We investigated the antiviral efficacy of synthetic cleistocalto n e A (1) against a contemporary recombinant RSV-A strain (0594 , genotype ON1) using an in vitro model system (Scheme 4). Presatovir, a known selective and potent inhibitor of RSVinduced cell fusion, served as a positive control. We confirm e d Presatovir's high antiviral efficacy in infected Vero cells, with an IC50 of 0.0056 µM. Cleistocaltone A (1) also inhibited RSV infection, with a modestly higher IC50 of 54.55 µM. Although the IC50 of synthetic cleistocaltone A (1) is rough l y tenfold higher than that of the isolated compound (IC50 = 6.75 ± 0.75 µM), it remains within a relevant range.