Vincamine, from an antioxidant and a cerebral vasodilator to its anticancer potential

Vincamine is a naturally occurring indole alkaloid showing antioxidant activity and has been used clinically for the prevention and treatment of cerebrovascular disorders and insufficiencies. It has been well documented that antioxidants may contribute to cancer treatment, and thus, vincamine has been investigated recently for its potential antitumor activity. Vincamine was found to show cancer cell cytotoxicity and to modulate several important proteins involved in tumor growth, including acetylcholinesterase (AChE), mitogen-activated protein kinase (MAPK), nuclear factor-κB (NF-κB), nuclear factor erythroid 2-related factor 2 (Nrf2), and T-box 3 (TBX3). Several bisindole alkaloids, including vinblastine and vincristine and their synthetic derivatives, vindesine, vinflunine, and vinorelbine, have been used as clinically effective cancer chemotherapeutic agents. In the present review, the discovery and development of vincamine as a useful therapeutic agent and its antioxidant and antitumor activity are summarized, with its antioxidant-related mechanisms of anticancer potential being described. Also, discussed herein are the design of the potential vincamine-based oncolytic agents, which could contribute to the discovery of further new agents for cancer treatment.

Panchromatic dirhodium photocatalysts for dihydrogen generation with red light

A series of three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂ (1, DPhB = diphenylbenzamidine; bncn = benzocinnoline), cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ (2, DPhTA = diphenyltriazenide), and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ (3, DPhF = N,N′-diphenylformamidinate) shown to act as single-molecule photocatalysts for H₂ production was evaluated. Complexes 1–3 are able to generate H₂ in the absence of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-reduced, activated form of the catalyst that is able to generate H₂. The present work shows the effect of substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes 1–3 operate following a double reductive quenching mechanism and, importantly, are active with red light. This work lays the foundation for the design of single-molecule photocatalysts that operate from the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed and utilized for solar energy conversion.

Three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂, cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ are shown to act as single-molecule photocatalysts for H₂ production.

α-Pyrone and Sterol Constituents of Penicillium aurantiacobrunneum, a Fungal Associate of the Lichen Niebla homalea

Four new metabolites, 4-epi-citreoviridin (1), auransterol (3), and two analogues (2 and 4) of paxisterol (6), together with two known metabolites (15R*,20S*)-dihydroxyepisterol (5) and (6), were isolated from cultures of the fungal associate, Penicillium aurantiacobrunneum, of the lichen Niebla homalea, endemic to California and Baja California. The structures of all compounds were determined by comprehensive spectroscopic and spectrometric methods, as well as single-crystal X-ray diffraction for the determination of the absolute configuration of 3. Compound 1 showed selective cytotoxicity toward MCF-7 breast and A2780 ovarian cells with IC₅₀ values of 4.2 and 5.7 μM, respectively.

Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes

Much of the recent work on catalytic hydroboration of alkenes has focused on simple alkenes and styrene derivatives with few examples of reactions of 1,3-dienes, which have been reported to undergo mostly 1,4-additions to give allylic boronates. We find that reduced cobalt catalysts generated from 1,n- bis-diphenylphosphinoalkane complexes [Ph₂P-(CH₂) _n-PPh₂]CoX₂; n = 1-5) or from (2-oxazolinyl)phenyldiarylphosphine complexes [(G-PHOX)CoX₂] (G = 4-substituent on oxazoline ring) effect selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to a variety of 1,3-dienes depending on the ligands chosen. Conditions have been found to optimize the 1,2-additions. The reactive catalysts can be generated from the cobalt(II)-complexes using trimethylaluminum, methyl aluminoxane, or activated zinc in the presence of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBARF). The complex, (dppp)CoCl₂, gives the best results (ratio of 1,2- to 1,4-addition >95:5) for a variety of linear terminal 1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)CoX₂] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted 1,3-dienes, but, surprisingly, selective 1,2-additions with 2-substituted or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized (X-ray crystallography) Co(I)-complexes, (dppp)₃Co₂Cl₂ and [( S,S)-BDPP]₃Co₂Cl₂, do not catalyze the reaction unless activated by a Lewis acid or NaBARF, suggesting a key role for a cationic Co(I) species in the catalytic cycle. Regio- and enantioselective 1,2-hydroborations of 2-substituted 1,3-dienes are best accomplished using a catalyst prepared via activation of a chiral phosphinooxazoline-cobalt(II) complex with zinc and NaBARF. A number of common functional groups, among them, -OBn, -OTBS, -OTs, N-phthalimido- groups, are tolerated, and er's > 95:5 are obtained for several dienes including 1-alkenylcycloalk-1-enes. This operationally simple reaction expands the realm of asymmetric hydroboration to provide direct access to a number of nearly enantiopure homoallylic boronates, which are not readily accessible by current methods. The resulting boronates have been converted into the corresponding alcohols, potassium trifluororoborate salts, N-BOC amines, and aryl derivatives by C-BPin to C-aryl transformation.

