Unsaturated Ring Systems Research Articles

6 P-Cymene Formation by Oxidation of Cyclohexadiene Monoterpenes – a Pitfall in Air Sampling of Terpenes

Abstract It has been known for a long time that certain terpenes are sensitive to oxidation. Especially in the presence of small amounts, as is the case in the environment or indoors, changes occur due to UV light or ozone exposition. Various oxidized products have also been detected when sampling low terpene concentrations using Tenax® tubes and subsequent thermal desorption. Here we show that carrying out common air sampling of monoterpenes in the lower ppm range on charcoal tubes, the atmospheric oxygen is sufficient to oxidize the most sensitive terpenes to a considerable extent, forming almost exclusively p-cymene. As expected, the extent of aromatization was strongest in the terpenes with a double unsaturated ring system such as α- or γ-terpinene or α-phellandrene while p-menthene or limonene were not affected. We studied the influence of sampling time, temperature, humidity and storage time of the coated tubes. Simultaneously conducted experiments with thermal desorption in the ppb range showed a significantly reduced p-cymene formation compared to the activated charcoal results. Our findings were confirmed by air samples of terpenes taken during the grinding of nutmegs. The concentrations of p-cymene determined were significantly higher than the terpene pattern of nutmeg oil would have suggested. Thus, the detection of p-cymene during air sampling may be an artifact caused by the presence of cyclohexadiene monoterpenes. This is particularly important because p-cymene is often used as an internal standard in terpene analysis thus preventing its artifact formation from being noticed.

Dihydropyrimidinones as potent anticancer agents: Insight into the structure–activity relationship

Cancer is a serious disease that has been around for a long time but currently has no sustainable solution. Several medications currently available offer an opportunity for the manifestation of cancer treatment; however, the "search for better" has led to the development and study of a variety of new scaffolds. Dihydropyrimidinones (DHPMs) are a privileged scaffold, prominent for their versatile range of biological activities. In recent years, the anticancer potential of these unsaturated pyrimidine ring systems has been traversed, along with their synthesis methods and the interlinked mechanisms leading to the anticancer activity. This review summarizes the structure-activity relationship of DHPMs as potential anticancer agents. This study is a short review of their synthesis, mechanism of action, and structure-activityrelationships (SARs) that are answerable for the anticancer activity of DHPMs and have been thoroughly researched and assessed.

Composite Correlated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions.

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, SiMe3 , and SO2 Me substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH2 with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline and their derivatives may be able to support ring opening metathesis polymerization and warrant further study.

PIPERIDINES DECORATED BY SMALL (CYCLO)ALKYL SUBSTITUENTS

The microreview describes recently developed methods for the synthesis of piperidines bearing small (cyclo)alkyl substituent via functionalization of preexisting unsaturated ring system.

Piperidines decorated by small (cyclo)alkyl substituents (microreview)

The microreview describes recently developed methods for the synthesis of piperidines bearing small (cyclo)alkyl substituent via functionalization of preexisting unsaturated ring system.

Synthesis, SAR and in vitro therapeutic potentials of thiazolidine-2,4-diones

BackgroundThiazolidinedione is a pentacyclic moiety having five membered unsaturated ring system composed with carbon, oxygen, nitrogen and sulfur molecules at 1 and 3 position of the thiazole ring and widely found throughout nature in various form. They favourably alter concentration of the hormones secreted by adipocytes, particularly adiponectin. They also increase total body fat and have mixed effects on circulating lipids. Thiazolidinedione nucleus is present in numerous biological moieties and has different pharmacological activities likes, e.g. antimalarial, antimicrobial, antimycobacterial, anticonvulsant, antiviral, anticancer, anti-inflammatory, antioxidant, anti-HIV (human immunodeficiency virus) and antituberculosis.Results and discussionThe synthesized compounds were screened for their in vitro antimicrobial potential against Gram (positive and negative) bacterial and fungal strains by tube dilution technique. In this series, compound 10 exhibited significant antimicrobial activity against B. subtilis and S. aureus with MIC = 4.2 × 10−2 µM/ml, compound 15 showed significant activity against K. pneumonia with MIC = 2.60 × 10−2 µM/ml and compound 4 displayed potent antibacterial activity against E. coli with MIC = 4.5 × 10−2 µM/ml. Compound 10 had most potent antifungal activity against C. albicans and A. niger with MIC = 4.2 × 10−2 µM/ml. Compounds 12 and 15 were found as most active antidiabetic agents having IC50 = 27.63 μg/ml and 22.35 μg/ml, respectively, using DPPH assay. Antioxidant activity results indicated that compounds 3 and 9 displayed good antioxidant agent with IC50 = 29.04 μg/ml and 27.66 μg/ml respectively, using α amylase assay.ConclusionAll the synthesized derivatives exhibited good antimicrobial, antidiabetic and antioxidant activities using specific methods then compared with mentioned standard drugs. Especially, compounds 3, 4, 9, 10, 12 and 15 displayed highest activity. Structure activity relationship demonstrated that presence of electron withdrawing group (o-NO2, p-Cl, p-Br) enhanced the antibacterial activity against E. coli as well as increased the antioxidant activity while the presence of electron releasing group (o/p-OCH3, 3,4,5-trimethoxy) enhanced the antibacterial activity against S. aureus, B. subtilis, S. typhi, K. pneumonia, C. albicans and A. niger as well as the antidiabetic activity.

