Bibenzyl derivatives from the orchid Dendrobium amoenum P.L Majumder*, S. Guha, S. Sen Department of Chemistry, University College of Science, 92 Acharya Prafulla Chandra Road, Calcutta 700 009, India Received 6 May 1999; accepted 9 June 1999 Abstract Reinvestigation of the orchid Dendrobium amoenum aorded two new bibenzyl derivatives, amoenylin and isoamoenylin, and their known structural analogues, 3,4 ’-dihydroxy-5-methoxybibenzyl and 4,4 ’-dihydroxy-3,3 ’,5-trimethoxybibenzyl (moscatilin), besides the sesquiterpenoids, amotin, amoenin and flaccidin reported earlier from this orchid. The structures of amoenylin and isoamoenylin were established as 4-hydroxy-3,4 ’,5-trimethoxybibenzyl and 3 ’-hydroxy-3,4,5-trimethoxybibenzyl, respectively, mainly from spectral evidence. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Dendrobium amoenum; Orchidaceae; Amoenylin; Isoamoenylin; Moscatilin and 3,4 ’-dihydroxy-5-methoxybibenzyl; Bibenzyl deriva- tives; Amotin and amoenin; Sesquiterpenes; Flaccidin; 9,10-Dihydrophenanthropyran derivative 1. Introduction As part of a general programme of research on orchids, we have chemically reinvestigated the orchid Dendrobium amoenum which has aorded two new bibenzyl derivatives, designated amoenylin and isoa- moenylin, and two of their known structural ana- logues, moscatilin (4g) (Majumder & Sen, 1987) and 3,4 ’-dihydroxy-5-methoxybibenzyl (4e) (Crombie & Crombie, 1982; Crombie & Jamieson, 1982), besides the picrotoxin group of sesquiterpenoids, amotin (1) and amoenin (2) (Dahmen & Leander, 1978) and the 9,10-dihydrophenanthropyran, flaccidin (3) (Veerraju et al., 1989) reported earlier from the same orchid. The above group of authors reported the isolation of flacci- din (3) from this orchid giving a new name to it (amoenumin) and claiming it to be a new compound which was, however, isolated from Coelogyne flaccida about one year back (Majumder & Maiti, 1988). While the structure of 4e was further confirmed by its 13 C- NMR spectral data, those of amoenylin and isoamoe- nylin were shown to be 4a and 4c, respectively, mainly from the following spectral evidence. 2. Results and discussion Both amoenylin (4a) and isoamoenylin (4c) were shown to have the same molecular formula C 17 H 20 O 4 from elemental analysis and their mass spectrometri- cally derived molecular weight 288. Both 4a and 4c showed typical benzenoid UV absorptions [4a: l max 208.5 and 274 nm log e 4.20 and 3.47); 4c: l max 217.5 and 280 nm log e 4.04 and 3.90)] which are similar to those of bibenzyl derivatives (Majumder & Ghosal, 1994; Majumder, Roychoudhury & Chakraborty, 1997). The phenolic nature of the compounds was indi- cated by their characteristic colour reactions with FeCl 3 (violet) and phosphomolybdic acid reagent (intense blue), alkali induced bathochromic shifts of their UV maxima and their IR absorption bands [4a: n max 3400 cm 1 (OH); 4c: n max 3410 cm 1 (OH)]. The presence of a single phenolic hydroxyl group in each of 4a and 4c was confirmed by the formation of their respective monoacetyl derivatives 4b and 4d, both hav- ing the same molecular formula C 19 H 22 O 5 (M + at m/z 330). The 1 H-NMR spectrum of each of 4a and 4c showed signals for one phenolic hydroxyl proton (4a: d 5.30; 4c: d 5.03; each disappeared on deuterium exchange), three aromatic methoxyl groups [4a: d 3.71 (3H, s) and Phytochemistry 52 (1999) 1365–1369 0031-9422/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0031-9422(99)00370-2 * Corresponding author.