scholarly journals Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

Molecules ◽  
2021 ◽  
Vol 26 (4) ◽  
pp. 804
Author(s):  
Marcel Bauch ◽  
Werner Fudickar ◽  
Torsten Linker

Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Yanming Cai ◽  
Jiaju Fu ◽  
Yang Zhou ◽  
Yu-Chung Chang ◽  
Qianhao Min ◽  
...  

AbstractSingle-atom catalysts (SACs) are promising candidates to catalyze electrochemical CO2 reduction (ECR) due to maximized atomic utilization. However, products are usually limited to CO instead of hydrocarbons or oxygenates due to unfavorable high energy barrier for further electron transfer on synthesized single atom catalytic sites. Here we report a novel partial-carbonization strategy to modify the electronic structures of center atoms on SACs for lowering the overall endothermic energy of key intermediates. A carbon-dots-based SAC margined with unique CuN2O2 sites was synthesized for the first time. The introduction of oxygen ligands brings remarkably high Faradaic efficiency (78%) and selectivity (99% of ECR products) for electrochemical converting CO2 to CH4 with current density of 40 mA·cm-2 in aqueous electrolytes, surpassing most reported SACs which stop at two-electron reduction. Theoretical calculations further revealed that the high selectivity and activity on CuN2O2 active sites are due to the proper elevated CH4 and H2 energy barrier and fine-tuned electronic structure of Cu active sites.


Author(s):  
Ibon Alkorta ◽  
José Elguero

AbstractThis communication gives an overview of the relationships between four reactions that although related were not always perceived as such: SN2, Walden, Finkelstein, and Menshutkin. Binary interactions (SN2 & Walden, SN2 & Menshutkin, SN2 & Finkelstein, Walden & Menshutkin, Walden & Finkelstein, Menshutkin & Finkelstein) were reported. Carbon, silicon, nitrogen, and phosphorus as central atoms and fluorides, chlorides, bromides, and iodides as lateral atoms were considered. Theoretical calculations provide Gibbs free energies that were analyzed with linear models to obtain the halide contributions. The M06-2x DFT computational method and the 6-311++G(d,p) basis set have been used for all atoms except for iodine where the effective core potential def2-TZVP basis set was used. Concerning the central atom pairs, carbon/silicon vs. nitrogen/phosphorus, we reported here for the first time that the effect of valence expansion was known for Si but not for P. Concerning the lateral halogen atoms, some empirical models including the interaction between F and I as entering and leaving groups explain the Gibbs free energies.


Author(s):  
Nooreddine Iskandar ◽  
Tatiana Rahbany ◽  
Ali Shokor

Abstract Background: Due to the common instability caused by political and security issues, Lebanese hospitals have experienced acts of terrorism multiple times. The most recent Beirut Explosion even forced several hospitals to cease operations for the first time in decades—but studies show the preparedness levels for such attacks in similar countries are low. Objective: The aim of this study is to explore the experience of Lebanese hospitals with terrorist attacks. Methods: This qualitative study used semi-structured interviews with various stakeholders to assess their experience with terrorist bombings. Data was analyzed using the thematic analysis method. Results: The researchers found that Lebanese hospitals vary greatly in their structures and procedures. Those differences are a function of 3 contextual factors: location, culture, and accreditation status. Hospitals found near ‘dangerous zones’ were more likely to be aware and to have better response to such events. A severe lack of communication, unity of command, and collaboration between stakeholders has made the process fragmented. Conclusion: The researchers recommend a larger role for the Ministry of Public Health (MOPH) in this process, and the creation of a platform where Lebanese organizations can share their experiences to improve preparedness and resilience of the Lebanese healthcare system in the face of terrorism.


