Conformational mutarotation

If the left- and right-handed crystals could be separated, by the method of Pasteur, the phenomenon of "conformational mutarotation might be observed. 

Maltose, l- 4- -link in, 998 molecular model of, 998 mutarotation of, 998 structure of, 998 Manicone, synthesis of. 805 Mannich reaction. 915 Mannose, biosynthesis of, 1011 chair conformation of, 126 configuration of, 982 molecular model of, 126 Margarine, manufacture of, 1063 Markovnikov. Vladimir Vassilyevich. 192... 

In the case of glucose, the mutarotation gives 36 percent a, 64 percent p, and negligible strciight chain. The unequal distribution of the two anomers is due to the fact that the -OH on the anomeric carbon of the p form is equatorial, which for a chair conformer is more stable. The -OH on the anomeric carbon in the a anomer is axial, which means this anomer is slightly less stable. 

It is most straightforward to begin with a-L-sorbopyranose (1) (see Figure 1) because this sugar shows virtually no mutarotational change in water (I, 7) thus, it remains basically in the a-pyranose form. By analogy with the established (8) structure of the crystalline material, this sugar exists in solution in the 1C(l) pyranose chair conformation (1) because all substituents are thereby optimally oriented—i.e., the... 

This is one example of the second criterion mentioned previously. However, the other OH substituents, depending upon the furanose ring conformation, are either eclipsed or in the anti conformation. In the former they are disposed to form a strong intramolecular hydrogen bond in the latter they are incapable of such bonding. Further evidence to support the contention that free / -D-fructofuranose is nearly tasteless is seen in the thermal mutarotation (3) of D-fructose. As the temperature of D-fruc-... 

Carbodiimide Polymers An optically active carbodiimide, (/ )-152 ([a]365 +7.6°), gives a polymer by polymerization using a titanium (IV) isopropoxide catalyst (Scheme 11.9) [203], The polymer showed optical activity essentially identical to the monomer however, on heating, the polymer indicated mutarotation and specific rotation reached a plateau value of [a]363 -157.5°, which is considered to be based on excess helical sense of the main chain. The mutarotation has been ascribed to a conformational transition from a kinetically controlled one to a thermodynamically controlled one. Excess single-handed helical conformation can be induced for polyfdi-/ -hexyl carbodiimide) by protonating the polymer with chiral camphorsul-fonic acid. 

The Mutarotation of 2-Deoxy-/3-D-erythro-pentose ( 2-Deoxy-/f-D-ribosc ). Conformations Kinetics, and Equilibria, ... 

Early steroid chemists characterised cholesterol and cholest-5-ene as their 5,6-dibromides, but the work was complicated by the isolation of two isomeric dibromides in each case. It was recognised [i] that one isomer of each pair was stable and dextrorotatory, while the other was unstable andlaevorotatory. Solutions of each isomer in chloroform were found to exhibit mutarotation [2], corresponding to the establishment of equilibria between isomeric pairs of dibromides, and it is now recognised that the position of equilibrium is determined by conformational strains, mainly arising from the large syn diaxial interaction between a 6jS-bromo substituent and the Ca9)-methyl group (Fig. 43) of, p. 10). 

At pH 4.4, 5-thio-D-xylopyranose (215) shows a slow mutarotation, namely, [a]p +202 - +178° (half-time, 10 hours). At pH 6.6, however, the half-time is only about 10 minutes. The direction of the rotation shows that 215 crystallizes in the a-D form. The nuclear magnetic resonance spectrum of 215 in deuterium oxide shows the presence of the H-1 proton of both anomers. The diaxial coupling of 8.2 Hz for the H-1 proton at the higher field (t 5.25) corresponds to the )8-D anomer in the CJ(d) conformation of the molecule having a sulfur-containing ring. 

Lactose, mutarotation of, 12,16,23 —, 6,l, 6 -tri-0-p-tolylsulfonyl-, 244 Lactulose, conformational equilibria, 32 Levoglucosan... 

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