Enantioselective Carbometalation of Cinnamyl Derivatives: New Access to Chiral Disubstituted Cyclopropanes— Configurational Stability of Benzylic Organozinc Halides

✍ Scribed by Stephanie Norsikian; Ilan Marek; Sophie Klein; Jean F. Poisson; Jean F. Normant

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 254 KB
• Volume: 5
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/(sici)1521-3765(19990702)5:7<2055::aid-chem2055>3.0.co;2-9

✦ Synopsis

A stoichiometric or catalytic amount of (À)-sparteine can serve as a promoter for the enantioselective carbolithiation of cinnamyl derivatives by primary and secondary organolithium compounds. The enantiofacial choice of the addition reaction is dependent on the stereochemistry of the initial double bond. The resulting benzylic organolithium compounds can be derivatized to a linear phenylated chain that bears two contiguous stereogenic centers with given configurations. The use of the dimethyl acetal of the (E)-cinnamyl alcohol allows the highest enantioselective carbolithiation and by simply warming the reaction mixture to room temperature, the resulting benzylic organo-lithium intermediate undergoes a 1,3elimination to give the chiral disubstituted cyclopropane in high enantiomeric excess (90 ± 95 % ee). Another significant finding is the observation that the Li ± Zn transmetalation in a benzylic species occurs with inversion of configuration, and the corresponding acyclic benzylic zinc halides have observable configurational stability at À 30 8C.