Trialkylsiloxy aldehydes 4a,b add functionalized diorganozincs in the presence of catalytic amounts of (1R, 2R)-bis(trifluoromethanesulfonamido)cyclohexane (8 mol %), affording protected 1,2-diols in 39-89% yield and 70-94% ee. The conversion of the monosilylated 1,2-diols to functionalized chiral epoxides has been demonstrated.The enantioselective preparation of scalemic 1,2-diols has been extensively investigated in the past 10 years. 1,2-Diols are versatile chiral building blocks1 and have found numerous applications as chiral auxiliaries or ligands of metal catalysts.2 The catalytic asymmetric epoxidation3•4 and dihydroxylation5•6 of olefins have been especially successful preparation methods of 1,2-diols. Alternative approaches such as the catalytic enantioselective reduction of -alkoxy ketones7 or the catalytic asymmetric addition of an organometallic to an a-alkoxy aldehyde (1) have not been extensively investigated (Scheme 1).Recently, we have shown that functional diorganozincs 2 can be added to various types of aldehydes in the presence of (lR,2R)-bis(trifluoromethanesulfonamido)cyclohexane (3)8 leading to polyfunctional secondary9 or aJlylic10 alcohols and aldol products11 with good to excellent enantioselectivity. Herein, we report that diorganozincs 2 prepared via an iodine-zinc exchange reaction9 add to -triorganosiloxy aldehydes 4a,b in the presence of catalytic amounts of 3 (8 mol %) and titanium(IV)
reactions of organo-metal compounds reactions of organo-metal compounds O 0350
-153Enantioselective Preparation of Polyfunctional Secondary Allylic Alcohols Using Functionalized Dialkylzincs Prepared by a Copper(I)-Catalyzed Iodine-Zinc Exchange Reaction.-The iodine-zinc exchange reaction between the alkyl iodides (I) and diethylzinc is effectively catalyzed by Cu(I) salts to give the dialkylzinc compounds (II) which are subsequently coupled with α, β-unsaturated aldehydes such as (III). In the presence of chiral titanium catalysts, secondary allylic alcohols such as (IV) are obtained with enantiomeric excesses up to 98 percent. -(ROZEMA, M. J.; EISENBERG, C.; LUETJENS, H.; OSTWALD, R.; BELYK, K.; KNOCHEL, P.; Tetrahedron Lett. 34 (1993) 19, 3115-3118; Fachber. Chem., Philipps-Univ.,
A New Catalytic Asymmetric Approach to Polyfunctional Aldol Products Mediated by Zinc Organometallics.-The 3-substituted propanals (I) are reductively alkylated with the dialkylzinc agents (II) in the presence of a freshly prepared chiral titanium catalyst. The chiral alcohols (S)-(III) are obtained with a high enantiomeric excesses and are further converted to the protected aldols (V). -(KNOCKEL, P.; BRIEDEN, W.; ROZEMA, M. J.; EISENBERG, C.; Tetrahedron Lett. 34 (1993) 37, 5881-5884; Fachber.
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