Synthesis and antimalarial and antituberculosis activities of a series of natural and unnatural 4-methoxy-6-styryl-pyran-2-ones, dihydro analogues and photo-dimers.

Previous studies have identified the 3,6-dialkyl-4-hydroxy-pyran-2-one marine microbial metabolites pseudopyronines A and B to be modest growth inhibitors of Mycobacterium tuberculosis and a range of tropical diseases including Plasmodium falciparum and Leishmania donovani. In an effort to expand the structure-activity relationship of this compound class towards infectious diseases, a library of natural product and natural product-like 4-methoxy-6-styryl-pyran-2-ones and a subset of catalytically reduced examples were synthesized. In addition, the photochemical reactivity of several of the 4-methoxy-6-styryl-pyran-2-ones were investigated yielding head-to-head and head-to-tail cyclobutane dimers as well as examples of asymmetric aniba-dimer A-type dimers. All compounds were evaluated for cytotoxicity and activity against M. tuberculosis, P. falciparum, L. donovani, Trypanosoma brucei rhodesiense and Trypanosoma cruzi. Of the styryl-pyranones, natural product 3 and non-natural styrene and naphthalene substituted examples 13, 18, 21, 22 and 23 exhibited antimalarial activity (IC(50) <10 µM) with selectivity indices (SI) >10. Δ(7) Dihydro analogues were typically less active or lacked selectivity. Head-to-head and head-to-tail photodimers 5 and 34 exhibited moderate IC(50)s of 2.3 to 17 µM towards several of the parasitic organisms, while the aniba-dimer-type asymmetric dimers 31 and 33 were identified as being moderately active towards P. falciparum (IC(50) 1.5 and 1.7 µM) with good selectivity (SI ~80). The 4-tert-butyl aniba-dimer A analogue 33 also exhibited activity towards L. donovani (IC(50) 4.5 µM), suggesting further elaboration of this latter scaffold could lead to the identification of new leads for the dual treatment of malaria and leishmaniasis.

Extending the range of neutral N-donor ligands available for metal catalysts: N-[1-alkylpyridin-4(1H)-ylidene]amides in palladium-catalyzed cross-coupling reactions.

N-[1-Alkylpyridin-4(1H)-ylidene]amides (PYAs) are a new class of easily prepared, neutral N-donor ligands that share some features in common with N-heterocyclic carbenes. They are strongly electron-donating toward metal centers, and a palladium(II) complex of one of these ligands has been shown to successfully catalyze both the Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions.

Noncovalent endo-binding of fullerenes to diprotonated bisporphyrins.

Noncovalent binding of fullerenes to bisporphyrins was studied in the gas phase by energy-dependent collision-induced dissociation (CID) with Xe under single-collision conditions. The electrospray ionization mass spectra of calix[4]arene-linked bisporphyrins show that bisporphyrins take up to 3-4 protons, depending on the type of meso-substituents. Of the protonated bisporphyrins, the diprotonated species form stable 1:1 complexes with fullerenes (C(60) and C(70)). CID cracking patterns of the diprotonated bisporphyrins indicate that each monomeric porphyrin moiety is singly protonated. CID yield-energy curves obtained from the 1:1 diprotonated bisporphyrin-fullerene complexes suggest that a fullerene occupies the endo-binding site intercalated between the two singly protonated porphyrin moieties. In the cases of 1:2 diprotonated bisporphyrin-fullerene complexes, CID results show that one fullerene binds inside (endo-binding) and the other outside (exo-binding). The exo-binding mode is energetically almost identical to the binding of fullerenes to singly protonated porphyrin monomers. The endo-binding energy is at least twice the exo-binding energy. To gain insights into the binding mode, we optimized structures of diprotonated bisporphyrins and their 1:1 endo-complexes with fullerenes, and calculated the endo-binding energy for C(60), C(70) (end-on), and C(70) (side-on). The endo-binding of fullerenes to diprotonated bisporphyrins nearly doubles the π-π interactions while reducing the electrostatic repulsion between the two singly protonated porphyrin moieties. The side-on binding of C(70) is favored over the end-on binding because the former exerts less steric strain to the lower rim of calixarene.

Preparation and antitumour properties of the enantiomers of a hypoxia-selective nitro analogue of the duocarmycins.

