CRYSTALLOGRAPHIC STUDIES ON UREA CYTOKININS

Some heteroaromatic ureas, such as N-(2-chloro-4-pyridyl)-N'-phenylura (I) and N-(4-pyridyl-N'-phenylurea (II), exhibit strong cytokinin activity. However, the structure-activity relationship of urea cytokinins remains unclear. To elucidate this problem, we have performed X-ray crystal structure analyses of the above-mentioned compounds. It was found that all the free urea molecules take an extended trans conformation, whereas the N-methylurea molecule which is completely inactive takes a folded cis conformation in the crystalline state. We concluded that the trans extended conformation is required for cytokinin activity in 4-pyridylurea derivatives. However, the difference between the potent cytokinin activities of I and II as compared with the much weaker activities of IV and III, respectively, could not be explained in terms of the steric structures around the urea moiety.