Photoreactivity of LY277359 maleate, a 5-hydroxytryptamine3 (5-HT3) receptor antagonist, in solution

Compound LY277359 maleate undergoes a photoinduced solvolysis reaction in water to generate the corresponding hydroxylated product and release chloride. Attempts to stabilize a parenteral formulation of the compound led to an investigation of possible reaction mechanisms. The data are consistent with a mechanism involving homolytic cleavage of the aryl-chloride bond followed by electron transfer to give an aryl cation intermediate. The cation thus formed reacts with surrounding nucleophiles to give the substituted product. A kinetic expression for reaction rate was derived from the mechanism, and various components of the rate constant were evaluated experimentally. The reaction is slowed with the addition of chloride, presumably via a common ion effect (enhanced retroreaction). In the absence of added chloride, the reaction can be described kinetically by an initiation term. An inner filter effect is also observed, where increasing amounts of the hydroxylated product slow the reaction. Experimental data for observed rate constants as a function of starting concentration and light intensity are fit with good correlation to an equation describing the filter effect. Additional studies evaluated the effects of various structural features of the parent compound on the rate of the reaction in glass containers. It was determined that reactivity was dependent on two features: (1) the ortho positioning of the carboxyl and ether groups, which shifted an absorption band above the container cutoff; and (2) the para orientation of the chloro group to the ether, which is para activating in the photoexcited state.