Hydrolysis and photolysis of 4-Acetoxy-4-(benzothiazol-2-yl)-2,5-cyclohexadien-1-one, a model anti-tumor quinol ester

Iron(III)-Catalyzed Regioselective Synthesis of Electron-Rich Benzothiazoles from Aryl Isothiocyanates via C-H Functionalization

We report herein a direct synthetic route for the preparation of 2-arylbenzothiazoles using aryl isothiocyanates and electron-rich arenes. The synthetic route involves triflic acid promoted addition of the arenes to aryl isothiocyanates followed by FeCl₃-catalyzed C-S bond formation via C-H functionalization. The approach provides the advantage of synthesis of benzothiazoles without the conventional use of aryl aldehyde/carboxylic acid precursors employing the less expensive iron(III) catalyst.

A simple and sensitive fluorescent sensor platform for Al3+ sensing in aqueous media and monitoring through combined PET and ESIPT mechanisms: practical applications in drinking water and bio-imaging

A novel hydrazide-based probe was designed and prepared as a fluorogenic “turn-on” sensor for Al³⁺ sensing in aqueous media.

Characterization of the 4-(benzothiazol-2-yl)phenylnitrenium ion from a putative metabolite of a model antitumor drug

The 4-(benzothiazol-2-yl)phenylnitrenium ion 11 is generated from hydrolysis or photolysis of O-acetoxy-N-(4-(benzothiazol-2-yl)phenyl)hydroxylamine 8, a model metabolite of 2-(4-aminophenyl)benzothiazole 1 and its ring-substituted derivatives that are being developed for a variety of medicinal applications, including antitumor, antibacterial, antifungal, and imaging agents. Previously, we showed that 11 had an aqueous solution lifetime of 530 ns, similar to the 560 ns lifetime of the 4-biphenylylnitrenium ion 12 derived from the well-known chemical carcinogen 4-aminobiphenyl. We now show that the analogy between these two cations extends well beyond their lifetimes. The initial product of hydration of 11 is the quinolimine 16, which can be detected as a long-lived reactive intermediate that hydrolyzes in a pH-dependent manner into the final hydrolysis product, the quinol 15. This hydrolysis behavior is equivalent to that previously described for a large number of ester metabolites of carcinogenic arylamines, including 4-aminobiphenyl. The major azide trapping product (90% of azide products) of 11, 20, is generated by substitution on the carbons ortho to the nitrenium ion center of 11. This product is a direct analogue of the major azide adducts, such as 22, generated from trapping of the nitrenium ions of carcinogenic arylamines. The azide/solvent selectivity for 11, k(az)/k(s), is also nearly equivalent to that of 12. A minor product of the reaction of 11 with N(3)(-), 21, contains no azide functionality but may be generated by a process in which N(3)(-) attacks 11 at the nitrenium ion center with loss of N(2) to generate a diazene 25 that subsequently decomposes into 21 with loss of another N(2). The adduct derived from attack of 2'-deoxyguanosine (d-G) on 11, 28, is a familiar C-8 adduct of the type generated from the reaction of d-G with a wide variety of arylnitrenium ions derived from carcinogenic arylamines. The rate constant for reaction of d-G with 11, k(d-G), is very similar to that observed for the reaction of d-G with 12. The similar lifetimes and chemical reactivities of 11 and 12 can be rationalized by B3LYP/6-31G(d) calculations on the two ions that show that they are of nearly equivalent stability relative to their respective hydration products. The calculations also help to rationalize the different regiochemistry observed for the reaction of N(3)(-) with 11 and its oxenium ion analogue, 13. Since 8 is the likely active metabolite of 1 and a significant number of derivatives of 1 are being developed as pharmaceutical agents, the similarity of the chemistry of 11 to that of carcinogenic arylnitrenium ions is of considerable importance. Consideration should be given to this chemistry in continued development of pharmaceuticals containing the 2-(4-aminophenyl)benzothiazole moiety.

Indirect and direct detection of the 4-(benzothiazol-2-yl)phenylnitrenium ion from a putative metabolite of a model anti-tumor drug

2-(4-Aminophenyl)benzothiazoles related to 1 are potentially important pharmaceuticals. Metabolism apparently involves oxidation and esterification to 3. In water, hydrolysis and photolysis of 3 generates the nitrenium ion 4 that can be detected indirectly by N(3)(-) trapping and directly by UV-vis spectroscopy following laser flash photolysis. The transient, with lambda(max) 570 nm, and a lifetime of 530 ns, reacts with N(3)(-) at a diffusion-controlled rate and generates the quinol 6 by reaction with water.

Multiple decomposition pathways for the oxenium ion precursor O-(4-(4'-methylphenyl)phenyl)-N-methanesulfonylhydroxylamine

Although O-arylhydroxylamine derivatives have been claimed to be sources of oxenium ions in a large number of studies, it is not clear that the products of these reactions are due to oxenium ions. Previously, we had shown through azide trapping studies that the quinol ester 2a and the title compound 3a generate the oxenium ion 1a. The ester 2a exclusively generates 1a in water and is also a photoprecursor of 1a in water. This is not true of 3a. The oxenium ion pathway accounts for a significant fraction of the reaction of 3a under neutral and acidic pH conditions, but there are three other pathways that account for hydrolysis of 3a. Both 3a and its conjugate base 3a(-) are present in aqueous solution under mild pH conditions. In addition to the oxenium ion product 4a, two other significant products are generated: the phenol 6a and the rearrangement product 8a. Both 4a and 8a are generated exclusively from 3a, whereas 6a is generated from both 3a and 3a(-). Azide trapping studies show that 6a and 8a are not generated from the oxenium ion. The phenol 6a is generated by two paths, one involving an apparent radical intermediate 7a and the other through a stepwise alpha-elimination pathway through 3a(-). The rearrangement product 8a is generated either through a concerted rearrangement or via an ion-pair rearrangement. Photolysis of 3a does not generate 1a. The only products of photolysis of 3a in water are 6a (major) and 8a (minor). The weak O-N bond of 3a is susceptible to homolysis under photolysis conditions, and the radical 7a is observed after laser flash photolysis of 3a. The cation 1a that is observed during laser flash photolysis experiments on 2a cannot be detected during similar experiments on 3a. These results suggest that the previous attribution of oxenium ions as the source of the decomposition products of other O-arylhydroxylamine derivatives in aromatic solvents via thermolysis or acid-catalyzed decomposition may not be correct.