C2 Bond Research Articles

Enzymatic and Chemical Cross-Linking of Bacterial Cellulose/Fish Collagen Composites—A Comparative Study

This article compares the properties of bacterial cellulose/fish collagen composites (BC/Col) after enzymatic and chemical cross-linking. In our methodology, two transglutaminases are used for enzymatic cross-linking—one recommended for the meat and the other proposed for the fish industry—and pre-oxidated BC (oxBC) is used for chemical cross-linking. The structure of the obtained composites is characterized by scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and Fourier transform infrared spectroscopy, and their functional properties by mechanical and water barrier tests. While polymer chains in uncross-linked BC/Col are intertwined by H-bonds, new covalent bonds in enzymatically cross-linked ones are formed—resulting in increased thermal stability and crystallinity of the material. The C2–C3 bonds cleavage in D-glucose units, due to BC oxidation, cause secondary alcohol groups to vanish in favor of the carbonyl groups’ formation, thus reducing the number of H-bonded OHs. Thermal stability and crystallinity of oxBC/Col remain lower than those of BC/Col. The BC/Col formation did not affect tensile strength and water vapor permeability of BC, but enzymatic cross-linking with TGGS improved them significantly.

X-ray charge density study of the 6-methyluracil derivative in the crystal: Revealing, consequences, and multipole refinement of minor static disorder

The accurate X-ray diffraction experiment was performed on the crystal of 1,6-dimethyl-3-propargyl-2,4-pyrimidinedione at 100 K. The static electron density (ED) was obtained via multipole refinement against these data. The presence of low-intense but nonrandom positive residual density peaks revealed that the structure was complicated by the minor conformational static disorder. Further analysis of the number and positions of critical points (CPs) (3, +3) of Laplacian of ED ∇2ρ(r) and the ratio of eigenvalues of its Hessian matrix unambiguously proved that the multipole-modeled ED of the main disorder component was contaminated by the second one. A method of visualizing the Laplacian minima as CP-ellipsoids with semiaxes inversely proportional to the eigenvalues of Hessian matrix of ∇2ρ(r) was applied to evaluate the correctness of modeled ED. Extra Laplacian CPs (3, +3) were attributed to the incorrect multipole populations, while the elongated shape of the corresponding discoid CP-ellipsoids was mainly considered to be the effect of troublesome kappa-parameters. A number of different multipole models were constructed in order to minimize the undesirable effects of disorder on the multipole ED. The two-component multipole model showed that the heterocyclic moiety of the minor component with an occupancy of 2% underwent the 180° rotation about the N3–C7 bond axis. Furthermore, the partial transfer of multipole parameters from the model refined against theoretical structure factors was utilized on the most problematic atoms, which massively contributed to disorder treatment. This approach allowed us to restore the ED of the main component in the crystal environment, which quality was gauged by the quantum-topological analysis as well as by its conformity to the data obtained from ab initio periodic calculations. Attempts to speculate on reasons for the limited disordering observed were made: theoretical calculations suggest that moderate hydrogen bonds support the disordering in the structure, while multiple weak noncovalent contacts, e.g., pi interactions, may provide the restrictions on the 50:50 disorder.

Selectivity control in inverse electron demand Diels–Alder reaction of o-Quinone methides catalyzed by chiral N,N′-Dioxide–Sc(III) complex

The reaction mechanism and origin of asymmetric induction in inverse electron demand Diels-Alder (IEDDA) reaction of ortho-quinone methide (o-QM) and fulvene mediated by chiral N,N'-dioxide–Sc(III) catalyst were rationalized using B3LYP-D3(BJ) functional with def2-TZVP basis set. The uncatalyzed IEDDA reaction was concerted but highly asynchronous with activation barriers of 29.8∼31.8 kcal mol−1. Good linear relationship between the Hammett substituent constant (σP) of o-QM and the activation barrier (ΔG≠) of DA reaction was discovered. The secondary orbital interaction (SOI) between the conjugated diene of o-QM and fulvene moiety stabilized the endo-transition state, contributing to high endo-selectivity. The catalytic asymmetric IEDDA reaction occurred via a stepwise mechanism, including the construction of Cβ−C4 bond, followed by the formation of Cα−O1 bond. The bulky substituents (i.e., adamantyl or triphenylmethyl) in amide moiety of ligand furnished sufficient steric shielding for re-face of diene, inducing the attack of fulvene from si-face in endo-pathway. The substituent at exocyclic methylene of the unsymmetrical fulvene was crucial for the adjustment of E/Z selectivity. The steric repulsion between cyclohexyl group in fulvene and aromatic ring in o-QM raised the destabilizing strain energy (ΔEstrain) at the transition state in Z-configuration, contributing to the predominant E-product.

