Structure and further fragmentation of significant [a3 + Na − H]+ ions from sodium‐cationized peptides

A good understanding of gas‐phase fragmentation chemistry of peptides is important for accurate protein identification. Additional product ions obtained by sodiated peptides can provide useful sequence information supplementary to protonated peptides and improve protein identification. In this work, we first demonstrate that the sodiated a₃ ions are abundant in the tandem mass spectra of sodium‐cationized peptides although observations of a₃ ions have rarely been reported in protonated peptides. Quantum chemical calculations combined with tandem mass spectrometry are used to investigate this phenomenon by using a model tetrapeptide GGAG. Our results reveal that the most stable [a₃ + Na ? H]⁺ ion is present as a bidentate linear structure in which the sodium cation coordinates to the two backbone carbonyl oxygen atoms. Due to structural inflexibility, further fragmentation of the [a₃ + Na ? H]⁺ ion needs to overcome several relatively high energetic barriers to form [b₂ + Na ? H]⁺ ion with a diketopiperazine structure. As a result, low abundance of [b₂ + Na ? H]⁺ ion is detected at relatively high collision energy. In addition, our computational data also indicate that the common oxazolone pathway to generate [b₂ + Na ? H]⁺ from the [a₃ + Na ? H]⁺ ion is unlikely. The present work provides a mechanistic insight into how a sodium ion affects the fragmentation behaviors of peptides. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical studies of spin state‐specific [2 + 2] and [5 + 2] photocycloaddition reactions of n‐(1‐penten‐5‐yl)maleimide

N‐alkenyl maleimides are found to exhibit spin state‐specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high‐level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N‐alkenyl maleimide in the S₁ and T₁ states as well as relevant photophysical processes. It is found that in the S₁ state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T₁ state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo‐addition reaction. Mechanistically, both S₁ [5 + 2] and T₁ [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S₁ [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S₁ intermediate, which then decays to the S₀ state as a result of an S₁ → S₀ internal conversion. In the T₁ [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T₁ → S₀ intersystem crossing process to the S₀ state. In the S₀ state, the second C C bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.     

Catalytic [2 + 2 + 2] cycloaddition polymerization of diyne–nitrile monomers in the presence of CoCl2‐6H2O/diphosphine/Zn

Cobalt‐catalyzed [2 + 2 + 2] cocycloaddition reaction of 1,6‐diynes and nitriles to generate substituted pyridines has been applied to the polymerization of diyne–nitrile monomers, the reaction of which proceeded smoothly in a step‐growth fashion to provide linear polymers comprising pyridine structures in the main chain. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 345–351     

The [4 + 2] Cycloaddition of 2‐Pyrone in Total Synthesis

Diels‐Alder reaction is one of the most important reactions in organic synthesis and has witnessed great achievements in total syntheses of complex natural products. In this review, we will focus on the [4 + 2] cycloaddition that utilizes 2‐pyrone as the diene component. Major advances of its applications in the total syntheses of natural products in the past fifty years will be discussed here.     

Facile Synthesis of Spirooxindole‐Cyclohexenes via Phosphine‐Catalyzed [4 + 2] Annulation of α‐Substituted Allenoates

A phosphine‐catalyzed [4 + 2] annulation of α ‐substituted allenoate with exocyclic alkene moiety of oxindoles or indan‐1,3‐diones has been developed. Thus, under the catalysis of PPh₃ (20 mol%), a series of spirooxindole‐ or spiroindan‐1,3‐dione‐cyclohexenes have been obtained in moderate to excellent yields and regioselectivity from the annulations of α ‐methyl allenoates with 3‐methyleneoxindoles or 2‐methyleneindan‐1,3‐diones. This method offers an easy access to structurally novel spirocyclohexenes.     

