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

Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road,
Kolkata-700 009, India
E-mail : priyalalm@hotmail.com
Manuscript received 15 September 2008, accepted 30 September 2008 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 CuCI(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.