Stilbenoids and sesquiterpene derivatives of the orchids Gastrochilum calcoelaria and Dendrobium amoenum : Application of 2D NMR spectroscopy in structural elucidation of complex natural products

Systematic chemical investigation of the Indian orchid Gastrochilum calcoelaria D.Don for the first time afforded two new bibenzyl derivatives, designated gastrochilin and gastrochilinin, besides the compounds confusarin [2,7-dihydroxy-3,4,8-trimethoxyphenanthrene] and 2,7-dihydroxy-3,4,6-trimethoxyphenanthrene. The orchid Dendrobium amoenum Wall was chemically investigated earlier. Further chemical investigation of this Indian orchid has resulted in the isolation of yet another new bibenzyl derivative amodenylinin, besides the previously reported stilbenoids amoenylin [4-hydroxy-3,4',5-trimethoxybibenzyl], isoamoenylin [3'-hydroxy-3,4,5-trimethoxybibenzyl], moscatilin [4,4'-dihydroxy-3,3',5-trimethoxybibenzyl], batatasin-III [3,3'-dihydroxy-5-methoxybibenzyl], 3,4'-dihydroxy-5-methoxybibenzyl, the two phenanthrenes confusarin and 2,7-dihydroxy-3,4,6-trimethoxyphenanthrene, the two phenanthropyran derivatives imbricatin [2,7-dihydroxy-6-methoxy-9,10-dihydro-5H-phenanthro[4,5-bcd]pyran] and flaccidin [2,6-dihydroxy-7-methoxy-9,10-dihydro-5H-phenanthro[4,5-bcd]pyran], the two sesquiterpenoids amotin and amoenin, p- hydroxybenzaidehyde and beta-sitosterol. The structures of gastrochilin, gastrochilinin and amoenylinin are established as 2'-hydroxy-3,3'-diniethoxy-4,5-methylenedioxybibenzyl, 2',3,3'-trimethoxy-4,5-methylenedioxybibenzyl and 3,3',4,4',5-pentamethoxybibenzyl, respectively, from spectral and chemical evidence. Additional spectral evidence for 3,4'-dihydroxy-5-methoxybibenzyl and detailed 2D NMR spectral analysis of amotin are provided for further confirmation of their assigned structures.