Glycoside isomerization

A-l-Allyl-2-formylpyrrole, cyclohydrocarbonylation, 11, 521 Allyl glycosides, isomerization, 10, 91 Allyl halides, in copper-mediated substitutions, 9, 549-550 Allylic alcohols... 

Latent/active strategy using vinyl glycosides isomerization of 3-buten-2-yl 2,3,4,6-tetra-O-benzyl-P-D-glucopyranoside [97]. 

Maltose, obtained by the hydrolysis of starch, and cellobiose, by the hydrolysis of cellulose, are isomeric disaccharides. In both maltose and cellobiose two D-glucopyra-nose units are joined by a glycosidic bond between C-1 of one unit and C-4 of the other. The two are diastereomers, differing only in the stereochemistry at the anomeric car bon of the glycoside bond maltose is an a-glycoside, cellobiose is a p-glycoside. 

An isomeric sugar, D-javose, is a constituent of two cardenolide glycosides (strophanthojavoside and antiarojavoside) found (42) in the seeds of Antiaris toxicaria Lesch. Degradative studies indicated (42) that javose had the structure 6-deoxy-2-0-methyl-D-allose (17) and this assignment was confirmed by two stereospecific syntheses. 

The present work involves the study of methyl glycosides and O-isopropylidene ketals of various isomeric deoxy sugars by mass spectrometry. Several of the compounds selected for the present study have free hydroxyl groups, and interpretation of their mass spectra shows the scope of the study of these and related deoxy sugar derivatives by mass spectrometry without prior substitution of all hydroxyl groups. Some of the candidates (compounds 4, 7, 8 and 10) are structurally related to biologically-derived deoxy sugars. 

Note The colors of the chromatogram zones produced by the mandelonitrile glycosides change with time Those of the D-isomers turn steel blue within an hour, while the 1 isomers turn reddish-brown during this time. It is possible to distinguish between D-and L-forms on the basis of these typical color differences and color changes. Both isomeric forms have a dark brown color after about 24 hours. 

A novel latent-active glycosylation strategy was reported. This strategy is based on a rhodium-catalyzed isomerization of substituted allyl glycosides followed by a Lewis acid-mediated glycosylation reaction (Scheme 46).76 77... 

In 1964, Kochetkov, Khorlin and Bochkov reported that the reaction of 1,2-alky-lorthoacetates with alcohols in the presence of catalytic amounts of HgBr2 and pTsOH furnished acetylated 1,2-trans glycosides or isomeric orthoesters depending on the reaction conditions [4]. Polar solvents (nitromethane, acetonitrile) and large amounts of catalyst promoted glycosylation (a, Scheme 5.5), whereas solvents of low polarity (dichloroethane) and the use of small amounts of catalyst favored transorthoesterification (b, Scheme 5.5) [16]. 

Sulphonic esters have been obtained from the sulphonyl chlorides in high yields under mild conditions for a range of alcohols and phenols [e.g. 18, 19]. Of particular value is the protection of glycosides possessing a free hydroxyl group and hydroxy-steroids, which are tosylated readily under phase-transfer conditions [20-22]. Alkyl sulphinites have been obtained in a similar manner [23]. Alternatively, preformed tetra-rt-butylammonium sulphonates or their alkali metal salts have also been alkylated with haloalkanes or alkyl fluorosulphonates [24,25]. In contrast with more classical procedures, tosylation of alcohols, which are susceptible to E/Z-isomerism, e.g. Z-alk-2-en-l-ols, occurs with retention of their stereochemistry under phase-transfer catalysis [26]. 

Note that harsher conditions may lead to further changes, e.g. epimerization at C-3 in fmctose, plus isomerization, or even reverse aldol reactions (see Section 10.3). In general, basic conditions must be employed with care if isomerizations are to be avoided. To preserve stereochemistry, it is usual to ensure that free carbonyl groups are converted to acetals or ketals (glycosides, see Section 12.4) before basic reagents are used. Isomerization of sugars via enediol intermediates features prominently in the glycolytic pathway of intermediary metabolism (see Box 10.1). 

---