Cytoplasmic dynein plays important roles in membrane transport, mitosis, and other cellular processes. A few small-molecule inhibitors of cytoplasmic dynein have been identified. We report here the first synthesis of purealin, a natural product isolated from the sea sponge Psammaplysilla purea, which is known to inhibit axonemal dynein. Also described are the first syntheses, by modular amide coupling reactions, of the natural product purealidin A (a component of purealin) and a small library of analogues. The library was examined for inhibition of cytoplasmic dynein heavy chain and cell growth. The compounds showed effective antiproliferative activity against a mouse leukemia cell line but selective activities against human carcinoma cell lines. Purealin and some of the analogues inhibited the microtubule-stimulated ATPase activity of recombinant cytoplasmic dynein heavy chain motor domain. The inhibitory effect of purealin was concentration dependent and uncompetitive, supporting the hypothesis that it does not compete with the binding of ATP.