Furanosides anomerization

Chiral tricyclic compounds have been prepared by thermal and Eu(fod)j-catalyzed cycloadditions of furanosidic a,-unsaturated aldehydes 26-29 (Figure 3.4) with cyclopentadiene (18) [31]. The diastereofacial selectivity depends markedly on the stereochemistry of the anomeric benzyloxy and methoxy groups. 

The preparation of glycosyl fluorides is described next. Aiming to have a convenient glycosyl donor convertible into 1,2-CM-furanosides, Mukaiyama and coworkers prepared 2,3,5-tri-O-benzyl-yS-D-ribofuranosyl fluoride (36y3) by treatment of a protected D-ribofuranose (35) with 2-fluoro-1 -meth-ylpyridinium tosylate (FMPTs) the total yield was raised by anomerizing the simultaneously produced a-1-fluoride (36a, 7, p 66, 72.f 24 Hz) to 36 (7, F 63.5 Hz, 72,F very small ) by treatment with BF3-OEt2. 

The stereochemistry of the reaction of glycosyl radicals is strongly influenced by the anomeric effect. Glucopyranosides and manno-pyranosides afford stereo selectively the a-C-glycosyl compounds whereas in furanosidic structures the stereochemistry is not always predictable. 

The C-l chemical-shifts of furanosides are generally at lower field than those of their anomeric counterparts in the pyranose series.13 Sometimes, an immediate identification may be made when very low-field signals of 8C 107 or more are present, for example, for /3-galacto-furanoside14-17 and a-arabinofuranoside.18 Generally, characteristic signals of furanoside-ring carbon atoms are present at low field (8C 80 -85) also. 

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)... 

Figure 6.25 Synthetic applications of pseudo-anomeric difluoromethylene furanosides. ...

Table 29 1H NMR Data for Anomeric Methyl Furanosides (64MI31002>...

A detailed study of the methyl glycosidation of the four aldopentoses has been made by Bishop and Cooper,16 42 with the aid of gas chromatography of the esters of the resulting pentofuranosides and pentopyranosides. Three competing reactions were established for the formation of the furanosides (1) an irreversible formation of furanosides from the free sugars (2) an anomerization of the furanosides and (3) conversion of the furanosides into the pyranosides. (A fourth reaction, anomerization of the pyranosides, will not be considered here.)... 

In all cases, the rate of anomerization is very high, relative to the rate of conversion into pyranosides this rate of anomerization is the major factor in determining the relative yields of the ct- and /8-furanosides from a reaction mixture, and not the relative proportions of these two anomers initially formed from the free sugar. The ratio of anomers was found to be approximately constant throughout the course of the reaction (1.7 1 for the 0- and a-n-xylofuranosides).42... 

The rates of methyl glycosidation are approximately proportional to the concentration of acid catalyst. In the early part of the glycosidation, a single reaction is predominant and can be characterized as first order. A cyclic, carbonium intermediate (13) was proposed for the mechanism of the anomerization of the furanosides. 

Overend and coworkers4 have studied the action of acidic methanol on methyl /3-D-glucofuranoside, with two concentrations of acid catalyst (see Table VI). The rapid anomerization in methanol under the conditions studied gave only furanosides. Use of l4C-labeled methanol gave labeled furanosides, showing participation of the solvent in the anomerization. These authors suggested the reaction (14) (15) to explain the role of methanol. 

The slower conversion of the anomerized D-glucofuranosides into D-gluco-pyranosides was studied with a higher concentration, of acid. The concentration of furanosides was followed by determining the amount of formaldehyde formed by periodate oxidation. It was concluded that the same acyclic ion (16) is formed from both furanoid anomers, and that ring closure gives a mixture of pyranosides similar to that found in the equilibrium mixture of pyranosides. 

Methanesulfonic acid. 4 The substrate is the 0-D-furanoside the data are for 10 (k + kt), determined polarimetrically the anomerized mixture contains 63% of the ti and 37% of the a form. The substrate is the anomerized mixture the data are for 10 kt, the overall rate constant, determined from the formaldehyde formed on oxidation with periodate. 

The formation of the tranu-l, 2-glycofuranosides is effected in neutral solution, where anomerization cannot occur. Treatment of 3,4-O-isopropyl-idene-2,5-di-O-methyl-L-rhamnose diethyl dithioacetal with mercuric chloride in boiling methanol gave" a 61% yield of the -L-furanoside no trace of the 0-l anomer was found. In this experiment, with an acidic solution (no mercuric oxide present), anomerization could have led to the formation of the more stable anomer. In the normal reactions cited, with maintenance of a neutral solution, no anomerization can occur, and yet the more stable anomer, presumably the kinetic product, is still the major anomer formed. 

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