Aldoses 2-6-anhydro

An intramolecular ether (commonly called an intramolecular anhydride), formally arising by elimination of water from two hydroxy groups of a single molecule of a monosaccharide (aldose or ketose) or monosaccharide derivative, is named by attaching the (detachable) prefix anhydro- preceded by a pair of locants identifying the two hydroxy groups involved. 

Mutarotation of Various Free or Partially Substituted 2,5-Anhydro-aMefce/do-aldoses... 

Also relevant to this matter is the fact that the n.m.r. spectra of the free 2,5-anhydro-aMe/iydo-aldoses in deuterium oxide rarely show106 the low-field signal characteristic of the free aldehyde group. Such a signal is generally visible when the spectrum (in chloroform-d) of compounds that are partially substituted is recorded, but its intensity is often weak. 

These compounds, namely 2,5-anhydro- and 3,6-anhydro-aldoses may exist in bicyclic form, including the 2,5-dioxabicyclo[2.2.1]heptane (97), 2,6-dioxabicyclo[3.2.1]octane (98), and 2,6-dioxabicyclo[3.3.0]octane (99) basic skeletons, respectively, or as monocyclic oxolane derivatives possessing a free aldehyde group. In the latter case, a suitably oriented hydroxyl group is not disposed to form a furanose or pyranose ring with the free aldehyde group. 

Anhydro-C-(p-methoxyphenyl)aldoses and anhydro- 1,3-dideoxy-ketoses were found to be compounds usually produced on heating free and partially protected aldoses with p-methoxybenzoyl- and acetyl-methylenetriphenylphosphoranes, respectively, in N,N-di-methylformamide. Many of them were isolated by thin-layer chromatography on alumina, although, on several occasions, these compounds were available only as mixtures of the five- and six-membered anhydro derivatives, because of their similar chromatographic mobilities. Five-membered anhydro derivatives are favored, but exclusive formation of six-membered anhydro derivatives was observed when five-membered ring-closure was impossible. Only L-rhamnose on reaction with acetylmethylenephosphorane formed a six-membered derivative, 196b. 

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