Divergent Syntheses of Indoles and Quinolines Involving N1-C2-C3 Bond Formation through Two Distinct Pd Catalyses.

Pd-catalyzed annulative couplings of 2-alkenylanilines with aldehydes using alcohols as both the solvent and hydrogen source have been developed. These domino processes allow divergent syntheses of two significant N-heterocycles, indoles and quinolines, from the same substrate by tuning reaction parameters, which seems to invoke two distinct mechanisms. The nature of the ligand and alcoholic solvent had a profound influence on the selectivity and efficiency of these protocols. Particularly noteworthy is that indole formation was achieved by overcoming two significant challenges, regioselective hydropalladation of alkenes and subsequent reactions between the resulting Csp3-Pd species and less reactive imines.

Fluorine Effects for Tunable C–C and C–S Bond Cleavage in Fluoro-Julia–Kocienski Intermediates

Classical Julia–Kocienski fluoroolefination represents an indispensable platform for the construction of monofluoroalkenes. Nevertheless, its complex multistep mechanistic manifold along with the u...

Carbon content control of silicon oxycarbide film with methane containing plasma

Deposition of silicon oxycarbide (SiCOH) thin films by remote plasma atomic layer deposition was performed. In the experiment, the recipe was composed by adjusting the ratio of Ar and CH4 plasmas to control the carbon content in the SiCOH thin film. Octamethyl cyclotetrasiloxane was used as a precursor during the deposition process at 200, 300, and 400 °C. Ar plasma was used as an activant and CH4 plasma was used as a reactant. Plasma and deposition temperatures cause a significant impact on the physical and electrical properties of the film. When CH4 plasma was used during the deposition process, the film contained carbon and exhibited a low dielectric constant. In addition, when CH4 plasma is used as a reactant, Si–C bonds in the thin film form pores and lower ionic polarization to lower the dielectric constant. Fourier-transform infrared spectroscopy data indicate that the higher the ratio of CH4 plasma, the more the cage structure in the thin film. The cage structure contributes to lowering the dielectric constant of the thin film. The film deposited with Ar plasma has the dielectric constant of 3.2 and the film deposited with CH4 plasma has the dielectric constant of 2.6. In both plasma conditions, the dielectric constant was lower than the SiO2 film with the dielectric constant of 3.9. On the other hand, x-ray photoelectron spectroscopy analysis showed that SiO1–C3 and SiC4 bonds appeared in the film deposited with CH4 plasma, which did not appear in the film deposited with Ar plasma. These bonds affected the physical and electrical properties of the thin film.

Bioinspired Divergent Oxidative Cyclizations of Geissoschizine: Total Synthesis of (–)‐17‐nor‐Excelsinidine, (+)‐16‐epi‐Pleiocarpamine, (+)‐16‐Hydroxymethyl‐Pleiocarpamine and (+)‐Taberdivarine H

