Synthesis and Structure–Activity Correlation of a Brunsvicamide‐Inspired Cyclopeptide Collection

Cyanobacterial cyclopeptides : A series of analogues of the cyanobacterial cyclopeptide brunsvicamide A was prepared by effective solid‐support‐based total synthesis. Variations in stereochemistry revealed the importance of the D ‐lysine and the L ‐isoleucine residues for the substrate‐competitive inhibitory activity of brunsvicamide A against carboxypeptidase A.The brunsvicamides are modified cyclopeptides from cyanobacteria, cyclised through the ε ‐amino group of a D ‐lysine unit. They are functionalised with urea groups and show potent carboxypeptidase inhibitory activities. In order to unravel the structural parameters that determine their activities, a collection of brunsvicamide analogues with varied amino acid structures and stereochemistries was synthesised by a combined solution‐ and solid‐phase approach. Biochemical investigation of the compound collection for carboxypeptidase A inhibition revealed that the presence of D ‐lysine and L ‐isoleucine in the urea section is important for inhibition. It was found that brunsvicamide A is a substrate‐competitive inhibitor of carboxypeptidase A. These findings are in agreement with the substrate specificity of the enzyme and were rationalised by computational studies, which showed the high relevance of the lysine stereochemistry for inhibitory activity.