Mutarotation optical rotation studies

It is of interest at this point to mention that the optical rotation of L-sorbose has been studied extensively. Lobry de Bruyn and Van Ekenstein21 reported that no mutarotation was observable however, Pigman and Isbell56 later discovered that L-sorbose does possess a small complex mutarotation. For example, at 20°C., L-sorbose (c, 11.3 l, 4) gave an initial observed rotation (°S) of —57.124, which changed to — 57.498 in 2.69 minutes and attained the final value of —57.768 after two hours. Similar results were observed at 0°C. but at a much slower rate. These authors were of the opinion that the smallness of this mutarotation was due to the fact that the equilibrium solution of L-sorbose is... 

Mutorotation continues to receive attention. Studies of the mutarotation of 2,3,4,6-tetra-O-methyl-a-D-glucopyranose in aqueous dioxan and aqueous DMSO by optical rotation over a wide range of water concentration, with and without catalysts, have shown that the uncatalysed reaction transition-state contains one mole of sugar and two moles of water in an acyclic bonded structure (6). In the pyridine-catalysed reaction one mole of water may be replaced by one mole of pyridine (Scheme 3). The evidence suggests that the reaction takes place by an intimate step-wise mechanism rather than synchronously. 

The mutarotations of the sugars listed in Table III and those for many other sugars follow the first-order equation. The activation energy averages about 17,000 cal./mole this value corresponds to an increase in rate of 2.5 times for a 10° rise in temperature. The conformity of the mutarotation data to the first-order equation makes it probable that the main constituents of the equilibrium solution are the a- and jS-pyranose modifications. The actual composition may be calculated from the optical rotations of the equilibrium solution when the rotations of the pure a- and /3-isomers are known. Data of this type are included in Table III. Independent confirmation of the composition of the equilibrium solutions is provided by studies of the rates of bromine oxidation of the sugars, the results of which are also found in Table III. 

As would be expected, deuterium exchange between nitromethane and D2O is catalyzed by acetate ions, and the rate of exchange is equal to the rate of bromination under the same conditions (Reitz, 43). Similar correspondences would be expected in the rate of racemization of optically active nitro compounds, but here the position has been complicated by reports (Kuhn and Albrecht, 45 Shriner and Young, 46) that these compounds do not lose their activity completely on conversion to the ion. However, it has recently been shown (Kornblum et al., 47, 48) that the residual activity in these observations was due to the presence of alkyl nitrate as an impurity. The mutarotation of a-nitro camphor represents a similar type of reaction it is catalyzed both by acids and bases (Lowry, 49), and a quantitative study of acid catalysis in chlorobenzene solution has been made (Bell and Sherred, 50). Lowry and most subsequent writers have supposed that the observed change of rotation is due to conversion to the aci-nitro form (II), i.e.,... 

Carbodiimide 152 gives helical polymer 153 through living polymerization with titanium and copper catalysts.The conformation of a polycarbodiimide has been studied by means of NMR. An optically active carbodiimide, (R)-154 ([a]D+7.6°), gives polymer 155 by polymerization using a titanium catalyst. The polymer showed optical activity essentially identical to that of the monomer however, on heating, the polymer indicated mutarotation and the specific rotation reached a plateau value of [a]o -157.5° probably based on excess helical sense of the main chain. The mutarotation has been ascribed to a... 

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