Imino acids, chiral purity

A group at the Academy of Sciences in Moscow 197) has synthesized chiral threonine. Derivatives of cyclic imino acids form copper complexes with glacine and carbonyl compounds. Hydroxyethylation with acetaldehyde and decomposition of the resulting complexes produced threonine with an optical purity of up to 97-100% and with threo/allo ratios of up to 19 1 197). The chiral reagents could be recovered and re-used without loss of stereoselectivity. The mechanism of this asymmetric synthesis of amino acids via glacine Schiff base/metal complexes was also discussed 197). 

Recently, asymmetric PTC using chiral quaternary ammonium salt 110 has proven to be an effective method for the enantioselective a-arylation of a-imino acid derivatives 108 via asymmetric nucleophilic aromatic substitution, to give a,a-disubstituted a-amino acids 111 in good to high enantiomeric purity (Scheme 8.22) [86]. 

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... 

Example 4.22 An axially chiral binaphthyl unit also characterizes chiral catalyst 41 for the asymmetric Mannich reaction presented in Scheme 4.53. The sufibnamido group acts as the proton donor to the imino N atom. Aldehydes are alkylated by an A-pam-methoxyphenyl (PMP) derivative of oxalaldehyde as an imino component. anft -Isomers of a-amino-y-keto acids TM 4.18a-e with (25,37 ) absolute configurations are obtained as the prevailing products with very high optical purity (Scheme 4.53) [37]. 

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