PHOTOHALOGENATION OF GLYCOPYRANOSYL HALIDES: AN EXPEDIENT ROUTE TO C-1 GEM DIHALOGENATED SUGARS

Article abstract of DOI:10.1016/S0040-4020(01)81310-7

Free-radical halogenation of peracetylated α and β-glycopyranosyl halides of ⁴C₁-D chair conformation takes place at C-1 or C-5 with an α-stereoselectivity.However the less reactive α-chloride and bromide moieties at first undergo a dehydrohalogenation reaction. β-Chlorides when treated with N-bromosuccinimde give new C-1 gem chlorobromo sugars in 65-70 percent yield while new C-5 halogenated compounds are obtained predominantly with β -fluorides or when chlorination is carried out with sulfuryl chloride SO2Cl2.The peracetylated C-1 gem chlorobromo derivative of gluco configuration can be cleanly dehydrobrominated to yield a C-1 chlorinated glucal.It also reacts chemio and stereoselctively in the presence of silver flouride to give the corresponding new C-1 gem chlorofluoro or difluoro derivatives, either in 70 percent yield, depending on the stoechiometry.All these new compounds exhibit a good to excellent stability. 19F-n.m.r. of three peracetylated 1-halogeno- β -D-glucopyranosyl fluorides shows that the J_F,C coupling constants increase with the increasing electronegativity of the geminal axial halogen while J_F,H decreases.