Partially Acylated Carbohydrates

4-hydroxyl group was prepared in a similar manner from methyl /3-n-glu-copyranoside. However, these substances are of limited use for further synthetic work since the secondary 4-hydroxyl group reacts very slowly. Other partially acetylated compounds behave similarly, the 

A similar reaction sequence led to the synthesis of riboflavin 5-phosphate and, earlier, to the 6-phosphate of methyl a-D-glucopyranoside.  

The following types of partially acylated carbohydrates may be prepared with the aid of tritylation. 

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It is likely that other partially acylated carbohydrates which are obtainable by the tritylation method and contain only one free hydroxyl group (particularly if this is a primary one), may also be used for the synthesis of oligosaccharides. 

In carbohydrate trityl ethers and the partially acylated carbohydrate derivatives obtained from them, the free hydroxyl groups may be etheri-fied with other alcohols. This method has been used repeatedly, particularly for the preparation of partially methylated compounds. With the alkali resistant trityl ethers, any alkaline alkylation method may be used, for example, dimethyl sulfate and alkali, or methyl iodide and silver oxide. In choosing an alkylation method for partially acylated compounds, one must be selected which does not cause acyl migration or saponification. 

Many partially alkylated carbohydrates may be obtained in an analogous manner by using trityl ethers and the partially acylated carbohydrates obtainable from them. 

Pozsgay, V, A simple method for avoiding alkylthio group migration during the synthesis of thioglycoside 2,3-orthoesters. An improved synthesis of partially acylated 1-thio-a-L-rhamnopyrano-sides, Carbohydr. Res., 235, 295-302, 1992. 

Transesterification takes place under both acidic and basic conditions, so that acyl substituents in partially esterified carbohydrates are prone to migration (Figure 6.27). Presumably for steric reasons, acyl groups tend to migrate to primary positions - the well-known migration of acyl groups from the 4- to the... 

Methylation of carbohydrates containing base-labile substituents using a combination of diazomethane, di-chloromethane, and boron trifluoride etherate is knovm to proceed without concomitant migration of acyl groups (17, 18). Consequently, this method was chosen for methylation of various, partially acylated derivatives of sucrose (19) Methylation of 1, 2,3,3, 4, 6 -hexa-0-acetylsucrose (G) with a freshly prepared solution of Jiazomethane in drchloromethane and boron trifluoride etherate for 0.5 hr at -5° after chromatographic separ-... 

ACOCH2CF3, porcine pancreatic lipase, THE, 60 h, 77% yield. This enzymatic method was used to acetylate selectively the primary hydroxyl group of a variety of carbohydrates. The selective enzymatic acylation of carbohydrates has been partially reviewed. ... 

Various epimerases acting on carbohydrate derivatives and acyl-CoA derivatives were demonstrated, purified, and characterized as reviewed previously1184. Lactate race-mase (E.C. 5.1.2.1) is the first racemase to he discovered (1 41. The mechanism of lactate racemase reaction was studied with the enzyme preparations partially purified from Clostridium butyricumll8S. Hiyama et al.[1861 highly purified the enzyme from Lactobacillus sake, but little is known about its enzymological properties. In contrast, mandelate racemase (E. C. 5.1.2.2) is the enzyme best characterized among various racemases and epimerases its tertiary structure and functional groups that participate directly in catalysis has been clarified. 

Akiyama, K., K. Kawazu, and A. Kobayashi. 1995. A novel method for chemo-enzymic synthesis of elicitor-active chitosan oligomers and partially A-deacetylated chitin oligomers nsing A-acylated chitotrio-ses as substrates in a lysozyme-catalyzed transglycosylation reaction system. Carbohydr. Res. 279 151-160. 

Sashiwa, H. and Shigemasa, Y. 1999. Chemical modification of chitin and chitosan. 2 Preparation and water soluble property of N-acylated or N-alkylated partially deacetylated chitins. Carbohydr. Polym. 39 127-138. 

Near total 1,2- and 1, 3-asymmetric induction was observed in reactions of a- and -alkoxyacyl cyanides respectively, with the tertiary cyanohydrins to obtained being configurationally stable. Secondary cyanohydrins were obtained after addition of cyanosilanes to the acyl cyanides under similar conditions but with only partial asymmetric induction. The potential application of these methods to the synthesis of carbohydrates was noted. 

When acyl groups are introduced into carbohydrates by the esterification of hydroxyl groups, some degree of reciprocal relation should exist between the extent of esterification and the intensity of the periodic acid - Schiff reaction. Whenever the hydroxyl groups are sufficiently numerous, partial esterification may take place without any great decrease in the relative... 

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