Migration alkylthio groups

Pozsgay, V, A simple method for avoiding alkylthio group migration during the synthesis of thioglycoside 2,3-orthoesters. An improved synthesis of partially acylated 1-thio-a-L-rhamnopyrano-sides, Carbohydr. Res., 235, 295-302, 1992. 

In the field of reactions of thioglycosides the photobromination of phenylthio compounds has been covered in a review. As has been noted before, and has been used usefully in synthesis of 2-deoxy compounds, the alkylthio group of thioglycosides may migrate to C-2 under certain conditions. Thus, as well as compound 93 giving the expected 2,3-orthoester on treatment with alkyl orthoesters in acid conditions, it gave compound 94... 

Further examples of 1,2-alkylthio group migration in the attempted selective 2-benzoylation of alkyl 1-thioglycopyranoside 2,3-diols, such as compound 17, via hydrolysis of their 2,3-orthoesters have been reported (see Vol.25, Chapter 7, ref.4). The problem was, however, avoided either by use of very low concentrations of p-TsOH or by thorough removal of the alcohol released in the orthoester formation, or by performing the hydrolysis of the orthoester in acetonitrile instead of DMF. ... 

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