Methyl furanosides conformation

Five-membered furanoid rings occur in envelope (E) or twist (T) conformations, which are in a rapid equilibrium at room temperature. Both conformers help to avoid energy-rich eclipsed orientation of neighboring substituents, which would occur in the planar conformations. The preferred conformation in methyl furanosides is E in which Cl is above the oxygen-C(2,3,4) plane and the hydroxymethyl group is in axial position. This conformer is enforced by the... 

Hudson s Rules of isorotation have been applied to furanosides and a correlation with known conformations of methyl aldofuranosides made. The molecular rotations and differences in molecular rotation were tabulated for all aldo-pentoses, aldohexoses, and hexuloses as their methyl furanosides, and hence their conformations were deduced. The jS-D-pentofuranosides were mainly in the T2 form with the 0-methyl group quasi-axial and the side-chain quasi-equatorial, whereas the a-D-pentofuranosides were either in the E form or a mixture of T forms. The optical rotations for all a-D-furanosides are negative, but for 3-d-furanosides they are all positive. ... 

From their kinetic results Bishop and collaborators >8) calculated the velocity constants of the furanoside anomerisations of their seven aldoses (Table 1), and rationalised them in terms of ring conformations and group interactions. Thus, for example, in the extreme cases methyl a-D-arabinofuranoside (4), having the fewest non-bonded steric interactions, is the most stable pentofuranoside, while methyl p-o-lyxofurano-side (5), having the least stable ring, reacts most rapidly. 

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... 

The data in Table VIII are presented in the order of the conformational stability observed for methyl glycosidation (see Table I on p. 99 and Table II on p. 105). In each of the three cases where data for both anomers are available, the trans-1,2 anomer is the more stable, and a lower rate-constant is observed. This is the same pattern as that observed in Table II. However the order of conformational stability given earlier is not observed here. The arabinofuranosides show the maximum stability, and the D-galactofuranosides and L-fucofuranosides, having similar conformations, show a similar stability, However, the furanosides of D-lyxose, n-mannose, and i>rhamnose show an unexpectedly high stability which is almost as great as those of the furanosides of the first-mentioned sugars. These data lead to the conclusion that the conformational stability for transition complexes... 

The rate of hydrolysis of polysaccharides is affected by several factors. Because of substituent interaction effects, furanosides are hydrolyzed much more rapidly than the pyranoside analogues. Differences in the hydrolysis rates of diastereomeric glycosides are significant. For example, the relative hydrolysis rates of methyl-a-D-gluco-, manno-, and galactopyranosides are 1.0 2.9 5.0. This can be related to the stabilities of the respective conjugate acids, which are transformed into the half-chair carbonium ions at different rates. Also, substituents bound to the C-2 position obviously prevent the formation of the half-chair conformation. 

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