Formation of Furanosides

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

The ready formation of furanosides from D-glucurono-6,3-lactone is aided by the presence of the lactone ring. 

With the aid of column chromatography, Mowery4 studied the rate of formation of furanosides and pyranosides from n-arabinose. The rate of furanoside formation was extrapolated back to zero time, and an initial rate was obtained (see Table III). The less stable anomer, the /3-furanoside, was initially formed to the greater extent in the final equilibrium mixture, the more stable a-n anomer was preponderant. 

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.)... 

The more probable formation of furanosides as compared with pyranosides, especially at lower temperatures, has been discussed by Shafiz-adeh.6 Specific alignment and orientation of the group on C-5 is required... 

The introduction of a second, five-membered ring has already been discussed in reference to the ready formation of furanosides from d-glucurono-M, 3-lactone.14 The rate constant, given in Table VIII for methyl 2, fwli-O-mcthyl-a-D-glucofuranosidurorio-G,3-lactone is much smaller than that for ethyl /3-D-glucofuranoside. The effect is also shown in the ready... 

It has been assumed that the initial formation of furanosides results from the greater reactivity of the furanose sugars present in the equilibrium mixture. In this respect, it should be noted that, although the furanose sugars are certainly derived from the equilibration of pyranose forms, these speculations have not considered the excellent likelihood of the furanosides being formed from the transitory precursor of the furanose sugars in equilibrium. 

D-mannose derivatives.437 All such reactions that they examined with sulfonates of methyl 6-deoxy-2,3-0-isopropylidene-a-D-manno-pyranoside (175) resulted in the formation of furanoside products, and this result probably accounts for some of the conflicting accounts of desulfonylation of the L enantiomorph of 175 (R = Ts).322,438 With acetate ion in boiling N,N-dimethylformamide, the methanesulfonate (175, R = Ms) gave, after saponification, a 65% yield of a mixture of methyl 6-deoxy-2-3-0-isopropylidene-a-D-talofuranoside (176, R = OH) with methyl 6-deoxy-2-3-0-isopropylidene-/3-L-allofurano-side (177, R = OH) in the ratio of 7 1. Displacements atC-5 of the sul-fonylated furanose derivatives 176 and 177 (R = OMs,OTs, or OBs)... 

Recently for the first time, a highly stereoselective surfactant-catalyzed INAC reaction in aqueous media, leading to the exclusive formation of a single isomer has been reported (726a). Either oxepane (mode A) or pyran (mode B) (Scheme 2.230) is formed from 3-O-allyl furanoside derivatives, which constitute... 

Such lactones had already been exploited in Rennes by Ferrieres et al. for the stereoselective formation of alkyl furanosides from the readily available D-glucuronic acid 20.27 Similarly, the analogous... 

There seems to be, therefore, no firm evidence to allow a distinction to be made between the furanose and hemiacetal mechanisms of furanoside formation, nor can it be assumed that one of these routes will be consistently favoured it is highly probable that different sugars will favour the different pathways to varying degrees. 

The selectivity for the formation of the p-D-pyranoside (equatorial) stereoisomer in the reaction originated from a thermodynamic control. Indeed, starting from D-glucose, after 24 h at room temperature, a mixture of the four possible a,p-furanosides 46 and 47, and a,p-pyranosides stereoisomers 48 and 45a, in which... 

Participation of the ring-oxygen atom has also been postulated,124 to account for the stereospecific formation of L-mannopyranoside (40) and D-allofuranoside (41) products in the deamination, by way of 39b, of methyl 4-amino-4,6-dideoxy-2,3-0-isopropylidene-a-L-man-nopyranoside (39a). Ring contraction to give furanoside products pre-... 

V. Pozsgay and A. Neszmelyi, Ring contraction by carbon participation in a hexopyranoside ring Formation of benzyl 2-O-benzyl-3,5-did6oxy-3-C-benzyloxymethyl-a-L-lyjC0-pento-furanoside, Tetrahedron Lett. 27 211 (1980). 

As in the pyranoside series, the substitution at C-2 of furanoside derivatives is also difficult (87], Treatment of the 2-imidazylate ester of a benzyl 5-deoxy-a-D-hexofurano-side derivative with tetrabutylammonium azide or benzoate in refluxing toluene led to the formation of substitution products in 82 and 53% yields, respectively [88] (Scheme 11). 

Extrapolation of rates back to zero time also showed a small but definite formation of pyranosides from the free sugar, in addition to conversion from the furanosides. The reversibility of the conversion of furanosides into pyranosides was definitely demonstrated by obtaining the final equilibrium composition from two directions, namely, from the a-D-pyranoside and from the free sugar. 

Only with o-lyxose was there any evidence of pyranoside formation early in the reaction. A slight reverse reaction, furanoside to pentose, was also noted for this sugar. This behavior is in agreement with the low conformational stability of the D-lyxofuranosides. However, no initial formation of D-arabinopyranosides was noted. 

A kinetic study of the methyl glycosidation of D-mannose was also made by Mowery.48 The more stable anomer, the a-D-mannofuranoside, was formed at a higher initial rate (see Table III) the proportions of both furanosides in the final equilibrium mixture was too small to permit accurate comparison of isomer distribution. The conformational stability of the D-mannofuranosides may be compared with that of the D-lyxo-furanosides the furanoid structures are similar, except for the bulky two-carbon group at 04 of the hexoside. This similarity is shown in the very small proportion of n-lyxofuranosides (see Table V) and of D-manno-furanosideo (see Table III) in the final equilibrium mixtures, and also in the initial formation of D-mannopyranosides48 and of D-lyxofuranosides.u... 

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