Synthesis and Properties of Benzo[d]dithieno[b,f]borepins

Borepin is a seven-membered unsaturated ring system containing a tricoordinate boron. Although many borepin compounds annulated with aromatic systems have been reported to date, only one [b,d,f]-annulated tetracyclic borepins has been synthesized. In the present study, we synthesized benzo[d]dithieno[b,f]borepins as a new class of building blocks for functional materials motivated by the following structural features: (1) the high chemical stability of dithieno[b,f]borepin species; (2) the highly conjugated tetracyclic system with a borepin ring; and (3) electronic structures that are easily tuned by the introduction of functional groups on the annulated benzene ring. The prepared borepins were stable in air both in solution and as solids, and the electronic states could be finely tuned by changing the substituents on the benzene ring. Interestingly, DFT calculations revealed that the LUMO and LUMO+1 energy levels were strongly affected by the benzo[d]-annulation, depending on the boron-bridged positions ...

Single-Pot Asymmetric Approach toward Enantioenriched Quaternary Stereocenter-Containing Alkylidenecyclobutanes.

Enantioenriched alkylidenecyclobutanes possessing a quaternary stereogenic center, usually difficult to access, have been synthesized by combining a double boron-homologation and an allylboration through a highly efficient and diastereoselective one-pot process. Starting from commercially available substrates, this protocol represents a simple way of accessing chiral unsaturated four-membered ring systems with excellent stereoisomeric ratios.

Structure-based design and synthesis of tricyclic IAP (Inhibitors of Apoptosis Proteins) inhibitors

The design and synthesis of a series of novel tricyclic IAP inhibitors is reported. Rapid assembly of the core tricycle involved two key steps: Rh-catalyzed hydrogenation of an unsaturated bicyclic ring system and a Ru-catalyzed ring closing alkene metathesis reaction. The final Smac mimetics bind to cIAP1 and XIAP BIR3 domains and elicit the desired phenotype in cellular proliferation assays. Dimeric IAP inhibitors were found to possess nanomolar potency in a cellular proliferation assay and favourable in vitro drug-like properties.

Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to the preparation of new organic dyes. Although numerous strategies have been reported to access naphthalene scaffolds, many procedures still present limitations in terms of incorporating functionality, which in turn narrows the range of available substrates. The development of versatile methods for direct access to substituted naphthalenes is therefore highly desirable. The Diels-Alder (DA) cycloaddition reaction is a powerful and attractive method for the formation of saturated and unsaturated ring systems from readily available starting materials. A new microwave-assisted intramolecular dehydrogenative DA reaction of styrenyl derivatives described herein generates a variety of functionalized cyclopenta[b]naphthalenes that could not be prepared using existing synthetic methods. When compared to conventional heating, microwave irradiation accelerates reaction rates, enhances yields, and limits the formation of undesired byproducts. The utility of this protocol is further demonstrated by the conversion of a DA cycloadduct into a novel solvatochromic fluorescent dye via a Buchwald-Hartwig palladium-catalyzed cross-coupling reaction. Fluorescence spectroscopy, as an informative and sensitive analytical technique, plays a key role in research fields including environmental science, medicine, pharmacology, and cellular biology. Access to a variety of new organic fluorophores provided by the microwave-assisted dehydrogenative DA reaction allows for further advancement in these fields.

ChemInform Abstract: Copper‐Catalyzed Coupling Reaction of Unactivated Secondary Alkyl Iodides with Alkyl Grignard Reagents in the Presence of 1,3‐Butadiene as an Effective Additive.

