A convenient synthesis of dihydropyranoxanthones
Abstract
The condensation of hydroxyxanthones, viz. 1-hydroxy-, 1,3-dihydroxy-, 1-hydroxy-3-methoxy-, 1,3-dihydroxy-5-methoxy-, and 1,3-dihydroxy-7-methoxy-xanthone, with isoprene in the presence of orthophosphoric acid, to give the corresponding dihydropyranoxanthones, is described. The resulting dihydropyranoxanthones or their methyl ethers have been dehydrogenated with 2,3-dichloro-5,6-dicyano-p-benzoquinone. The synthesis of the methyl ethers of naturally occurring osajaxanthone, 6-desoxyjacareubin, and 6-desoxyisojacareubin has also been achieved.