NBS, as brominating agent

Silver ion makes possible in the second reaction an isnmeri/ation-free hydrolysis of the S,0 acetal to ketone 32. Generation of the cnol triflate 33 is accomplished in the third step with the Hendrickson-Me M it rry reagent (Tf2NPh).10 Addition of an alcohol produces the potassium aikoxide, which because of its lower basicity permits isomerization to the thermodynamically favored enolate. Chemose-lective reaction to bromohydnn 9 is achieved in the last step with NBS as brominating agent in aqueous THF. NBS acts here as a source of cationic bromine in an ionic mechanism. The intermediate bromonium ion forms preferentially at a) electron-rich double bonds and h) the sterieally least hindered double bond. It also opens in such a way as to provide the most stable carbocation. 

Davies et al. synthesized H-T2-H 2 in 50% yield reacting the ethanolamine ester of di(2-thienyl)borinic acid with iV-bromosuccinimide (NBS) as brominating agent. The investigation aimed at the use of the easily isolable borinic acids as intermediates in synthetic organic chemistry [57]. 

---