Bifunctional Lewis acid-nucleophile-based asymmetric catalysis: mechanistic evidence for imine activation working in tandem with chiral enolate formation in the synthesis of beta-lactams.
J Am Chem Soc
; 127(4): 1206-15, 2005 Feb 02.
Article
em En
| MEDLINE
| ID: mdl-15669860
ABSTRACT
We report a mechanistically based study of bifunctional catalyst systems in which chiral nucleophiles work in conjunction with Lewis acids to produce beta-lactams in high chemical yield, diastereoselectivity, and enantioselectivity. Chiral cinchona alkaloid derivatives work best when paired with Lewis acids based on Al(III), Zn(II), Sc(III), and, most notably, In(III). Homogeneous bifunctional catalysts, in which the catalyst contains both Lewis acidic and Lewis basic sites, were also studied in detail. Mechanistic evidence allows us to conclude that the chiral nucleophiles form zwitterionic enolates that react with metal-coordinated imines. Alternative scenarios, which postulated metal-bound enolates, were disfavored on the basis of our observations.
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Base de dados:
MEDLINE
Assunto principal:
Alcaloides de Cinchona
/
Beta-Lactamas
/
Iminas
/
Metais
Idioma:
En
Ano de publicação:
2005
Tipo de documento:
Article