Racemic 2-{[1-(chloromethyl)-5-nitro-3-{5-[2-(dimethylamino)ethoxy]indol-2-carbonyl}-1,2-dihydro-3H-benzo[e]indol-7-yl]sulfonyl}aminoethyl dihydrogen phosphate, a synthetic nitro derivative of the duocarmycins, is a hypoxia-selective prodrug active against radiation-resistant tumour cells at nontoxic doses in mice. An intermediate in the synthesis of this prodrug was resolved by chiral HPLC and the absolute configuration assigned by X-ray crystallography. The intermediate was used to prepare the prodrug's enantiomers, and also the enantiomers of the active nitro and amino metabolites. In vitro analysis in the human cervical carcinoma cell line SiHa showed that both nitro enantiomers are hypoxia-selective cytotoxins, but the "natural" S enantiomer is at least 20-fold more potent. Examination of extracellular amino metabolite concentrations demonstrated no enantioselectivity in the hypoxia-selective reduction of nitro to amino. Low levels of amino derivative were also found in aerobic cell suspensions, sufficient to account for the observed oxic toxicity of the nitro form. At an equimolar dose in SiHa-tumour bearing animals, the (-)-R enantiomer of the prodrug was inactive, while the (+)-S enantiomer caused significantly more hypoxic tumour cell kill than the racemate. At this dose, the combination of (+)-S-prodrug and radiation eliminated detectable colony-forming cells in four out of five treated tumour-bearing animals.

Density functional theory study on the mechanism of the reductive cleavage of CO2 by a bis-beta-diketoiminatediiron dinitrogen complex.

Density functional theory has been used to analyze the detailed reaction mechanism for the reductive cleavage of CO(2) by a dinitrogen bridged bis-beta-diketoiminatediiron complex, L(tBu)Fe-N(2)-FeL(tBu) (I), recently reported by Holland and co-workers. A number of pathways have been investigated and the most likely mechanism correlates well with experimental evidence. A rationale has been provided for the binding of CO(2), the release of CO, and the ready formation of CO(3)(2-). Our results show that the insertion of CO(2) into the diiron complex is the rate determining step of the reductive cleavage reaction. An intramolecular reduction step from the reduced dinitrogen bridge is proposed which serves to increase the activation of CO(2). This is followed by an intersystem crossing from the septet to the nonet state which acts as a driving force for the subsequent release of CO. The overall reductive cleavage reaction is exergonic by 120 kJ/mol, and further reaction of the released CO with the starting diiron complex is also predicted to be strongly exergonic.

Reductive coupling and protonation leading to diboron corroles with a B-H-B bridge.

The reaction of PhBCl(2) with free base triarylcorroles results in a spontaneous reduction to give diboron corrole complexes (PhBHBPh)(Cor) in which a proton has been captured to form a bridging B-H-B group encapsulated within the corrole ligand. The proposed mechanism is supported by the reaction of PhBF(2) with H(3)Cor to give (PhBF)(BPh)(Cor) in which no reduction has occurred.

Nucleation of antiferromagnetically coupled chromium dihalides: from small clusters to the solid state.

The nucleation of chromium dihalide clusters is investigated by studying clusters of the form Cr(n)X(2n) (n

(1R,6R,13R,18R)-(Z,Z)-1,18-Bis[(4R)-2,2-dimethyl-1,3-dioxolan-4-yl]-3,16-dimethyl-ene-8,20-diaza-dispiro-[5.6.5.6]tetra-cosa-7,19-diene.

The crystal structure of the title compound, C(34)H(54)N(2)O(4), has been solved in order to prove the relative and absolute chirality of the newly-formed stereocentres which were established using an asymmetric Diels-Alder reaction at an earlier stage in the synthesis. This unprecedented stable dialdimine contains a 14-membered ring and was obtained as the minor diastereoisomer in the Diels-Alder reaction. The absolute stereochemistry of the stereocentres of the acetal functionality was known to be R based on the use of a chiral (R)-tris-ubstituted dienophile derived from enanti-opure (S)-glyceraldehyde. The assignment of the configuration in the dienophile and the title di-aldimine differs from (S)-glyceraldehyde due to a change in the priority order of the substituents. The crystal structure establishes the presence of six stereocentres all attributed to be R. The 14-membered ring contains two aldimine bonds [C-N = 1.258 (2) and 1.259 (2) Å]. It adopts a similar conformation to that proposed for trans-trans-cyclo-tetra-deca-1,8-dienes.

