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 <b>21</b>, and its structure was elucidated by NMR analysis. The results confirm that compound <b>21</b> is not the natural product of zelkovamycin H. We deduced that during the synthesis of peptide <b>12</b>, the D-Abu residue epimerized to the L-Abu form, leading to the formation of peptide <b>20</b>, which blocked our efforts during the synthesis of zelkovamycin H. Two more analogues, <b>22</b> and <b>23,</b> were synthesized by changing the structure of amino acid residues using the same strategy. The anticancer activity of analogues <b>21</b>-<b>23</b> against Huh-7 cells was evaluated in vitro; however, their IC₅₀ values were >50 μM.