Organocatalysis by Hydrogen Bonding Networks
293
Scheme 7. Kinetic resolution of some 4-aryl- and 4-alkyl-substituted oxazinones
It was mentioned in the beginning that azlactones and oxazinones
are activated amino acid derivatives. In the KR discussed here, the remaining oxazinone enantiomer can be reacted further, in the crude reaction mixture, with nucleophiles. For example, treatment with resinbound and N-terminally nonprotected peptides results in coupling with
a β-amino acid. Heating of the homogeneous crude reaction mixture
(typically toluene as solvent) with dilute aqueous hydrochloric acid results in hydrolysis of the unreacted oxazinone enantiomer and precipitation of the corresponding N-acyl β-amino acid. The latter can be isolated
in excellent enantiomeric purity by simple filtration. The filtrate contains the β-amino acid ester of opposite configuration (Berkessel et al.
2005).
293
Scheme 7. Kinetic resolution of some 4-aryl- and 4-alkyl-substituted oxazinones
It was mentioned in the beginning that azlactones and oxazinones
are activated amino acid derivatives. In the KR discussed here, the remaining oxazinone enantiomer can be reacted further, in the crude reaction mixture, with nucleophiles. For example, treatment with resinbound and N-terminally nonprotected peptides results in coupling with
a β-amino acid. Heating of the homogeneous crude reaction mixture
(typically toluene as solvent) with dilute aqueous hydrochloric acid results in hydrolysis of the unreacted oxazinone enantiomer and precipitation of the corresponding N-acyl β-amino acid. The latter can be isolated
in excellent enantiomeric purity by simple filtration. The filtrate contains the β-amino acid ester of opposite configuration (Berkessel et al.
2005).