DIASTEREOSELECTIVE BROMINATION OF (R)-N-CINNAMOYL-4-PHENYLOXAZOLIDIN-2-ONE IN THE PRESENCE OF LEWIS-ACIDS

The stereochemistry of addition of Br-2 to E- and Z-(R)-N-cinnamoyl-4- phenyloxazolidin-2-ones was studied. It was established that both the E-and Z-isomers give only two out of four possible diastereoisomers in the presence of Lewis acids (BPr3 or AlBr3). The absolute configurati ons of the diastereoisomers [(2S',3R') and (2R',3S') of the side chain ] were established by X-ray structural analysis. The stereochemistry o bserved is a consequence of the stepwise bromination and the absence o f bridging bromine atoms along the reaction coordinate. In the case of the Z-isomer, the diastereoselectivity of the reaction was high, wher eas it is low in the case of the E-isomer. It was suggested that at th e first stage of addition of Br-2 at the C=C bond, the attack of the B r+ cation occurs at the alpha position, and the second stage of transf er of Br- occurs with the participation of boron or aluminum complexes in the intermediate state of the bromination reaction. This hypothesi s as well as the results of calculations of the initial conformations of the substrates provide an explanation of the regularities observed.