AbstractDerivatives (VIIIa) and (VIIIb) can be easily converted into the corresponding unsaturated ring systems by metathesis.

Spectroscopic Study of some Schiff Bases Derived from Dibenzoylmethane

A series of Schiff bases 7-11 derived from dibenzoylmethane have been prepared. The UV, IR, 1H NMR and mass spectra revealed theses compounds were existed mainly as the keto-enamine tautomer in the solution. The absorption bands which appeared in the range λmax = 376-406 nm were assigned to the electronic transitions which arised from the central hydrogen bonded chelated unsaturated ring system in this tautomer. The appearance of the broad singlet near d = 13 ppm due to the N-H proton and a singlet near d = 6 ppm due to the –C=C-H proton inaddition to benzoyl fragment ion signal m/z =105 in the mass spectra supported the above suggested products.

Can π6 + π4 = 10? Exploring Cycloaddition Routes to Highly Unsaturated 10-Membered Rings

This paper uses DFT and G3(MP2) calculations to examine whether unbridged 10-membered rings can be made by (pi)6 + (pi)4 cycloadditions to (Z)- and (E)-hexatrienes, hexa-1,5-dien-3-ynes, (Z)-hexa-1,3-dien-5-ynes, hexa-1,2,3,5-tetraenes, and (Z)-hexa-3-ene-1,5-diynes. Cycloadditions to four 4pi reactants, buta-1,3-diene, butenyne, butatriene, and butadiyne, are explored. Thirty different basic cycloadditions are identified, and all are shown to be exothermic according to G3(MP2) calculations; strain energies in the products are comparable with that of cyclodecane itself, despite the presence of trans-alkene, alkyne, allene, cumulene, and s-trans diene moieties. The major obstacles to the isolation of 6 + 4 cycloaddition products are competing (pi)4 + (pi)2 cycloadditions and, especially, rapid Cope rearrangement of the products, but, in many cases, the judicious introduction of substituents can overcome these problems so that practical syntheses should be possible. Reactions between (E)-hexa-1,3,5-triene and s-trans-buta-1,3-diene are shown to have substantially lower activation energies than those involving (Z)-hexa-1,3,5-triene reacting with either s-cis- or s-trans-buta-1,3-diene. Conformationally locked derivatives of s-cis,s-cis (E)-hexa-1,3,5-trienes can lead to derivatives of (Z,Z,E)-cyclodeca-1,3,7-triene that are stable to Cope rearrangement, and reactions should proceed at close to ambient temperatures with suitable activating groups. We predict that it should be possible to prepare suitably substituted derivatives of at least 11 more highly unsaturated ring systems: (5Z,7Z)-cyclodeca-1,2,5,7-tetraene, (1Z,3Z)-cyclodeca-1,3-dien-7-yne, (2Z,7E)-cyclodeca-1,2,3,7-tetraene, (Z)-cyclodeca-1,2,3-trien-7-yne, (4Z,8E)-cyclodeca-1,2,4,8-tetraene, (Z)-cyclodeca-1,2,4,5,7-pentaene, (Z)-cyclodeca-1,2,4-trien-8-yne, (1Z,7E)-cyclodeca-1,7-dien-3-yne, (R,S,E)-cyclodeca-1,2,4,5,8-pentaene, cyclodeca-1,2,4,5,8,9-hexaene, and (R,S)-cyclodeca-1,2,4,5-tetraen-8-yne. In three other cases, we predict that cycloaddition will be followed by unusual and intriguing rearrangements. Cycloadditions can be accelerated by the presence of electron-withdrawing groups in either the 6pi or 4pi reactants. Transannular cyclizations of some products may lead to interesting stereocontrolled routes to 6,6- and/or 5,7-bicyclic structures.

Association of Intramolecular Furan Diels-Alder Reaction and N-Acylim­­i­n­ium Alkylation for the Synthesis of Pentacyclic Precursor of Aromathecins

A new approach for the synthesis of isoindoloquinoline and aromathecin templates is presented. These were obtained in a few steps starting from inexpensive reagents by two different strategies. The key step for both sequences was the IMFDA reaction, leading diastereoselectively to the formation of the unsaturated DE ring system of the expected alkaloid skeletons.

