Amino acids, preparation esters, from azlactones

N-acyldehydrodipeptides were readily prepared either by the condensation of N -acyldehydro-a-amino acids with a-amino acid esters or by the reaction of the azlactones of dehydro-a-amino acid with a-amino acid esters (eq. 1). Asymmetric hydrogenation of the N-acyldehydrodipeptides thus obtained (eq. 2) was carried out by using rhodium complexes with a variety of chiral diphosphines such as -Br-Phenyl-CAPP (3), Ph-CAPP (3), (-)BPPM (4), (+)BPPM (4), (-)DIOP ( ), (+)DIOP ( ), diPAMP (6), Chiraphos (7), Prophos (S), BPPFA (9) and CBZ-Phe-PPM (Fig. 1)(10). The chiral catalysts were prepared in situ from chiral diphosphine ligand with [Rh(NBD)2l -CIO4 (NBD = norbomadiene). Typical results are summarized in Tables I-V. 

The dipeptide azlactone (208) has been converted to the protected unsaturated dipeptide (209) by oxidation with DDQ followed by methanolysis. Two reports have appeared on the conversion of a-keto-acids and esters into a,j3-unsaturated amino-acid derivatives. In the first the anhydride (210) is prepared from an a-keto-acid, then converted into (211) with ethanol. The second example... 

---