We report a full account of our efforts towards bioinspired oxidative cyclizations of geissochizine and analogs to mimic the biosynthesis of the mavacuran, akuammilan, and excelsinidine groups of monoterpene indole alkaloids. The construction of the A,B,C,D ring system of geissoschizine was first achieved by merging two known syntheses of this alkaloid. Modified Ma's oxidative conditions (KHMDS/I2) applied directly to geissoschizine induced formation of the N4–C16 bond encountered in the excelsinidines core. Identical conditions applied to C16‐dimethylmalonate‐containing N4‐quaternized substrates ended in the formation of the mavacurans core (N1–C16 bond). With this unified oxidative cyclization strategy: (–)‐17‐nor‐excelsinidine, (+)‐16‐epi‐pleiocarpamine, (+)‐16‐hydroxymethyl‐pleiocarpamine, 16‐formyl‐pleiocarpamine and (+)‐taberdivarine H were synthetized. We also report a shortened total synthesis of 16‐epi‐pleiocarpamine compared to our preliminary communication from a C16‐monoester analog. Alternatively, 17‐nor‐excelsinidine was synthesized via an intramolecular nucleophilic substitution of a 7‐membered ring α‐chlorolactam prepared from 16‐desformyl‐geissoschizine.

An Ab Initio Computational Study of Electronic and Structural Factors in the Isomerization of Donor-Acceptor Stenhouse Adducts.

In this work, the photochemically and thermally induced isomerization of multiple donor-acceptor Stenhouse adducts (DASAs) of the first, second, and third generation is studied by means of state-of-the-art ab initio electronic structure methods leading to new insight into multiple facets of the reaction mechanism. Importantly, prior to any studies of the reaction mechanism, a set of test calculations demonstrate the suitability of the applied ADC(2) and CC2 methods in the present context. An important aspect in this regard is the availability of electronic energies and gradients under implicit consideration of solvent effects. On the basis of calculated reaction energies and barriers as well as a thorough analysis of the wave function compositions, interesting features of the reaction mechanism are deduced. For example, the closed form of second- and third-generation DASAs can be significantly stabilized by π - π interactions between the donor and acceptor termini when certain structural requirements are fulfilled. The central point of this work concerns the delicate balance between neutral and zwitterionic resonance structures that governs the relative barrier height for the crucial C2-C3 and C3-C4 bond rotations. Finally, a set of calculations on yet unreported derivatives highlights how this balance and hence the barrier heights can be tuned through variation of the donor-acceptor strength as well as the solvent polarity.

Molecular mechanism of Hetero Diels-Alder reactions between (E)-1,1,1-trifluoro-3-nitrobut-2-enes and enamine systems in the light of Molecular Electron Density Theory

The molecular mechanism of the reaction of (E)-1,1,1-trifluoro-3-nitrobut-2-ene 1 with 3,3-dimethyl-2-morpholinobutene 2 has been studied within the Molecular Electron Density Theory (MEDT). This theoretical study confirm the possibility of the formation of zwitterionic structures in the first reaction stage. Interestingly, that localized zwitterions are however not common intermediates for identified in the postreaction mixture products. The further Bonding Evolution Theory (BET) study show, that the key, HDA reaction takes place in one-step but in a non-concerted manner since three stages are clearly identified. First the C5–C6 double bond breaks, then the C1–C2 bond breaks, and in the last phase we observed the formation of the C1–C6 single bond and V(C2) and V’ (C2) pseudoradical centers in Z1 molecule. In turn, the molecular mechanism of the conversion of zwitterion Z1 to product 3 can be divided also in three groups in which we observed the disappearance of the two pseudoradical centers and formation O4–C5 single and C3–N3 double bonds, respectively.

From Metallacycle-Mediated Annulative Cross-Coupling to Steroidal Tetracycles through Intramolecular C9-C10 Bond Formation.

While semisynthesis is a common platform for medicinal investigation of steroidal systems, varying the nature of substitution and stereochemistry at C9 and C10 remains challenging. It is demonstrated here that de novo synthesis, enabled by a metallacycle-centered annulation reaction, provides a uniquely effective means of addressing this problem. In short, double asymmetric Friedel-Crafts cyclization proved most effective for establishing anti- relative stereochemistry (with respect to C13), while an intramolecular Heck reaction reliably delivered the syn- diastereomers with high selectivity. In addition, these studies reveal that this oxidative rearrangement is effective for establishing a C10 quaternary center boasting variable alkyl or aryl substitution.

