Roger Hunter

Hydrocarbon formation from methanol and dimethyl ether: a review of the experimental observations concerning the mechanism of formation of the primary products

Abstract In recent years a considerable amount of research effort has been directed at gaining an understanding of the mechanism of the formation of the primary products in the methanol conversion reaction. This paper reviews the experimental data accumulated to date and discusses the mechanisms that have been proposed with respect both to the formation of the initial carboncarbon bond and the formation of methane. Four mechanisms are discussed in detail, namely: (a) formation of a carbene from methanol by α-elimination, (b) intermolecular reaction of dimethyloxonium methylide, (c) involvement of a radical pathway and (d) deprotonation of a surface bonded methyloxonium ion to give a surface-bonded oxonium methylide. These mechanisms are discussed against the background of the experimental data that has been obtained either against or in support of these proposals.

Protein kinase C-mediated phosphorylation and activation of PDE3A regulate cAMP levels in human platelets.

The elevation of [cAMP]i is an important mechanism of platelet inhibition and is regulated by the opposing activity of adenylyl cyclase and phosphodiesterase (PDE). In this study, we demonstrate that a variety of platelet agonists, including thrombin, significantly enhance the activity of PDE3A in a phosphorylation-dependent manner. Stimulation of platelets with the PAR-1 agonist SFLLRN resulted in rapid and transient phosphorylation of PDE3A on Ser312, Ser428, Ser438, Ser465, and Ser492, in parallel with the PKC (protein kinase C) substrate, pleckstrin. Furthermore, phosphorylation and activation of PDE3A required the activation of PKC, but not of PI3K/PKB, mTOR/p70S6K, or ERK/RSK. Activation of PKC by phorbol esters also resulted in phosphorylation of the same PDE3A sites in a PKC-dependent, PKB-independent manner. This was further supported by the finding that IGF-1, which strongly activates PI3K/PKB, but not PKC, did not regulate PDE3A. Platelet activation also led to a PKC-dependent association between PDE3A and 14-3-3 proteins. In contrast, cAMP-elevating agents such as PGE1 and forskolin-induced phosphorylation of Ser312 and increased PDE3A activity, but did not stimulate 14-3-3 binding. Finally, complete antagonism of PGE1-evoked cAMP accumulation by thrombin required both Gi and PKC activation. Together, these results demonstrate that platelet activation stimulates PKC-dependent phosphorylation of PDE3A on Ser312, Ser428, Ser438, Ser465, and Ser492 leading to a subsequent increase in cAMP hydrolysis and 14-3-3 binding.

Interference with Hemozoin Formation Represents an Important Mechanism of Schistosomicidal Action of Antimalarial Quinoline Methanols

Background The parasitic trematode Schistosoma mansoni is one of the major causative agents of human schistosomiasis, which afflicts 200 million people worldwide. Praziquantel remains the main drug used for schistosomiasis treatment, and reliance on the single therapy has been prompting the search for new therapeutic compounds against this disease. Our group has demonstrated that heme crystallization into hemozoin (Hz) within the S. mansoni gut is a major heme detoxification route with lipid droplets involved in this process and acting as a potential chemotherapeutical target. In the present work, we investigated the effects of three antimalarial compounds, quinine (QN), quinidine (QND) and quinacrine (QCR) in a murine schistosomiasis model by using a combination of biochemical, cell biology and molecular biology approaches. Methodology/Principal Findings Treatment of S. mansoni-infected female Swiss mice with daily intraperitoneal injections of QN, and QND (75 mg/kg/day) from the 11th to 17th day after infection caused significant decreases in worm burden (39%–61%) and egg production (42%–98%). Hz formation was significantly inhibited (40%–65%) in female worms recovered from QN- and QND-treated mice and correlated with reduction in the female worm burden. We also observed that QN treatment promoted remarkable ultrastructural changes in male and female worms, particularly in the gut epithelium and reduced the granulomatous reaction to parasite eggs trapped in the liver. Microarray gene expression analysis indicated that QN treatment increased the expression of transcripts related to musculature, protein synthesis and repair mechanisms. Conclusions The overall significant reduction in several disease burden parameters by the antimalarial quinoline methanols indicates that interference with Hz formation in S. mansoni represents an important mechanism of schistosomicidal action of these compounds and points out the heme crystallization process as a valid chemotherapeutic target to treat schistosomiasis.

Garlic-derived anticancer agents: Structure and biological activity of ajoene

Garlic has been used throughout the centuries to treat infections, heart disease, and cancer. Ajoene is one of the main compounds formed from heating crushed garlic as a mixture of E‐ and Z‐isomers (E‐ and Z‐4,5,9‐trithiadodeca‐1,6,11‐triene 9‐oxide). Ajoene possesses a broad spectrum of biological activities that include anticancer activity. Its cytotoxicity towards cancer cells is postulated to occur via an apoptotic mechanism involving activation of the mitochondrial‐dependent caspase cascade. Structure‐activity studies on ajoene and ajoene analogues have revealed that the Z‐isomer is moderately more active than the E‐isomer at inhibiting in vitro tumor cell growth, suggesting that specific protein interactions may be important. Substitution of the terminal end allyl groups in ajoene for alkyl, aromatic, or heteroaromatic groups produces some analogs with superior in vitro anticancer activity to ajoene, opening up the way to developing ajoene‐based anticancer agents.