Crystal Structures and Human Leukemia Cell Apoptosis Inducible Activities of Parthenolide Analogues Isolated from Piptocoma rufescens

The molecular structures of three parthenolide analogues, (-)-goyazensolide (1), (-)-15-deoxygoyazensolide (2), and (-)-ereglomerulide (3), isolated from the leaves of Piptocoma rufescens in a previous study were determined by X-ray analysis, and the absolute configuration of (-)-goyazensolide (1) was confirmed crystallographically using Cu Kα radiation at low temperature. Compounds 1-3, (+)-rufesolide A (4), and commercial parthenolide were found to be growth inhibitory toward MOLM-13 and EOL-1 human acute myeloid leukemia cells using PKC412 (midostaurin) as the positive control, with 1-3 being more active than parthenolide. Also, compounds 1-4 exhibited synergistic effects when tested with PKC412, but parthenolide did not show this type of activity. At a concentration lower than 2.0 μM, both 1 and 2 induced approximately 50% of the cells to become apoptotic at a late stage of the cell cycle, but no similar apoptotic effects were observed for 3, 4, or parthenolide. Leukemia cell apoptosis was induced by these compounds through the activation of caspase-3 and the inhibition of NF-κB, as indicated by immunoblotting analysis, and compounds 1 and 2 seem to be promising leads for development as potential antileukemic agents.

Catalytic Enantioselective Hetero-dimerization of Acrylates and 1,3-Dienes

1,3-Dienes are ubiquitous and easily synthesized starting materials for organic synthesis, and alkyl acrylates are among the most abundant and cheapest feedstock carbon sources. A practical, highly enantioselective union of these two readily available precursors giving valuable, enantio-pure skipped 1,4-diene esters (with two configurationally defined double bonds) is reported. The process uses commercially available cobalt salts and chiral ligands. As illustrated by the use of 20 different substrates, including 17 prochiral 1,3-dienes and 3 acrylates, this hetero-dimerization reaction is tolerant of a number of common organic functional groups (e.g., aromatic substituents, halides, isolated mono- and di-substituted double bonds, esters, silyl ethers, and silyl enol ethers). The novel results including ligand, counterion, and solvent effects uncovered during the course of these investigations show a unique role of a possible cationic Co(I) intermediate in these reactions. The rational evolution of a mechanism-based strategy that led to the eventual successful outcome and the attendant support studies may have further implications for the expanding use of low-valent group 9 metal complexes in organic synthesis.

An Intramolecular CAr-H•••O=C Hydrogen Bond and the Configuration of Rotenoids

Over the past half a century, the structure and configuration of the rotenoids, a group of natural products showing multiple promising bioactivities, have been established by interpretation of their NMR and electronic circular dichroism spectra and confirmed by analysis of single-crystal X-ray diffraction data. The chemical shift of the H-6' ¹H NMR resonance has been found to be an indicator of either a cis or trans C/D ring system. In the present study, four structures representing the central rings of a cis-, a trans-, a dehydro-, and an oxadehydro-rotenoid have been plotted using the Mercury program based on X-ray crystal structures reported previously, with the conformations of the C/D ring system, the local bond lengths or interatomic distances, hydrogen bond angles, and the H-6' chemical shift of these compounds presented. It is shown for the first time that a trans-fused C/D ring system of rotenoids is preferred for the formation of a potential intramolecular C_6'-H_6'•••O=C₄ H-bond, and that such H-bonding results in the ¹H NMR resonance for H-6' being shifted downfield.

Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the 1MLCT S1 State

Four photophysically interesting dimolybdenum paddlewheel compounds are synthesized and characterized: I and II contain amide ligand (N,3-diphenyl-2-propynamide), and III and IV contain thioamide ligand (N,3-diphenyl-2-propynethioamide). I and III are trans-Mo₂L₂(O₂C-TⁱPB)₂-type compounds, and II and IV are Mo₂L₄-type compounds, where O₂C-TⁱPB is 2,4,6-triisopropylbenzoate. I-IV display strong light absorption due to metal to ligand charge transfer (MLCT) transitions from molybdenum to the amide/thioamide ligands. Charge transfer dynamics in the MLCT excited states of I-IV have been examined using femtosecond transient absorption (fs-TA) spectroscopy and femtosecond time-resolved infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands show two forms of regioarrangements in the paddlewheel compounds. Analyses of the ν(C≡C) bands in the fs-TRIR spectra of I and II show similar electron density distribution over ligands in their ¹MLCT S₁ states where only two amide ligands are involved and the transferred electron is mainly localized on one of them. The fs-TRIR spectra of III and IV, however, show different charge distribution patterns where the transferred electron is fully delocalized over two thioamide ligands in III and partially delocalized in IV. Fast interligand electron transfer (ILET) was recognized as the explanation for the various charge distribution patterns, and ILET was shown to be influenced by both the ligands and the ligand arrangements.

4-Nitrophenyl- and 4'-nitro-1,1'-biphenyl-4-carboxylates attached to Mo2 quadruple bonds: ground versus excited state M2δ-ligand conjugation

From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.

Potent cytotoxic arylnaphthalene lignan lactones from Phyllanthus poilanei

Two new (1 and 2) and four known arylnaphthalene lignan lactones (3-6) were isolated from different plant parts of Phyllanthus poilanei collected in Vietnam, with two further known analogues (7 and 8) being prepared from phyllanthusmin C (4). The structures of the new compounds were determined by interpretation of their spectroscopic data and by chemical methods, and the structure of phyllanthusmin D (1) was confirmed by single-crystal X-ray diffraction analysis. Several of these arylnaphthalene lignan lactones were cytotoxic toward HT-29 human colon cancer cells, with compounds 1 and 7-O-[(2,3,4-tri-O-acetyl)-α-L-arabinopyranosyl)]diphyllin (7) found to be the most potent, exhibiting IC50 values of 170 and 110 nM, respectively. Compound 1 showed activity when tested in an in vivo hollow fiber assay using HT-29 cells implanted in immunodeficient NCr nu/nu mice. Mechanistic studies showed that this compound mediated its cytotoxic effects by inducing tumor cell apoptosis through activation of caspase-3, but it did not inhibit DNA topoisomerase IIα activity.

N-[2-(9H-Carbazol-9-yl)ethyl]-4-(methylsulfonyl)aniline

In the title mol­ecule, C₂₁H₂₀N₂O₂S, the dihedral angle between the mean plane of the carbazole ring system [maximum deviation = 0.021 (4) Å] and the benzene ring is 80.15 (6)°. In the crystal, mol­ecules are linked by N—H⋯O and weak C—H⋯O hydrogen bonds into a C(8) chain along [001].

