Cytokinin activity of some substituted ureas and thioureas

Abstract

N,N$^{\prime}$-Diphenylurea was shown to have reproducible cytokinin activity. Some 500 ureas, mainly of the N-monosubstituted and N,N$^{\prime}$-disubstituted types, were tested and about one half of these were active. Attempts were made to correlate chemical structure with biological activity. Although there are some exceptions to nearly every generalization it has been possible to formulate some principles. (1) Phenyl urea was the simplest active compound. (2) An HNCONH bridge conferred higher activity than an HNCSNH linkage and any other tested arrangement of the bridge gave inactive compounds. (3) Compounds in which both amino hydrogen atoms on one or both sides of the bridge were substituted were of low activity or were inactive. (4) Many compounds of the type RNHCONH$_{2}$ in which R = a substituted phenyl ring were tested. Ring substitution generally increased the activity and the highest activity was associated with meta substitution and the lowest with ortho. Compounds with electron-attracting substituents were generally more active than those with electron-donating substituents. Pyridyl compounds were active but compounds with non-planar rings were inactive. (5) In compounds of the type RNHCONHR$^{\prime}$ in which R and R$^{\prime}$ were phenyl or substituted phenyl groups the highest activities were usually found in compounds with one unsubstituted phenyl ring. Those with two substituted phenyl groups generally had lower activity. Some ureas showed detectable activity at 0$\cdot $1 parts/10$^{6}$. This was about four times less active than kinetin when tested in the tobacco pith assay.