Quinoline Antimalarials Containing a Dibemethin Group Are Active against Chloroquinone-Resistant Plasmodium falciparum and Inhibit Chloroquine Transport via the P. falciparum Chloroquine-Resistance Transporter (PfCRT)

A series of 4-amino-7-chloroquinolines with dibenzylmethylamine (dibemethin) side chains were shown to inhibit synthetic hemozoin formation. These compounds were equally active against cultures of chloroquine-sensitive (D10) and chloroquine-resistant (K1) Plasmodium falciparum. The most active compound had an IC(50) value comparable to that of chloroquine, and its potency was undiminished when tested in three additional chloroquine-resistant strains. The three most active compounds exhibited little or no cytotoxicity in a mammalian cell line. When tested in vivo against mouse malaria via oral administration, two of the dibemethin derivatives reduced parasitemia by over 99%, with mice treated at 100 mg/kg surviving the full length of the experiment. Three of the compounds were also shown to inhibit chloroquine transport via the parasites chloroquine-resistance transporter (PfCRT) in a Xenopus oocyte expression system. This constitutes the first example of a dual-function antimalarial for which the ability to inhibit both hemozoin formation and PfCRT has been demonstrated directly.

Hydrocarbon formation from methylating agents over the zeolite catalyst ZSM-5. Comments on the mechanism of carbon–carbon bond and methane formation

The mechanism of hydrocarbon formation from methanol has been studied by reacting methylating agents MeX (X = OH, I, OSO3Me) over the acidic form of the pentasil zeolite ZSM-5 and the sodium form of its conjugate base. At >0.1% conversions the product distributions obtained are similar, indicating that similar mechanisms for carbon–carbon bond formation operate with the three reagents. Hydrocarbon formation (CH4 and C2H4) from Me2SO4 over Na-ZSM-5 was observed and is considered to be strong evidence against the involvement of dimethyloxonium methylide as an intermediate since such a species cannot be formed with this reactant under basic conditions. Further evidence against the involvement of a dimethyloxonium methylide is given using reactivity comparisons of oxygen and sulphur-containing reagents. MeI and Me2SO4 were found to be less reactive than MeOH and at low conversion the primary hydrocarbon products observed, CH4and C2H4, suggest that these are the primary products formed in methanol conversion. Under conditions where conversion increased with reaction time, the methane/alkenes ratio decreased markedly and this is considered to be inconsistent with a mechanism involving a surface carbene intermediate formed in a one-step process from the reagent MeX. Instead it is proposed that the crucial first step in the mechanism is the formation of a surface methoxyl species which is primarily responsible for CH4 formation at low conversions. Deprotonation of the surface methoxyl species generates a surface-bonded oxonium methylide which subsequently reacts to form the initial carbon–carbon bond. This mechanistic proposal is both consistent with and unifies the available experimental data obtained for this reaction to date.

Thiolsulfinate allicin from garlic: inspiration for a new antimicrobial agent.

Consideration of the underlying features responsible for garlic‐allicins antimicrobial activity as well as its instability has prompted an investigation into substituted S‐aryl alkylthiolsulfinates as a class of garlic mimic with enhanced stability. Synthesis of the targets has inspired the development of new methods for synthesizing unsymmetrical aralkyl disulfides, which are then oxidized to the targets. Some simple representatives have been synthesized, setting the scene for a full SAR study of this relatively unexplored class of thiolsulfinate.

Substituted ajoenes as novel anti-cancer agents

A new synthesis of the ajoene pharmacophore core is presented involving the regioselective radical addition of a thiyl radical to a terminal alkyne as the key step. The synthesis allows structural variation of the two end groups on sulfur, and a range of novel derivatives varying the R(1) group (sulfoxide end) has been prepared and tested against CT-1 transformed fibroblast cells for anti-cancer activity. The results indicate comparable or even improved activity compared to the parent natural product ajoene isomers. This opens up the way to systematically studying the biology of the ajoene core.

Conjugates of plumbagin and phenyl-2-amino-1-thioglucoside inhibit MshB, a deacetylase involved in the biosynthesis of mycothiol

N-Acetylglucosaminylinositol (GlcNAc-Ins)-deacetylase (MshB) and mycothiol-S-conjugate amidase (Mca), structurally related amidases present in mycobacteria and other Actinomycetes, are involved in the biosynthesis of mycothiol and in the detoxification of xenobiotics as their mycothiol-S-conjugates, respectively. With substrate analogs of GlcNAc-Ins, MshB showed a marked preference for inositol as the aglycon present in GlcNAc-Ins. The inhibition of MshB and Mca by 10 thioglycosides, 7 cyclohexyl-2-deoxy-2-C-alkylglucosides, and 4 redox cyclers was evaluated. The latter contained plumbagin tethered via 2 to 5 methylene carbons and an amide linkage to phenyl-2-deoxy-2-amino-1-thio-alpha-d-glucopyranoside. These proved to be the most potent amongst the 21 compounds tested as inhibitors of MshB. Their inhibitory potency varied with the length of the spacer, with the compound with longest spacer being the most effective.

Anti-Proliferative Activity of Synthetic Ajoene Analogues on Cancer Cell-Lines

The ability of garlic preparations to inhibit cancer cell-growth has been attributed to a group of structurally-related organosulfur compounds found in the crushed clove. Historically, interest has centred on three such compounds as allicin, diallyl disulfide and diallyl trisulfide, with less interest on E- and Z-ajoene. A recently developed synthetic route from our laboratory for preparing ajoene analogues allows access to derivatives containing the sulfoxide / vinyl disulfide core whilst varying the terminal end-group functionality. A small library has been synthesized and an advanced lead with p-methoxybenzyl end groups (8) identified. Data on the in vitro anti-proliferation activity of compound (8) is presented here on six cancer cell-lines in comparison with that of Z- and E-ajoene to reveal an enhancement in activity of up to twelvefold. In addition, a modest selectivity is observed for tumour over normal cell-lines of up to threefold. Data on ajoene and its derivatives is presented in the context of chemosensitization in drug-resistance, and ideas on ajoenes mode of action at the molecular level are presented and discussed.