General Methods for the Direct Halogenation of Alcohols

A general and important method for activating alcohols toward nucleophilic substitution is by converting them into alkoxyphosphonium ions  

The use of triphenylphosphine-carbon tetrachloride to convert lincomycin 2 into clindamycin 3 has already been mentioned the 7-bromo aud 7-iodo analogues of 3 were also prepared by treatment of lincomycin hydrochloride with triphenylphosphine and carbon tetrabromide or carbon tetraiodide, with acetonitrile as the solvent [6]. 

There are several methods for the conversion of alcohols into iodides using phosphorus-containing reagents. Two important reagents (Rydon reagents) are methyltriphenoxy-phosphonium iodide 6 and iodotriphenoxyphosphonium iodide 7, respectively formed 

There are many examples of the reaction of carbohydrates with Rydon reagents [16] the reaction is controlled by steric factors. Thus, no reaction occurred between 1,2-0-isopropylidene-5,6-di-0-methyl-a-D-glucofuranose and either 6 or bromotriphenoxyphos-phonium bromide, presumably because of the steric hindrance caused by the trioxabicyclo [3.3.0]octane ring-system, whereas methyl 2,5,6-tri-O-methyl-p-D-glucofuranoside reacted with 6 to give a 3-deoxy-3-iodo derivative in 31% yield. 

In the last-cited work [28], methyl 4,6-dichloro-4,6-dideoxy-a-D-galactopyranoside 

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