Photohalogenation of glycopyranosyl halides: An expedient route to c-1 gem dihalogenated sugars
1989; Elsevier BV; Volume: 45; Issue: 13 Linguagem: Inglês
10.1016/s0040-4020(01)81310-7
ISSN1464-5416
AutoresJean‐Pierre Praly, Laurent Brard, Gérard Descotes, Loı̈c Toupet,
Tópico(s)Glycosylation and Glycoproteins Research
ResumoFree-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-bromosuccinimide give new C-1 gem chlorobromo sugars in 65–70 % 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 stereoselectively in the presence of silver fluoride to give the corresponding new C-1 gem chlorofluoro or difluoro derivatives, either in 70 % 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 JF,C3 coupling constants increase with the increasing electronegativity of the geminal axial halogen while JF,H decreases.
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