A new Lewis acid-catalyzed atom transfer radical cyclization reaction of unsaturated α-bromo oxazolidinone imides is reported. In the presence of Lewis acids such as Mg(ClO4)2 and Yb(OTf)3, a series of trans cyclic products was obtained in high yield (up to 87%) between 0°C and room temperature. The loading of strong Lewis acids, such as Yb(OTf)3, can be reduced to 0.1 equiv. without significantly compromising the yield. Excellent diastereoselectivity could be achieved by using 1,2-stereocontrol or a chiral oxazolidinone auxiliary. For substrates 1e and 1f bearing a β-methyl substituent and the chiral auxiliary, (S)-(-)-4-benzyl-5,5-dimethyl-2-oxazolidinone, respectively, the diastereomeric ratio of the products was greater than 50:1.