From cyclic dehydrodipeptides to uncommon acyclic peptide mimetics

1-Acetyl-3-arylmethylene-2,5-piperazinediones gave N-3-arylpyruvylamino esters by acid-promoted alcoholysis under thermal conditions or microwave irradiation. Compounds obtained from 1-acetyl-3-(o-nitro)arylmethylene-2,5-piperazinediones gave, after reduction of the nitro group and spontaneous cyclization, N-2-indolylcarbonylamino acid derivatives. A similar alcoholysis/reduction sequence applied to dehydrodipeptides bearing a 3(4)-nitroaryl substituent gave N-3(4)-aminophenyl-α-ketopropionylamino acid derivatives. Coupling of the free amino group with Boc-protected amino acids gave tripeptide mimetics with a 6-aminoindole-2-carboxylic or a 3(4)-aminophenyl-α-ketopropionic acid as the second residue.