Mutarotation in solution

Once optical resolution is achieved, measurements of the ease of racemization can provide a qualitative picture of the severity of the molecular distortion. Bell and Waring (1949), who coined the phrase molecular overcrowding , were able to detect mutarotation in solutions of the morphine salt of 3,4,-5,6-dibenzophenanthrene-9,10-dicarboxylic... 

Another controversial aspect of the bishydrazone structure concerns the hydrazone residues. The bishydrazone was proposed to have the structure 46, which mutarotates in solution to 47 (47). More recently, on the basis of a comparative study of the spectroscopic properties of the bis(phenylhydrazone) with some related compounds, the bishydrazone was assigned the structure 2,3-dideoxy-3-phenylazo-2-phenylhydrazino-L-threo-hex-2-enone-l,4-lactone (48) (48). However, this latter structure was inconsistent with its NMR spectra (49). 

When racemic 173 (R = Me) was hydrolyzed in the presence of chymo-trypsin, the resulting optically active acid 173 (R = H) exhibited an ORD absorption spectrum characteristic of L-phenylalanine the starting ester possessed an axially oriented carbomethoxy group.338-362 Atropisomerism and conformational asymmetry of a precisely definable nature in a substrate are therefore recognized by chymotrypsin. X-ray diffraction studies confirmed that the chymotrypsin-active isomer has an axial ester moiety in the solid state, and that the ester mutarotates in solution to a CD-inactive isomer, whose ester group is in the equatorial position.363... 

Mutarotation in Solution. In solution, equilibrium exists between the linear conformation and the two anomeric forms of pyranose and fiiranose cyclic hemiacetal forms (Fig. 9) the percentage of each form as well as the ratio a/p is characteristic of the monosaccharide considered. 

Fig. 9. Mutarotation in solution representation of the different species in equilibrium for D-glucose (see Table 2).

Properties. Physical properties of the three crystalline forms of dextrose are Hsted in Table 1. In solution, dextrose exists in both the a- and P-forms. When a-dextrose dissolves in water, its optical rotation, [cc], diminishes gradually as a result of mutarotation until, after a prolonged time, an... 

Delphinine, C34H47O9N (Walz) or C33H45O9N (J. and C.). The alkaloid crystallises in rhombs, or six-sided plates, m.p. 198-200°, [a]f ° + 25° (EtOH), shows mutarotation in alcoholic solution, and forms an acid oxalate, B. H2C2O4, m.p. 168° dry), a hydrochloride, B. HCl, m.p. 208-210°, and a monobenzoyl derivative, m.p. 171-3°. On alkaline hydrolysis it yields one molecule each of acetic and benzoic acids. The basic, hydrolytic product of this action is delphonine, C24H3g07N, which is amorphous, but can be distilled at a bath temperature of 140° and a pressure of 0-001 to 0-0001 mm. The brittle, possibly semi-crystalline resin so obtained, has m.p, 76-8° and [a]f ° - - 37-5 (EtOH). 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

The isomerism of a- and jS-glucose is to be attributed to the spatially different arrangement of the H and OH-groups attached to the asymmetric carbon atom 1. This atom is asymmetric in the cyclic lactol formula (Tollens). The mutarotation of the sugars, i.e. the gradual change to the final stationary value of the optical rotation, is to be explained by an equilibrium occurring in solution between the various... 

The oxidation of a-D-glucose occurs at less than 1% of the rate of oxidation of the /3 anomer. Because these two forms exist in solution in equilibrium in the proportion of 36% (a) and 64% (/3), mutarotation of the a to the /3 form must be allowed to reach equilibrium in the sample and standards for consistent... 

Having accomplished the resolution, it was found, however, that only one brucine salt could be isolated and that it mutarotated rapidly in solution from an initial reading of [a]o+30 35 to a final activity of... 

Certain procedures make it possible to obtain the a and 3 anomers of glucose in pure form. A 1-molar solution of a-D-glucose has a rotation value [a]o of +112°, while a corresponding solution of p-D-glucose has a value of +19°. These values change spontaneously, however, and after a certain time reach the same end point of +52°. The reason for this is that, in solution, mutarotation leads to an equilibrium between the a and p forms in which, independently of the starting conditions, 62% of the molecules are present in the P form and 38% in the a form. 

In aqueous solution the free nitriles mutarotate in a complex manner that indicates the existence of at least three compounds in the equilibrated solutions. This reaction has been interpreted as a reversible transformation of the nitriles into other substances. 

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