Iron(II) tris-[N4-substituted-3,5-di(2-pyridyl)-1,2,4-triazole] complexes: structural, magnetic, NMR, and density functional theory studies.

Eight mononuclear iron(II) complexes of N(4)-3,5-di(2-pyridyl)-1,2,4-triazole (Rdpt) ligands have been prepared and characterized. In all cases the iron(II)/ligand ratio used is 1:3, giving red complexes of the general formula [Fe(II)(Rdpt)(3)](BF(4))(2) x solvents, in 55-89% yield. The ligands differ only in the nature of the N(4)-substituent (amino, pyrrolyl, iso-butyl, methyl, phenyl, para-tolyl, 3,5-dichlorophenyl, and 4-pyridyl; for ligands adpt, pldpt, ibdpt, medpt, phdpt, ptdpt, Cldpt, and pydpt, respectively) allowing substituent effects on the properties of the resulting iron(II) complexes to be probed. The low temperature crystal structures of seven of the complexes reveal low spin iron(II) environments. Packing analyses reveal anion-pi and acetonitrile-pi interactions involving the tetrafluoroborate counteranions and interstitial acetonitrile molecules, respectively. Both "pi-pockets" and "pi-sandwiches" are observed. Solid state magnetic susceptibility measurements (4-300 K) indicate the iron(II) is low spin (LS) in all complexes at all temperatures studied, except for [Fe(II)(pldpt)(3)](BF(4))(2) x 1 1/2 H(2)O which has the beginnings of spin crossover (SCO) at elevated temperatures. Downfield shifts and peak broadening observed in the variable temperature (1)H NMR studies indicate that in d(3)-nitromethane solution the LS [Fe(II)(Rdpt)(3)](2+) complexes are in equilibrium with a trace of a high spin (HS) species. (15)N NMR spectra (measured and calculated) of the ligands reveal that altering the N(4)-substituent changes the chemical shift of the N(1) triazole and pyridine nitrogen atoms, allowing probing of the relationship between ligand substituent and the nature of the coordinating nitrogen atoms.

3-Allyl-2-hydr-oxy-5,6,8-trimethoxy-naphthalene-1,4-dione.

In the crystal structure of the title compound, C(16)H(16)O(6), a pair of naphthoquinone rings are linked via O-H⋯O-C hydrogen bonds in a nearly orthogonal arrangement. This dimeric unit is linked to a neighbouring dimer by π-π stacking inter-actions between the naphthoquinone rings, where the distance between the mean plane of the naphtoquinone backbones is 3.468 Å, and O-H⋯O-C hydrogen bonds.

(±)-N-(3-Hydr-oxy-1,2-diphenyl-prop-yl)-4-methyl-benzene-sulfonamide.

In the title compound, C(22)H(23)NO(3)S, the relative stereochemistry of the two stereogenic centres is anti with respect to the H atoms. The mol-ecular packing of the crystal shows a double-strand arrangement, consisting of one strand of (S*,S*) enanti-omers and one strand of (R*,R*) enanti-omers. Both strands lie parallel to each other along the a axis. Each strand is made up of dimers in which the mol-ecules are connected to each other via an inter-molecular O-H⋯O hydrogen bond between the hydroxyl groups and an O-H⋯π inter-action with the aromatic ring. These units are then connected to neighbouring dimers via N-H⋯O hydrogen bonds and C-H⋯O interactions. Intramolecular C-H⋯O interactions are also observed.

(4R)-4-[(1R)-1,2-Dihydroxy-ethyl]-1-[(1R)-1-phenyl-ethyl]pyrrolidin-2-one.

The title compound, C(14)H(19)NO(3), was obtained as one of the two isomers of a Sharpless asymmetric dihydroxy-lation reaction of (1S)-1-[(1R)-1-phenyl-ethyl]-4-vinyl-pyrrolidin-2-one. The absolute stereochemistry of this isomer was determined from the known stereochemistry (R) at the bridge C atom between the phenyl and pyrrolidine rings. The mol-ecules form one-dimensional tapes along the b axis via hydrogen bonding between the carbonyl O atom and the alcohol groups of neighbouring mol-ecules. These assemble into sheets via inter-digitative stacking of the phenyl rings and C-H⋯O inter-actions.

