Possible Explanation for Yamamoto's Results CO2R CO2R CO2R RO2C RO2C RO2C MINOR MAJOR red conformation but tercel H CO2R CO2R disfavored and stereoelectronical due to steric interactions with the -Cu, CH2I2 Possible Explanation for Mash's Ketals.0 Eto, reflux OBn Bno Bno ated away from BnO-CH and chronically aligned:π-σ*C-landσCZn-π 09B-08329/9812:30PMR X RO2C R1 O O CO2R2 CO2R2 R1 O O CO2R2 CO2R2 O O H R RO2C Zn I I Zn I RO2C O O H R RO2C O O CO2R H CO2R Zn I I Zn I MAJOR O O Possible Explanation for Yamamoto's Results CO2R CO2R Et2Zn, CH2I2 hexane, -20°C to 0°C H R • Sterically favored conformation and stereoelectronically alligned: π − σ∗ C-I and σ C-Zn - π∗ • Sterically disfavored and stereoelectronically misaligned MAJOR X R RO2C O O H RO2C Zn I I Zn I RO2C O O H R RO2C MINOR • Sterically favored conformation but stereoelectronically misaligned for cyclopropanation O O CO2R H CO2R R Zn I I Zn O O CO2R H CO2R R • Disfavored due to steric interactions with the ester group MINOR O O O O OBn OBn OBn OBn MINOR O O O CH2OBn n Bn n Zn I I Zn-Cu, CH2I2 Et2O, reflux O O O CH2OBn • Chelation reduces the electrophilicity of the Zn reagent enough to slow cyclopropanation from this face of the olefin Bn Possible Explanation for Mash's Ketals X O O Zn I I Zn I BnO BnO O O BnO BnO MAJOR • Coordinated away from BnO-CH2 group and stereoelectronically aligned: π − σ∗ C-I and σ C-Zn - π∗ 09B-08 3/29/98 12:30 PM