Caeruleanone A, a rotenoid with a new arrangement of the D-ring from the fruits of Millettia caerulea

Caeruleanone A (1), a novel rotenoid with an unprecedented arrangement of the D-ring, was isolated with another two new analogues, caeruleanones B (2) and C (3), together with 11 known rotenoids from the fruits of Millettia caerulea. The structures of the new compounds were determined by spectroscopic data analysis, with that of 1 being confirmed by single-crystal X-ray diffraction. Compounds 2 and 3 displayed potent mitochondrial transmembrane potential inhibitory and quinone reductase induction activities.

(E)-3-(9-Ethyl-9H-carbazol-3-yl)-1-(2-meth-oxy-phen-yl)prop-2-en-1-one

In the title mol­ecule, C₂₄H₂₁NO₂, the dihedral angle between the carbazole ring system [with a maximum deviation of 0.052 (2) Å] and the benzene ring is 38.6 (1)°. In the crystal, weak bifurcated (C—H)₂⋯O hydrogen bonds link the mol­ecules into chains along [100].

Bimetallic catalysis in the highly enantioselective ring–opening reactions of aziridines

Structural and kinetic evidence for bimetallic mechanism.

Single-site bismuth alkoxide catalysts for the ring-opening polymerization of lactide

Salen bismuth alkoxides, where the salen ligand contains 2,4-di-tert-butylphenoxy groups and one of ethylene, cyclohexane or ortho-phenyl as a backbone have been prepared from reactions involving Bi[N(SiMe₃)₂]₃ and the free salen ligand followed by alcoholysis (ButOH, PriOH and 2,6-But₂C₆H₃OH). The molecular structures of the salen ligand with the cyclohexyl back-bone have been determined for the complexes salenBiCl and salenBiOC₆H₃-2,6-But₂. The chloro compound is a dimer with chloride bridges while the phenoxide is monomeric with an unusually distorted five-coordinate geometry. The phenoxide and tert-butoxide complexes have been employed in the ring-opening polymerization of lactides (L- and rac-) to give polylactides, PLAs. With rac-LA heterotactic PLA is formed preferentially, Pr = ~0.9, in dichloromethane or toluene at room temperature. The reaction is first order in [Bi] and is notably faster than most aluminum and zinc initiators as well as tin(II) octanoate. These results are discussed in terms of a recent report on the polymerization of LA by Peptobismol® and bismuth subsalicylate.

Photoinduced ligand exchange and DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2]+ with long wavelength visible light

The complex cis-[Ru(phpy)(phen)(CH3CN)2](+) (phpy=2-phenylpyridine, phen=1,10-phenanthroline) was investigated as a potential photodynamic therapy (PDT) agent. This complex presents desirable photochemical characteristics including a low energy absorption tail extending into the PDT window (600-850nm) and photoinduced exchange of the CH3CN ligands, generating a species analogous to the chemotherapy drug cisplatin. Furthermore, photochemical reactivity can be controlled through selective irradiation into the Ru-phen singlet metal-to-ligand charge transfer ((1)MLCT) band (λirr=500 nm) of [Ru(phpy)(phen)(CH3CN)2](+) in the presence of excess t-butylammonium chloride (TBACl) resulting in efficient photoinduced production of [Ru(phpy)(phen)(CH3CN)Cl] (Φ=0.25). This lower energy irradiation resulted in greater quantum yield of photosubstitution when compared to direct irradiation into the Ru-phpy (1)MLCT peak (λirr=450 nm; Φ=0.08) in CH2Cl2. It was found that the lower quantum yield observed for irradiation into the Ru→phpy(-)(1)MLCT band results from significant orbital mixing of the phpy(-) ligand with the t2g-type filled set in the metal, giving this state significant ligand-centered character. Lastly, this complex produced a decrease in the mobility of linearized ds-DNA when irradiated with λirr≥420nm, indicative of covalent binding by the transition metal complex similar to that observed for cisplatin. No change in mobility was found for the same samples kept in the dark indicating, unlike cisplatin, DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2](+) only occurs with the activation of light. These observations support the use of cis-[Ru(phpy)(phen)(CH3CN)2](+) as a potential PDT agent by the photoinduced generation of a cisplatin analog.

