D. A. Evans Olefin bromination-2 Chem 206 Bromination of Cyclohexene Derivatives Pasto, JACS 1970, 92, 7480 Representative Examples of Diastereoselective Bromination Diaxial opening of bromonium ions may be viewed as an extension of the Furst-Plattner Rule for epoxide ring opening.(Lecture-2 OMe Case A MeoH me3C\ Me3C Pyr-Br Br3 B→AB47%Hoe2dedg44 Br MeOh Me3C H Me3C 53% Minor product Major Product (70%) Pyr-Br Br3 How to generate either epoxide from a conformatinaly biased olefin 3C Me exclusive product RCO3H Epoxidation controlled by steric effects imposed by cis-fused ring OBr 8+ How do we construct the other epoxide diastereomer?? Me3C H syn-Unreactive anti-Reactive H20OB5 仓 Me3C、 Me observed Me HOMe syn-Unreactive From Case A one assumes that both bromonium ions are formed however for the syn isomer to react, ring opeing must proceed against the polarization due to Methyl substituent. OH both bromohydrin afford same productH Me3C H MeOH MeOH H Me3C Me MeOH H Me3C Br H H H Me3C Br H H H Me3C OMe Br Me H H Me3C Br OMe H H H Me3C OMe Br H H Me H Me H H H RCO3H Me R R H H H Br Br H H H Me3C Br Me H H Me3C Br Me H H Me3C Br Me H HOMe H Me3C Br Me H OMe –H+ Me H Br2 H2O Me H O H H Br2 HOAc Me H O Me H Br OH H H Me H Br H2O Br Me R R H H H Br H H Me R R H H H H H Br Br Me H Br OH H H Me H Br H2O D. A. Evans Olefin Bromination-2 Chem 206 Bromination of Cyclohexene Derivatives Pasto, JACS 1970, 92, 7480 Pyr–Br+ Br3 – 47% 53% Diaxial opening of bromonium ions may be viewed as an extension of the Furst-Plattner Rule for epoxide ring opening. (Lecture-2) Pyr–Br+ Br3 – exclusive product syn-Unreactive Case A Case B From Case A, one assumes that both bromonium ions are formed; however, for the syn isomer to react, ring opeing must proceed against the polarization due to Methyl substituent. anti-Reactive Representative Examples of Diastereoselective Bromination syn-Unreactive not observed d+ d+ d+ Major Product (70%) Minor Product (7%) House 2nd Ed, pg 424 How to generate either epoxide from a conformatinaly biased olefin Epoxidation controlled by steric effects imposed by cis-fused ring How do we construct the other epoxide diastereomer?? base minor major both bromohydrins afford same product