Properties of the Amide Bond Involving Proline 4,5-methanologues: an Experimental and Theoretical Study

Herein we disclose a combined experimental and theoretical study on the conformational properties of amide bonds involving diastereomeric 4,5-methano-L-proline congeners as N-acetyl ethyl esters and dimeric amide ethyl esters as substrates for cyclization to diketopiperazines. The results are discussed in light of density functional theory calculations aimed at delineating the importance of the n* stabilization of the trans-amide conformations. The transitions states for the cyclization of diastereomeric 4,5-methano-L-proline ethyl esters to diketopiperazines were calculated and correlated to the experimental studies.