Christopher J. Moody

Indole synthesis – something old, something new

Indoles, both naturally occurring and synthetic, exhibit wide-ranging biological activity. Unusual and complex molecular architectures occur among their natural derivatives. As a result, this important ring system continues to attract attention from the international chemical community, and new methodologies for the construction of this ever relevant heteroaromatic ring continue to be developed. Unfortunately, many methods frequently start from ortho-substituted anilines, thereby greatly restricting the availability of starting materials. A more general approach would start from a mono-functionalized arene such as an aniline or halobenzene, followed by cyclization with C–C or C–N bond formation to an unactivated C–H bond. Such methods are the subject of this perspective.

Rhodium carbene routes to oxazoles and thiazoles. Catalyst effects in the synthesis of oxazole and thiazole carboxylates, phosphonates, and sulfones.

Dirhodium tetraacetate catalyzed reaction of alpha-diazo-beta-keto-carboxylates and -phosphonates with arenecarboxamides gives 2-aryloxazole-4-carboxylates and 4-phosphonates by carbene N-H insertion and cyclodehydration. In stark contrast, dirhodium tetrakis(heptafluorobutyramide) catalysis results in a dramatic change of regioselectivity to give oxazole-5-carboxylates and 5-phosphonates. Alpha-diazo-beta-ketosulfones behave similarly and give 5-sulfonyloxazoles upon dirhodium tetrakis(heptafluorobutyramide) catalyzed reaction with carboxamides. The analogous reactions of thiocarboxamides give the corresponding thiazole-5-carboxylates, -phosphonates, and -sulfones.

Relationship between NAD(P)H:quinone oxidoreductase 1 (NQO1) levels in a series of stably transfected cell lines and susceptibility to antitumor quinones

To investigate the importance of NAD(P)H:quinone oxidoreductase 1 (or DT-diaphorase; NQO1) in the bioactivation of antitumor quinones, we established a series of stably transfected cell lines derived from BE human colon adenocarcinoma cells. BE cells have no NQO1 activity due to a genetic polymorphism. The new cell lines, BE-NQ, stably express wild-type NQO1. BE-NQ7 cells expressed the highest level of NQO1 and were more susceptible [determined by the thiazolyl blue (MTT) assay] to known antitumor quinones and newer clinical candidates. Inhibition of NQO1 by pretreatment with an irreversible inhibitor, ES936 [5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl]indole-4,7-dione], protected BE-NQ7 cells from toxicity induced by streptonigrin, ES921 [5-(aziridin-1-yl)-3-(hydroxymethyl)-1,2-dimethylindole-4,7-dione], and RH1 [2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone]. RH1 was evaluated further by clonogenic assay for cytotoxic response and was more cytotoxic to BE-NQ7 cells than to BE cells. Cytotoxicity was abrogated by inhibition of NQO1 with ES936 pretreatment. Using a comet assay to evaluate DNA cross-linking, BE-NQ7 cells demonstrated significantly higher DNA cross-links than did BE cells in response to RH1 treatment. DNA cross-linking in BE-NQ7 cells was observed at very low concentrations of RH1 (5 nM), confirming that NQO1 activates RH1 to a potent cross-linking species. Further studies using streptonigrin, ES921, and RH1 were undertaken to analyze the relationship between NQO1 activity and quinone toxicity. Toxicity of these compounds was measured in a panel of BE-NQ cells expressing a range of NQO1 activity (23-433 nmol/min/mg). Data obtained suggest a threshold for NQO1-induced toxicity above 23 nmol/min/mg and a sharp dose-response curve between the no effect level of NQO1 (23 nmol/min/mg) and the maximal effect level (>77 nmol/min/mg). These data provide evidence that NQO1 can bioactivate antitumor quinones in this system and suggest that a threshold level of NQO1 activity is required to initiate toxic events.

Keap Calm, and Carry on Covalently

The Nrf2-Keap1 system plays a major role in cellular defense against oxidative stress. Upon exposure to electrophiles, the cysteine-rich protein Keap1 is covalently modified, and it is this modification of Keap1 that allows the accumulation and subsequent nuclear translocation of Nrf2 where it induces the transcription of over 100 protective genes. This mechanism can be exploited in drug discovery approaches to diseases such as chronic kidney disease (CKD), chronic obstructive pulmonary disease (COPD), asthma, and neurodegenerative diseases like multiple sclerosis (MS) and Parkinsons, utilizing the modification of Keap1 by electrophiles, compounds that would not normally be considered useful in drug discovery programs. This Perspective discusses the development of potential therapies based on potent electrophiles, such as isothiocyanates and Michael acceptors, that, far from being associated with toxic events, can actually initiate a range of beneficial protective pathways.

Gliotoxin is a dual inhibitor of farnesyltransferase and geranylgeranyltransferase I with antitumor activity against breast cancer in vivo

Gliotoxin is a natural mycotoxin with immunosuppressive and antimicrobial activity. Inhibition of farnesyltransferase (IC50 80 µM) and geranylgeranyltransferase I (IC50 17 µM) stimulated interest in the potential antitumor activity of this epidithiodioxopiperazine. Gliotoxin inhibited proliferation of six breast cancer cell lines in culture with mean±SD IC50 289±328 µM (range 38–985 µM); intracellular farnesylation of Lamin B and geranylgeranylation of Rap1A were inhibited in a dose-dependent manner. In randomized controlled studies using the N-methyl-N-nitrosourea rat mammary carcinoma model, gliotoxin had pronounced antitumor activity in vitro and little systemic toxicity when administered to 10 animals at 10 mg/kg by subcutaneous injection weekly for 4 wk compared with 10 controls. Single doses up to 25 mg/kg were well tolerated. The present studies confirm that gliotoxin is a dual inhibitor of farnesyltransferase and geranylgeranyltransferase I with pronounced antitumor activity and favorable toxicity profile against breast cancer in vitro and in vivo.

