Mutarotation spectroscopy

The synthesis of the glucosidic alkaloid buchananine has been reported (Scheme 5). The penultimate product (21), though crystalline, was shown by n.m.r. spectroscopy to be a mixture of a- and / -anomers, confirmed chemically by the last step, which led to a separable mixture of two tetra-acetates. This same mixture of acetates resulted when 1,2,3,4-tetra-O-acetyl-D-glucopyranose was esterified with nicotinyl chloride. It is concluded that buchananine is itself a mixture of a- and / -anomers (21), as a consequence of its ability to catalyse its own mutarotation.31... 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

Of the new topics, the application of quantitative vapor analysis has been chosen for inclusion, largely because measurements in this phase are encountered rather infrequently in the spectroscopy of carbohydrates and the measurement of mutarotation, because it serves as a good illustration of the practical technique of aqueous solution measurements and because it may stimulate new interest in studies in this environment. 

The above equilibria are confirmed by the complex, positive mutarotation in ethanol. First, a slow 3-D-furanose interconversion to equilibrium occurs, and then a very slow interconversion to equilibrium with the -D-pyranose form. The crystalline compound is, therefore, a /3-d anomer. This compound is interesting in that the identity of the forms taking part in the mutarotation can be ascertained, and the relative amounts of each determined, by nuclear magnetic resonance spectroscopy. Furthermore, the compound is unusual in existing in solution as the furanose forms, mostly. 

Inversion at Co —SR R is fast (0.1-10s ) but diastereoisomeric systems have been detected by H and C NMR spectroscopy. Thus (305) shows clearly resolved signals of the tren ligand arising from the chiral thioether. " Also the related t-(N,S)[Co (R)cys (tren)] chelate shows immediate H-NMR spectral changes on deprotonation of the dangling equatorial carboxylate arising from rapid (< 5 s) mutarotation of the (/ )S(J )C, (5)S(Ji)C diastereoisomeric mixture. Thus inversion appears to be 10 -10 times more rapid than for uncoordinated sulfonium salts, a result in keeping with other M—SR R systems. ... 

The tautomeric equilibria of aldopentoses, aldohexoses and hexuloses in DMSO have been examined by C-n.m.r. spectroscopy the mutarotation kinetics of aqueous D-glucose have been investigated by i.r. methods, and Raman spectroscopic data on the tautomeric transformations of glucose in aqueous solution over a wide pH range confirmed that the ring-chain tautomerism depends on pH." The ring shapes of the two fiiranose and two pyranose forms of psicose in aqueous solution, riiown by n.m.r. spectroscopy to be present in almost equal proportions, have been analyzed by MM3."... 

The nature of the cleavage mechanism has been studied using n.m.r.-spectroscopy leading to the proposal that the a-glycoside is cleaved to yield a-Neu5Ac which then leaves the active site of the sialidase molecule and mutarotates in water to give over 90% of the p-anomer (Holmquist and Ostman 1975, Friebolin et al 1980). 

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