Mechanism Underlying Anti-Markovnikov Addition in the Reaction of Pentalenene Synthase

Most terpene synthase reactions follow Markovnikov rules for formation of high-energy carbenium ion intermediates. However, there are notable exceptions. For example, pentalenene synthase (PS) undergoes an initial anti-Markovnikov cyclization reaction followed by a 1,2-hydride shift to form an intermediate humulyl cation with positive charge on the secondary carbon C9 atom of the farnesyl diphosphate substrate. The mechanism by which these enzymes stabilize and guide the regioselectivity of secondary carbocations has not heretofore been elucidated. In an effort to better understand these reactions, we grew crystals of apo-PS, soaked them with the nonreactive substrate analogue 12,13-difluorofarnesyl diphosphate, and determined the X-ray structure of the resulting complex at 2.2 Å resolution. The most striking feature of the active site structure is that C9 is perfectly positioned to make a C-H···π interaction with the side chain benzene ring of residue F76; this would enhance hyperconjugation to stabilize a developing cation at C10 and thus support the anti-Markovnikov regioselectivity of the cyclization. The benzene ring is also positioned to catalyze the migration of H to C10 and stabilize a C9 carbocation. On the opposite face of C9, further cation stabilization is possible via interactions with the main chain carbonyl of I177 and the neighboring intramolecular C6═C7 bond. Mutagenesis experiments also support a role for residue 76 in these interactions, but most interesting is the F76W mutant, whose crystal structure clearly shows C9 and C10 centered above the fused benzene and pyrrole rings of the indole side chain, respectively, such that a carbocation at either position could be stabilized in this complex, and two anti-Markovnikov products, pentalenene and humulene, are formed. Finally, we show that there is a rough correlation (although not absolute) of an aromatic side chain (F or Y) at position 76 in related terpene synthases from Streptomyces that catalyze similar anti-Markovnikov addition reactions.

Investigation on electronic and mechanical properties of penta-graphene nanotubes

Penta-graphene nanotubes (PGNTs), a type of novel one-dimensional materials, express excellent electronic, thermal and mechanical properties with great potential for inspiring unconventional optoelectronic devices design. In this work, electronic properties and mechanical behaviors of PGNTs under axial tension and compression loadings were investigated using density functional theory and molecular dynamics method. Several models of PGNTs were constructed to highlight important insights about band structure, charge population, atom orbital and fracture behavior. It is found that the band gap decreases relatively as the size increases. The band gap in (9,9) PGNT decreases with the compressive strain, while it increases with the tensile strain. Charge transformation can be found with the strain variation and observed using the HOMO–LUMO orbitals. The mechanical parameters are fluctuated with the PGNTs diameter. During the fracture process of (9,9) PGNT, the C1–C1 and C1–C2 bonds act as the main stress-bearing bonds. With the continuous tensile strain increase, an eight-member ring forms at the critical state and acts as a defect. The breaking of bonds occurs then around this newly born defect, which leads to the final fracture of the tube. The results of this investigation bring a better understanding of the PGNTs intrinsic properties.

Anti-inflammatory and analgesic activities of Neolamarckia cadamba and its bioactive monoterpenoid indole alkaloids

