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.     

N-(2,2-Dimethyl-2-(2-nitrophenyl)acetyl)-4-aminocyclophosphamide as a potential bioreductively activated prodrug of phosphoramide mustard

N-(2,2-Dimethyl-2-(2-nitrophenyl)acetyl)-4-aminocyclophosphamide isomers (DMNA-NH-CPA, 4) were synthesized stereospecifically from Boc-l-Hse(OBn)-OH and the degradation of the corresponding reduced amine 5a was investigated by UV/vis spectroscopy and LC/MS. The rate of cyclization of 5a was found to increase with decreasing pH, with half-lives ranging from 3.2 to 54 min at pH 4–7.4, suggesting that the cyclization is catalyzed by the hydronium ions. LC/MS analysis of the degradation products of 5a indicates that 4-aminocyclophosphamide is rapidly released from 4 upon reductive activation under acidic conditions and further decomposes into the cytotoxic phosphoramide mustard. These results validated 4-aminocyclophosphamide as a prodrug form of phosphoramide mustard and suggest that compound 4 can potentially be used as a prodrug of phosphoramide mustard for bioreductive activation.     

Synthesis of a Fluorobenzoxazine Derivative and Its Analogues

Syntheses of a fluorobenzoxazine derivative, fluorobenzothiazine derivative and fluoroquinoxaline derivative are described. These compounds were synthesized by reductive cyclization of the corresponding fluorodinitrobenzene derivatives. The fluorobenzoxazine derivative and its analogues are useful intermediates for agro-chemicals.     

Synthesis of pyrrolo[2,1-c][1,4]benzodiazepines and their conjugates by azido reductive cyclization strategy as potential DNA-binding agents

Synthesis of pyrrolo[2,1-c][1,4]benzodiazepines via azido reductive cyclization process employing FeCl3-NaI reagent system. This methodology has been extended for the preparation of new nicotinamido-pyrrolobenzodiazepine hybrids linked through piperazino-alkane-oxy spacers that exhibit good DNA binding affinity.     

Solid-phase synthesis of pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones

The solid-phase synthesis of biologically important pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones using Wang resin through amide formation and reductive cyclization procedures is described. Further, N10-substituents have been introduced in the final products and these have been cleaved from the solid support in good yields.     

Electrochemical studies of the redox behavior of ubiquinone-1

The electrochemical reduction of ubiquinone has been studied in acetonitrile in the presence of acids of widely varying proton donor strength. The final product of two-electron reduction is the hydroquinone. No evidence could be found to suggest that the corresponding chromanol, the product expected from reductive cyclization, is formed in the electroreduction of ubiquinone-1.     

Oxidative folding and N-terminal cyclization of onconase

Cyclization of the N-terminal glutamine residue to pyroglutamic acid in onconase, an anti-cancer chemotherapeutic agent, increases the activity and stability of the protein. Here, we examine the correlated effects of the folding/unfolding process and the formation of this N-terminal pyroglutamic acid. The results in this study indicate that cyclization of the N-terminal glutamine has no significant effect on the rate of either reductive unfolding or oxidative folding of the protein. Both the cyclized and uncyclized proteins seem to follow the same oxidative folding pathways; however, cyclization altered the relative flux of the protein in these two pathways by increasing the rate of formation of a kinetically trapped intermediate. Glutaminyl cyclase (QC) catalyzed the cyclization of the unfolded, reduced protein but had no effect on the disulfide-intact, uncyclized, folded protein. The structured intermediates of uncyclized onconase were also resistant to QC catalysis, consistent with their having a native-like fold. These observations suggest that, in vivo, cyclization takes place during the initial stages of oxidative folding, specifically, before the formation of structured intermediates. The competition between oxidative folding and QC-mediated cyclization suggests that QC-catalyzed cyclization of the N-terminal glutamine in onconase occurs in the endoplasmic reticulum, probably co-translationally.     

An efficient, asymmetric solid-phase synthesis of benzothiadiazine-substituted tetramic acids: potent inhibitors of the hepatitis C virus RNA-dependent RNA polymerase

An efficient, asymmetric solid-phase synthesis of benzothiadiazine-substituted tetramic acids is reported. Starting from commercially available chiral Fmoc-protected alpha-amino acids loaded onto Wang resin, Fmoc removal, reductive amination followed by amide bond formation, and base-catalyzed cyclization with simultaneous cleavage from the resin provided the desired products. Compounds described are potent inhibitors of the hepatitis C virus RNA-dependent RNA polymerase.     

Solid-phase synthesis of new pyrrolobenzodiazepine-chalcone conjugates: DNA-binding affinity and anticancer activity

A new class of C8-linked pyrrolo[2,1-c][1,4]benzodiazepine-chalcone conjugates have been prepared by employing a solid-phase synthetic protocol. In this strategy an intramolecular aza-Wittig reductive cyclization approach has been utilized. Interestingly, some of these molecules have shown enhanced DNA-binding affinity and promising anticancer activity on a large number of human cancer cell lines.     

