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Gly c o s yl halid es

Sugar derivatives in which the anomeric hydroxy group is replaced by a halogen atom are named as glycosyl halides. According to the halogen atom X there are glycosyl fluorides, glycosyl chlorides , glycosyl bromides and glycosyl iodides. Gly c o s yl halid es.

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Gly c o s yl halid es

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  1. Sugar derivativesin which the anomeric hydroxy group isreplaced by a halogen atom are named asglycosyl halides. According to the halogen atom X there are glycosyl fluorides, glycosyl chlorides,glycosyl bromides and glycosyl iodides. Glycosylhalides Stability of acetylated glycosyl halides(R=CH3CO) decreases from glycosyl fluorides to glycosyl iodides; their reactivity increases in the same order. While glycosyl iodides are very labile, glycosyl fluorides are stable and as the only glycosyl halides are known with unprotected hydroxy groups (X=F, R=H). Due to their strong anomeric effects, the axial anomer (usually -anomer) is more stable than the equatorial anomer. Acetylated glycosyl bromides are usually prepared by reaction of a sugar peracetates withhydrogen bromidein acetic acid. They can also be obtained from glycosides, oligosaccharides or polysaccharides by cleavage of glycosidic bonds with hydrogen bromidein acetic anhydride (per-O-acetylated glycosyl bromides), eventually with gaseous hydrogen fluoride (unsubstituted glycosyl fluorides). Acylated glycosyl halides other than bromides are prepared from the bromides by displacement reactions with respective chlorides, iodides, or fluorides. Widely used in synthesis of sacharides; they are starting compounds for preparation of glycosides, glycans, glycosyl amines, 1-thioaldoses, 1-thio glycosides, oligosaccharides, etc.

  2. Preparation of glycosyl halides AlCl3, CHCl3 HBr, HOAc 20 ºC, ½ h 20 ºC, 20 h Koenigs-Knorr procedure of synthesis of glycosides

  3. Synthesis of disaccharidesby the Koenigs-Knorr procedure Mol. sieve, 80 C 80 ºC 78 % PhMe, 60 ºC 81 %

  4. Glycosylimidates or NaH Depending on the base usedfor generation ofthe anomeric alkoxide,its addition to trichloroacetonitrile provides eitherα-or β-glycosylimidates.

  5. Ac (CH3CO) or another protecting group Also sugar acetates can be utilized for preparation of glycosyl imidates.Their acetoxy group bounded to the anomeric carbon is selectively deacetylated by treatment with hydrazinium acetate in DMF.Thus obtained sugar derivatives, O-protected at the other hydroxy groups, can be transformed to the corresponding glycosyl imidates similarly as the O-benzyl protected sugar derivatives, i.e., by the procedure shown in the foregoing reaction scheme.

  6. Synthesis of disaccharidesby the Schmidt (glycosylimidate) procedure

  7. General synthetic usefulness of glycosylimidates i) (BnO)2P(O)OH, CH2Cl2, 22 ºC, 2 h ii) , BF3.Et2O, CH2Cl2, 22 ºC, 2 h iv) PrSH, BF3.Et2O, CH2Cl2, mol. sieve iii) , BF3.Et2O, CH2Cl2

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