Identification of the Radical Anions of C2N4S2and P2N4S2Rings by In Situ EPR Spectroelectrochemistry and DFT Calculations

The previously unknown radical anions of unsaturated E2N4S2 ring systems (E=RC, R2NC, R2P) can be generated voltammetrically by the one-electron reduction of the neutral species and, despite half-lives on the order of a few seconds, have been unambiguously characterized by electron paramagnetic resonance (EPR) spectroelectrochemistry using a highly sensitive in situ electrolysis cell. Cyclic voltammetric studies using a glassy-carbon working electrode in CH3CN and CH2Cl2 with [nBu4N][PF6] as the supporting electrolyte gave reversible formal potentials for the [E2N4S2]0/- process in the range of -1.25 to -1.77 V and irreversible peak potentials for oxidation in the range of 0.66 to 1.60 V (vs the Fc+/0 couple; Fc=ferrocene). Reduction of the neutral compound undergoes an electrochemically reversible one-electron transfer, followed by the decay of the anion to an unknown species via a first-order (chemical) reaction pathway. The values of the first-order rate constant, kf, for the decay of all the radical anions in CH2Cl2 have been estimated from the decay of the EPR signals for (X-C6H4CN2S)2*-, where X=4-OCH3 (kf=0.04 s(-1)), 4-CH3 (kf=0.02 s(-1)), 4-H (kf=0.08 s(-1)), 4-Cl (kf=0.05 s(-1)), 4-CF3 (kf=0.05 s(-1)), or 3-CF3 (kf=0.07 s(-1)), and for [(CH3)3CCN2S]2*- (kf=0.02 s(-1)), [(CH3)2NCN2S]2*- (kf=0.05 s(-1)), and [(C6H5)2PN2S]2*- (kf=0.7 s(-1)). Values of kf for X=4-H and for [(CH3)2NCN2S]2*- were also determined from the cyclic voltammetric responses (in CH2Cl2) and were both found to be 0.05 s(-1). Possible pathways for the first-order anion decomposition that are consistent with the experimental observations are discussed. Density functional theory calculations at the UB3LYP/6-31G(d) level of theory predict the structures of the radical anions as either planar (D2h) or folded (C2v) species; the calculated hyperfine coupling constants are in excellent agreement with experimental results. Linear correlations were observed between the voltammetrically determined potentials and both the orbital energies and Hammett coefficients for the neutral aryl-substituted rings.

Cascade cyclization: an easy access to highly unsaturated polycyclic ring systems through a tandem stille/[4 + 2] reaction under mild conditions.

The synthesis of several polycyclic compounds 1a-c, 2, and 3 has been performed through a tandem Stille/[4 + 2] cascade reaction from cyclic bis(enoltrifluomethanesulfonate) 4a-c, 5, and 6, respectively. The reaction proceeds very efficiently in a one-pot operation at roomtemperature in DMF in the presence of a catalytic amount of Pd(CH(3)CN)(2)Cl(2) and LiCl.[reaction: see text]

A High-Affinity Fluorenone-Based β2-Adrenergic Receptor Antagonist with a Photoactivatable Pharmacophore

To develop molecules capable of directly probing the catechol binding region of the beta(2)-adrenergic receptor (beta(2)AR), novel benzophenone- and fluorenone-based beta(2)AR antagonists were prepared as potential photoaffinity probes. While the benzophenone-containing ligands bound with relatively modest affinity, one of the fluorenone-based compounds, 4-(2-hydroxy-3-isopropylaminopropoxy)-7-amino-6-iodofluorenone+ ++ (iodoaminoflisopolol, IAmF), showed very high affinity for the beta(2)AR, inhibiting [(125)I]ICYP binding with an apparent K(i) of approximately 1 x 10(-)(9) M. In comparison to the benzophenone ligands, the fluorenone ligands have one additional carbon-carbon bond that creates a planar unsaturated ring system and leads to a large increase in receptor binding affinity. Unlike previous beta(2)AR photoaffinity ligands, an attractive and unique feature of the fluorenone derivative IAmF is that the large planar unsaturated ring (believed to correspond to the catechol end of other beta(2)AR ligands) serves as both the binding pharmacophore and the photoreaction center for this molecule. With this potential for directly probing the catechol binding region of the beta(2)AR, we synthesized and tested IAmF in carrier-free radioiodinated form ([(125)I]IAmF). When photoreduction was conducted at 350 nm for 20 min, [(125)I]IAmF was able to produce cross-linked products in both triethylamine and methanol, with a reactivity pattern similar to that found in benzophenone photochemistry. As a final test of suitability as a photoaffinity label, specific labeling of the beta(2)AR in membranes (protectable by 10 microM alprenolol) was demonstrated. [(125)I]IAmF represents a new class of beta(2)AR photoaffinity labels that can directly probe the catechol-analogous antagonist pharmacophore binding site in the beta(2)AR ligand binding pocket.

