New Chiral Didehydroamino Acid Derivatives from a Cyclic Glycine Template with 3,6-Dihydro-2H-1,4-oxazin-2-one Structure: Applications to the Asymmetric Synthesis of Nonproteinogenic α-Amino Acids

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Abstract
New chiral (Z)-α,β-didehydroamino acid (DDAA) derivatives with 3,5-dihydro-2H-1,4-oxazin-2-one structure 11a−f have been stereoselectively prepared after condensation of chiral glycine equivalent 7 with aldehydes in the presence of K2CO3 under mild solid−liquid phase-transfer catalysis reaction conditions. These new systems have been used in diastereoselective cyclopropanation reactions using Corey's ylide for the asymmetric synthesis of 1-aminocyclopropane-1-carboxylic acids (ACCs) such as allo-corononamic and allo-norcoronamic acids. The hydrogenation reaction of these systems at ambient pressure in the presence of formaldehyde affords saturated oxazinones and N-methylated oxazinones which have been transformed into the N-methyl-α-amino acids (N-MAAs) (S)-2-(methylamino)butanoic acid and (S)-N-methylleucine. In addition, the parent α,β-didehydroalanine derivative 11g has been prepared by a direct aminomethylation−elimination sequence from 7 and Eschenmoser's salt and has been used in Diels−Alder cycloaddition with endo selectivity for the synthesis of the enantiomerically pure bicyclic α-amino acids (−)-2-aminobicyclo[2.2.1]heptane-2-carboxylic and (−)-2-aminobicyclo[2.2.2]octane-2-carboxylic acids.

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