Idose aqueous solution

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in the axial position, the two groups can react to form an acetal. This is called the anhydro form of the sugar. (It has lost water.) The anhydro form of D-idose is shown here. In an aqueous solution at 100 °C, about 80% of D-idose exists in the anhydro form. Under the same conditions, only about 0.1% of D-glucose exists in the anhydro form. Explain. 

For the same conformational reasons, the P anomers of d aldoses dominate in aqueous solution when the Cf atom has the D configuration (ribose, xylose, allose, glucose, gulose, galactose). The a anomers, however, predominate when the atom has the l configuration (arabinose, lyxose, altrose, mannose, idose, talose). 

Studies of the D-aldohexapyranoses showed that the classical C chair is the most likely conformer for all of those -pyranoses. The or-anomers have one more axial group in the Cf orm, and so allose and altrose are likely to have C4 con-formers present as well. or-D-Idose in aqueous solution should contain equal amounts of both chairs and the °S2 form as well. 

By treatment of D-gulose (XVII) or D-idose (XVIII) with warm aqueous barium hydroxide solution Van Ekenstein and Blanksma were able to isolate D-sorbose (II) with a melting point of 165° and [< ]d + 42.9° in water. The aldoses were prepared from D-gulonic and D-idonic lactones obtained by the cyanohydrin synthesis from D-xylose. 

While the aforementioned methodologies constitute the most useful and general synthetic procedures for the preparation of 1,6-anhydropyranoses, a number of alternative routes have been described [27]. As mentioned previously, solutions of a number of sugars in aqueous acid at equilibrium contain considerable amounts of 1,6-anhydro sugars. A simple synthesis of tri-0-acetyl-l,6-anhydro-/J-L-idopyranose has been reported by Stoffyn and Jeanloz that utilizes the acid-catalyzed dehydration of L-idose to form the anhydro ring [28]. 

The 1C conformation (51) of most of the D-hexoses is highly unfavored, because of the large, axial, hydroxymethyl substituent at C-5. Stabilization of the structure of D-idose is effected by the formation of the 1,6-anhydro derivative (52), which is formed in about 80% yield by thermodynamic equilibration in aqueous acid solution, the highest yield of 1,6-anhydride for any of the hexoses. It is obvious that, in 6-deoxy-(n or L)-idose, no such anhydride formation can occur and, since the 5-(7-methyl group requires somewhat less space than the 5-(hydroxymethyl) group of the hexose, a considerable proportion of the 1C conformation of 6-deoxy-D-idose may be present in this sugar. 

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