Carbohydrates Containing Nitrogen or Sulfur in the “Hemiacetal” Ring

In 5‐amino‐5‐deoxy‐and 5‐thioaldoses, a 6‐membered hemiacetal ring can be formed by incorporation of a nitrogen or sulfur atom into the ring instead of the oxygen present in pyranoses. In the case of 5‐(N‐acylamido) ‐5‐ deoxyaldoses, the nucleophilic strength of the amide grouping is lowered, with the result that ring closure occurs only when sterically favored, or when a side reaction with formation of furanose is not possible. On the other hand, 5‐amino‐5‐deoxyaldoses containing a free amino group readily from 6‐membered “hemiacetal” rings. The resulting piperidinoses possess properties that are unusual in sugars, since they contain the 2‐hydroxypiperidine ring system. Piperidinoses are stable to alkalis and labile to acids; they rearrange in the presence of acids to give Amadori compounds, or lose three molecules of water to give derivatives of β‐hydroxypyridines. Owing to the high reactivity of the mercapto group, 5‐thioaldoses from only sugars with rings containing sulfur, the behavior of which resembles that of the oxygen analogues. 4‐Amino‐4‐deoxy‐ and 4‐thioaldoses can form 5‐membered rings containing nitrogen and sulfur.