 Stereospecificity of reaction between olefin and hypobromous acid (HOBr) in base, "N"-bromosuccinimide (NBS), is determined by the formation of a bromonium intermediate; the electrophilic bromonium cation approaches the ring's less sterically hindered "α"-face and is attacked by the π-electron density of the alkene.The hydroxide ion then attacks from above the ring ("β"-face) at the 11-carbon, resulting in a structure (1.18) by the stereospecific addition of hydroxyl and bromine across the double bond.Addition of sodium hydroxide results in deprotonation of the 11"α"-hydroxyl, and the subsequent structure undergoes an intramolecular SN2 epoxy ring formation.