1988 ◽  
Vol 53 (21) ◽  
pp. 5155-5158 ◽  
Author(s):  
M. H. Lin ◽  
J. E. Silver ◽  
W. J. Le Noble
Keyword(s):  
The Face ◽  

2007 ◽  
Vol 2 ◽  
pp. 117739010700200 ◽  
Author(s):  
Tamara Zoltan ◽  
Franklin Vargas ◽  
Carla Izzo

We have determined and quantified spectrophotometrically the capacity of producing reactive oxygen species (ROS) as 1O2 during the photolysis with UV-A light of 5 new synthesized naphthyl ester derivates of well-known quinolone antibacterials (nalidixic acid (1), cinoxacin (2), norfloxacin (3), ciprofloxacin (4) and enoxacin (5)). The ability of the naphthyl ester derivatives (6-10) to generate singlet oxygen were detecting and for the first time quantified by the histidine assay, a sensitive, fast and inexpensive method. The following tendency of generation of singlet oxygen was observed: compounds 7 >10 > 6 > 8 > 9 >> parent drugs 1-5.


2007 ◽  
Vol 62 (11-12) ◽  
pp. 833-838 ◽  
Author(s):  
Violeta B. Velikova ◽  
Aglika M. Edreva ◽  
Tsonko D. Tsonev ◽  
Hamlyn G. Jones

This paper demonstrates for the first time that plant metabolites of the phenylamide type, conjugates of putrescine with hydroxycinnamic acids (p-coumaric, caffeic and ferulic), possess 1O2 quenching properties. Data were obtained confirming that their acidic parent compounds were also able to quench 1O2, as did polyamines (putrescine, spermidine and spermine), and that this ability depends on the number of amino groups. Potentiation of the 1O2 quenching ability of the conjugates relative to both parent components was established. The importance of polyamines and phenylamides in the plant non-enzymatic antioxidant defence at sites of intensive 1O2 generation, such as the photosynthetic centers, was suggested.


Author(s):  
И.А. Куклин ◽  
Н.П. Малишевская ◽  
М.М. Кохан ◽  
Г.Д. Сафонова ◽  
О.Г. Римар ◽  
...  

В статье представлен краткий обзор литературы о клинических проявлениях, современных методах диагностики и лечения чрезвычайно редкого заболевания – множественной эруптивной кератоакантомы типа Гржебовски. Заболевание характеризуется возникновением сотен или тысяч генерализованных зудящих узелков на коже лица, туловища, конечностей, половых органов, слизистой полости рта и гортани в течение короткого времени. Заболевание развивается в возрасте старше 40 лет, имеет рецидивирующее течение, возможна спонтанная инволюция отдельных элементов с формированием участков депигментации или атрофических рубцов. Авторы приводят случай собственного клинического наблюдения множественной эруптивной кератоакантомы типа Гржебовски у пациента 50 лет, который обратился на консультацию к дерматовенерологу в клинику Уральского НИИ дерматовенерологии и иммунопатологии. Длительность болезни на момент обращения составляла 2,5 года, ее возникновению предшествовало неоднократное посещение больным стран с избыточной инсоляцией, что может рассматриваться в качестве этиологического фактора развития заболевания. Диагноз впервые был заподозрен на консилиуме дерматовенерологов на основании клинической картины заболевания и в дальнейшем верифицирован данными патоморфологического исследования биоптата пораженной кожи. Показана эффективность применения ароматических ретиноидов в лечении множественной эруптивной кератоакантомы типа Гржебовски и приведены побочные эффекты, которые развились у больного при самостоятельном увеличении дозы ацитретина до 70 мг/сутки. В статье констатируется чрезвычайно редкая встречаемость данного заболевания, о чем свидетельствует приведенный клинический случай, диагностируемый впервые более чем за 90-летнюю историю существования института. Подчеркивается важность консолидации клинического опыта нескольких ведущих дерматовенерологов для диагностики множественной эруптивной кератоакантомы типа Гржебовски. The article presents a brief review of the literature about clinical manifestations, modern methods of diagnosticsand treatment of an extremely rare disease – generalized eruptive keratoacanthomas of Grzybowski. The disease is characterized by the appearance of hundreds or thousands of generalized itchy nodules on the skin of the face, trunk, limbs, genitals, oral mucosa and larynx within a short time. The disease develops at the age of over 40, has a recurrent course, spontaneous involution of individual elements with the formation of areas of depigmentation or atrophic scars is possible. The authors describe a case of their own clinical care of generalized eruptive keratoacanthomas of Grzybowski in a 50-year-old patient who consulted a dermatovenerologist at the clinic of the Ural Research Institute of Dermatovenerology and Immunopathology.The duration of the disease at the time of visit to a doctor was 2,5 years, its occurrence was preceded by repeated visits to countries with excessive insolation, which can be considered as an etiological factor in the development of the disease. The diagnosis was first suspected at a boarddermatovenerologists based on the clinical picture of the disease and subsequently verified by the data of a pathomorphological examination of the biopsy of the affected skin.The effectiveness of the use of aromatic retinoids in the treatment of generalized eruptive keratoacanthomas of Grzybowski is shown and the side effects that developed in the patient with an independent increase in the dose of acitretin to 70 mg/day are given. The article states the extremely rare occurrence of this disease, as evidenced by the above clinical case, diagnosed for the first time in more than 90 years of the institute's existence. The importance of consolidating the clinical experience of several leading dermatovenerologists for the diagnostics of generalized eruptive keratoacanthomas of Grzybowski is emphasized.