Cordifolide A, a sulfur-containing clerodane diterpene glycoside from Tinospora cordifolia

Cordifolide A (1), a novel unprecedented sulfur-containing clerodane diterpene glycoside, together with other two new diterpene glycosides, cordifolides B (2) and C (3), and four known analogues, was isolated from a methanol-soluble extract of the stems of Tinospora cordifolia. The structures of the new compounds were determined on the basis of spectroscopic data interpretation, with that of cordifolide A (1) confirmed by a single-crystal X-ray crystallographic analysis. All isolates were evaluated for their in vitro immunomodulatory activity using mouse bone marrow-derived dentritic cells (BMDCs).

Cytotoxic and NF-κB inhibitory sesquiterpene lactones from Piptocoma rufescens

Six new (1-6) and eight known germacranolide-type sesquiterpene lactones, along with several known phenylpropanol coumarates and methylated flavonoids, were isolated from the leaves of Piptocoma rufescens, collected in the Dominican Republic. The new compounds were identified by analysis of their spectroscopic data, with the molecular structure of 3 being established by single-crystal X-ray diffraction. The absolute configurations of the sesquiterpene lactones isolated were determined from their CD and NOESY NMR spectra, together with the analysis of Mosher ester reactions. Bioassay screening results showed the majority of the sesquiterpene lactones isolated (1-13) to be highly cytotoxic toward the HT-29 human colon cancer cell line, with the most potent compound being 15-deoxygoyazensolide (10, IC(50), 0.26 µM). In addition, several of the sesquiterpene lactones exhibited NF-κB (p65) inhibitory activity.

Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M[quadruple bond]M): a comparison of their chemical and photophysical properties

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO(2)H, and 2-selenophene carboxylic acid, SpCO(2)H in toluene, the new compounds trans-M(2)(T(i)PB)(2)(O(2)CFu)(2) (1a M = Mo, 2a M = W) and trans-M(2)(T(i)PB)(2)(O(2)CSp)(2) (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by (1)H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo(2)(T(i)PB)(2)(O(2)CSp)(2) (1b) has been characterized by single crystal X-ray crystallography. These data are compared with those previously reported for related 2-thiophene carboxylate derivatives: M(2)(T(i)PB)(2)(O(2)CTh)(2). The physico-chemical data correlate well with electronic structure calculations performed on model compounds. All compounds have detectible S(1) photoexcited states with lifetimes that vary from ∼5 ps to < 1 ps. The molybdenum compounds have T(1) states with microsecond lifetimes that are assigned as MMδδ* whereas the T(1) states for tungsten are (3)MLCT with lifetimes on the order of nanoseconds. In all cases, shorter lifetimes were seen in complexes containing heavier atoms.

[Ru(bpy)2(5-cyanouracil)2]2+ as a potential light-activated dual-action therapeutic agent

The cation cis-[Ru(bpy)(2)(5CNU)(2)](2+) (bpy = 2,2'-bipyridine; 5CNU = 5-cyanouracil) was synthesized and investigated for use as a potential light-activated dual-action therapeutic agent. The complex undergoes efficient photoinduced 5CNU ligand exchange for solvent water molecules, thus simultaneously releasing biologically active 5CNU and generating [Ru(bpy)(2)(H(2)O)(2)](2+). The latter binds covalently to ds-DNA, such that photolysis results in the generation of 3 equiv of potential therapeutic agents from a single molecule.

A low-temperature phase of bis(tetrabutylammonium) octa-μ(3)-chlorido-hexachlorido-octahedro-hexatungstate

The title compound, (C(16)H(36)N)(2)[W(6)Cl(14)], undergoes a reversible phase transition at 268 (1) K. The structure at 150 and 200 K has monoclinic (P2(1)/c) symmetry. Both crystallographically independent tungsten chloride cluster anions sit on crystallographic inversion centers [symmetry codes: (-x, -y + 1, -z) and (-x + 1, -y + 2, -z)]. Two previous studies at room temperature describe the structure in the space group P2(1)/n with a unit-cell volume approximately half the size of the low-temperature unit cell [Zietlow, Schaefer et al. (1986). Inorg. Chem. 25, 2195-2198; Venkataraman et al. (1999). Inorg. Chem. 38, 828-830]. The unit cells of the room- and low-temperature polymorphs are closely related. The hydrocarbon chain of one of the tetrabutylammonium cations is disordered at both 150 and 200 K.

