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Two new 'quinone methide' diterpenes with a cassane skeleton were isolated from the root bark of Bobgunnia madagascariensis (DESV.) J.K. KIRKBR. & WIERSEMA (Leguminosae). The two compounds occurred as a diastereoisomer mixture 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-phenanthro[1,2-c]pyran-6,12-dione by spectroscopic methods and by comparison with the 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 experiments, and H-1-NMR spectra of the separated diastereoisomers 1 and 2 were 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 fungi, in particular the yeast Candida albicans. |
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