ASYMMETRIC-SYNTHESIS USING DIISOPROPYL TARTRATE MODIFIED (E)-CROTYLBORONATES AND (Z)-CROTYLBORONATES - PREPARATION OF THE CHIRAL CROTYLBORONATES AND REACTIONS WITH ACHIRAL ALDEHYDES

Diisopropyl tartrate modified (E)- and (Z)-crotylboronates 2 and 3 are easily prepared with very high isomeric purity (⩾98% E for 2; ⩾99% Z for 3) via the metalation of (E)- and (Z)-2-butene with n-BuLi and KOtBu in THF followed by treatment of the (E)- and (Z)-crotylpotassiums 4 and 5 with (i-PrO)3B, aqueous hydrolysis, and esterification with diisopropyl tartrate (DIPT). (E)-Crotylboronate 2 has been prepared on 400-mmol scales with excellent results. A procedure for the preparation and standardization of solutions of 2 and 3 is also described. Crotylboronates 2 and 3 undergo highly diastereo and enantioselective reactions with aliphatic (linear or a-monobranched; 72-91% ee), aromatic and α,β-unsaturated aldehydes (55-74% ee). The reaction diastereoselectivity closely parallels the reagent isomeric purity especially for reactions performed at-78 °C. The enantioselectivity is best in toluene for all substrates except benzaldehyde where best results are obtained in THF. The relative and absolute stereochemistry of 9 of the 14 homoallyl alcohols synthesized in this study were assigned by correlation with epoxyalcohols prepared by the Sharpless asymmetric epoxidation. The results in each case are consistent with the asymmetric crotylboration reaction proceeding preferentially through transition state A. That is, assuming that R of RCHO has priority over the crotyl group that is transferred, (R,R)-2 and (R,R)-3 invariably produce homoallyl alcohols with 5 configuration at the carbinol center. The enantiomeric set of homoallylic alcohols are available simply by using reagents (S,S)-2 and (S,S)-3 prepared from (S,S)-DIPT. © 1990, American Chemical Society. All rights reserved.