Synthesis and molecular structure of hexaruthenium clusters containing unsaturated ring systems derived from 1-ethynylcyclopentanol, 1-ethynylcycloheptanol and 1-ethynylcyclooctanol

Six hexaruthenium compounds which are identified as [Ru6C(CO)13(μ-CO){μ3-η1,η2,η3-C5H7CCC(H)C5H8}] 1 (12%), [Ru6(CO)12(μ-CO)2{μ4-η1,η1,η2,η4-CC(H)C5H6}2] 2 (8%), [Ru6C(CO)14(μ-CO){μ2-η1,η3-C(H)C7H11}] 3 (8%), [Ru6(CO)14{μ4-η1,η1η2,η4-CC(H)C7H10}2] 4 (10%), [Ru6C(CO)13(μ-CO){μ3-η1,η2,η3-C8H12C(H)C(H)C(H)C(OH)C8H14}] 5 (16%), and [Ru6(CO)14{μ4-η1,η1η2,η4-CC(H)C8H12}2] 6 (8%) have been synthesised by reaction of HCC(C5H8OH), HCC(C7H12OH) and HCC(C8H14OH), respectively, with triruthenium dodecacarbonyl in cyclohexane under refluxing conditions. All these compounds have been fully characterised by spectroscopic and X-ray diffraction methods. The structures of 1, 3 and 5 are based on a Ru6 octahedral skeleton containing a μ6-carbide. Both clusters 1 and 5 involve the coupling of two functionalized alkyne molecules to give an unusual hexatetraene chain with the elimination of water molecules. Complex 3 consists of one alkyne moiety bonded to a Ru3 triangular face via one σ and a π-allyl bond. An interesting feature in clusters 2, 4 and 6 is the formation of a metallacyclic five-membered ring with a μ4-η1,η1,η2,η4 mode, which is derived from the CC triple bond and CH bond activations. Reaction of [Ru3(CO)10(NCMe)2] with 1-ethynylcyclopentanol affords another new cluster, [Ru3(CO)9(μ-CO)(μ3-η1,η1,η2-HCCC5H8OH)] 7, which consists of an alkyne ligand bound to the triruthenium cluster unit via a typical μ3-(η2-||) coordination mode.

Mechanism of Formation of Novel Covalent Drug·DNA Interstrand Cross-Links and Monoadducts by Enediyne Antitumor Antibiotics

The potent enediyne antitumor antibiotic C1027 has been previously reported to induce novel DNA interstrand cross-links and drug monoadducts under anaerobic conditions [Xu et al. (1997) J. Am. Chem. Soc. 119, 1133-1134]. In the present study, we explored the mechanism of formation of these anaerobic DNA lesions. We found that, similar to the aerobic reaction, the diradical species of the activated drug initiates anaerobic DNA damage by abstracting hydrogen atoms from the C4', C1', and C5' positions of the A1, A2, and A3 nucleotides, respectively, in the most preferred 5'GTTA1T/5'ATA2A3C binding sequence. It is proposed that the newly generated deoxyribosyl radicals, which cannot undergo oxidation, likely add back onto the nearby unsaturated ring system of the postactivated enediyne core, inducing the formation of interstrand cross-links, connecting either A1 to A2 or A1 to A3, or drug monoadducts mainly on A2 or A3. Comparative studies with other enediynes, such as neocarzinostatin and calicheamicin gamma1I under similar reaction conditions indicate that the anaerobic reaction process is a kinetically competitive one, depending on the proximity of the drug unsaturated ring system or dioxygen to the sugar radicals and their quenching by other hydrogen sources such as solvent or thiols. It was found that C1027 mainly generates interstrand cross-links, whereas most of the anaerobic lesions produced by neocarzinostatin are drug monoadducts. Calicheamicin gamma1I was found to be less efficient in producing both lesions. The anaerobic DNA lesions induced by enediyne antitumor antibiotics may have important implications for their potent cytotoxicity in the central regions of large tumors, where relative anaerobic conditions prevail.

New rearranged products from the methylation of 13-oxobaccatin III

Treatment of 7-triethylsilyl-13-oxobaccatin III ( 1) with sodium hydride and methyl iodide gave the methylated ten-membered ring compound 2 that rearranged, via intramolecular aldol condensation, to unsaturated decalin ring system ( 3, 4, 5). These molecules were characterized by 1D and 2D NMR tecniques.