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From the seedpods of Tephrosia elata, a new β-hydroxydihydrochalcone named (S)-elatadihydrochalcone was isolated. In addition, the known flavonoids obovatachalcone, obovatin, obovatin methyl ether and deguelin were identified. The structures were determined on the basis of spectroscopic evidence. The crude extract and the flavonoids obtained from the seedpods of this plant showed antiplasmodial activities. The literature NMR data on β-hydroxydihydrochalcones is reviewed and the identity of some of the compounds assigned β-hydroxydihydrochalcone skeleton is questioned.
From the stem bark of Erythrina sacleuxii two new isoflavanones, (R)-5,7-dihydroxy-2',4',5'- trimethoxyisoflavanone (trivial name, (R)-2,3-dihydro-7-demethylrobustigenin) and (R)-5-hydroxy- 2',4',5'-trimethoxy-2'',2''- dimethylpyrano[5'',6'':6,7]isoflavanone (trivial name, (R)-saclenone) were isolated. In addition the known compounds shinpterocarpin, 2,3-dehydrokievitone, abyssinone V, abyssinone V-4'-methyl ether, erythrinasinate and 4'-O-methylsigmoidin B were isolated. The structures were determined on the basis of spectroscopic evidence.
The synthesis of six analogs of N,N;-diacetylchitobiose is reported, including a novel transglycosylation reaction for the preparation of S-aryl thioglycosides. The conformations of the compounds were studied by a combination of NMR spectroscopy and molecular modeling, using force field calculations. In the case of the S-aryl thioglycosides with exclusively S-glycosidic linkages, dihedral angles of the disaccharidic S-glycosidic bonds, ;; and ;; and of the S-arylglycoside bonds, ; and ;, were found to be similar, whereas they were different in mixed glycosides and in a thiazoline derivative. An adequate correlation between the calculated H,H-distances of the local minima and the measured NOE contacts was achieved by applying population-weighted averages over participating conformers based on weighted relative energies.
The soln.-state conformations of N,N',N''-triacetyl chitotriose (1) and other potential chitinase inhibitors 2-4 were studied using a combination of NMR spectroscopy (NOESY) and mol. mechanics calcns. Detn. solely of the global energy min. conformation was found to be insufficient for an agreement with the NMR results. An appropriate consistency between the NMR exptl. data and theor. calcns. was only reached by assessing the structures as population-weighted av. conformers based on Boltzmann distributions derived from the calcd. relative energies. Analogies, but also particular differences, between the synthetic compds. 2-4 and the naturally-occurring N,N',N''-triacetyl chitotriose were found. Furthermore, the conformation of compds. 1 and 2 when bound to hevamine was also studied using transferred NOESY expts. and the binding process was found to impart a level of conformational restriction on the ligands. The preferred conformation as detd. for 1 in the bound state to hevamine belonged to one of the conformational families found for the compd. when free in soln., although full characterization of the bound-state conformations was impeded due to severe signal overlap. Satn. transfer difference NMR expts. were also employed to analyze the binding epitopes of the bound compds. We thus detd. that it is mainly the acetyl amido groups of the trisaccharide and the heterocyclic moiety which are in close contact with hevamine.