Stereochemistry of Metalation and Alkylation of Chiral Oxazolines. A 13C Nuclear Magnetic Resonance Study of Lithio Oxazolines1

Low-temperature 13C NMR spectra of lithiated chiral oxazolines 2 and 3 exhibit two isomers in ratios of 92 ± 3% Z and 8 ± 3% E. These ratios are indicative of kinetic products which do not equilibrate over the temperature range —106 to — 28 °C and time ranging from 0.5 to 221 h. The studies were conveniently performed using enriched 13C labeling in the lithio oxazolines. Alkylation of these lithio anions with incremental equivalents of alkyl halides indicated that the two metalated species are disappearing at the same rate supporting the contention that the high % ee of the chiral acids produced in this reaction are directly derived from the ratio of lithio anions in solution.