A-Glucopyranose

Figure Bl.11.10. 400 MHz H NMR spectrum of nrethyl-a-glucopyranose (structure as in figure Bl.l 1.12)...

Analogously, for preparation of racemic carba-a-glucopyranose 49 from 52, esterification of (—)-52 furnished the ester 95, which was transformed into compound 96 by debromination with zinc dust and acetic acid. Stereoselective hydroxylation of 96 with osmium tetraoxide and hydrogen peroxide, followed by acetylation, gave compound 97. Lithium aluminum hydride reduction of 97, and acetylation of the product, gave pentaacetate 98, which was converted into 99 by hydrolysis. ... 

Acarbose is a glucopyranose derivative that acts by inhibiting intestinal a-gluco-sidase. This delays carbohydrate absorption and reduces the postprandial (1.5 hours after food) blood glucose levels and is used in combination with other sulfonyl-ureas. Acarbose may cause GI disturbances, flatulence, abdominal distortion, diarrhea, and pain. Acarbose should be avoided during pregnancy, as it affects the fetus. Acarbose is contraindicated in inflammatory bowel disease and hepatic dysfunction. 

Fig. 2 Chemical structure of P-CD. Arrows indicate the 2-, 3-, and 6-hydroxyls of a glucopyranose unit. (Adapted from Ref ll)...

Fig. 28. Gas chromatogram of trimethylsilyl derivatives of an equilibrated mixture of galactose, glucose, and methyl a-D-mannopyranoside (2 2 1, w/w). The numbered peaks refer to (f) methyl a-o-mannopyranoside (2) a possible furanose form of galactose (3) a-D-galactopyranose (4) a- -glucopyranose (5) yS-o-galacto-P3u-anose (6) /8-n-glucopyranose. For GLC conditions see text. Reproduced from Copenhaver (CIO) with permission.

A disaccharide with many of the same properties as sucrose is trehalose, which consists of two a-glucopyranose units in 1,1 linkage (p. 168). The bios5mthetic pathway from UDP-glucose and glucose 6-P parallels that for s5mthesis of sucrose (Eq. 

Figure Bl.11.12. H- H COSY-45 2D NMR spectrum of methyl-a-glucopyranose (structure shown). The coupling links and the approximate couplings can be deduced by inspection.

In general, only one of these four isomers crystallizes at a given condition (e.g., a-glucopyranose from water at high temperature or p-glucopyranose at low temperature). It is either the predominant species that crystallizes or the least soluble one in the solvent used. Crystallization then gives rise to optical mutarotation, which is caused by establishment of the equilibrium mixture in a solution from which one component is removed. In the literature crystal structures of pyranoses predominate with only a few samples of crystalline fura-noses. The furanose isomers of D-ribose and 2-deoxy-D-ribose, for example, have never been isolated and crystallized since configurational interconversion of cyclic compounds in solution tends to inhibit crystallization. For this reason, the 1-0-methyl acetals, which cannot isomerize, crystallize more readily than half-acetals. 

Finally, the ferrocenyl complexes were decomposed photochemically and thermally. Thermolyses can be performed in this case because the decomposition temperature of the azide (96) (80 °C) is much lower than those employed for other diazirines and azides. The results obtained from photolyses and thermolyses do not differ significantly. Here again, a-CD causes the most drastic changes because of the complete encapsulation of the guest in a 1 2 complex (Scheme 10.27). In accord with the other reactions performed in a-CD, the main reaction pathway is hydrogen abstraction from the host. Upon thermolysis ferrocenyl amine (112) is obtained in a yield up to 60%. More remailcably, ferrocenyl nitrene (111) seems to react in very low yields with a-CD. However, the structure of the reaction product 113 has not been fully established yet and is quite unexpected because 113 is the result of a glucopyranose-furanose conversion. In contrast, the products obtained by thermolysis of ferrocenyl nitrene in the soUd state, namely... 

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