Synthesis and carbon‐13 nuclear magnetic resonance spectra of Cyclo( D ‐leucyl‐ L ‐leucine) and Cyclo( L ‐leucyl‐ L ‐leucine)

Cyclo(D‐Leu‐L‐Leu) and cyclo(L‐Leu‐L‐Leu) were synthesized, and their carbon‐13 nmr spectra at 65 MHz were examined in dimethylsulfoxide and trifluoroacetic acid solutions. The chemical shift data are consistent with a boat or “twisted” boat conformation of the diketopiperazine ring in both solvents. There was no indication of protonation of the cyclic dipeptides by trifluoroacetic acid. Attempts at polymerizing the cyclic dipeptides were unsuccessful.