Reaction of reductively activated mitomycin C with aqueous bicarbonate: Isolation and characterization of an oxazolidinone derivative of cis-1-hydroxy-2,7-diaminomitosene

The reductive activation of mitomycin C in aqueous bicarbonate buffer resulted in the formation of a previously unknown compound, characterized as an oxazolidinone derivative of cis-1-hydroxy-2,7-diaminomitosene. This compound is the result of a cyclization reaction of bicarbonate with the aziridine ring of aziridinomitosene, and was observed at bicarbonate concentrations close to those present in physiological plasma.     

N-(sulfonamido)alkyl[tetrahydro-1H-benzo[e]indol-2-yl]amines: potent antagonists of human neuropeptide Y Y5 receptor

[3a,4,5,9b-Tetrahydro-1H-benzo[e]indol-2-yl]amines were prepared via reductive amination and concomitant cyclization of alpha-cyanomethyl-beta-aminotetralins. N-acylation with omega-sulfonamido-carboxylic acids and subsequent reduction afforded a series of N-(sulfonamido)alkyl[tetrahydro-1H-benzo[e]indol-2-yl]amines, which bound to the human neuropeptide Y Y5 receptor with nanomolar affinity.     

Synthesis of 2-aminomethyl-4-phenyl-1-azabicyclo[2.2.1]heptanes via LiAlH4-induced reductive cyclization of 2-(4-chloro-2-cyano-2-phenylbutyl)aziridines and evaluation of their antimalarial activity

2-(4-Chloro-2-cyano-2-phenylbutyl)aziridines were employed for the one-step stereoselective construction of both endo- and exo-2-aminomethyl-4-phenyl-1-azabicyclo[2.2.1]heptanes as new azaheterobicyclic scaffolds via a double LiAlH₄-induced reductive cyclization protocol. Antiplasmodial assessment of these 1-azabicyclo[2.2.1]heptanes revealed moderate to good activities in the micromolar range, with the exo-isomers being the most promising structures. Furthermore, the proposed mode of action was supported by ligand docking studies, pointing to a strong binding interaction with the enzyme plasmepsin II.     

Novel Gly building units for backbone cyclization: synthesis and incorporation into model peptides

We report the preparation of novel building units for backbone cyclization that have the general formula Fmoc-N[CH(R)CO₂Al]Gly-OH. These building units were prepared by the reductive alkylation method using allyl esters of several amino acids as starting material and hence, respectively, contain the side chain of these amino acids. These N-alkylated Gly building units were incorporated in model backbone cyclic peptides. The resulting crude backbone cyclic peptides were obtained in high degree of purity according to HPLC and mass spectrometric analyses.     

A heptaketide naphthaldehyde produced by a polyketide synthase from Nectria haematococca

Bostrycoidin and fusarubin are biologically active fungal polyketides produced by Nectria haematococca. This azaanthraquinone and naphthoquinone are thought to be biosynthesized via formation of a C(14) heptaketide aldehyde as a common key intermediate. A BLAST search against the genome of N. haematococca revealed one candidate gene (NECHADRAFT_101778, NhPKS1), which encodes a multi-domain polyketide synthase (PKS) with a thiol reductase (TR) domain that would facilitate the reductive release of the intermediate to produce a free aldehyde. To investigate the possible involvement of NhPKS1 in the biosynthesis of bostrycoidin and fusarubin, NhPKS1 was heterologously expressed in Aspergillus oryzae, and shown to produce a heptaketide 3-acetonyl-1,6,8-trihydroxy-2-naphthaldehyde as a single product. Thus, NhPKS1 catalyzes a C-2/C-11 and C-4/C-9 aldol-type cyclization of a linear intermediate followed by a subsequent reductive product release to yield the naphthaldehyde. The results indicate NhPKS1 is the enzyme involved in the biosynthesis of bostrycoidin and fusarubin.     

The synthesis and evaluation of 10- and 12-membered ring benzofused enediyne amino acids

The enediyne moiety is a versatile functional group found in natural anticancer and anti-infective agents, undergoing the Bergman cyclization reaction to afford a diradical which cleaves double-stranded DNA. We have incorporated the enediyne group into 10- (4-10) and 12-membered ring (11) cyclic amino acids and dipeptides, respectively, and explored their relative reactivity toward cyclization, varying N-substitution in the case of the 10-membered ring substrate, which gave the expected cyclization products in good yields when using either thermal conditions in the presence or absence of microwave irradiation. The N-tosyl substituted derivative (4) was shown to nick double-stranded supercoiled DNA. N-Arylsulfonyl substitution on the ring promoted the cyclization, when compared to N-mesyl or acyl substitution, possibly because of a pi-pi stacking effect as an endo-relationship of the aryl group with the enediyne was demonstrated in both the solid state and in solution. The 12-membered ring enediyne dipeptide (11) was inert to the Bergman cyclization under a variety of conditions. When this substrate was irradiated with ultraviolet light, regio- and stereospecific reduction was observed in which one of the alkynes was reduced to a Z-olefin (47).     

