Stereoselective synthesis of alcohols, XXXV. Addition ofE- andZ-crotylboronates to chiral α-substituted aldehydes

✍ Scribed by Brinkmann, Heinrich ;Hoffmann, Reinhard W.

• Publisher: Wiley (John Wiley & Sons)
• Year: 1990
• Tongue: English
• Weight: 714 KB
• Volume: 123
• Category: Article
• ISSN: 0009-2940
• DOI: 10.1002/cber.19901231223

✦ Synopsis

Allylboration reaction / Asymmetric induction / Calculations, force-field / Transition-state models

The direction (Cramlanti-Cram) and the extent of asymmetric induction on addition of crotylboronates to a range of chiral aldehydes was investigated. A reversal in the sense of the asymmetric induction on changing from the Z-to the E-crotylboronate was found for aldehydes having polar ct substit-uents and for some of the nonpolar chiral aldehydes. Forcefield calculations on transition-state models reproduced these results in the majority of the cases investigated despite of unaccounted short comings of the method.

Among the well studied addition reactions of organometallic reagents to chiral aldehydes 1 those of crotylboronates are exceptional, because the sense of the aldehydebased asymmetric induction depends on the structure of the achiral crotylboronate. While E-crotylboronates 2 lead unconspicuously to the usual Cram product, the addition of the Z-crotylboronates 3 violates Cram's rule') and generates the anti-Cram diastereomer in preference 3,4).