Asymmetric Memory at Labile, Stereogenic Boron:  Enolate Alkylation of Oxazaborolidinones

Abstract

Oxazaborolidinones 3, 25, 32, 42, 49, and 53 can be obtained as single diastereomers by crystallization-induced asymmetric transformation (AT). Asymmetric memory is maintained in the derived enolates because the stereogenic boron 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), phenylglycine (43), and valine (54) without significant loss of boron configuration. The phenylglycine-derived oxazaborolidinone alkylation products 44 and 45 slowly undergo boron epimerization at room temperature, and the C-allyl product 44b partially racemizes during hydrolytic cleavage, apparently by a 2-aza-Cope rearrangement. These complications were not encountered with phenylalanine derivatives. Preparatively useful results were obtained with oxazaborolidinones 3 and 32, derived from phenylalanine. AT favors a different boron 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 same phenylalanine enantiomer.

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Organocatalyzed Asymmetric Conjugate Addition of Heteroaryl and Aryl Trifluoroborates: a Synthetic Strategy for Discoipyrrole D

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