New antifungal 'quinone methide' diterpenes from Bobgunnia madagascariensis and study of their interconversion by LC/NMR

Two new 'quinone methide' diterpenes with a cassane skeleton were isolated from the root bark of Bobgunnia madagascariensis (DESV.) J.K. KIRKBR. & WIE RSEMA (Leguminosae). The two compounds occurred as a diastereoisomer mixtur e 1/2, Their structure was established as (4R,6aS,10aS)- and (4S,6aS,10aS)- 1,4,6a,7,8,9,10,10a-octahydro-4,11-dihydroxy-7,7,10a-trimethyl-2H-phenanthr o[1,2-c]pyran-6,12-dione by spectroscopic methods and by comparison with th e data obtained for another 'quinone methide' diterpene isolated previously from the same plant and identified by single-crystal X-ray analysis. H-1- and C-13-NMR signals were assigned by extensive in-mixture 2D correlation e xperiments, and H-1-NMR spectra of the separated diastereoisomers 1 and 2 w ere recorded by on-now LC/NMR. The interconversion of 1 and 2 was followed by repeated stop-flow LC/NMR experiments over a two-hour period, Compounds 1 and 2 showed moderate antifungal properties towards human pathogenic fung i, in particular the yeast Candida albicans.