Iheyamide A (<b>1</b>) is an antitrypanosomal linear peptide isolated from a <i>Dapis</i> sp. marine cyanobacterium by our group in 2020, and based on structure-activity relationships of its natural analogues, the C-terminal pyrrolinone moiety has been identified as the phamacophore for its antiparasitic activity. Further, we isolated this pyrrolinone moiety by itself as a new natural product from the marine cyanobacterium and named it iheyanone (<b>2</b>). As expected, iheyanone (<b>2</b>) showed antitrypanosomal activity, but its potency was weaker than iheyamide A (<b>1</b>). To clarify more detailed structure-activity relationships, we completed a total synthesis of iheyamide A (<b>1</b>) along with iheyanone (<b>2</b>) and evaluated the antitrypanosomal activities of several synthetic intermediates. As a result, we found that the longer the peptide chain, the stronger the antitrypanosomal activity. As iheyamide A (<b>1</b>) showed selective toxicity against <i>Trypanosoma brucei rhodesiense</i>, these findings can provide design guidelines for antitrypanosomal drugs.