Ethnopharmacological relevanceNeolamarckia cadamba has been used traditionally to treat inflammation, fever, and pruritus in the Dai ethnopharmacy in Yunnan province, P.R. China. However, according to literature survey, the action basis of anti-inflammatory and analgesic activities of this plant were rarely reported, which accounts for the original intentions of this investigation. Aim of the studyThe study aimed to investigate the anti-inflammatory and analgesic action of methanolic extract (ME), ethyl acetate (EA), and aqueous (AQS) fractions of N. cadamba and further explore the accurate compounds responsible for the activities of EA fraction. Materials and methodsThe in vivo anti-inflammatory and analgesic activities of ME, EA, and AQS fractions at the doses of 200 and 400 mg/kg and two major constituents (compounds 5 and 7) at 50 and 100 mg/kg via intragastrically administrated, respectively, were evaluated by carrageenan-induced paw edema and acetic acid-stimulated writhing animal models. Aspirin (ASP) was used as the positive control at the dose of 200 mg/kg. The monoterpenoid indole alkaloids (MIAs) in EA fraction were phytochemically studied utilizing chromatographic techniques, and their structures and absolute configurations were established on the basis of multiple spectroscopic analyses and quantum computational chemistry method. Moreover, the in vitro anti-inflammatory activities of all the isolates were assessed by suppressing releases of LPS-activated inflammatory mediators (TNF-α, IL-1β, and COX-2) in RAW 264.7 macrophage cells at a concentration of 10 μg/mL. Dexamethasone (DXM) was used as the positive control. ResultsThree fractions (ME, EA, and AQS) significantly ameliorated the paw edema caused by carrageenan and reduced the number of writhing induced by acetic acid in comparison to the control group at the doses of 200 and/or 400 mg/kg (in vivo). Subsequent phytochemical investigation of EA fraction led to the structural characterization of four new monoterpenoid indole alkaloids, neocadambines A–D (1–4), as well as eight known analogues (5–12). Neocadambine A possesses a novel 14-nor-MIA skeleton that could be derived from the corynantheine-type MIAs via oxidative cleavage of C3–C14 bond and subsequently degradation of C14. Moreover, the structure of a bioactive known MIA, cadambine acid (6), was reassigned by analysis of its NMR spectroscopic data. Further biological assays revealed that the major constituent 3β-dihydrocadambine (7) significantly relieved the paw edema and decreased the number of writhing at 100 mg/kg in vivo. In addition, most of the isolates displayed remarkable in vitro anti-inflammatory effects by inhibiting the secretion of aforementioned inflammatory mediators (COX-2, IL-1β, and TNF-α) at a concentration of 10 μg/mL, and compounds 4, 7, and 9 showed better anti-inflammatory effects than that of positive control, dexamethasone. ConclusionsThis study further validated the anti-inflammatory and analgesic activities of N. cadamba, and revealed that monoterpenoid indole alkaloids could partly contribute to the efficacy of this ethnodrug. The major constituent 3β-dihydrocadambine (7) showed significant anti-inflammatory activities both in vitro and in vivo, which suggested that it could be a promising anti-inflammatory lead compound. Our findings provided scientific justification to support the traditional application of N. cadamba for treating inflammatory and nociceptive disorders.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans.

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C–C and/or C–O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a–O bond formation: (route a). (2.2) O–C2 bond formation: (route b). (2.3) C2–C3 bond formation: (route c). (2.4) C3–C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3–C3a bond formation (route a + d). (3.2) O–C2 and C2–C3 bond formation: (route b + c). (3.3) O–C2 and C3–C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Synthesis and Transformations of the Adduct of 1-Phenyltricyclo[4.1.0.02,7]heptane with 2-Bromoethanesulfonyl Bromide

The addition of 2-bromoethanesulfonyl bromide to the central C1–C7 bond of 1-phenyltricyclo[4.1.0.02,7]heptane proceeds in a highly anti-stereoselective fashion to form 1 : 1 adducts with the norpinic structure. The action of some bases and nucleophiles on the anti-addition product was studied. It was found that, depending on the reaction conditions, the adduct undergoes solvolysis, 1,2-dehydrobromination, and further addition of nucleophile molecule to the double bond of the vinylsulfonyl moiety.

On Radical Reactions of 1-Bromotricyclo[4.1.0.02,7]heptane with Phenylethynyl Sulfones

Phenylethynyl sulfones of the general formula RSO2C≡CPh (R = p-Tol, Ph, CH3) undergo a photochemically induced anti-stereoselective addition to the central C1–C7 bond of 1-bromotricyclo[4.1.0.02,7]heptane to form monoadducts of the bicyclo[3.1.1]heptane (norpinic) series containing a syn-sulfonyl group in position 7 and a phenylethynyl moiety on C6. Treatment of the synthesized adducts with potassium tert-butylate in THF leads to 1,3-dehydrobromination to give sulfonyl-substituted 1-(phenylethynyl)tricyclo[4.1.0.02,7]heptanes.