Author(s):  
Houchao Xu ◽  
Carsten Schotte ◽  
Russell Cox ◽  
Jeroen Dickschat

The non-canonical fungal α-humulene synthase was investigated through isotopic labelling experiments for its stereochemical course regarding inversion or retention at C-1, the face selectivity at C-11, and the stereoselectivity of...


2008 ◽  
Vol 86 (4) ◽  
pp. 298-304 ◽  
Author(s):  
Erwin Buncel ◽  
Sam-Rok Keum ◽  
Srinivasan Rajagopal ◽  
Eric Kiepek ◽  
Robin A Cox

Extension of our studies of the generic Wallach rearrangement (of azoxybenzene to 4-hydroxyazobenzene) to the heteroaromatic series (azoxypyridines and axoxypyridine N-oxides) has revealed some dramatic reactivity differences, particularly for the α and β compounds. We have studied the 3-isomers and the 4-isomers in each series, each with α and β forms, eight compounds in all, in the 100 wt% sulfuric acid region of acidity. In those cases in which a product could be observed, the α and β isomers both give the same one, the corresponding 4′-hydroxyazo compounds. All the compounds react much more slowly than does azoxybenzene itself, presumably because of the extra positive charge present in the substrates, but the β isomers have half-lives of seconds and the α isomers half-lives of hundreds of hours in the 100 wt% H2SO4 acidity region. The α compounds have measurable pKBH+ values, but the β compounds do not, exhibiting only a medium effect in the acidity region in which the α compounds protonate. This means that for the β compounds, the protonated intermediates must be much less stable and the postulated reaction intermediates must be much more stable than for the α compounds. To clarify this, we have obtained Mulliken charge distributions for the various species concerned, calculating the charge carried by each half of the molecule, larger charge separations being taken to indicate lesser stability. As far as we can establish, this is the first time that this technique has been used to indicate the stabilities of carbocationic species.Key words: azoxypyridines, azoxypyridine N-oxides, Wallach rearrangement, excess acidity, basicities, theoretical calculations, charge distributions, reactivities.


1970 ◽  
Vol 48 (14) ◽  
pp. 1664-1674 ◽  
Author(s):  
D. W. Lepard

This paper presents a method for calculating the relative intensities and Raman shifts of the rotational structure in electronic Raman spectra of diatomic molecules. The method is exact in the sense that the wave functions used for the calculations may belong to any intermediate case of Hund's coupling schemes. Using this method, theoretical calculations of the pure rotational and electronic Raman spectrum of NO, and the pure rotational Raman spectrum of O2, are presented. Although a calculated stick spectrum for NO was previously shown by Fast et al., the details of this calculation are given here for the first time.


Sign in / Sign up

Export Citation Format

Share Document