Theoretical study in [C2H4–Tl]+ and [C2H2–Tl]+ complexes

We studied the attraction between [C₂H_n] and Tl(I) in the hypothetical [C₂H_n–Tl]⁺ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C–Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree–Fock (HF), second‐order Møller–Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance R_e (C–Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C₂H_n]–Tl⁺ interaction may be related mainly to charge‐induced dipole and dispersion terms, both involving the individual properties of the olefinic π‐system and thallium ion. However, the charge‐induced dipole term (R^?4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Glycopentosides D – F,Three New Phenolic Glycosides from Glycosmis pentaphylla

A phytochemical investigation of the BuOH‐soluble fraction of the EtOH extract from the stems of Glycosmis pentaphylla resulted in the isolation of three new phenolic glycosides, glycopentosides D–F ( 1 – 3 , resp.). Their structures were determined by using spectroscopic analysis including UV, ¹H‐ and ¹³C‐NMR, DEPT, COSY, ROESY, HMBC, HSQC, HR‐ESI‐MS, and acid hydrolysis.     

Geometric and electronic structures of silicon fluorides (N = 4 – 6) and potential energy surfaces for dissociation reactions → SiF4 + F– and → + F–

The geometric and electronic structures of a series of silicon fluorides (n = 4 ? 6) were computationally studied with the aid of density functional theory (DFT) method with B3LYP and M06‐2X functionals and coupled cluster (CCSD and CCSD(T)) methods with 6‐311++G(d,p) basis set. The nature of the Si‐F bonds in these compounds was analyzed in the framework of the natural bond orbital theory and natural resonance theory. Energy characteristics (heats of reactions and energy barriers) of the dissociation reactions → SiF₄ + F^– and → + F^– were calculated using the DFT and CCSD methods. The potential energy surface of elimination of a fluoride anion from has a specific topology with valley‐ridge inflection points corresponding to bifurcations of the minimal energy reaction path. © 2016 Wiley Periodicals, Inc.     

Molecular dynamics and metadynamics simulations of [2 + 2] photocycloaddition

Triplet state mechanism of [2 + 2] photocycloaddition forming a cyclobutane ring from two ethylenes is investigated in the context of photocatalysis. High‐level ab initio calculations are combined with ab initio adiabatic molecular dynamics and ab initio metadynamics for rare events modeling. In a photocatalytic scheme, a reactant reaches the triplet state either via intersystem crossing (ISC) or triplet sensitization. The model system adopts a biradical structure, which represents energy intersection with the ground state. The system either completes cyclization or undergoes fragmentation into two olefinic units. The potential and free energy surfaces of the cyclobutane/ethylenes system are mapped with multireference approaches describing possible reaction pathways. To obtain a full picture of a double bond photoreactivity, ab initio adiabatic dynamical calculations were used to estimate reaction yields and to model the effects of excess energy. The potential use of density functional theory based approaches for [2 + 2] photocycloaddition was investigated for future simulations and design of realistic photocatalytic systems. Dynamical aspects of [2 + 2] photocycloaddition via a triplet state manifold are investigated by combining ab initio multireference methods and ab initio molecular dynamics and metadynamics. The reaction pathways are studied for a model system of two ethylenes forming a cyclobutane ring to provide a basis for further studies on design of photocatalytic systems.     

[4 + 2] Heterocyclization for Efficient Formation of Substituted Quinoxalines through Carbon–Oxygen Bonds Cleavage

A new domino strategy for efficient synthesis of highly functionalized quinoxaline derivatives via [4 + 2] heterocyclization involving ring‐opening of oxirane process has been developed. The reaction promoted by Cs₂CO₃ was easy to perform in a simple operation from common and inexpensive starting materials. The bisfunctionalization of quinoxaline framework including C2 benzylation and C3 arylation was readily achieved in domino fashion that involved the cleavage of three C–O bonds of 1,3‐diaryl‐2,3‐epoxypropan‐1‐one.     

The Carbonyl Chlorine(I) Cation [ClCO]+

The first carbonyl halogen cation in the condensed phase , the superelectrophilic [ClCO]⁺ ion, has been clearly and unambiguously established by its complete, fully assigned vibrational spectrum. The cation is obtained by the reaction of COCl₂ with SbF₅ at room temperature in the form of a suspension, in which it is stabilized by oligomeric fluoroantimonate ions.     

