Disaccharides peracetates

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). 

The disaccharide donor was also synthesized by enzymatic degradation of chitin followed by condensation with suitable allosamizoline acceptor (Scheme 11). Enzymatic degradation of chitin using chitinase (EC 3.2.1.14, Streptomyces griseus) followed by acetylation afforded the peracetate 109. Treatment of 109 with PhSTMS afforded 110 in 87% yield. 

Ytterbium(III) triflate in methanol is an efficient catalyst for cleaving methoxy-acetates (e.g., 23 - 24). The methoxyacetate protecting group was introduced 30 years ago (C.B. Reese and J.C.M. Stewart, Tetrahedron Lett., 1968,4723) but has found little application so far, due to difficulties associated with its removal. Efficient, selective anomeric deacetylation of peracetylated reducing disaccharides has been achieved with hydrazine hydrate (1.2 equivalent) in acetonitrile. It has been observed that the BF3.Et20-catalysed glycosidation of several pento- and hexo-pyranose peracetates with simple alcohols is accompanied by selective deacetylation at the 2-position. ... 

Peracetates of aldoses, disaccharides, and methyl glycosides have been obtained in good yields by reaction of the trimethylsilylated sugars with iron(III) chloride-acetic anhydride without change in their stereochemistry. The same reaction on ketoses produced dehydration products only. 

Reactions carried out on disaccharide derivatives included the conversion of peracetates to l-phenylthio- 3-derivatives by use of phase-transfer methods and the epimerization at C-2 by calcium -amine systems (CaCl2.2H20, EtjN, for example). By this means maltose, lactose, isomaltose and melibiose were converted into the corresponding glucosyl-D-mannoses. An extensive range of variously substituted maltosyl fluorides have been made from maltose derivatives as substrates for cyclodextrin transferase. Only the 6 -methyl ether and 6 -acetate were transformed into cyclic products. In the course of the work the thio-derivatives (83) and (84) were made. ... 

Partial deacetylation of thioglycoside peracetates during synthesis has been reported above and many disaccharides and oligosaccharides mentioned in Chapters 3 and 4 have been made by use of such compounds. 

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