Agrostophyllanthrol and Isoagrostophyllanthrol, Two Novel Diastereomeric Phenanthropyran Derivatives (I) from the Orchid Agrostophyllum khasiyanum.

TLDR: Agrostophyllanthrol and isoagrostophyllanthrol, two diastereomeric phenanthropyran derivatives, were isolated from the orchid Agrostophylla khasiyanum which earlier afforded eleven stilbenoids as discussed by the authors.

Abstract: Agrostophyllanthrol and isoagrostophyllanthrol, two novel diastereomeric phenanthropyran derivatives were isolated from the orchid Agrostophyllum khasiyanum which earlier afforded eleven stilbenoids, viz. the two dimeric phenanthrene derivatives agrostonin [2,2',7,7'-tetrahydroxy-4,4',6,6'-tetrainethoxy-1,1'-biphenanthryl] and agrostonidin [2,2',6,6'-tetrahydroxy-4,4',7,7'-tetramethoxy-1,1'-biphenanthryl], eight phenanthropyran derivatives imbricatin [2,7-dihydroxy-6-methoxy-9,10-dihydro-5H-phenanthro[4,5-bcd]pyran], agrostophyllidin [7-hydroxy-2,6-dimethoxy-9,10-dihydro-5H-phenanthro[4,5-bcd]pyran], agrostophylloxidin [7-hydroxy-2,5,6-trimethoxy-5H-phenanthro[4,5-bcd]pyran], flaccidinin [2,6-dihydroxy-7-methoxy-5H-phenanthro[4,5-bcd]pyran-5-one], isoflaccidinin [2,7-dihydroxy-6-methoxy-5H-phenanthro[4,5-bcd]pyran-5-one], agrostophyllin [7-hydroxy-2,6-dimethoxy-5H-phenanthro[4,5-bcd]pyran], agrostophyllone [7-hydroxy-2,6-dimethoxy-5H-phenanthro[4,5-bcd]pyran-5-one], agrostophylloxin [2,6,7-trimethoxy-5H-pheaanthro[4,5-bcd]pyran-5-one] and the bibenzyl derivative moscatalin [4,4'-dihydroxy-3,3',5-trimethoxybibenzyl]. The structures of agrostophyllanthrol and isoagrostophyllanthrol were established as 7-hydroxy-2,6-dimethoxy-5H-phenanthro[4,5-bcd]pyran-5(equat)ol and 7-hydroxy-2,6-dimethoxy-5H-phenanthro[4,5-bcd]pyran-5(ax)ol, respectively from various spectral and chemical evidence. Agrostophyllanthrol and isoagrostophyllanthrol, thus, represent two novel examples of otherwise conformational isomers becoming highly stable diastereomers due to unusual steric hindrance to conformational flipping.