Synthesis and evaluation of substrate analogue inhibitors of trypanothione reductase.

Trypanothione reductase (TR) is found in the trypanosomatid parasites, where it catalyses the NADPH-dependent reduction of the glutathione analogue, trypanothione, and is a key player in the parasite's defenses against oxidative stress. TR is a promising target for the development of antitrypanosomal drugs; here, we report our synthesis and evaluation of compounds 3-5 as low micromolar Trypanosoma cruzi TR inhibitors. Although 4 and 5 were designed as potential irreversible inhibitors, these compounds, as well as 3, displayed reversible competitive inhibition. Compound 3 proved to be the most potent inhibitor, with a K(i) = 2 µM.

Transition-metal-free formal Sonogashira coupling and alpha-carbonyl arylation reactions.

Transition-metal-free formal Sonogashira coupling and alpha-carbonyl arylation reactions have been developed. These transformations are based on the nucleophilic aromatic substitution (S(N)Ar) of beta-carbonyl sulfones to electron-deficient aryl fluorides, producing a key intermediate that, depending on the reaction conditions, gives the aromatic alkynes or alpha-aryl carbonyl compounds. The development of these reactions is presented and, based on investigations under basic and acidic conditions, mechanisms have been proposed. To develop the formal Sonogashira coupling further, a milder, two-step protocol is also disclosed that expands the reaction concept. The scope of these reactions is demonstrated for the synthesis of Sonogashira and alpha-carbonyl arylated products from a range of electron-deficient aryl fluorides with a variety of functional groups and aryl-, heteroaryl-, alkyl-, and alkoxy-substituted sulfone nucleophiles. These transition-metal-free reactions complement the metal-catalyzed versions in terms of substitution patterns, simplicity, and reaction conditions.

Organocatalysis with endogenous compounds: towards novel non-enzymatic reactions.

The aldol reaction of the endogeneous compounds acetone and methylglyoxal has been studied using organocatalysis in relation to biologically relevant non-enzymatic reactions. Under preparative conditions, 3-hydroxy-2,5-hexadione, known as Henze's ketol, is formed in high yield and with enantioselectivities up to 88% ee. Furthermore, Henze's ketol is also formed under simulated physiological conditions at micromolar scale, indicating that this reaction might take place in living organisms.

Stereochemistry of 1,2-elimination reactions at the E2-E1cB interface--tert-butyl 3-tosyloxybutanoate and its thioester.

Experimental data on the stereoselectivity of base-catalyzed 1,2-elimination reactions that produce conjugated carbonyl compounds are scarce in spite of the importance of these reactions in organic and biochemistry. As part of a comprehensive study in this area, we have synthesized stereospecifically-deuterated beta-tosyloxybutanoate esters and thioesters and studied the stereoselectivity of their elimination reactions under non-ion pairing conditions. With the availability of both the (2R*,3R*) and (2R*,3S*) diastereomers the innate stereoselectivity could be determined unambiguously. (1)H and (2)H NMR data show that these substrates produce 5-6% syn elimination, the usual amount for acyclic substrates undergoing E2 reactions. Contrary to earlier suggestions, activation by a carbonyl group has virtually no influence upon the stereoselectivity. Elimination of the (2R*,3R*) diastereomer of the beta-tosyloxyester and thioester produces 21-25% of the (Z)-alkene, much more than observed with a poorer beta-nucleofuge. A relatively large amount of (Z)-alkene product seems to be a good marker for an E2 pathway, in which the transition state is E1cB-like, rather than an E1cB(irrev) mechanism. Syn KIE values were higher than those for anti elimination for the esters as well as the thioesters. Experimental challenges to the synthesis of stereospecifically-deuterated beta-tosyloxyesters are discussed.

Differential inhibition of class I and class II 5-enolpyruvylshikimate-3-phosphate synthases by tetrahedral reaction intermediate analogues.

The shikimate pathway enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase or EPSPS) is best known as the target of the herbicide glyphosate. EPSPS is also considered an attractive target for the development of novel antibiotics since the pathogenicity of many microorganisms depends on the functionality of the shikimate pathway. Here, we have investigated the inhibitory potency of stable fluorinated or phosphonate-based analogues of the tetrahedral reaction intermediate (TI) in a parallel study utilizing class I (glyphosate-sensitive) and class II (glyphosate-tolerant) EPSPS. The (R)-difluoromethyl and (R)-phosphonate analogues of the TI are the most potent inhibitors of EPSPS described to date. However, we found that class II EPSPS are up to 400 times less sensitive to inhibition by these TI analogues. X-ray crystallographic data revealed that the conformational changes of active site residues observed upon inhibitor binding to the representative class I EPSPS from Escherichia coli do not occur in the prototypical class II enzyme from Agrobacterium sp. strain CP4. It appears that because the active sites of class II EPSPS do not possess the flexibility to accommodate these TI analogues, the analogues themselves undergo conformational changes, resulting in less favorable inhibitory properties. Since pathogenic microorganisms such as Staphylococcus aureus utilize class II EPSPS, we conclude that the rational design of novel EPSPS inhibitors with potential as broad-spectrum antibiotics should be based on the active site structures of class II EPSP synthases.

The synthesis and inhibitory activity of dethiotrypanothione and analogues against trypanothione reductase.

Trypanothione reductase (TR) catalyzes the NADPH-dependent reduction of trypanothione disulfide (1). TR plays a central role in the trypanosomatid parasite's defense against oxidative stress and has emerged as a promising target for antitrypanosomal drugs. We describe the synthesis and activity of dethiotrypanothione and analogues (2-4) as inhibitors of Trypanosoma cruzi TR. The syntheses of these macrocycles feature ring-closing olefin metathesis (RCM) reactions catalyzed by ruthenium catalyst 17. Derivative 4 is our most potent inhibitor with a Ki=16 microM.

Interaction of phosphonate analogues of the tetrahedral reaction intermediate with 5-enolpyruvylshikimate-3-phosphate synthase in atomic detail.

The enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) catalyzes the penultimate step of the shikimate pathway and is the target of the broad-spectrum herbicide glyphosate. Since the functionality of the shikimate pathway is vital not only for plants but also for microorganisms, EPSPS is considered a prospective target for the development of novel antibiotics. We have kinetically analyzed and determined the crystal structures of Escherichia coli EPSPS inhibited by (R)- and (S)-configured phosphonate analogues of the tetrahedral reaction intermediate. Both diastereomers are competitive inhibitors with respect to the substrates of the EPSPS reaction, shikimate-3-phosphate (S3P) and phosphoenolpyruvate (PEP). Remarkably, the (S)-phosphonate (K(iS3P) = 750 nM), whose configuration corresponds to that of the genuine tetrahedral intermediate, is a much weaker inhibitor than the (R)-phosphonate analogue (K(iS3P) = 16 nM). The crystal structures of EPSPS liganded with the (S)- and (R)-phosphonates, at 1.5 and 1.9 A resolution, respectively, revealed that binding of the (R)-phosphonate induces conformational changes of the strictly conserved residues Arg124 and Glu341 within the active site. This appears to give rise to substantial structural alterations in the amino-terminal globular domain of the enzyme. By contrast, binding of the (S)-phosphonate renders the enzyme structure unchanged. Thus, EPSPS may facilitate the tight binding of structurally diverse ligands through conformational flexibility. Molecular docking calculations did not explain why the (R)-phosphonate is the better inhibitor. Therefore, we propose that the structural events during the open-closed transition of EPSPS are altered as a result of inhibitor action.