Comparative NMR study of rhamnobioses—applications

Abstract

L-Rhamnose is widely distributed in many plant families, where it occurs either alone as in quercetin, associated with D-glucose or D-galactose as in rutin or robinin, or as rhamnobioses in heterosides such as alaternin, xanthorhamnin and catharticin.NMR spectroscopy, both proton' and 13C2-4, has played an important role in determining the structure of the aglycone in flavonoids but it is more difficult to identify the sugar sequence, the point of attachment of the oligosaccharide to the aglycone and the position of the interglycosidic linkages.L-Rhamnose is also found in structural units of bacterial capsular polysa~charides,~.~ e.g. Streptococcus, Pneumococcus and Klebsiella. In studies concerned with their antigenic behaviour, and with the synthesis or analysis of immunodominant fragments, it is important to know the exact carbohydrate structures involved. For these reasons we have described the synthesis and NMR parameters of several di-and tri-saccharides containing L-rhamnose. This paper compares the proton and I3C NMR values of L-rhamnose (1) and three a-rhamnobioses, 0 -a -L-rhamnopyranosyl-( 1 -+ 2) -L-rhamnopyranose (2) and the (1 -+ 3) (3) and (1 -+ 4) (4) isomers (Fig. 1). Application of the data tabulated here has permitted the complete elucidation of the structure of a xanthorhamnin (Fig. 2) and, also, the decision to be made between two possible structures of an oligosaccharide obtained by bacteriophage degradation of the capsular polysaccharide in Klebsiella serotype K32.

Structurally diverse metabolites from the rare actinobacterium Saccharothrix xinjiangensis

“…COSY correlations from H-2'' to H-6'', in conjunction with the HMBC correlation of H-5'' to C-1'' suggested the presence 6-deoxyhexose sugar moiety. This was identified to be α-rhamnopyranose by comparison of the 1 H and 13 C spectroscopic data with those reported in the literature, 16 Lahoum et al 17 have recently published two bipyridine glycosides isolated from another Saccharothrix strain. One of the glycosides, cyanogriside J, has a similar structure to 1.…”

Two new flavone glycosides from Chenopodium ambrosioides growing wildly in Egypt

“…The 7-O-glycosylation was deduced from the UV data and confirmed by the presence of a signal C -7 at δ = 162.1 [15][16][17]. The (1→2) interglycosidic linkages were deduced from the downfield shift of carbon at position 2 (C-2'' at δ = 77.3 and C-2''' at δ = 77.2) and upfield shift of carbons at position 3 (C-3'' at δ = 71.1 and C-3''' at δ = 71.6) of the two non-terminal rhamnopyranoses [18]. This was ascertained from the NOESY spectrum which illustrated a relation between H-1''' at δ = 5.52 and H-2'' at δ = 3.81 and another relation between H-1'''' at δ = 5.28 and H-2''' at δ = 3.96.…”

“…The 7-O-glycosylation was deduced from the UV data and confirmed by the presence of a signal C-7 at δ = 161.4. The downfield shift of C-2'' of the internal rhamnopyranose moiety at δ = 79.5 indicated that the interglycosidic linkage between the two rhamnopyranose moieties is a 1→2 linkage [18]. HRESI-MS spectrum showed an ion peak at m/z 579.1692 corresponding to [ .…”

Methyltrioxorhenium‐Catalyzed Epoxidation‐Methanolysis of Glycals under Homogeneous and Heterogeneous Conditions

“…These epoxides afforded the derivatives 20a [44] and 21a [45] via S N 2 ring opening with MeOH at the anomeric carbon. The minor derivatives 20b [46] and 21b [47] are derived from the diastereomeric b-epoxides. Finally, the MTO-catalyzed epoxidation of d-arabinal derivatives occurred with high stereoselectivity.…”