Glycosylation oxazolines

A reaction which is analogous to the orthoester synthesis but which utilises glycosyl oxazolines affords means of synthesising, 2-trans-2 amino-2-deoxy-glycoside derivatives. Treatment of the gluco compomd (17) with phenol in the presence of toluene- -sulphonic acid thus gave the glycoside (18), and it was also utilised in a nucleoside synthesis 48). 

Treatment of 5 with 4 M HC1, followed by treatment with isothiocyanate 379 in pyridine-DMF, produced 380. This was treated with yellow HgO to afford the desired glycosyl oxazoline 381. Finally, deprotection of the trehazolin tribenzyl ether 381 afforded trehazolin (2) (Scheme 47).49... 

Oxazoline glycosyl donors are generally prepared from the corresponding 2-acylamido-2-deoxyglycosides by the activation of the C-l leaving group using an... 

Warren and associates (18) have prepared a glycosyl azide derivative (15) of a heptasaccharide. This glycosyl azide was obtained from O-a-D-manno-pyranosyl-( 1 — 6)-0-[a-D-mannopyranosyl-( 1 — 3)-(9-a-D-mannopyrano syl-(l— 6)-0-a-D - mannopyranosyl - (1 — 3)] - O-fi- d - mannopyranosyl -(1 — 4) - O - (2 - acetamido - 2 - deoxy-/ -d-glucopyranosyl) - (1 — 4) - 2 -acetamido-2-deoxy-D-glucopyranose (12) by treatment of its peracetylated derivative 13 with trimethylsilyl trifuoromethanesulfonate, followed by reaction of the intermediary oxazoline 14 with trimethylsilyl azide. Reduction of the glycosyl azide 15 in the presence of Lindlar catalyst gave the glycosyla-mine derivative 16. The condensation of 16 with 1-tert-butyl N-(9-fluoren-... 

SYNTHESIS OF LINEAR AND HYPERBRANCHED STEREOREGULAR AMINOPOLYSACCHARIDES BY OXAZOLINE GLYCOSYLATION... 

Synthesis of Linear and Hyperbranched Stereoregular Aminopolysaccharides by Oxazoline Glycosylation... 

The oxazoline method is one of the excellent glycosylations for high stereoselectivity, in which the stereospecific glycosylation of alcohol with... 

Synthesis of natural-type aminopolysaccharide having dibenzylchitin structure was achieved by the polymerization of a sugar oxazoline monomer, 1 having one hydroxy group at position 4 (Scheme 4) [9]. The polymerization was carried out with an acid catalyst in 1,2-dichloroethane solvent at reflux temperature. All the H-NMR, C-NMR, and IR spectra as well as elemental analysis data of the isolated polysaccharide supported that the polymerization proceeded by the stereoregular glycosylation to give (1 4)-... 

The preparation of several glucofuran [2, l-t/] oxazolines 35 and 36 from reaction of 2-amino-2-deoxy-D-glucose 34 with HF has been described. Compounds 35a and 35b are formed when the reaction is carried out in formic acid, whereas the orthoesters 36a-c are formed when the reaction is carried out using anhydrides. Further reaction of 35 and 36 with methanol gives methyl glycosides. Thus, 35 and 36 may hnd use as potential glycosyl donors for the synthesis of 2-amino-2-deoxy sugars (Scheme 8.14). " ... 

Activation of the hydroxyl group as an acetate leaving group to promote oxazoline formation has been applied extensively in carbohydrates to afford p-glycosylation with high selectivity. A Lewis acid such as ferric chloride (FeCL), tin chloride (SnCLt), or TMSOTf is usually added to facilitate cyclization. Several recent examples are shown in Scheme 8.20. Compound 54 has been further elaborated to 1,2-dideoxynojmmycin 54a, a potent p-Al-acetylglycosamine inhibitor. 

Colombo and co-workers also developed an oxazoline glycosylation method wherein an acetate is replaced by a vinyl ether. Activation of 55 with iodine in the presence of DBU gave the oxazoline 56. Glycosylation of 56 with a second sugar moiety using TMSOTf afforded the disaccharide 56a in 79% yield (Scheme 8.21). 

Other applications of oxazolines have also been discovered. Anomeric oxazolines have now emerged as useful glycosyl acceptors in the glycosylation of sugars. 2-Alkenyloxazolines have been found to undergo asymmetric Michael addition and hetero-Diels-Alder reactions. Further explorations in these areas of oxazoline chemistry will undoubtedly continue and the list of new applications will grow. 

Figure 4. Preparation of glycosyl phosphates by the oxazoline procedure.

---