Anomeric bromide intermediate

The reaction of phosphines and alkyl halides presents an alternative way to generate phosphonium electrophiles (Scheme 3.8). In particular, the combination of a phosphine and carbon tetrabromide (the Appel reaction) allows for in situ formation of a phosphonium dibromide salt (48, X = Br). Treatment of a hemiacetal donor 1 with the phosphonium halide 48 initially provides the oxophosphonium intermediate 38 (X = Br). However, the oxophosphonium intermediate 38 can react with bromide ion to form the anomeric bromide intermediate 49 (X = Br) with concomitant generation of phosphine oxide. With the aid of bromide ion catalysis (i.e. reversible, catalytic formation of the more reactive P-anomeric bromide 50) [98], the nucleophile displaces the anomeric bromide to form the desired glycoside product 3. The hydrobromic add by-product is typically buffered by the presence of tetramethyl urea (TMU). 

For the synthesis of CIO carba derivatives, a better leaving gronp than hydroxyl is nsnally preferred. The anomeric finoride , bromide, acetate, benzoate and phenyl-snlphonyl gronps have all been nsed and some of the chemistry that has been achieved is shown in Scheme 25. AllylsUanes tend to provide /S-confignred prodncts by axial attack on the oxoninm ion species (Scheme 25C) whereas larger nncleophiles snch as electron-rich aromatics add by attack on the a face (Scheme 25D). For nitrogen nncleophiles, Haynes and CO workers have employed the CIO anomeric bromide intermediate that gives the ClO-a hemi-aminal prodncts (Scheme 25E), some of which exhibit remarkable stability (Chart 1) . ... 

Notes and discussion. This procedure is an oxalyl bromide-mediated bromination of the free anomeric hydroxyl of tetra-O-benzylglucose. Presumably, an anomeric bromooxalate ester is a primary intermediate in the process, which is then displaced by bromide ion to yield the anomeric bromide. As such the oxalyl bromide is also acting as a source of in situ generated HBr. Once again the a-bromide is the major,... 

In GDP-Fuc and derivatives the anomeric phosphate group is P-linked to the C-1 atom. Peracetate protection of the fucose enables the quantitative synthesis of the a-bromide intermediate which is converted to the desired P-phosphate almost exclusively (see Scheme 7). 

The first poly-O-acetylaldosyl azide was synthesized by Bertho by treating tetra-O-acetyl-a-D-glucopyranosyl bromide with a metal azide. In addition to the aldosyl azide derivatives that are known (see Table IV), derivatives of 2-amino-2-deoxy-D-glucosyl azide have been prepared - (see Table V). If the trans hydroxyl group at C-2 is not acetylated, an intermediate epoxide of the type suggested for the aldosyl fluorides can form, and the azide obtained has the. same anomeric configuration as the original halide, as. shown by the formulas. The acetylated azides, like the... 

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