A-D -Mannose

Chiral nitrogen chelates derived from sugars were prepared by Ruffo [48], introducing diimines and diamines functionalities on inexpensive monoses, a-D-glucose and a-D-mannose. 

The problems of anomeric equilibrium may be avoided by investigating 2-ketoses. Both a hydroxyl group and a hydroxymethyl group are attached to the anomeric carbon atom in such sugars, and the bulky hydroxymethyl group favors the equatorial position. These authors measured c.d. spectra for three ketoses, the 2-(hydroxymethyl) derivatives of a-L-xylose, a-D-xylose, and a-D-mannose, in aqueous solution. 

Leloir110(a) has described a conversion of D-mannosyl phosphate to D-mannose 6-phosphate by phosphoglucomutase, and here, too, the reaction requires catalytic quantities of a-D-glucose 1,6-diphosphate. The role of the latter is probably to generate a-D-mannose 1,6-diphosphate as follows. 

Con A Jack bean seeds a-D-Mannose, a-D-glucose a-D-Methyl mannose... 

Signals of rhamnomannans and acidic rhamnomannan were assigned as follows. The C-l resonances of mono- and di-rhamnosyl side-chains were determined by comparison with those of 3-O-a-L-rhamnopyranosyl-a-D-mannose (C-l, 98.0) and O-a-L-rhamnopyrano-syl-(l— 2)-0-o -L-rhamnopyranosyl-(l—>3)-a-D-mannose (C-l", 103.7 C-l, 96.8). C-l signals of the main chain should be similar in shift to that12 of an unsubstituted, a-D-( 1— 6)-linked D-mannopyranan, Sc... 

This enzyme [EC 2.7.7.13], also known as GTP-mannose-1-phosphate guanylyltransferase, catalyzes the reaction of GTP with a-D-mannose 1-phosphate to produce GDP-mannose and pyrophosphate (or, diphosphate). The bacterial enzyme can also use ITP and dGTP as the phospho-ryl donors. 

C. Nobrega and J. T. Vazquez, Conformational study of the hydroxymethyl group in a-D-mannose derivatives, Tetrahedron Asymmetry, 14 (2003) 2793-2801. 

Glycosyl halides (7a-e) were stereoselectively transformed into l,2-tra s-thio-glycoses by i) (8a-d, 8j) a two-step procedure via the pseudothiourea derivatives [9,10a] the substitution of halide by thiourea is mostly a S l-type reaction since acetylated 1-thio-a-D-mannose (8b) was obtained from acetobromoman-nose (7b) [9cj ii) (8e-i) using thiolates in protic and aprotic solvents [10], or under phase transfer catalysis conditions [11]. Another approach involved the reaction of thioacetic acid with 1,2-trans-per-O-acetylated glycoses catalyzed with zirconium chloride [12]. The 1,2-trans-peracetylated 1-thioglycoses (8e-h) were obtained in high yield. No anomerized products could be detected in these reactions (Fig. 1). 

Results of an experiment with an oligosaccharide free from amino acid were less conclusive, but were not incompatible with the structure shown. The oligosaccharide was prepared from a separated glycoprotein fraction having a D-mannose hexosamine ratio of 5 3. a-D-Mannosidase and 2-acetamido-2-deoxy-/3-D-glucosidase were used alternately. Initially, a-D-mannosidase removed about 1.5 D-mannose residues per molecule. Subsequent treatment of the residual oligosaccharide with 2-acetamido-2-deoxy-/3-D-glucosidase released rather less than one proportion of hexosamine residue, and then another 1.5 residues of D-mannose became open to attack by a-D-mannosidase. 

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