Studies on the structure and stereochemistry of cytotoxic furanonaphthoquinones from Tabebuia impetiginosa: 5- and 8-hydroxy-2-(1-hydroxyethyl)naphtho[2,3-b]furan-4,9-diones

Abstract

Chemical investigation of the bark of Tabebuia impetiginosa(Bignoniaceae) afforded cytotosic furanonaphthoquinones, including 5-hydroxy-2-(1-hydroxyethyl)naphtho[2,3-b]furan-4.9 dione and its 8-hydroxy isomer. The position of the phenolic group in these two compounds was established by X-ray crystallography. The furanonaphthoquinones were found to be mixtures of both enantiomers in diffferent proportions, i.e., (for the 5-hydroxyisomer)R:S1:23 and (for the 8-hydroxy isomer)R:S 3:1. The synthesis of the racemic naphthofurans, their optical resolution into enantiomerically pure forms, and the determination of their absolute stereochemistry are described. The structure of kigelinone was also established to be that of the 5-hydroxy isomer, not the 8-hydroxy one, through this study.