Glycosides furanosides

There also appear to be systematic differences in the orders of elution for pyranoside and furanoside derivatives. For the fully methylated glycosides, furanosides precede pyranosides in the arabinose, galactose, and fructose series the order is reversed for these derivatives of xylose and glucose. ... 

Two polymorphs of a,a-trehalose octacetate monohydrate, 4,4 -dideoxy-a,a- ty/o-trehalose, methyl 2,3,4,6,2, 4, 6 -hepta-C -acetyl-a-laminaribinoside, 2 3,3 -dideoxy-a,a-flrflbjno-trehalosc. 3 Methyl 2,3-0-isopropylidene-5-0-tosyl-P-D-ribofuranoside 2, the 7-carbon glycoside 3 (see Chapter 17), C-glycosidic furanoside 4 (see Chapter 14) the oxazoline derivative 5 and methyl 1,6-di-0-(4-bromobenzoyl)-3,4-di-0-(4-methoxycinnamoyl)-P-D-fructofuranoside. Di-(3-deoxy-D-g/yccro-pcntulose)-l,2 2,r-dianhydridc, methyl 4,6-benzylidene-a-D-allopyranoside, methyl-2,3,4-tri-0-acctyl-a-L-rhamnopyranoside, l 2-(indol-3-yl)ethyl 2,3,4-tri-O-acetyl-a-L-... 

LiBF4, CH3CN, 70°, 3-8 h, 81-90% yield. This system of reagents also cleaves benzylidene acetals. Conventional reagents failed to cleave these glycosides. It is interesting to note that the /3-anomers are cleaved more rapidly than the a-anomers and that the furanoside derivatives are not cleaved. 

Special topics (synthesis of P-mannosides, furanosides, sialosides, glycosides of aminosugars and deoxysugars, if applicable)... 

Fischer was intrigued by the fact that emulsin caused hydrolysis of both / -glucosides and / -galactosides but had no effect on either the a- or / -xylo-sides (33). Since, at the time, Fischer expected glycosides to be furanosides, he suggested that both the enzymes required the presence of a free hydroxyl group at position 5 of a hexoside. 

Newer Observations on the Synthesis of O-Glycosides 1. Formation of Furanosides )... 

On the grounds that furanosides anomerise and hydrolyse very much more readily than do the corresponding pyranosides. Bishop eind Cooper assumed that the first step in the glycosidation process is the methanol-ysis of the furanose form of the free sugar, and they visualised, without evidence, a unimolecular process proceeding by way of a stabilised cyclic ion (1). In support of this they observed 5) that for xylose, lyxose and ribose the furanoside formation rates (3,1,12 respectively) correlated with the furanoside contents at equilibrium (see Table 3) and hence, presum-... 

This last observation that the a-furanoside predominates initially might, however, also help to substantiate the furanose to furanoside mechanism since the more stable p-furanose would give mainly the a-glycoside in a synchronous displacement reaction it would, alternatively, seem unlikely that, if such a displacement occurred by way of the cyclic ion (1), the reaction would result in a predominance of a-furanoside. 

Bishop and his colleagues were again able to calculate the relative reactivities of the various methyl aldopjnranosides in anomerisations, and to obtain equilibrimn values for anomers, but they did not siscertain the mechanisms of the reactions involved other experiments have however helped to make this the best understood step in the glycosidation process. As with the furanosides, the reaction can proceed by way of... 

Hydroxyethyl-D-glucose (Intramolecular glycosidation) a-Furanoside and the anomeric pyrano-sides were obtained No kinetic information available p-fura-noside likely to be highly unstable because of trans-in%ion of a five- and a six-membered ring 23)... 

The kinetics of the reaction have been monitored350 polarimetri-cally, and by separation of the products, after various intervals, by g.l.c. of their per(trimethylsilyl) ethers. L-Fucose dissolved completely in boiling methanol within 30 min, even in the absence of the resin, and the initial solution contained 5% of L-fucofuranoses, 38% of a-L-fucopyranose, and 57% of /3-L-fucopyranose. Immediately upon adding the resin, glycoside formation occurred. After 1 min, all four isomers appeared, in approximately equimolar ratios and the furano-side content attained a maximum after 15 min (18% of a- and 34% of /3-furanoside, with 23% of a- and 21% of /3-pyranoside). The final equilibrium mixture (after 12 h at the b.p., with the resin) contained 6% of a- and 13% of /3-fucofuranoside, and 54% of a- and 27% of /3-fuco-pyranoside. 

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