rnate.I4] The Grignard reagent 3 consists of an equilibrium mixture of enantiomers, diastereomers, and conformational isomers owing to rotation around formal single bondsrs] and 1,3-metallotropic shift.[61 Borate 4 reacts preferentially with 3 b, brr7] to afford an 89 % yield of the pentenylboronic esters 1 and 5, which have (Z) configurations (cf. Ref. [S]). v 5 Furthermore, the chiral borate 4 differentiates between the enantiomers 3b,b', so that 1 can be obtained with a diastereomeric excess of 94 % by kinetic resolution of a racemate. This was established by reaction of 1 with benzaldehyde and determination of the enantiomeric purity of alcohol 2. The glycol from which 4 is derived and thus the reagent 1 are readily accessible in either enantiomeric Experimental Procedure A 0.85 M solution (6.0 mL, 5.1 mmol) of (1-methyl-2-buteny1)magnesium chloride (3) in THF was added dropwise over 15 min to a solution of (4S,5s)-2-isopropyloxy-4,5-dicyclohexyl-1,3,2-dioxaborolane (4; prepared from 1,2-dicycIohexylethane-l,2-diol and triisopropyl borate by distillative removal of the isopropanol) in 20 mL of THF at -78 "C. The solution was allowed to warm overnight to room temperature and the products were hydrolyzed by addition of a saturated solution (1 5 mL) of NH,CI. The aqueous phase was extracted with 3 x 20 mL of ether and the organic phase was dried over MgSO, and concentrated. The colorless oily residue was chromatographed with CH,Cl, (column 30 x 2 cm, silica gel 60), affording 1.38 g (89 YO) of boronic ester 1. For spectroscopic data and determination of the enantiomeric purity by addition to benzaldehyde, see 12).