomers of the hydroxylated tetrahydrofuran 6 and the diastereomeric pair of benzopyran aldehydes 8, was identified in the equilibrium mixture established when hemiacetals 4 were in solution.This identification was carried out by 1H and 13C NMR techniques using 13C double labeled isotopomers of compound 4.Theoretical calculations performed at the semiempirical level (PM3) for all of the possible diastereomers involved in the equilibrium mixture afforded a good correlation between the respective heats of formation and the relative abundance determined by NMR.On the other hand, the intervention of aldehydes 8 in the equilibrium mixture was confirmed by their independent synthesis and the observation that they show identical NMR spectra to that exhibited by hemiacetals 4.Finally, the synthetic value of this equilibrium as an alternative entry to the furo<2,3-b>benzofuran fragment present in aflatoxins has become evident by its application to the convenient preparation of diacetate 17, an intermediate in the total synthesis of racemic aflatoxin B1 reported by Buechi and Weinreb. (J.Am.Chem.Soc. 1971, 93, 746-752).