Total Synthesis of Sequential Retro-Peptide Oligomers.

The total synthesis of sequential oligomers of retro-peptides of the general formula Boc(-gGly-mAib)(n)-OBn, starting from dimethylmalonic acid, has been carried out in good overall yield. The key step is the formation of the gGly unit >N-CH2-N<, which is easily obtained in a one-pot/threestep reaction from BnO-mAib-Gly-OH and diphenylphosphoryl azide. The synthesis of the sequential oligomers of RCO(-gAla-mAib)(n)-OR' was also attempted, but the oligomerization step failed because the gAla moiety readily decomposes or racemizes.