Hydrolysis and Photolysis of 4-Acetoxy-4-(benzothiazol-2-yl)-2,5-cyclohexadien-1-one, a Model Anti-Tumor Quinol Ester

Abstract

4-Acetoxy-4-(benzothiazol-2-yl)-2,5-cyclohexadien-1-one, 1, a quinol derivative that exhibits significant anti-tumor activity against human breast, colon, and renal cancer cell lines, undergoes hydrolysis in aqueous solution to generate an oxenium ion intermediate, 3, that is selectively trapped by N(3)(-) in an aqueous environment. The 4-(benzothiazol-2-yl) substituent slows the rate of ionization of 1 compared to analogues with 4-phenyl or 4-(p-tolyl) substituents, 4a or 4b. However, once generated, 3 is somewhat more selective than the 4-phenyl-substituted cation 5a. Calculations performed at the B3LYP/6-31G(d) level agree that the 4-(benzothiazol-2-yl) substituent does significantly stabilize 3. The structure of the major isolated azide adduct, 4-(6-azidobenzothiazol-2-yl)phenol, 9, confirms that the positive charge is highly delocalized in 3. The results of hydrolysis of 1 show that the 4-(benzothiazol-2-yl) substituent has a significant inductive electron-withdrawing effect as well as a significant resonance effect that is electron-donating. Photolysis of 1 in aqueous solution generates the quinol 2 as one of several photolysis products. The presence of the quinol suggests that photolysis also leads, in part, to generation of 3, but photoionization of 1 is significantly less efficient than is the case for the esters 4a and 4b. This study proves that 3 is generated by ionization of 1 in an aqueous environment. A significant number of other 2-benzothiazole derivatives that are not quinols, including ring-substituted derivatives of 2-(4-aminophenyl)benzothiazole 15, are under development as anti-tumor agents as well. The possible generation of the reactive intermediate 17 by hydrolysis of the putative metabolite 16 is under investigation.

Indirect and Direct Detection of the 4-(Benzothiazol-2-yl)phenylnitrenium Ion from a Putative Metabolite of a Model Anti-Tumor Drug

“…The azide adduct identified in that study is 10 , and k az / k s for 9 at 80 °C is 310 M −1 10. The structure of 10 demonstrates that the charge in 9 is highly delocalized, and k az / k s comparisons to other oxenium ions show that the azide/solvent selectivity of 9 is similar to the 4-biphenylyloxenium ion, 11 10.…”

“…The structure of 10 demonstrates that the charge in 9 is highly delocalized, and k az / k s comparisons to other oxenium ions show that the azide/solvent selectivity of 9 is similar to the 4-biphenylyloxenium ion, 11 10. Since 4 , 7 , 9 , and 11 react with N 3 − at or near the diffusion controlled limit, the aqueous solution lifetimes of 4 and 7 and also of 9 and 11 are very similar 10,12,13. These results show that the 4-(benzothiazol-2-yl) group behaves as a significantly delocalizing and stabilizing substituent for both oxenium and nitrenium ions.…”

“…Kinetics of the decomposition of 3 (2.5 × 10 −5 M) at pH 7.1 in phosphate buffer, and the formation of the major hydrolysis product 6 (Scheme 3, identified by HPLC and 1 H NMR comparison to an authentic sample10) monitored by UV spectroscopy, are described by two pseudo-first-order rate constants, k o and k 1 . HPLC studies (Figure 1A) show that the larger rate constant, k o governs the decay of 3 , while the appearance of 6 is biphasic, and is fit well by a rate equation for two consecutive first-order reactions.…”

Aryl Nitrenium Ions in Aqueous Solution

“…156a The same quinol is generated from hydration of the related oxenium ion 119 that is obtained by hydrolysis of the quinol ester 118. 157 This ion is also surprisingly stabilized by the 4-(benzothiazol-2-yl) substituent. 157 The reactions of 116 with N 3 À and dG generate products that are analogous to those obtained from 64n.…”

“…157 This ion is also surprisingly stabilized by the 4-(benzothiazol-2-yl) substituent. 157 The reactions of 116 with N 3 À and dG generate products that are analogous to those obtained from 64n. 156b SCHEME 4.32.…”