Agrostophyllanthrol and isoagrostophyllanthrol, two novel diastereomeric phenanthropyran derivatives from the orchid Agrostophyllum khasiyanum

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.