Synthesis of hexa-, tetra-, and dihydroxanthene derivatives from salicylaldehyde aminals

Salicylaldehyde aminals react with cyclohexanone upon heating to form tetra hydroxanthene derivatives. The structure of one of these derivatives, viz., 5,7-dichloro-4a-morpholino-1,2,3,4-tetrahydro-4aH-xanthene, was establishe d by X-ray diffraction analysis. The scheme of the reaction was suggested, which involves cycloaddition of intermediate o-methylene-quinone (from amin al) and enamine (from cyclohexanone). The reactions of salicytaldehyde amin als with enamines that formed from cyclohexanone can successively afford de rivatives of hexahydroxanthene, tetrahydroxanthene, and dihydroxanthene. Pr ocedures were developed for the synthesis of these compounds. N-Substituted 4a-amino-7-nitro-1,2,3,4-tetrahydro-4aH-xanthenes were also prepared by th e reactions of dialkylammonium 2-formyl-4-nitrophenoxides with the above-me ntioned enamines.