Location of O-acetyl groups in the acidic d-xylan of mimosa scabrella (bracatinga). A study of O-acetyl group migration

Abstract

Extraction with dimethyl sulfoxide of wood-meal of the stem of bracatinga ( Mimosa scabrella), a south Brazilian hardwood, that was defatted and delignified by treatment with aqueous chlorine at 0–5° followed by extraction with cold ethanol, gave a soluble O-acetylated 4- O-methyl- d-glucurono- d-xylan having (1→4)-linked β- d-xylopyranosyl residues that were unsubstituted (65%) and 2- O-(14%), 3- O- (16%), and 2,3-di- O-acetylated (5%), as determined by methylation analysis. Another preparation obtained by use of refluxing ethanol in the delignification process showed neither removal nor migration of acetyl groups. By comparison with synthetic, partly O-acetylated d-xylans of known composition, 13C-n.m.r. spectroscopy indicated that O-acetyl group migration does not occur during treatment with cold aqueous chlorine, refluxing ethanol, or water at 70°. Methyl 2- O-acetyl-4- O-methyl-β- d-xylopyranoside ( 6) was also unaffected by aqueous chlorine. O-Acetyl group migration took place more readily in aqueous and dimethyl sulfoxide solutions of 6 than of O-acetyl- d-xylans. The lowest temperatures at which migration was observed in monosaccharides was at 50 and 70° for solutions in D 2O and (CD 3) 2SO, respectively.