▁small ▁amount ▁of ▁one ▁en anti omer ▁of ▁CBS ▁catal yst ▁( a ▁Core y – I ts uno ▁reduction ). ▁The ▁choice ▁of ▁ch ir ality ▁of ▁the ▁catal yst ▁thus ▁gives ▁select ivity ▁for ▁one ▁or ▁the ▁other ▁s tere och em istry ▁of ▁the ▁alco hol ▁in ▁the ▁product . ▁ ▁This ▁ 2 - h ydro xy ▁structure ▁is ▁then ▁react ed ▁with ▁az ide ▁and ▁a ▁nucle oph il ic ▁base . ▁The ▁mult iste p ▁reaction ▁mechanism ▁begins ▁with ▁de pro ton ation ▁of ▁the ▁alco hol , ▁followed ▁by ▁the ▁o xygen - an ion ▁attack ing ▁the ▁adjacent ▁tr ich lor om eth yl ▁position ▁to ▁form ▁an ▁epo x ide . ▁The ▁az ide ▁then ▁opens ▁this ▁ring ▁by ▁an ▁SN 2 ▁reaction ▁to ▁give ▁a ▁ 2 - az ido ▁structure ▁whose ▁s tere och em istry ▁is ▁invert ed ▁compared ▁to ▁the ▁original ▁ 2 - h ydro xy . ▁The ▁o xygen , ▁having ▁become ▁attached ▁to ▁the ▁first ▁carbon ▁of ▁the ▁chain ▁during ▁the ▁epo x ide ▁formation , ▁simultaneously ▁disp laces ▁a ▁second ▁ch lor ine ▁atom ▁there ▁to ▁form ▁an ▁a cy l ▁ch lor ide . ▁An ▁additional ▁nucle oph il ic ▁react ant , ▁such ▁as ▁hydro x ide ▁or ▁an ▁alk ox ide , ▁then ▁triggers ▁an ▁a cy l ▁substitution ▁there ▁to ▁produce ▁a ▁car box y lic ▁acid ▁or ▁est er . ▁Vari ous ▁other ▁nucle oph iles ▁can ▁be ▁used ▁to ▁generate