Unprecedented SnCl2-Mediated Cyclization of Nitro Arenes via N−N Bond Formation†

[reaction: see text] A mild, efficient, one-pot protocol for the cyclization of nitro-aryl substrates using SnCl(2) has been described. The mechanistic course of the reaction suggests the involvement of a hydroxylamine intermediate leading to an intramolecular cyclization via N-N bond formation. The versatility of the methodology has been demonstrated by using two nitro-aryl substrates derived from dihydroisoquinolines and dihydro-beta-carbolines. The intramolecular cyclization led to the formation of indazoles in high yields and purities.

5'-(2-Nitrophenylalkanoyl)-2'-deoxy-5-fluorouridines as potential prodrugs of FUDR for reductive activation

Four 5'-(2-nitrophenylalkanoyl)-2'-deoxy-5-fluorouridines (1a-d) were designed and synthesized as potential prodrugs of FUDR for reductive activation. Two methyl groups were introduced alpha to the ester carbonyl to increase both the rate of cyclization activation and the stability of the conjugates towards serum esterases. Chemical reduction of the nitro group into an amino leads to cyclization and release of the active FUDR. Kinetic analysis of the cyclization activation process indicates that the two methyl groups alpha to the ester carbonyl restrict the rotational freedom of ground state molecule and promote the cyclization reaction. However, the two methyl groups also were found to render the conjugates as poor substrates of E. coli B nitroreductase. Conjugate 1c, without the two methyl groups, was reduced by E. coli B nitroreductase (t(1/2)=8 h) to give two products, a N-hydroxyl lactam and the drug FUDR, suggesting that the enzymatic reduction and subsequent cyclization activation proceeded through the hydroxylamine intermediate. These results indicate that cyclization activation will occur once the nitro group is reduced either to an amino or to a hydroxylamino group. The fact that the amino intermediates cyclized easily to release the incorporated drug FUDR suggests the feasibility of using peptide-linked acyl 2-aminophenylalkanoic acid esters as potential prodrugs for proteolytic activation.

5'-[2-(2-Nitrophenyl)-2-methylpropionyl]-2'-deoxy-5-fluorouridine as a potential bioreductively activated prodrug of FUDR: synthesis, stability and reductive activation

5'-[2-(2-Nitrophenyl)-2-methylpropionyl]-2'-deoxy-5-fluorouridine was synthesized as a potential bioreductively activated prodrug of 5-fluoro-2'-deoxyuridine (FUDR). The target compound was stable in both phosphate buffer and human serum and was found to release quickly the parent drug FUDR in quantitative yield upon mild chemical reduction.

A 2-nitroimidazole carbamate prodrug of 5-amimo-1-(chloromethyl)-3-[(5,6,7-trimethoxyindol-2-yl)carbony l]-1,2-dihydro-3H--benz[E]indole (amino-seco-CBI-TMI) for use with ADEPT and GDEPT

The synthesis of a 2-nitroimidazol-5-ylmethyl carbamate prodrug 10 of the potent minor groove alkylating agent amino-seco-CBI-TMI 3 is described. Chemical, radiolytic, and enzymic reductions of a model 2-nitroimidazol-5-yl carbamate 8 show release of the amine effector upon reduction. Prodrug 10 gives a ten fold increase in cytotoxicity against human ovarian carcinoma SKOV3 cells in the presence of E. coli B nitroreductase (NTR) and a 21-fold increase in cytotoxicity against a SKOV3 cell line (SC3.2) transfected with the gene for NTR. The cytotoxicity of 10 increased 15- to 40-fold under hypoxia. Prodrug 10 has significant potential as a prodrug for use in ADEPT and GDEPT applications, and as a hypoxia-selective cytotoxin.

N-Substituted 2-(2,6-dinitrophenylamino)propanamides: novel prodrugs that release a primary amine via nitroreduction and intramolecular cyclization

A series of N-dinitrophenylamino acid amides [(4-CONHZ-2, 6-diNO2Ph)N(R)C(X,Y)CONHPhOMe] were prepared as potential bioreductive prodrugs and reduced radiolytically to study their rates of subsequent intramolecular cyclization. Compounds bearing a free NH group (R = H) underwent rapid cyclization in neutral aqueous buffers (t1/2 < 1 min) following 4-electron reduction, with the generation of a N-hydroxydihydroquinoxalinone and concomitant release of 4-methoxyaniline. Amine release from analogous N-methyl analogues (R = Me) was relatively slow. These results are consistent with intramolecular cyclization of a monohydroxylamine intermediate. The high rates of cyclization/extrusion by these very electron-deficient hydroxylamines suggest that the process is greatly accelerated by the presence of an H-bonding "conformational lock" between the anilino NH group and the adjacent o-nitro group (Kirk and Cohen, 1972). Changes in the phenylcarboxamide side chain or in C-methylation in the linking chain had little effect on the rate of cyclization. The model compounds had 1-electron reduction potentials in the range appropriate for cellular reduction (-373 mV for a measured example) and appeared suitable for development as prodrugs that release amine-based effectors following enzymic or radiolytic reduction. Prodrug examples containing 4-aminoaniline mustard and 5-amino-1-(chloromethyl)benz[e]indoline alkylating units were evaluated but were not activated efficiently by cellular nitroreductases. However, cell killing by the radiation-induced reduction of the latter prodrug was demonstrated.