Annulated diketopiperazines from dipeptides or Schöllkopf reagents via tandem cyclization-intramolecular N-arylation

Facile CuI-mediated N-arylation of diketopiperazine using the Fukuyama modification of the Ullmann-Goldberg reaction can be exploited in new approaches to enantiopure polycyclic diketopiperazines from easily assembled dipeptides or functionalized Schöllkopf reagents.

Chemistry of BDI*M2+ complexes (M = Mg, Zn) and their role in lactide polymerization where BDI* is the anion derived from methylenebis(C-(t)Bu, N-2,6-diisopropylphenyl)imine BDI*H

The bulky diimine CH(2){C((t)Bu)N-2,6-Pr(i)(2)C(6)H(3)}(2), BDI*H, is shown to exist in two isomers trans-trans, 1a and cis-trans, 1b that interconvert slowly on the NMR time-scale. Treatment of the diimine with (n)BuLi in hexane proceeds slowly to give the lithium beta-diketoiminate LiBDI*, 2, which upon hydrolysis yields the eneimine tautomer of BDI*H which has been characterized by X-ray studies in three rotamers, 3a, 3b and 3c. Dialkylmetal compounds (M = Mg, R = Bu; M = Zn, R = Et) react with either 1 or 3 in hydrocarbon and ether solvents to give BDI*Mg((n)Bu)THF, 4, and BDI*ZnEt, 5, which have been structurally characterized. M(N(SiMe(3))(2))(2) compounds where M = Mg, Zn or Ca failed to react with either 1 or 3 in hydrocarbon solvents. The reactions between ZnCl(2) and 2 yield BDI*ZnCl, 6, and a further reaction with LiNMe(2) yields BDI*ZnNMe(2), 7. The compounds 5 and 6 are shown to contain trigonal planar, M(2+) ions. Compounds 4, and 7 initiate ring-opening polymerization, ROP, of rac-lactide to give atactic polylactide, PLA. The rate of ROP depends on the metal M = Mg > Zn and is slower than that observed for the related beta-diketoiminate complexes where the bulky-(t)Bu group is replaced by Me.

Enolization regioselectivity involving stereoisomeric 4a-methyl-5-methoxyperhydrobenzo[7]annulen-2-ones

Efficient synthetic routes to the four isomers 17b-20b of the title ketone are described. Entry begins from the Wieland-Miescher homologue 3 whose pair of carbonyl groups are amenable to regiochemical manipulation. The compositions of the reaction mixtures generated under kinetic or thermodynamic control were defined by (1)H NMR analysis subsequent to chromatographic purification. The regiochemical trends are correlated with B3LYP/6-31G* calculations, the results of which conform to the preferred introduction of a 1,2- or 2,3-double bond.

Quadruply bonded dimetal units supported by 2,4,6-triisopropylbenzoates MM(TiPB)(4) (MM = Mo(2), MoW, and W(2)): preparation and photophysical properties

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

Effect of sulfonyl protecting groups on the neighboring group participation ability of sulfonamido nitrogen

The addition of elemental bromine dissolved in CH(2)Cl(2) to para-disubstituted benzodiazocines where X is the same (H, CH(3), Br, OMe, NO(2)) or a different substituent as X and Y (CH(3), Br; OMe, NO(2)) has been found to proceed in most cases with competition between two pathways. While conventional trans-1,2-addition operates predominantly, electron-releasing groups also foster a ring-contraction process with ultimate 1,3-positioning of the pair of bromine atoms. The observed regio- and stereoselectivities, confirmed where necessary by X-ray crystallographic analysis, establish the capability of sulfonamide nitrogen centers to engage in neighboring group participation.