Rigidanthrin, a new dimeric phenanthrene derivative of the orchid Bulbophyllum rigidum

Abstract

: Rigidanthrin, a new dimeric phenanthrene derivative, was isolated from the orchid Bulbophyllum rigidum which also afforded the known monomeric phenanthrenes nudol and gymnopusin and the simple aromatic compound ethyl 4-hydroxy-3-methoxycinnamate. The structure of rigidanthrin was established as 2,2',7,7'-tetrahydroxy-3,3',4,4'-tetramethoxy-1,1'-biphenanthryl from various spectral and chemical evidence. The structure of rigidanthrin was finally confirmed by regio- as well as regio- and enantio-selective biomimetic synthesis from its monomeric congener nudol (2,7-dihydroxy-3,4-dimethoxyphenanthrene) by oxidative phenol-coupling reaction with phosphomolybdic acid (PMA) on silica gel surface and CuCl(OH).(-)-(S)-proline methyl ester, respectively, in very good yields. The optical purity of (-)-rigidanthrin obtained in the latter case was found to be 95.79%. The co-occurrence of rigidanthrin with its monomer nudol in the same orchid Bulbophyllum rigidum provides a strong circumstantial evidence in support of the proposed biogenesis of the naturally occurring biphenanthryl derivatives which are assumed to have been formed from their corresponding monomers by enzymatic oxidative phenol-coupling reaction.