Rearrangement pyranose-furanose

If, in an attempt to obtain free 6-amino-6-deoxy-L-xt/Io-hexulose, the isopropylidene compound 80 is hydrolyzed at 65° with 2 M hydrochloric acid, an almost quantitative yield of 3-hydroxy-2-pyrldinemeth-anol (86) hydrochloride is obtained instead. The formation of 86 can result only through the intermediate 6-amino-6-deoxy-L-xyfo-hexulo-pyranose (83). The furanose (81) first formed is in equilibrium with the pyranose (83). The latter is dehydrated in acid solution to 82 which, under acid catalysis, rearranges to the intermediate 84. In the following steps, the allylic hydroxyl groups on C-4 and C-5 are readily removed, and aromatization to the pyridine derivative (86) ensues. 

Pyrolysis of peracetylated pyranose monosaccharides has been studied, and found to proceed via initial loss of the 1-acetoxy group followed in some cases by ally lie rearrangement. In contrast, peracetylated furanoses lose two acetoxy groups to give substituted furans, e.g. equation (59). ... 

Acetolysis of fully acetylated methyl pyranosides of 4-amino-4-deoxy-D-glucose and -D-galactose results in partial rearrangement to N-in-ring furanoses which, it has been shown, arise directly from the glycosides and not from initially formed acetylated pyranoses. ... 

This rearrangement involves the opening of the pyranose ring between C-2 and C-3, quite possibly via an ene-diol intermediate, to produce a primary alcohol at C-3 and an aldehyde at C-2. By rearrangement at C-3 and C-4, via an ene-diol, an aldehyde is formed at C-3 and the C-4-oxo-group is lost. The formation of a furanose ring is possible, by the linkage of C-2 and C-4 by an aldol condensation, in which C-3 becomes an aldehyde carbon. Reduction at C-3 then yields UDPapiose. 

Treatment of l-0-(a-D-glucopyranosyl)-D-fructose with hydrogen fluoride in pyridine has afforded a l,T 2,2 dianhydride mixture with the fructose moiety in either the pyranose or furanose fonn. Some novel 1,6-anhydro-lactose derivatives for the synthesis of oligosaccharides containing N-acetyl-lactosamine residues are mentioned in Chapter 4, and a 3,6-anhydroglucofuranose derivative resulting from an unusual molecular rearrangement of a xanthate is described in Chapter 7. 

N.m.r. spectroscopy has been used to study the equilibrium existing between the isomeric forms of D-fructose (143 R = OH) and of each of the Amadori-rearrangement products (143) in [ Hs]pyridine. There was generally 46% of -D-pyranose, 7% of a-o-pyranose, 30% of -o-furanose, 12% of a-D-furanose, and 5% of the keto-form at equilibrium, the position of which varied only slightly depending on the substituent at C-1. The vacuum thermolysis of 1-deoxy-l-L-prolino-D-fructose has been examined as part of an investigation aimed at establishing the importance of Amadori compounds as precursors of... 

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