Synthesis and Anticancer Activity Assessment of Zelkovamycin Analogues

The zelkovamycin family is a class of cyclic octapeptides with potent antibacterial and antiviral activity. Due to their unique chemical structures and excellent bioactivity, zelkovamycins have consistently attracted the interest of synthetic chemists. However, only the total synthesis of zelkovamycin and zelkovamycin G has been reported until now. The current work presents, for the first time, the synthesis of zelkovamycin analogues, along with their anticancer activity assessment. Firstly, the corresponding chain peptide based on the amino acid sequence of zelkovamycin H was synthesized using the Fmoc solid-phase peptide strategy. This was followed by cyclization under high dilution conditions to obtain compound 21, and its structure was elucidated by NMR analysis. The results confirm that compound 21 is not the natural product of zelkovamycin H. We deduced that during the synthesis of peptide 12, the D-Abu residue epimerized to the L-Abu form, leading to the formation of peptide 20, which blocked our efforts during the synthesis of zelkovamycin H. Two more analogues, 22 and 23, were synthesized by changing the structure of amino acid residues using the same strategy. The anticancer activity of analogues 21–23 against Huh-7 cells was evaluated in vitro; however, their IC50 values were >50 μM.