Reaction of Tricyclo[4.1.0.02,7]heptane with 1-(Arenesulfonyl)-2-phenyldiazenes

Tricyclo[4.1.0.02,7]heptane reacted with 1-(arenesulfonyl)-2-phenyldiazenes by radical mechanism to give bicyclo[3.1.1]heptane derivatives. Unlike analogous reactions with alkenes, the addition of diazenes occurs readily without a catalyst and yields mainly arylazosulfonation products at the C1–C7 bond of tricyclo-[4.1.0.02,7]heptane. The addition products are capable of undergoing thermal prototropic isomerization to 7-endo-(arenesulfonyl)bicyclo[3.1.1]heptan-6-one phenylhydrazones.

Recent Advances in Synthetic Strategies to 2,3-Dihydrobenzofurans

This review gives an overview on recent developments in methods for the construction of compounds with the 2,3-dihydrobenzo­furan core in the period 2012 to 2019. Interest in 2,3-dihydrobenzofurans is constantly increasing. The methods are divided into intermolecular and intramolecular approaches. Intermolecular approaches are subdivided according to the parent intermediate for the key reaction, while intermolecular approaches are subdivided according by which bond is formed in the key reaction. The transformation of benzofurans to dihydrobenzofurans and other miscellaneous methods are also discussed. Approaches useful for the synthesis of natural products are emphasized.1 Introduction2 Intermolecular Approaches2.1 o-Quinone Methides and o-Quinones2.2 p-Quinone Methides and p-Quinones2.3 Nitrogen-Containing Phenols and Quinones2.4 o-Hydroxyphenylcarbonyl Derivatives and Phenols2.5 Miscellaneous3 Intramolecular Approaches3.1 O–C2 Bond Forming3.2 C2–C3 Bond Forming3.3 C3–Aryl Bond Forming3.4 O–Aryl Bond Forming4 From BF to DHB5 Rearrangements and Aromatizations

An Insight into the Reactivity of the Electrogenerated Radical Cation of Caffeine

Controlled potential electrolyses of caffeine (CAF) were carried out at a Pt electrode in undried acetonitrile (ACN) and ACN-H2O and the products of the anodic oxidation were analyzed by HPLC-PDA-ESI-MS/MS. A higher current efficiency occurred in ACN-H2O, but an analogous chromatographic outline was found in both media, evidencing a reactive pathway of the electrogenerated radical cation CAF•+ with water, added or in trace, as nucleophile. No dimeric forms were evidenced, excluding any coupling reactions. Neither was 1,3,7-trimethyluric acid found, reported in the literature as the main oxidative route for CAF in water. Four main chromatographic peaks were evidenced, assigned to four proposed structures on the base of chromatographic and spectral data: a 4,5-diol derivative and an oxazolidin-2-one derivative were assigned as principal oxidation products, supporting a mechanism proposed in a previous work for the primary anodic oxidation of the methylxanthines olefinic C4 = C5 bond. Two highly polar degradation products were also tentatively assigned, that seemed generating along two different pathways, one opening the imidazolic moiety and another one opening the purinic one.

Photosensitized Production of Singlet Oxygen via C60 Fullerene Covalently Attached to Functionalized Silica-coated Stainless-Steel Mesh: Remote Bacterial and Viral Inactivation

This study demonstrates the capability of a C60-based sensitizer to remotely inactivate bacteria and viruses in air. The nucleophilic addition of amine groups attached to SiO2 particles (used as free-standing support or as hosts electrosprayed on a stainless-steel mesh) to the double bonds of C60 led to the production of recyclable sensitizers. The high-yield production of singlet oxygen (1O2) from the immobilized C60 was established by studying the effects of 1O2 quenchers and enhancer on furfuryl alcohol (FFA) oxidation efficiency and wavelength-dependent FFA decay kinetics. We monitored the inactivation rates of Staphylococcus aureus and MS-2 bacteriophage in petri dishes at predetermined distances from the 1O2 source and identified the probability of remote microbial inactivation via 1O2, which occurred up to 10-15 cm from the source. This result accorded with epoxy moiety occurrence as an indication of singlet oxygenation in the Fourier Transform-Infrared spectra of cis-polybutadiene in remote positions.