2- amino-2-deoxy dehydration

Nuclear magnetic resonance (n.m.r.) studies have revealed that 4-amino-4-deoxy-L-xylose,35,350 4-amino-4,5-dideoxy-L-xylose,356 and 4-amino-4-deoxy-D-glucose34 (40) exist as equilibrium mixtures for example, of the pyrrolidine 40, the pyrroline 39 formed by its dehydration, and a dimer (41), the equilibrium lying strongly towards the last. Similar studies have shown that the hydrochlorides of 4-amino-... 

Amino-5-deoxy-D-xylopyranose (34 = 17) is like 2-piperidinol (2-hydroxypiperidine) in existing in a pH-dependent equilibrium with its dehydration product (33 = 16). The ultraviolet peak of the n-7T transition of the C=N chromophore of 33 is not suitable for structural elucidation. However, the asymmetric nature of this chromophore gives rise to a Cotton effect. A solution of free 5-amino-5-deoxy-D-xylose shows a negative Cotton effect at 300 nm that is well demonstrable by measurement of circular dichroism for this purpose, the optical rotatory dispersion is much less sensitive. The Cotton effect is ascribable to 33, as 34 and 22 would exhibit no Cotton effect in this region. Thin-layer chromatograms of 34 always show a second spot which, like 34, gives a red coloration with o-aminobenz-aldehyde this is, presumably, caused by 33. 

On hydrolysis with sulfurous acid, 5-amino-5-deoxy-l,2-0-isopro-pylidene-jS-D-arabinofuranose gives an acyclic bisulfite adduct which, on treatment with barium hydroxide, afiFords a solution of 5-amino-5-deoxy-D-arabinopyranose that behaves similarly to the analogous D-xylose compound. The Amadori rearrangement proceeds with somewhat more difiBculty. The equilibrium between the pyranose form and its dehydration product is recognizable by the presence of a positive Cotton efiFect (300 nm), a result predictable by theory, as 42 should represent the most favored conformation. 

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. 

Amino-6-deoxy-L-sorbopyranose (88 = 83) is in equilibrium with the dehydration product (87 = 82). As with the unsaturated compound from 5-amino-5-deoxy-D-xylose (33), this is recognized by its positive Cotton efiFect at 305 nm. The most favored half-chair conformation is shown in 87. In this conformation, C-5 is above the plane of the ring and, according to theory, this situation should lead to a positive Cotton efiFect. 

Another equilibrium partner of the form 102a is the cyclic Schiff base 101a, formed by dehydration. The C=N chromophore in 101a exhibits a weakly positive Cotton effect at 250 nm, by which the proportion of 101a present can be demonstrated. The Cotton effect disappears at pH values below 6. By comparison with 5-amino-5-deoxy-D-xylopyranose (17), 102a is definitely the more stable toward acids. Neither an Amadori rearrangement product, nor aromatization to a p)nTole derivative, is observed down to pH 1.0. 

The pyranose 35 and the furanose 164 are stable in neutral solution at room temperature, and are separable by column chromatography without equilibration. On heating, or by acid catalysis (0.1 M hydrochloric acid, room temperature, 35 hours), an equilibrium between forms 35 and 164 is established. From the optical rotation, the ratio of the six-membered to the five-membered ring is calculated to be about 2 1. Alkaline catalysis causes very rapid attainment of equilibrium, but, simultaneously, decomposition occurs. The 5-acetamidopyranose 35 is, after attainment of its equilibrium, stable toward acids, and shows no dehydration reaction to form 3-pyridinol. However, under conditions in which the N-acetyl group is hydrolytically removed (heating with 2 M hydrochloric acid), 35 is transformed into 3-pyridinol through the intermediate formation of free 5-amino-5-deoxy-D-xylopyranose. ... 

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