Zbigniew Ochal

Oxygenation of a Me2Zn/α‐Diimine System: A Unique Zinc Methylperoxide Cluster and Evidence for Its Sequential Decomposition Pathways

Interest in the reaction of alkylzinc complexes with O2 has persisted for over 150 years since the pioneering studies by Frankland. The nature of the products, however, has been the subject of intense controversy. The widely accepted freeradical chain-reaction mechanism for these reactions, as found in the vast majority of text books, assumes the initiation by adventitious alkyl radicals (RC) followed by a cascade of fast reactions with little opportunity for the detection of intermediates. Recently, however, we provided unambiguous proof that R2Zn compounds, or their adducts with Lewis bases, have a marked tendency to undergo oxidation of only one alkyl group under controlled conditions with subsequent formation of RZnOOR or RZnOR species, and simultaneously structurally characterized the first examples of zinc alkylperoxides. We also proposed a plausible hypothesis for the mechanism of the reaction of alkylzinc complexes with O2. [5,6] In recent years there has been an increased interest in various radical additions initiated by the R2Zn/O2 system, especially regarding organic substrates which contain donor sites capable of forming the Lewis acid/base adducts with R2Zn that are actually involved in the reaction with O2. [7] This latter fact is usually ignored, and another assumption made in this field, which seems irrefutable, involves an alkyl radical RC (generated through the oxygenation reaction) acting as the chain carrier. Moreover, the most effective initiation systems involve Me2Zn, [7f,g] which, according to recent findings, can be selectively transformed into MeZnOMe without the generation of free MeC radicals. In light of this fact, as well as the lack of structurally characterized ZnOOMe species, it seemed reasonable to wonder how the oxygenated products participate in radical reactions. To gain a more in-depth view of both the role of the supporting ligands and the character of the radical species formed in the reactions of Me2Zn with O2, we have turned our attention to a-diimines, which have been widely used in fundamental coordination chemistry as noninnocent ligands. In the field of zinc chemistry, van Koten et al. have extensively studied the reactions of R2Zn compounds with 1,4-diazabutadiene (R-DAB) ligands and have demonstrated convincingly that this reaction system smoothly forms both paramagnetic and diamagnetic species. Herein we report the synthesis and structural characterization of a novel zinc oxo(methylperoxide) cubane along with the MeOC radical entrapped product, the formation of which involves ZnO OMe bond homolysis. Previous studies have demonstrated that the treatment of Me2Zn with tBu-DAB in diethyl ether at ambient temperature results in the formation of [Me2Zn(tBu-DAB)] (1). [11] According to the authors, the four-coordinate adduct 1 is rather stable under these reaction conditions and only upon heating above 35 8C does it undergo an inner-sphere single electron transfer to give the radical pair [MeZn(tBu-DABC)(MeC)], which subsequently dimerizes to the C C coupled dinuclear compound [{MeZn(tBu-DABC)}2] (2 ; Scheme 1). In light of these findings, we decided to modify the reaction system slightly and, in the first instance, stirred equimolar amounts of tBu-DAB andMe2Zn in toluene at 22 8C for 7 h. [12]

Synthesis and structural studies of four- and five-coordinated monomeric alkylaluminum O,N-chelate compounds derived from 2-aminoacetophenone. Crystal and molecular structure of the five-coordinated MeAl[OC(Me)C6H4-2-NH]2 complex

Abstract The reaction of Me3Al with 1 and 2 equiv. of 2-aminoacetophenone yielded Me2Al[OC(Me)C6H4-2-NH] (1) and MeAl[OC(Me)C6H4-2-NH]2 (2), respectively. Compound 2 was also obtained in the reaction of 1 with 1 equiv. of 2-aminoacetophenone. The addition of 4-methylpyridine (py-Me) to 1 produced the Lewis acid-base adduct Me2Al[OC(Me)C6H4-2-NH]·py-Me (3). The resulting compounds have been characterized by elemental analysis, cryometric molecular weight determination and 1H and 27Al NMR spectroscopy. These studies revealed that compound 1 has a four-coordinated chelate structure in solution, while 2 and 3 are five-coordinated monomeric complexes. The molecular structure of 2 has been confirmed by single-crystal X-ray crystallography.

Derivatives of phenyl tribromomethyl sulfone as novel compounds with potential pesticidal activity.

Summary A halogenmethylsulfonyl moiety is incorporated in numerous active herbicides and fungicides. The synthesis of tribromomethyl phenyl sulfone derivatives as novel potential pesticides is reported. The title sulfone was obtained by following three different synthetic routes, starting from 4-chlorothiophenol or 4-halogenphenyl methyl sulfone. Products of its subsequent nitration were subjected to the SNAr reactions with ammonia, amines, hydrazines and phenolates to give 2-nitroaniline, 2-nitrophenylhydrazine and diphenyl ether derivatives. Reduction of the nitro group of 4-tribromomethylsulfonyl-2-nitroaniline yielded the corresponding o-phenylenediamine substrate for preparation of structurally varied benzimidazoles.

