Colouring matters of the West African red woods Pterocarpus osun and P. soyauxii. Structures of santarubins A and B

Abstract

The red heartwood of the West African trees Pterocarpus osun and P. soyauxii contains the pigments santarubins A and B and the known santalin A. Santarubins A and B have been identified as 6-(2,4-dimethoxybenzyl)-2,10-di-hydroxy-5-(4-hydroxy-3-methoxyphenyl)-1,3-dimethoxybenzo[a]xanthen-9-one (2a) and the corresponding 5-(3,4-dihydroxyphenyl) derivative (2b), respectively, on the basis of degradative and spectroscopic evidence, particularly the n.m.r. spectrum of the O-trideuteriomethyl derivatives in the presence of a lanthanoid shift reagent.