Glucitol conformation

Fig. 4. —Conformations of D-Glucitol. (a, Planar b, nonplanar, after rotation about C-2-C-3 c, nonplanar, after rotation about C-3-C-4 and C-4-C-5.)...

A less obvious instance is the acylation of l,5-anhydro-4,6-0-benzylidene-D-glucitol. The 1,5-anhydride has a chair conformation, and the 2- and 3-hydroxyl groups are equatorially attached trans to each other, and are, apparently, essentially equivalent91 However, the 2-benzoate and 2-p-toluenesulfonate were obtained almost exclusively on using the acyl chlorides in pyridine, although in the former acylation, an appreciable proportion of the dibenzoate was formed. 

Four X-ray investigations of isohexide derivatives have been published.39-42 The oldest of them gave very little information about structural details. Two crystallographically independent molecules of 1,4 3,6-dianhydro-2-0-(p-bromophenylsulfonyl)-D-glucitol 5-nitrate (30), having the same conformation, are in the asymmetric unit. 

Residue D356 is in the same conformation in all other MIPS structures save the 2-deoxy-glucitol-6-phosphate complex structure, a conformation that would occlude the 2-deoxy-glucitol-6-phosphate binding mode seen in this structure. This indicates that this mode of binding is inconsistent with all of these structures. 

All of this data lead to the conclusion that 2-deoxy-glucitol-6-phosphate is modeled in a catalytically incompetent conformation, possibly due to the low pH used to produce the crystals. The mechanism based on this structure must then also be disguarded (Jin and Geiger, 2003 Jin et al., 2004 Stein and Geiger, 2002). 

The extension of the reaction with sodium iodide to tosyl derivatives of other acetals of iditol, glucitol, and mannitol could provide evidence for the conformation of 2,4 3,5-diacetals of mannitol and, more important, lead to a clarification of the steric requirements of this reaction. The importance of 2,4 3,5-diacetals of talitol, at present unknown, is now clear they will have one axial and one equatorial terminal group (see LIU), and the trans ring junction imposes a rigidity of conformation greater than that of any of the diacetals with cis junctions. 

Complex-formation with a cation does not, in itself, affect the optical rotatory power of a carbohydrate. However, complex-formation is often accompanied by a change of conformation that causes a change in the optical rotation. For example, the rotation of D-glucitol and, to a lesser extent, of D-mannitol is affected by the presence of cations, in the order Na" " < Mg < Zn " " < Ba " < Sr " " < Ca (see Section III,1). The optical rotation of methyl jS-o-ribopyranoside and ) D-lyxopyranoside would, undoubtedly, be substantially changed by complex-formation, but this experiment has, apparently, not been reported. Therefore, a chmige of optic rotation on addition of a salt can be regarded as proof of complex-formation, but lack of such change does not necessarily indicate that no complex is formed. 

It would be of interest to study the crystal structures of alditol-cation complexes, because, in them, the conformation would differ from that of the free alditol. However, no suitable example is known. Attempts to prepare crystalline complexes from D-glucitol and various salts have not been successful. ... 

Figure 2.2 Conformations of D-glucitol and D-mannitol. Note the apparent toleration of vicinal hydroxyl groups being either gauche or trans, but the prohibition on f>-syn interactions of hydroxyl groups.

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