Antifungal Effect of Novel 2-Bromo-2-Chloro-2-(4-Chlorophenylsulfonyl)-1-Phenylethanone against Candida Strains

We investigated the antifungal activity of novel a 2-bromo-2-chloro-2-(4-chlorophenylsulfonyl)-1-phenylethanone (compound 4). The synthesis of compound 4 was commenced from sodium 4-chlorobenzene sulfinate and the final product was obtained by treatment of α-chloro-β-keto-sulfone with sodium hypobromite. The sensitivity of 63 clinical isolates belonging to the most relevant Candida species toward compound 4 using the method M27-A3 was evaluated. We observed among most of the clinical strains of C. albicans MIC ranging from 0.00195 to 0.0078 μg/mL. Compound 4 at 32 μg/mL exhibited fungicidal activity against nine Candida strains tested using the MFC assay. Compound 4 displayed anti-Candida activity (with clear endpoint) against 22% of clinical strains of Candida. Under compound 4, Candida susceptibility and tolerance, namely paradoxical effect (PG), was found for only two clinical isolates (C. glabrata and C. parapsilosis) and reference strain 14053 using both M27-A3 and MFC method. We found that compound 4 does not induce toxicity in vivo against larvae of Galleria mellonella (≥97% survival) and it displays reduced toxicity on mammalian cells in vitro (< CC20 at 64 μg/mL). Furthermore, XTT assay denoted clear metabolic activity of sessile cells in the presence of compound 4. Thus, the effect of compound 4 on formed C. albicans biofilms was minimal. Moreover, strain 90028 exhibited no defects in hyphal growth on Caco-2 monolayer under compound 4 influence at MIC = 16 μg/mL. The MIC values of compound 4 against C. albicans 90028, in medium with sorbitol did not suggest that compound 4 acts by inhibiting fungal cell wall synthesis. Our findings with compound 4 suggest a general strategy for antifungal agent development that might be useful in limiting the emergence of resistance in Candida strains.

Unprecedented Variety of Outcomes in the Oxygenation of Dinuclear Alkylzinc Derivatives of an N,N‐Coupled Bis(β‐diketimine)

Reactions between O2 and organometallics with non-redox-active metal centers have received continuous interest for over 150 years, although significant uncertainties concerning the character and details of the actual mechanism of these reactions persist. Harnessing dinuclear three-coordinate alkylzinc derivatives of an N,N-coupled bis(β-diketimine) proligand (LH2 ) as a model system, we demonstrate for the first time that a slight modification of the reaction conditions might have a dramatic influence on the oxygenation reaction outcomes, leading to an unprecedented variety of products originating from a single reaction system, that is, partially and fully oxygenated zinc alkoxides, zinc alkylperoxides, and zinc hydroxide compounds. Our studies indicate that accessibility of the three-coordinate zinc center by the O2 molecule, coupled with the lower reactivity of Zn-Me vs. Zn-Et units towards dioxygen, are key factors in the oxygenation process, providing a novel tetranuclear methyl(methoxy)zinc {[L][ZnMe][Zn(μ-OMe]}2 and zinc ethoxide {[L][Zn(μ-OEt)]2 }2 . Remarkably, oxygenation of three-coordinate alkylzinc [L][ZnR]2 complexes at ambient temperature afforded a unique hydroxide {[L][Zn(μ-OH)]2 }2 . Oxygenation of the [L][ZnEt]2 complex in the presence of 4-methylpyridine (py-Me) at low temperature led to the isolation of a dinuclear zinc ethylperoxide [L][Zn(OOEt)(py-Me)]2 , which nicely substantiates the intermediacy of an unstable zinc alkylperoxide in the formation of the subsequent zinc alkoxide and hydroxide compounds. Finally, our investigations provide compelling evidence that a non-redox-active metal center plays a crucial role in the oxygenation process through assisting in single-electron transfer from an M-C bond to an O2 molecule. Although the oxygenation of zinc alkyls occurs by radical pathways, the reported results stand in clear contradiction to the widely accepted free-radical chain mechanism.

Susceptibility of Candida albicans to New Synthetic Sulfone Derivatives

The influence of halogenated methyl sulfones, i.e. bromodichloromethyl‐4‐chloro‐3‐nitrophenyl sulfone (named halogenated methyl sulfone 1), dichloromethyl‐4‐chloro‐3‐nitrophenyl sulfone (halogenated methyl sulfone 2), and chlorodibromomethyl‐4‐hydrazino‐3‐nitrophenyl sulfone (halogenated methyl sulfone 3), on cell growth inhibition, aspartic protease gene (SAP4–6) expression, adhesion to epithelium, and filamentation was investigated. Antifungal susceptibility of the halogenated methyl sulfones was determined with the M27‐A3 protocol in the range of 16–0.0313 µg/mL. Adherence to Caco‐2 cells was performed in 24‐well plates; relative quantification was normalized against ACT1 in cells after 18 h of growth in YEPD and on Caco‐2 cells. SAP4–6 expression was analyzed using RT‐PCR. Structure–activity relationship studies suggested that halogenated methyl sulfone 1 containing bromodichloromethyl or dichloromethyl function at C‐4 (halogenated methyl sulfone 2) of the phenyl ring showed the best activity (100% cell inhibition at 0.5 µg/mL), while hydrazine at C‐1 (halogenated methyl sulfone 3) reduced the sulfone potential (100% = 4 µg/mL). SAP4–6 were up‐ or down‐regulated depending on the strains’ genetic background and the substitutions on the phenyl ring. Halogenated methyl sulfone 2 repressed germination and affected adherence to epithelium (P ≤ 0.05). The tested halogenated methyl sulfones interfered with the adhesion of Candida albicans cells to the epithelial tissues, without affecting their viability after 90 min of incubation. The mode of action of the halogenated methyl sulfones was attributed to the reduced virulence of C. albicans. SAP5 and SAP6 contribute to halogenated methyl sulfones resistance. Thus, halogenated methyl sulfones can inhibit biofilm formation due to their interference with adherence and with the yeast‐to‐hyphae transition.