(1R,1'R,3S,3'S)-5,5',10,10'-Tetra-meth-oxy-1,1',3,3'-tetra-methyl-3,3',4,4'-tetra-hydro-1H,1'H-8,8'-bi[benzo[g]isochromene].

In the title compound, C(34)H(38)O(6), the methyl groups on each pyran ring exhibit 1,3-cis stereochemistry, established during synthesis by pseudo-axial delivery of hydride during a lactol reduction step. In the crystal structure, the mol-ecule lies on a twofold rotation axis and the torsion angle about the central diaryl bond is 41.3 (1)°. The mol-ecules pack in a herringbone arrangement.

Ethyl 4-hydroxy-methyl-2-methyl-pyridine-5-carboxyl-ate.

The title compound, C(10)H(13)NO(3), was obtained as a by-product of the aldolization reaction of furo[3,4-c]pyridin-3(1H)-one with thio-phene-2-carboxaldehyde. The substituents on the pyridine ring are nearly coplanar, with an 8.1 (2)° rotation of the hydroxmethyl group from this plane. The mol-ecules assemble in the crystal structure as chains via O-H⋯N hydrogen bonding between the pyridine N atom and a neighbouring hydroxy-methyl OH group.

(1R,6R)-1-Methyl-8-aza-spiro-[5.6]dodecan-7-one.

The crystal structure of the title compound, C(12)H(21)NO, has been investigated to establish the absolute stereochemistry at position 1. The absolute stereochemistry at the quaternary centre at position 6 is established to be R using an asymmetric Birch reductive alkyl-ation reaction for which the stereochemical outcome is known. The crystal structure indicates the presence of two conformers of the bicyclic (1R,6R)-spiro-lactam ring system that differ in the conformation adopted by the six-membered ring. In one conformer, the meth-yl group adopts an axial position whereas in the other conformer, the same methyl group adopts an equatorial position. In both conformers, the seven-membered ring adopts a chair conformation. The two conformers of the bicyclic spiro-lactam are connected to each other via inter-molecular N-H⋯O hydrogen bonds forming a heterodimer. The asymmetric unit contains two such dimers.

Bis(N,N-diethyl-dithio-carbamato)(1,10-phenanthroline)cobalt(III) tetra-fluorido-borate.

The cationic complex in the structure of the title compound, [Co(Et(2)NCS(2))(2)(C(12)H(8)N(2))]BF(4), has a Co(III) atom with a distorted octa-hedral coordination formed by four S atoms of two diethyl-dithio-carbamate and two N atoms of 1,10-phenanthroline ligands. The crystal structure features head-to-tail stacking of the phenanthroline ligands. The tetra-fluorido-borate anions are positioned in the channels between the cation stacks running along the a axis, and form weak C-H⋯F interactions.

(±)-2'-Phenyl-cyclo-hexa-nespiro-4'-(aze-pano[1,2-b]isoxazolidine).

In the crystal structure of the racemic title isoxazolidine, C(19)H(27)NO, the relative stereochemistry between the phenyl group and the bridgehead H atom is shown to be syn. There are two mol-ecules in the asymmetric unit, one of which is the 7R*,13R* enanti-omer, and one of which is the 7S*,13S* enanti-omer. These enanti-omers adopt different orientations of the phenyl ring with respect to the isoxazolidine ring, with C-C-C-C torsion angles of 63.6 (4) and 86.8 (4)°, respectively. In both enanti-omers, the six-membered ring adopts a chair conformation, while the seven-membered ring adopts a twist-chair conformation.

(±)-Cyclo-hexane-1,2-diyl bis-(4-nitro-benzoate).

The crystal structure of the title compound, C(20)H(18)N(2)O(8), has been investigated to establish the relative stereochemistry between the ester groups. The cyclo-hexane ring adopts a chair conformation, in which the two ester groups occupy the adjacent equatorial positions in a trans relationship with each other. The mol-ecules assemble in the crystal as chains along the c axis via C-H⋯π inter-actions between the cyclo-hexane ring and a pair of nitro-phenyl rings of the neighbouring mol-ecule. Also observed are π-π stacking inter-actions between the nitro-phenyl rings of neighbouring chains, with a perpendicular distance between these rings of 3.409 Šand a slippage of 0.969 Å.