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Synthesis 2008(16): 2594-2602  
DOI: 10.1055/s-2008-1067172
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Asymmetric Synthesis of (2S,3S)- and (2R,3R)-α,β-Dialkyl-α-amino Acids via Alkylation of Chiral Nickel(II) Complexes of Aliphatic α-Amino Acids with Racemic α-Alkylbenzyl Bromides

Vadim A. Soloshonok*, Thomas U. Boettiger, Shawna B. Bolene
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, USA
Fax: +1(405)3256111; e-Mail: vadim@ou.edu;
Further Information

Publication History

Received 30 March 2008
Publication Date:
08 July 2008 (online)

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Abstract

This study has demonstrated that the stereochemical outcome of the direct alkylation of nickel(II) complexes derived from chiral Schiff bases of glycine, alanine, 2-aminobutyric acid, and leucine with racemic α-methylbenzyl bromide depends on the steric bulk of the corresponding amino acid residue. In particular, the alkylation of the alanine complex was found to proceed with a synthetically useful level (90% de) of stereoselectivity offering a concise synthesis of enantiomerically pure (2S,3S)- or (2R,3R)-α,β-dimethylphenylalanines.

Key words

sterically constrained amino acids - alkylation - asymmetric synthesis - kinetic resolution

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1

As we recently pointed out (see ref. 2), the terms unnatural, unusual, or nonproteinogenic, noncoded amino acids depend on the success of specific scientific achievements. For instance, amino acids containing the most xenobiotic element fluorine were shown to be synthesized by microorganisms (see ref. 3), and also new amino acids can be added to genetic code of microorganisms (see ref. 4). Therefore, the time-independent term tailor-made, meaning rationally designed/synthesized amino acids, in the absence of a better definition, is used in this paper and generally recommended for use in the corresponding literature.

25

As shown previously (see refs. 21-23), CD and ORD spectra of Ni(II) complexes of this type in neutral solutions exhibit two maxima in the region of metal d-d transition (Cotton effects at 450 and 550 nm). In the ORD spectra, the sign of Cotton effects in this region strictly depends upon a conformation of the polycyclic system of chelate rings. Thus, in the case of complexes containing α-monosub-stituted α-amino acid, the pseudoaxial orientation of the amino acid side chain, corresponding to α-l configuration of α-amino acid, causes a Cotton effect with a positive sign at the 500-700 nm region and negative sign at 400-450 nm. Consequently, a pseudoequatorial orientation of the amino acid side chain brings about opposite signs of the Cotton effects at 400-450 (positive) and at the 500-700 nm (negative) regions. As established in numerous studies, this general trend is not influenced by the structure and nature of the α-amino acid side chain, and the configuration of stereogenic centers within it.