The Biosynthesis of Phenalenones

Over twenty phenalenones and related compounds have been isolated from fungi and from higher plants. The route of biosynthesis of the compounds isolated from fungi is however quite different from that of the phenalenones produced by green plants. The fungal phenalenones atrovenetin, herqueinone and norherqueinone are typical acetate-derived polyketides, the biosynthetic interrelationships of which have been clarified by the recent isolation of deoxyherqueinone from Penicillium herquei. In the monocot, Haemodorum corymbosum, on the other hand, the phenalenone nucleus is synthesised from two C9 residues derived from shikimate, and a single acetate unit, possibly via a diarylheptanoid intermediate. INTRODUCTION The phenalene nucleus (I) was reported as a constituent of natural products in 1955 and 1956, by two groups concerned with structural investigations of apparently unrelated pigments produced by fungi imperfecti1'2 and by an Australian monocotyledonous plant3. These initial studies described the occurrence of the phenalenone (II) nucleus in three fungal pigments, atrovenetin (lila), norherqueinone (IVa) and herqueinone (IVb) from Penicillium herquei and P. atrovenetum, the isolation and interrelationships of which had been previously established4-6, and also the higher plant cellobioside haemocorin (Va) from the rhizome of Haemodorum corymbosum (Haemodoraceae).3