Pigments of fungi. LXIII - Synthesis of (1S,3R)- and (1R,3S)-austrocortilutein and the enantiomeric purity of austrocortilutein in some Australian Dermocybe toadstools

The naturally occurring tetrahydroanthraquinones (1S,3R)- and (1R,3S)-austr ocortilutein (1b) and (1d), respectively, are synthesized for the first tim e in enantiomerically pure form by Diels-Alder cycloaddition between the fu nctionalized butadiene derivative (4) and the corresponding monochiral tran s-1,3-dihydroxy-1,2,3,4-tetrahydro-5,8-naphthoquinone (5a) or (5b), themsel ves derived from citramalic acid. Separation of the four stereoisomeric aus trocortiluteins by using h.p.l.c. over a chiral stationary phase reveals th at the enantiomeric purity of the (1S, 3S) and (1R,3R)-quinones (1a) and (1 c) varies from species to species whereas the (1S, 3R)- isomer (1b) is, in the five cases examined, enantiomerically pure.