Nuclear Magnetic Resonance Study on Glycosyl Esters : Glycosyl Esters of 3-O-Acetyloleanolic Acid and Octanoic Acid

Abstract

α-L-Rhamnosyl, α-L-arabinosyl, 2'-O-β-D-glucosyl-α-L-arabinosyl and 2'-O-α-L-rhamnosyl-α-L-arabinosyl esters of octanoic aicd and β-D-xylosyl, α-L-rhamnosyl, α-L-arabinosyl, 2'-O-β-D-glucosyl-α-L-arabinosyl and 2'-O-α-L-rhamnosyl-α-L-arabinosyl esters of 3-O-acetyloleanolic acid were synthesized. The nuclear magnetic resonance spectra of sugar moieties of the esters of 3-O-acetyloleanolic acid, which is a representative sterically hindered carboxylic acid, were compared with those of the corresponding less-hindered glycosyl esters of octanoic acid. In the case of rhamnose and arabinose, the displacements of sugar carbon signals on acylation of the anomeric hydroxyl group are similar to those already observed for fatty acid glucosyl esters regardless of the steric hindrance of the acyl group. However, the 2-O-glycosylation shifts of 1-O-acyl-α-L-arabinopyranose were found to depend upon the steric hindrance of the acyl groups. The anomalous 2'-O-glycosylation shifts of arabinosyl carbon signals of α-L-arabinosyl 3-O-acetyl-oleanolate were not observed in the case of α-L-arabinosyl octanoate.