Peracetylated disaccharides

Peltier, J.M. MacLean, D.B. Szarek, W.A. Determination of the Glycosidic Linkage in Peracetylated Disaccharides Comprised of D-Glucopyranose Units by Use of Desorption Electron-Ionization Mass Spectrometry. Rapid Commun. Mass Spectrom. 1991,5,446-449. 

The conventional mass spectra obtained from saccharides transformed into their OTMS and -OCH3 derivatives were interpreted by using labeled derivatives. Systematic studies had never been performed on -OAc derivatives. However, in 1975, Das and Thayumanavan [230] studied various peracetylated disaccharides by their IKE spectra to resolve the ambiguity appearing in the El mass spectra of the isomeric peracetates of trehalose, sophorose, kojibiose, laminaribiose, maltose, melibiose and gentiobiose, all characterized by the same molecular weights (678). 

Bhaskar and Loganathan96 described O-peracetylation of monosaccharides, disaccharides, and methyl glycosides (94) with acetic anhydride under catalysis by acid zeolites. From the panel of zeolites tested (HY, HEMT, HZSM-5, HZSM-12, HZSM-22, and H-beta), the large-pore zeolite H-beta provided the best yields of the fully acetylated sugars, most of them being over 85% and up to 99%, with the pyranose forms 95 accounting for 66-100% of the reaction products (Scheme 22). 

SCHEME 22. Peracetylation of monosaccharides, disaccharides and methyl glycosides in the presence of H-beta zeolite. 

For the galactosylation reactions, 2,3,4,6-tetra-O-benzylgalactopyranosyl trichloroacetimidate 2 and dibenzylphosphite 10 were used as donors (Table 12.1). Under these conditions, the (5-(l—>3)- and (5-(l—>4)-linked disaccharides 16 and 17 were formed in only minor amounts (entries 1 and 2). To increase the proportion of the (5-anomers, the peracetylated galactopyranosyl trichloroacetimidate 11 was used as the donor. Reactions were poor in DMF, but in dioxane, a clean mixture of products was obtained. Two equivalents of donor were required to give a conversion of 20-30% of 1 to disaccharides 13-18. Peaks eluting when the column was washed with more polar solvent mixtures suggested that only traces of trisaccharides were formed, but the identity of these peaks was not investigated. 

Acetylation. Rapid peracetylation of carbohydrates can be effected in the presence of iodine. With more iodine and longer reaction times the selective acetolysis of primary benzyl ethers is achieved. Perbenzyl ethers of mono- and disaccharides undergo group exchange (OBn —> OAc) at the primary carbon atoms on treatment with ACjO-HOAc-ZnClj. 

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