ORIGINS OF STEREOSELECTIVITY IN ASYMMETRIC SYNTHESES USING CHIRAL OXAZOLINES
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The structures of chiral oxazoline lithio anions enriched in 13C have been studied by 13C NMR spectroscopy. No rotation about the carbon-carbon bond in the lithio azaallyl anion (5) from (4S,5S)-2-(1-methylethyl)-4-methoxymethyl-5-phenyl-2-oxazoline was observed in 2 h at -50 C. The ratio of the E (2) and Z (3) isomers formed by deprotonation of (4S,5S)-2-ethyl-4-methoxymethyl-5-phenyl-2-oxazoline varied as a function of the base used for deprotonation (base-solvent, 2:3; lithium diisopropylamide-tetrahydrofuran, 96:4; lithium diisopropylamide-dimethoxyethane, 96:4; n-butyllithium-tetrahydrofuran, 69:31; lithium diisopropylamide-hexamethylphosphoric triamide-tetrahydrofuran, 67:33; lithium diphenylamide-tetrahydrofuran, 80:20). Asymmetric syntheses employing chiral oxazoline lithio anions are discussed in terms of the stereoselectivity of the deprotonation reaction as well as that of the alkylation reaction. 1978, American Chemical Society. All rights reserved.