1—>3-axial-equatorial linked

Acetonitrile is another participating solvent, which in many cases leads to the formation of an equatorially linked glycoside [125-131], It has been proposed that these reactions proceed via an a-nitrilium ion intermediate. It is not well understood why the nitrilium ion adopts an axial orientation however, spectroscopic studies support the proposed anomeric configuration [130,131], It is known that nucleophilic substitution of the a-nitrilium ion by an alcohol leads to P-glycosidic bonds and the best P-selectivities are obtained when reactive alcohols at low reaction temperatures are employed. Unfortunately, mannosides give poor anomeric selec-tivities under these conditions. 

When a cyclohexane ring inverts, the substituents that were equatorial become axial and vice versa. This is fine for ris-decalin, which has an axial-equatorial junction, but it means that ring inversion is not possible for trans-decalin. For frans-decalin to invert, the junction would have to become axial-axial, and it s not possible to link the axial positions to form a six-membered ring. Cis-decalin, on the other hand, ring inverts just as fast as cyclohexane, ring inversion of cis-decatin... 

The exo-anomeric effect enforces a value of cp around 90° for anomeric axial linkages. This ensures a proclivity towards helical structures (as distinct from the ribbons of 1,4-diequatorially linked polysaccharides), as in starch and glycogen. p-(1 4)-Linked galactans, 1,4 axial-equatorial in the other sense, are known as hemicellulose components of compression and tension wood, the storage polysaccharides of lupins and as arabinogalactans attached to the rhamnogalacturonan I component of pectin, but have not been subject to conformational studies they appear to be biosynthesised from UDP-Gal. " ... 

In B, morpholine has a chair structure with the N—H bonds almost all disposed axially in CT and cyclohexane, the N—links are distributed axially equatorially roughly as 3 2... 

Similar synthetic approach were presented by Anandhan and Rajakumar." They synthesized and subsequently characterized novel gly-codendrimers and JV-allq ldendrimers with aromatic core using NMR techniques. De Castro et al. presented conversion of mannan-polysaccharides in mannose oligosaccharides with a thiopropargyl linker. Leino et al. ° performed synthesis and conformational analysis of phos-phorylated (3-(l->2) linked mannosides. The stereochemistiy at the anomeric centre was determined by NMR, which was a challenging task due to the fact that the H-1 and H-2 protons always displayed either axial-equatorial or equatorial-axial orientation. 

The CH fragment which is linked to the OH group (Sh = 5.45 ) can easily be located in the H and NMR spectra. The chemical shift values Sc =74.2 for C and Sh = 3.16 for //are read from the CH COSY plot. The H signal at S,i = 3.16 splits into a triplet (11.0 Hz) of doublets (4.0 Hz). The fact that an antiperiplanar coupling of 11 Hz appears twice indicates the diequatorial configuration (trans) of the two substituents on the cyclohexane ring 5. If the substituents were positioned equatorial-axial as in 4 or 5, then a synclinal coupling of ca 4 Hz would be observed two or three times. 

A single enzyme, inositol monophosphatase, leads to loss of the remaining phosphate and the regeneration of free inositol. This enzyme exhibits similar affinities for all five of the equatorial inositol monophosphate hydroxyls. Inositol 2-phosphate, which is not produced in this degra-dative pathway, is a poor substrate, a property that is probably attributable to its axial configuration. The enzyme is inhibited by Li+ in an uncompetitive manner i.e. the degree of inhibition is a function of substrate concentration. The putative link between the ability of Li+ to interfere with phosphoinositide turnover and its therapeutic efficacy in the treatment of bipolar disorders is discussed in Box 20-1 and Chapter 55. It should be noted that unlike most other tissues, brain can synthesize inositol de novo by the action of inositol monophosphate synthase, which cyclizes glucose 6-phosphate to form I(3)P. The enzyme has been localized immunohistochemically to the brain vasculature. 

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