Use of glycals as direct glycosyl donors haloglycosylation

Various pathways for the electrophilic addition to a glycal followed by nucleophilic opening of the resulting halonium ion can account for the different product distributions obtained from these reactions [74]. Irreversible formation of the halonium ion and subsequent nucleophilic displacement leads to the -gluco compound, while reversible formation of the halonium ion followed by slow nucleophilic trans diaxial opening leads to the a-manno compound. Common 1 sources used to synthesise a-manno species include Af-iodosuccinimide (NIS), and iodonium di- ym-coUidine perchlorate (IDCP). An iodonium source is particularly favoured over a bromonium source, as this more readily allows further subsequent functionalisation of the product often via radical chemistry. The yields of 

Synthesis of benzyl 3-0-(4-methoxybenzyl)-2,6-dideoxy-2-iodo-a-L-talopyrano-side [75]. 

Notes and discussion. The example given here is a typical procedure for iodoglycosylation. In this specific case, only one diastereomer is produced, resulting from frani-diaxial opening of the 3-iodonium ion. 

Anhydrous benzyl alcohol (1 ml, 9.6 mmol) A -iodosuccinimide (540 mg, 2.4 mmol) 

Round-bottomed flask (10 ml) with rubber septum and magnetic stirrer bar Magnetic stirrer 

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