New Synthetic Bromination Procedures for Use in Radiolabelling‐Chemistry: Reaction of C ‐Metallated Derivatives of Carbohydrates with Bromide, in the Presence of Mild Oxidizing Agents

In the presence of 2 mol‐equiv. of AcONa, both triglycosylborane 1 and dicyclohexylglycosylborane 2 readily react with bromine chloride generated in situ from bromide and N ‐chlorosuccinimide (NCS) to give the bromo‐sugar 3 (75 and 60%, resp.). The use of the BH 3 ‐THF/bromide/NCS/2 AcONa procedure permits the rapid, face‐specific synthesis of 6 (58% uptake of bromine) and face‐selective synthesis of 8/9 (71% uptake of bromine), from vinyl ether derivatives 5 and 7 , respectively. The dicyclohexylborane/bromide/NCS/2 AcONa procedure leads to the fast and quantitative conversion of 11 to the bromosugar 12 (91%). Hydroboration‐transmetallation sequences give access to C ‐mercuriated carbohydrates 13 (71%) and 14 (78%). The bis(glycosyl)mercury derivative 13 is spontaneously cleaved by reaction with one equivalent of bromide/chloramine‐T/aqueous HCl‐solution to give 3 (87%) and 14 (76%). Hydrostannylation of acetylenic sugar 15 gave the ( E )‐stannylvinyl derivative 16 as the major product. This latter precursor 16 is spontaneously cleaved by the bromide/chloramine‐T/aqueous HCl‐solution reagent to give the bromovinyl‐sugar 17 (96%).