Pigments of Fungi. LIX. Synthesis of (1S,3S)- and (1R,3R)-Austrocortilutein and (1S,3S)-Austrocortirubin from Citramalic Acid

The naturally occurring tetrahydroanthraquinone (1S,3S)-austrocortilutein (1) is synthesized for the first time in enantiomerically pure form by Diels–Alder cycloaddition between the functionalized butadiene derivative (8) and the chiral 1,3-dihydroxy-1,2,3,4-tetrahydro-5,8-naphthoquinone (9), the latter being derived from (R)-citramalic acid (3). The natural products (1S,3S)-austrocortirubin (2) and (1R,3R)-austrocortilutein (5) were also prepared for the first time by using the same strategy.