Chiral Auxiliary Methods: Acetals Et2zn, CH2l2 eaR220o0° CO2R2 Acetal Yield (% d e (%) R1=Me R2= fPr R2=i-Pr (Mori, A; Arai, I; Yamamoto, H. Tetrahedron, 1986, 42, 6458) Arai. I: Mon. A. Yamamoto. H. J. Am. chem. Soc. 1985. 107. 8254 Mash: Ketals for Cyclic Olefins Zn-Cu, CH2l2 OBn Et20, reflux Substrate de Ketones ingood yields e-99 No mention of stereochemical 0 Mash. E: Nelson K. J. Am. Chem. Soc. 1985. 107. 8256 09B-073/299812:26PMChiral Auxiliary Methods: Acetals Arai, I; Mori, A.; Yamamoto, H. J. Am. Chem. Soc. 1985, 107, 8254 R1 O O CO2R2 CO2R2 R1 O O CO2R2 CO2R2 O O CO2i-Pr CO2i-Pr O O Et2Zn, CH2I2 hexane, -20 °C to 0 °C CO2Et CO2Et • There was no mention of stereochemical rationale. However, later publications state that the mechanism of induction is unclear. (Mori, A; Arai, I; Yamamoto, H. Tetrahedron, 1986, 42, 6458) R1 = Me R2 = i- Pr R1 = n- Pr R2 = i - Pr R1 = Ph R2 = i- Pr 90 91 92 94 91 91 81 61 89 88 Acetal Yield (%) d.e. (%) O O O O OBn OBn OBn OBn O O O O Substrate n O O n d.e. MeO2C Yield n=1 n=2 n=3 O O Zn-Cu, CH2I2 Et2O, reflux 3 80 80 77 90 33 86 0 98 72 90 99 88 88 62 • Ketals formed from corresponding ketones in good yields (43-93%) • No mention of stereochemical rationale Mash, E.; Nelson, K. J. Am. Chem. Soc. 1985, 107, 8256 Mash: Ketals for Cyclic Olefins 09B-07 3/29/98 12:26 PM