Diastereoselective ring cleavage of azetidines with cyanogen bromide

N-alkyl azetidines bearing a chiral substituent at the nitrogen atom, and a variable substituent at C-3 as a prochiral centre, react readily with cyanogen bromide even at low temperatures to produce the corresponding 3-bromo-N-cyanamides as diastereoisomeric mixtures via ring opening. This reaction was found to be diastereoselective (up to 80:20 dr). In one case, the major diastereomer could be isolated by recrystallization, and the configuration of the newly created stereocentre was determined by X-ray crystallography.