NOVEL OXIDATIVE PHENOL-COUPLING REACTION WITH PHOSPHOMOLYBDIC ACID ON SILICA-GEL SUPPORT - REGIOSELECTIVE BIOMIMETIC SYNTHESIS OF DIMERIC PHENANTHRENE DERIVATIVES

Biomimetic synthesis of the naturally occurring dimeric phenanthrene derivatives, cirrhopetalanthrin (3a) and flavanthrin (7a), and their structural analogues 3c, 3e, 5a and 7c was achieved in very high yields by regioselective oxidative coupling of their respective phenolic monomers 2a, 6a, 2c, 2e, 4a and 6c with phosphomolybic acid on a silica gel support. Unlike coelonin (6a) and lusianthridin (6c) 2,7-dihydroxy-3,4,6-trimethoxy-9,10-dihydrophenthrene (6e) gave, on similar oxidation, the phenanthrene dimer 3e, instead of the expected 9,10-dihydrophenanthrene dimer 7e. Beta-Naphthol afforded the corresponding 1,1'-dimer 1a in almost quantitative yield. The structures of the new dimeric compounds were established from various spectral data of the compounds and their respective acetyl derivatives.