Mixed zincate preparation

The chiral organocopper compound (186) adds diastereoselectively to 2-methyl-2-cyclopentenone, allowing the preparation of optically active steroid CD-ring building blocks (Scheme 68).202-204 A related method was applied to a synthesis of the steroid skeleton via an intramolecular (transannular) Diels-Alder reaction of a macrocyclic precursor.203 Chiral acetone anion equivalents based on copper azaeno-lates derived from acetone imines were shown to add to cyclic enones with good selectivity (60-80% ee, after hydrolysis).206-208 Even better ee values are obtained with the mixed zincate prepared from (187) and dimethylzinc (Scheme 69). Other highly diastereoselective but synthetically less important 1,4-additions of chiral cuprates to prochiral enones were reported.209-210... 

C. Preparation of Mixed Zincates by Halogen-Zinc Exchange Reaction. . . 688... 

Mixed zincates R1 (R2)2ZnLi are prepared either from (R2)2Zn and R1 Li or by successive addition of R2Li (2 equiv) and R Li (1 equiv) to ZnX2. Sodium and potassium zincates are prepared from diorganozinc according to Wanklyn s procedure (equation 5)1. 

The more difficult cross-coupling reaction between Csp -Csp can be accomplished by mixed zincate compounds, prepared by the reaction of diorganozinc derivatives with Me2CuMgCl-MgCl(CN) (equation 59). Substitution reaction at Csp centers are also efficiently catalyzed by Ni salts when the alkyl halide contains a double bond in remote position (Scheme 22). Changing the double bond for a triple bond leads to an intramolecular carbozincation reaction (equation 60). 

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