Lyxose rates

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-... 

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... 

For some monosaccharides, the rate of mutarotation (K = k 3-kf) is found to obey a simple first-order rate law in which — d[a]/dt = A [a] — A2[P] (Scheme 1.4). Glucose, mannose, lyxose and xylose exhibit... 

Lemieux and Brice have measured the rate of exchange (in chloroform) of a large number of 1,2-trans sugar acetates with stannic trichloride acetate labeled with C. Their results are given in Table XVII. The low reactivities of the acetates of 8-D-ribose, a-D-lyxose, 8-D-allose, and a-D-mannose (rela-... 

NH4)6Mo7024 D-mannose, D-xylone, D-lyxose D-glucose N/Ac MeCN-H20 mixtures POM-acid catalyzed epimerization of rate constants given 542,543... 

Another conformationally unstable, anomeric pair of glycosides, the methyl pyranosides of a- and -D-lyxose, have calculated, composite rate-constants of 14.5 and 46.4, respectively, relative to the rate constant for methyl o-n-glucopyranoside (1.0). In this case, it is the /3-n glycoside which is hydrolyzed three times faster than the a-n glycoside, and the /3-d glycoside which possesses both an equatorial methoxyl group and a A2 instability factor. 

The isopropylidene group in methyl 2,3-0-isopropylidene-a-D-manno-pyranoside was hydrolyzed even more readily, with 0.01 N hydrochloric acid at 15°. The rates of hydrolysis of some isopropylidene derivatives of D-xylose, L-arabinose, D-ribose, and D-lyxose were examined,using 0.2 N oxalic acid at room temperature or at 65°. 

The similar polarographic behavior of semicarbazones, hydrazones, phenylhydrazones, and oximes of D-glucose, D-galactose, and n-mannose was described in another paper. For the aldoses studied, the equilibrium and rate constants for the formation of their oximes, semicarbazones, and hydrazones were determined at several different pH values, and it was found that their reactivity increases in the order D-glucose < D-galactose < D-mannose < D-xylose < n-arabinose < D-ribose < D-lyxose. ... 

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