Aldoses rotations

The following Tables list the aldose oxirane derivatives that have been prepared. The solvents used in determination of the specific rotation are given by A, acetone C, chloroform E, Ethanol E.A., ethyl acetate M, methanol P, pyridine and W, water. 

Reaction of an aldose with NaBH produces a polyol (alditol). Because an alditol has the same functional group at both ends, two different aldoses can yield the same alditol. Here, L-gulose and D-glucose form the same alditol (rotate the Fischer projection of L-gulitol 180° to see the identity). 

Glyoeraldehydo, the simplest aldose, has only one chiratity center and thm ha. i two enantiomeric [/nirror-unajfe) forms. Only Ibe dexrrorowttvy ei B tiomer occurs naturally, however. That is, a sample of naturally occurrinf glyccraldohyde placed in a polarimeter rotates plane-polarized light in t clockwise direction, denoted <+). 

In a number of amides, thioamides, and related systems, rotation about the single bond is hindered, and distinct geometric isomers can be observed and even isolated. This type of geometric isomerism is referred to as atropisomerism and results from resonance contributions by the nitrogen atom that imparts significant double bond character to the system, thus slowing rotation. Such is the case for the thioamide aldose... 

Synthesis of the 3,4,6-trimethyl ether of 2-deoxy-2-methylamino-n-gluconic acid was carried out along lines similar to those of the synthesis of the 3,6-dimethyl ether. By the addition of methylamine and hydrogen cyanide to 2,3,5-tri-O-methyl-n-arabinose, followed by hydrolysis, only one compound was formed, and this was related to n-glucosamine on the basis of its optical rotation. An attempt to obtain the aldose derivative by reduction with sodium amalgam was not successful, probably because of the impossibility of 5-lactone formation. 

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