Telomestatin: formal total synthesis and cation-mediated interaction of its seco-derivatives with G-quadruplexes.

The structurally unique natural product telomestatin incorporates seven oxazole rings and one sulfur-containing thiazoline in a macrocyclic arrangement. The compound is a potent inhibitor of the enzyme telomerase and therefore provides a structural framework for developing new potential therapeutic agents for cancer. An efficient formal total synthesis of telomestatin is reported in which the key steps are the use of dirhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate six oxazole rings, demonstrating the power of rhodium carbene methodology in organic chemical synthesis. CD spectroscopy establishes that seco-derivatives of telomestatin are potent stabilizers of G-quadruplex structures derived from the human telomeric repeat sequence. Mass spectrometry studies, confirmed by molecular dynamics simulations, provide the first evidence that high affinity binding to terminal G-tetrads in both 1:1 and 2:1 ligand complexes is mediated through the macrocycle coordinating a monovalent cation, with selectivity for the antiparallel structure.

Structure-based development of anticancer drugs: complexes of NAD(P)H:quinone oxidoreductase 1 with chemotherapeutic quinones.

BACKGROUND NAD(P)H:quinone acceptor oxidoreductase (QR1) protects animal cells from the deleterious and carcinogenic effects of quinones and other electrophiles. Remarkably, the same enzyme activates cancer prodrugs that become cytotoxic only after two-electron reduction. QR1s ability to bioactivate quinones and its elevated expression in many human solid tumors makes this protein an excellent target for enzyme-directed drug development. Until now, structural analysis of the mode of binding of chemotherapeutic compounds to QR1 was based on model building using the structures of complexes with simple substrates; no structure of complexes of QR1 with chemotherapeutic prodrugs had been reported. RESULTS Here we report the high-resolution crystal structures of complexes of QR1 with three chemotherapeutic prodrugs: RH1, a water-soluble homolog of dimethylaziridinylbenzoquinone; EO9, an aziridinylindolequinone; and ARH019, another aziridinylindolequinone. The structures, determined to resolutions of 2.0 A, 2.5 A, and 1.86 A, respectively, were refined to R values below 21% with excellent geometry. CONCLUSIONS The structures show that compounds can bind to QR1 in more than one orientation. Surprisingly, the two aziridinylindolequinones bind to the enzyme in different orientations. The results presented here reveal two new factors that must be taken into account in the design of prodrugs targeted for activation by QR1: the enzyme binding site is highly plastic and changes to accommodate binding of different substrates, and homologous drugs with different substituents may bind to QR1 in different orientations. These structural insights provide important clues for the optimization of chemotherapeutic compounds that utilize this reductive bioactivation pathway.

SYNTHESIS OF FUNCTIONALISED FLUORESCENT DYES AND THEIR COUPLING TO AMINES AND AMINO ACIDS

A series of novel functionalised benzophenoxazinones, analogues of Nile Red, is prepared and their fluorescence properties evaluated. The ring system is prepared by reaction of 5-diethylamino-2-nitrosophenol with 1,6-dihydroxynaphthalene followed by alkylation of the 2-hydroxy group with 6-bromohexanoic acid derivatives. Subsequent ester cleavage under a variety of conditions gives the acid 6. Replacement of the 9-diethylamino group with the N-ethyl-(3-sulfonylpropyl)amino group is carried out to increase water solubility and the resulting dye 14 has similar fluorescence properties. The acid 6 is coupled to a range of amino compounds.

Quinones as dienophiles in the Diels-Alder reaction: history and applications in total synthesis.

In the canon of reactions available to the organic chemist engaged in total synthesis, the Diels-Alder reaction is among the most powerful and well understood. Its ability to rapidly generate molecular complexity through the simultaneous formation of two carbon-carbon bonds is almost unrivalled, and this is reflected in the great number of reported applications of this reaction. Historically, the use of quinones as dienophiles is highly significant, being the very first example investigated by Diels and Alder. Herein, we review the application of the Diels-Alder reaction of quinones in the total synthesis of natural products. The highlighted examples span some 60 years from the landmark syntheses of morphine (1952) and reserpine (1956) by Gates and Woodward, respectively, through to the present day examples, such as the tetracyclines.

An Exploration of Ferrocenyl Ureas as Enantioselective Electrochemical Sensors for Chiral Carboxylate Anions

The syntheses of a series of chiral ureas containing the redox-active ferrocene group are described. Each of these bind chiral carboxylates in organic solvents through hydrogen-bonding interactions, as evidenced by spectroscopic and cyclic voltammetry measurements, the latter allowing these guests to be electrochemically sensed in solution. The enantioselectivity in the complexation of the protected amino acid N-benzenesulfonylproline by a ferrocenylbenzyl host is high enough to allow opposite enantiomers to be distinguished by electrochemical means.