Combined 1H, 13C and 11B NMR and mass spectral assignments of boronate complexes of D‐(+)‐glucose,D‐(+)‐mannose,methyl‐α‐D‐glucopyranoside,methyl‐β‐D‐galactopyranoside and methyl‐α‐D‐mannopyranoside

Complex formation between N‐butylboronic acid and D ‐(+)‐glucose, D ‐(+)‐mannose, methyl‐α‐D ‐glucopyranoside, methyl‐β‐D ‐galactopyranoside and methyl α‐D ‐mannopyranoside under neutral conditions was investigated by ¹H, ¹³C and ¹¹B NMR spectroscopy and gas chromatography–mass spectrometry (GC–MS) D ‐(+)‐Glucose and D ‐(+)‐mannose formed complexes where the boronates are attached to the 1,2:4,6‐ and 2,3:5,6‐positions of the furanose forms, respectively. On the other hand, the boronic acid binds to the 4,6‐positions of the two methyl derivatives of glucose and galactose. Methyl α‐D ‐mannopyranoside binds two boronates at the 2,3:4,6‐positions. ¹¹B NMR was used to show the ring size of the complexed sugars and the boronate. GC–MS confirmed the assignments. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis,Structural Aspects,Antimicrobial Activity,and Ion Transport Investigations of Four New [1 + 1] Condensed 12‐Membered Cyclophane Amides

Two new diamines ( a , b ) and their four [1 + 1] condensed macrocyclic peptides ( c–f ) were synthesized via dilution method affording the expected dilactams in reasonably high‐yield yields. The compounds were characterized by elemental analyses, mass, FT‐IR, ¹H, and ¹³C NMR spectral data. Mass spectra reveal their [1 + 1] cyclic condensation. The macrocycles having two peptide units in the ring would constitute three configurational isomers: cisoid–cisoid, transoid–transoid, and cisoid–transoid. Their ¹H and ¹³C NMR show one signal for each chemically equivalent SCH₂ and CONH and could be assigned symmetrical structures lacking any configurational isomerism. The ion transport activity of the compounds ( c ) and ( d ) on Na⁺ and K⁺ ions were evaluated by CH₂Cl₂ liquid–liquid membrane method using a U tube system. Slightly higher affinity for potassium than sodium was also observed, which is undoubtedly related to the matching size of the cavity between K⁺ and cyclic peptides. The antimicrobial and antifungal activities of four macrocyclic amides (c , d , e , and f) were illuminated using disk diffusion method in dimethyl sulfoxide as well as the minimal inhibitory concentration dilution method, against several bacteria and yeast cultures. Comparing all methods, in most cases, the ( e ) and ( f ) compound seems to show slightly higher biological activity than the ( c ) and ( d ) macrocycles. This may be because of the presence of extra chlorine atoms on the exterior part of the molecules.     

Mass spectrometry of steroid systems. XXIV–The origin of [M – 28]+ and [M – 43]+ ions in equilenin and its 14β-isomer

The formation of the [M? 43]⁺ ion in equilenin is due mainly to elimination of Me radical from the [M? CO]⁺ ion and, to a lesser extent, to CO loss from the [M? Me]⁺ ion. In 14β-isoequilenin the [M? CO]⁺ ion is absent, and the formation of [M? 43]⁺ occurs via the [M? Me]⁺ ion. This makes the determination of the mode of junction of the rings C and D in the equilenin series possible, using high resolution mass spectra, even when only one stereoisomer is available.     

Aphagranols D – H: Five New Limonoids from the Fruits of Aphanamixis grandifolia

Five new limonoids, aphagranols D–H ( 1 – 5 , resp.), were isolated from the EtOH extract of the fruits of Aphanamixis grandifolia by chromatographic methods. Their structures were established on the basis of extensive spectroscopic analyses. The absolute configuration of 1 was determined by comparison the experimental and calculated electronic circular dichroism (ECD) spectra. All of the isolates were evaluated for insecticidal activities.