Chiral oxime ethers in asymmetric synthesis. 3. Asymmetric synthesis of (R)-N-protected alpha-amino acids by the addition of organometallic reagents to the ROPHy oxime of cinnamaldehyde

A new asymmetric synthesis of a-amino acids is described in which the key step is the diastereoselective addition of organometallic reagents to (R)-O-(1-phenylbutyl)cinnamaldoxime 5 to give hydroxylamines 6. Subsequent reductive cleavage of the N-O bond in the hydroxylamine 6 followed by N-protection gave the carbamates 7, which upon oxidation with ruthenium(III) chloride/periodate gave the N-protected amino acids 8. The method was also adapted to the synthesis of a quaternary amino acid 15 from the ketoxime ether 9.