Aldoses conformations

From their kinetic results Bishop and collaborators >8) calculated the velocity constants of the furanoside anomerisations of their seven aldoses (Table 1), and rationalised them in terms of ring conformations and group interactions. Thus, for example, in the extreme cases methyl a-D-arabinofuranoside (4), having the fewest non-bonded steric interactions, is the most stable pentofuranoside, while methyl p-o-lyxofurano-side (5), having the least stable ring, reacts most rapidly. 

The chemistry of the 2,5-anhydrides of aldoses subsequently entered a prolonged lull, and Peat s review21 of 1946 in this Series does not report on any work later than 1925. The experimental basis of the deamination of amino sugars with nitrous acid was, nevertheless, established. The progress afterwards made in the conformational analysis of sugars made it possible for Shafizadeh22 to draw a parallel with the nitrous acid deamination of the aminocyclohexanols, and to rationalize the whole of these results. 

Fig. 2.2.6.S Identified acceptor substrates of recombinant sucrose synthase 1 expressed in E. coli. The aldoses are depicted in their furanose conformations based on the conclusion that SuSyl from E. coli accepts D-fructofuranose preferentially.

For ketoses, this simple method of calculation does not give the correct values for the a / -pyranose ratio. Apparently, the conformational interactions have different values in this case it is considered15 that both the st/n-axial and the gauche interactions are larger on the anomeric carbon atom, which carries two substituents, than they are on that in the aldoses. Owing to these larger interactions, the pyranose forms of the ketoses are less stable than those of the aldoses, and, consequently, for the ketoses... 

Fortunately, the rules of chemical reactivity and conformational analysis, coupled with the laws of thermodynamics, join forces to allow us to functionalize polyhydroxy aldehydes and ketones (aldoses and alduloses) in a selective and predictable fashion. 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

Most sugars occur in the chair conformation that places the largest number of substituents in equatorial positions and is therefore most stable thermodynamically. For D-aldoses this is usually the 4Q conformation ... 

OH group in an axial alignment which, assuming that the anomeric effect in keto-hexoses is of the same magnitude as in aldoses (5), supplements the conformational impact of the equatorial 1-carbinol group.)... 

Scheme 2. Mechanistic analysis of reaction pathways for NeuA catalysis that considers the crucial influences of C-3 and anomeric configurations and of chair conformations on the three-point attachment of aldoses as acceptor substrates, Si-face attack leads to regular (4S)-configurated adducts while re-face attack leads to inverted stereochemistry (abbreviations anm = anomerization, inv = ring inversion)...

Very many properties of oligosaccharides are determined by a unique group — the acetal fragment at Cl in aldoses or C2 in ketoses. This fragment shows unique effects the anomeric effect and exo anomeric effect which represent two important facts ruling the conformations of oligosaccharides [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. 

S) Ketose Structures.—The equilibria for ketoses differ markedly from those of aldoses, because the hydroxymethyl group attached to the anomeric carbon atom tends to assume an equatorial position. Maintenance of the anomeric hydroxymethyl group in the equatorial position keeps the anomeric hydroxyl group in the axial position. With a-D-sorbose (a-D-xylo-hexulose) and a-D-gluco-heptulose in the CA conformation, all large groups except the anomeric hydroxyl... 

Attention may be drawn to a new reagent, chromium trioxide in pyridine, that is said to oxidize primary alcohols to aldehydes. The potential importance of its application to bicyclic diacetals with free terminal groups is clear, both as a preparative method for aldoses and as a means of assigning conformations. 

Reeves has suggested that the spontaneous formation of 1,6-anhydro derivatives of idopyianose and altropyranose may be due to the conformational behavior of the aldoses his instability factors show that /3-Didose and /3-D-altrose will exist partly in the conformations favorable for closure of the 1,6-anhydro ring. This reasoning could lead to the further conclusions that D-talose, but probably not n-gulose, should fairly readily afford a 1,6-arihydro derivative of the pyranose form. 

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