Synthesis of cyclic peptides from unprotected precursors using removable N alpha-(1-(4-methoxyphenyl)-2-mercaptoethyl) auxiliary.

A new method to cyclize unprotected peptides is presented. The method involves the use of a 1-phenyl-2-mercaptoethyl derivative on the N-terminal glycine. This template acts as an auxiliary thiol-containing group in order to drive cyclization with a counterpart thioester moiety on the same molecule. Subsequent facile removal of the derivative generates products with only native peptide structure. The successful, high-yield cyclization of the peptide GSPYSSDTTPA is described.     

Syntheses of tetrahydroxyazepanes from chiro-inositols and their evaluation as glycosidase inhibitors

Two pairs of C(2)-symmetric tetrahydroxyazepanes [(-), (+)-1 and (-), (+)-2] have been synthesized from the enantiomeric chiro-inositols and evaluated as glycosidase inhibitors. Alternative syntheses of ido-tetrahydroxyazepanes (-)- and (+)-2 from myo-inositol were also developed. The key synthetic transformations were glycol fission and cyclization of the derived dialdehydes by double reductive amination. The D-manno-tetrahydroxyazepane [(-)-1] showed selective inhibition of alpha-L-fucosidase and beta-D-galactosidase, while the enantiomer [(+)-1] was a selective inhibitor of an alpha-D-galactosidase. In contrast, the L-ido-tetrahydroxyazepane (+)-2 was a broad spectrum hexosidase inhibitor, but showed none of the reported hexosaminidase inhibition. Its enantiomer (-)-2 is a poor hexosidase inhibitor.     

Biomimetic polyene cyclizations. The synthesis and cyclization of 2-(4-pentenyl)-2-cyclohexenol

This aim of this study was to ascertain whether the dienol 3 would undergo cyclization to yield products derived from the cation 4 in a manner similar to that of the lower homolog, 1 → 2. The dienol 3 was synthesized by the sequence outlined in Scheme 1. Thus alkylation of the potassium salt of 5 with 1-bromo-4-pentene (6) gave the bisenol ether 7. Hydrolysis of 7 followed by reduction of the resulting β-diketone afforded the desired dienol 3. On treatment with anhydrous formic acid at 23°C, dienol 3 underwent cyclization to give as the major product the tricyclic alcohol 8, evidently formed via further cyclization of the cation 4 (see Scheme 2). The structure 8 was confirmed by single-crystal X-ray diffraction analysis. The corresponding tricyclic chloride 9 could be obtained either directly from 8 upon treatment with thionyl chloride or by cyclization of 3 with stannic chloride in benzene. Pyrolysis of 9 with triphenyltin hydride gave the parent hydrocarbon 10.     

Synthesis and structure/NMDA receptor affinity relationships of 1-substituted tetrahydro-3-benzazepines

A novel synthesis of 1-substituted tetrahydro-1H-3-benzazepines 4 is described. Starting with (2-bromophenyl)acetaldehyde acetal 5, the nitrostyrene 9 was prepared in three steps allowing the addition of various nucleophiles to yield the nitroacetals 10. The one-pot Zn/HCl reductive cyclization of the nitroacetals 10 provided the 3-benzazepines 4, which were investigated for their affinity to the phencyclidine binding site of the NMDA receptor. A one-atomic spacer between the 3-benzazepine system and the phenyl residue in position 1 seems to be favorable for high NMDA receptor binding. In this series the benzazepine 4l substituted with the conformationally restricted and H-bond accepting acetanilide substituent in position 1 displays the highest NMDA receptor affinity (K(i)=89 nM).     

A C-ring regioisomer of the marine alkaloid meridine exhibits selective in vitro cytotoxicity for solid tumours.

9-Hydroxybenzo[b]pyrido[4,3,2-de](1,10)-phenantrolin-8-one (1), a regioisomer of the marine alkaloid meridine, was synthesized from 5,8-dimethoxy-6-nitro-4(1H)-quinolinone in eight steps and 23% overall yield. A shorter route was also investigated, based on the hetero Diels-Alder reaction between o-nitrocinnamaldehyde dimethylhydrazone and 4-halogen-6-bromo-5,8-quinolinequinones followed by reductive cyclization onto the C-5 carbonyl of the quinone. Compound 1 showed a remarkable in vitro cytotoxicity, with a pattern of selectivity towards solid tumours that is not found in the reference alkaloid, the activity against the human lung carcinoma (A-549) being particularly noteworthy. The activities of meridine and compound 1 as inhibitors of topoisomerase II were also significantly different.