The coupling reactions of 3-phenylpropanoic acid and N-carboxybenzyl α-amino acids with unprotected α-amino acids containing hydrophilic side chains such as aliphatic alcohol, aromatic alcohol, thiol, carboxylic acid, and amide afforded the corresponding amides in 66–96% yield without racemization via the corresponding mixed carbonic carboxylic anhydrides under basic conditions through an ecological green synthetic method.

Condensation of carboxylic acids 1 and 5 with unprotected α-amino acids 2 via activation by ethyl chloroformate and triethylamine proceeded effectively to afford the corresponding amides in 50–99% yields. Tripeptide 7c was obtained in 42% yield from the dipeptide 6c in a similar manner.

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Primary amides were easily prepared in 22–99% yields from the corresponding carboxylic acids 1 or 5 with NH4Cl via activation with ClCO2Et and Et3N. The enantiomers of the corresponding primary amides of Cbz-, Boc-, or Fmoc-α-amino acids can be separated by using a chiral column.

Carboxylic acids 1 and 7 reacted effectively with unprotected α-amino acids 2 containing another hydrophilic moiety under basic conditions via activation by ethyl chloroformate and triethylamine to afford the corresponding amides in 74–99% yields.

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The amidation of N-protected -amino acids proceeds without racemization. -(NOGUCHI, T.; SEKINE, M.; YOKOO, Y.; JUNG, S.; IMAI*, N.; Chem. Lett. 42 (2013) 6, 580-582, http://dx.