Fungal products. Part XIII. Xanthomegnin, viomellin, rubrosulphin, and viopurpurin, pigments from Aspergillus sulphureus and Aspergillus melleus

Abstract

The known fungal metabolites xanthomegnin (1) and viopurpurin (19) have been isolated from Aspergillus sulphureus together with the new pigments rubrosulphin (3,4,11,12-tetrahydro-9,17-dihydroxy-7-methoxy-3,12-dimethyl-2,13,16-trioxanaphth[1′,2′:5,6]indeno[2,1-a]anthracene-1,8,14,15-tetrone)(16) and viomellein {8-(3,4-dihydro-9,10-dihydroxy-7-methoxy-3-methyl-1-oxo-1H-naphtho[2,3-c]pyran-8-yl)-3,4-dihydro-6-hydroxy-9-methoxy-3-methylnaphtho[1,2-c]pyran-1,7,10-trione}; (10). The pigments (1), (10), and (19)were also isolated from Aspergillus melleus. Chemical and spectroscopic evidence for the proposed structures is presented. On this evidence structure (19) for viopurpurin is preferred to the alternative pyranobisnaphthopyran structures (22) or (23) previously suggested for this pigment.

N.m.r. evidence is presented showing that xanthomegnin (1) and viomellein (10) exist in solutions at room temperature as diastereoisomeric mixtures owing to restricted rotation about the dimeric linkage.