Structure-activity studies on organoselenium alkylating agents

A variety of organoselenium alkylating agents were synthesized, using 2-hydroxyethyl and 3-hydroxypropyl selenocyanate intermediates, and studied to determine their chemical reactivities with 4-(4-nitrobenzyl)pyridine (NBP) and cytotoxicities against CCRF-CEM, L1210/0, and L1210/L-PAM cells. The comparison between the 2-chloroethyl sulfides and selenides 1-4 revealed the markedly enhanced nucleophilicity of selenium (Se) over sulfur (S) by two or more orders of magnitude. This finding indicates that a major consideration in the design of antitumor alkylating organoselenides is the reactivity of selenium. A Taft plot of the experimental first-order rate constant, knbp, and sigma* in a series of 2-chloroethylseleno compounds gave a slope of -1.73 (rho*), with the exception of 2-chloroethyl 2-nitrophenyl selenide (10). The anomalous behavior of 10 is explained in terms of the ortho-nitro stabilization effect directly interacting with the selenium atom of ethyleneselenonium ion to form a 5-membered cyclic intermediate. In the same series, a 5000-fold difference in alkylating reactivity offered only a sixfold variation in cytotoxicity against CCRF-CEM cells. Increasing the alkylating chain length from ethlene to propylene units markedly reduced alkylating reactivities. In the CH3Se(CH2)n Cl series, 16 (n = 3) was 1.5 X 10(5) times slower than 2 (n = 2) in NBP alkylation, revealing that 3-chloro-n-propyl selenides are not chemically reactive enough to be biological alkylating agents despite the presence of the highly nucleophilic selenium atom. Replacement of chloride with mesylate in 3-substituted propyl selenides, such as 17 and 20, restored desirable reactivities and cytotoxicities.