Asymmetric memory at labile, stereogenic boron: Enolate alkylation of oxazaborolidinones

Oxazaborolidinones 3, 25, 32, 42, 49, and 53 can be obtained as single dias tereomers by crystallization-induced asymmetric transformation (AT). Asymme tric memory is maintained in the derived enolates because the stereogenic b oron resists equilibration with achiral, trivalent boron-containing species on the time scale of enolate alkylation with methyl iodide, allyl bromide, or benzyl bromide. Conditions were found for alkylating oxazaborolidinone enolates derived from phenylalanine (5, 33), alanine (18, 26), phenylglycin e (43), and valine (54) without significant loss of boron configuration. Th e phenylglycine-derived oxazaborolidinone alkylation products 44 and 45 slo wly undergo boron epimerization at room temperature, and the C-allyl produc t 44b partially racemizes during hydrolytic cleavage, apparently by a 2-aza -Cope rearrangement. These complications were not encountered with phenylal anine derivatives. Preparatively useful results were obtained with oxazabor olidinones 3 and 32, derived from phenylalanine. AT favors a different boro n configuration in the B-naphthyl analogue 32 compared to 3. This provides access to either quasi-enantiomeric enolate 5 or 33 by starting from the sa me phenylalanine enantiomer.