Effect of serine protease KEX2 on Candida albicans virulence under halogenated methyl sulfones

AIM The effect of KEX2 mutations on C. albicans virulence and resistance to halogenated methyl sulfones was assessed. MATERIALS & METHODS The mechanism of action of sulfones was studied using flow cytometry and microscopy. Expression of KEX2 and SAP5 was assessed using quantitative Real-Time-PCR. 2,3-Bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide and lactate dehydrogenase assays were elaborated to study, respectively, metabolism of Candida treated with sulfones and their cytotoxicity against tissues. Inflammatory response was detected by ELISA. RESULTS Lysosome permeabilization and dose-dependent programmed cell death under sulfones were noted. KEX2 induction depended on halogenomethylsulfonyl groups, which affected cell wall biosynthesis and adhesion. CONCLUSION Sulfones treatment reduced Candida pathogenicity in Galleria mellonella. Sulfones are an alternative for antifungal therapies due to their safety profile and antibiofilm activity.

Possible role of hydrolytic enzymes (Sap, Kex2) in Candida albicans response to aromatic compounds bearing a sulfone moiety

Hydrolytic enzymes e.g., Saps and Kex2 are, due to their role in Candida virulence, considered important targets for new synthetic inhibitors. MICTI and MICPI values indicate that disruption of SAP1-3 significantly increases the resistance of Candida mutants to β-ketosulfone (1). Contrariwise, sap123Δ showed sensitive phenotype to halogenated methylphenyl sulfone (2). Anticandidal potency of 2 differed in the Candida cells of kex2Δ. Sulfone is the most effective agent against the Candida albicans kex2Δ double mutant (MICTI of 0.5 µg mL−1). Up-regulation of KEX2 mediated the resistance of sap4-6Δ towards 2. Both sulfones tested reduced the adhesion of the wild type cells significantly (P ≤ 0.05). Contrariwise, sap123Δ showed significantly enhanced adhesion capability when 1 was used (P ≤ 0.05). Both sulfones had weak fungicidal effect on mature C. albicans biofilms. It was shown that the uptake of IP correlates with the membrane perturbations caused by 1 in the blastoconidial cells. Sulfones were found to disturb the basic developmental phases of biofilm growth: adhesion and morphogenesis. Altered KEX2 levels for 1 can be caused by the compensatory mechanism for the maintenance of cell wall integrity and morphogenesis. KEX2 decreases the antifungal activity of sulfones. Sulfones affecting the crucial virulence factors of Candida can even eliminate these fungal infections.

Dramatic structural changes of donor-functionalized alkoxides of aluminium upon replacement of the ester functionality by ketone

Upon replacement of the ester functionality by ketone functionality in α-hydroxy carbonyl compounds dramatic structural changes of donor-functionalized alkoxides of aluminium are observed: complete reversal of the stereochemistry of the aluminium alkoxide adduct formation, dissociation of the five-coordinate dimer to a monomeric four-coordinate chelate complex; the first example of an aluminium Cram-type chelate complex derived from the reaction of Et3Al with an equimolar amount of rac-acetoin is reported.

Novel Sulfones with Antifungal Properties: Antifungal Activites and Interactions with Candida spp. Virulence Factors

Since candidiasis is so difficult to eradicate with an antifungal treatment and the existing antimycotics display many limitations, hopefully new sulfone derivatives may overcome these deficiencies. It is pertinent to study new strategies such as sulfone derivatives targeting the virulence attributes of C. albicans that differentiate them from the host. During infections, the pathogenic potential of C. albicans relies on the virulence factors as follows: hydrolytic enzymes, transcriptional factors, adhesion, and development of biofilms. In the article we explored how the above-presented C. albicans fitness and virulence attributes provided a robust response to the environmental stress exerted by sulfones upon C. albicans; C. albicans fitness and virulence attributes are fungal properties whose inactivation attenuates virulence. Our understanding of how these mechanisms and factors are inhibited by sulfones has increased over the last years. As lack of toxicity is a prerequisite for medical approaches, sulfones (non-toxic as assessed in vitro and in vivo) may prove to be useful for reducing C. albicans pathogenesis in humans. The antifungal activity of sulfones dealing with these multiple virulence factors and fitness attributes is discussed.