Structures of the red sandalwood pigments santalins A and B
The red sandalwood (Pterocarpus santalinus) pigment santalin A has been identified as 6-(3,4-dihydroxybenzyl)-2,10-dihydroxy-5-(4-hydroxy-2-methoxyphenyl)-1,3-dimethoxybenzo[a]xanthen-9-one (2) on the basis of degradative and spectroscopic evidence. Santalin B (3) is the corresponding 6-(4-hydroxy-3-methoxybenzyl) derivative. N.m.r. data, particularly those obtained by use of the shift reagent Eu(dpm)3, have led to the assignment of a revised structure (24) to per-O-methylsantarubin. A common biogenetic scheme for santalins and santarubin is suggested.