Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 13 de 13
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Am Chem Soc ; 141(27): 10605-10609, 2019 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-31240909

RESUMO

Cationic polymerizations provide a valuable strategy for preparing macromolecules with excellent control but are inherently sensitive to impurities and commonly require rigorous reagent purification, low temperatures, and strictly anhydrous reaction conditions. By using pentacarbomethoxycyclopentadiene (PCCP) as the single-component initiating organic acid, we found that a diverse library of vinyl ethers can be controllably polymerized under ambient conditions. Additionally, excellent chain-end fidelity is maintained even without rigorous monomer purification. We hypothesize that a tight ion complex between the PCCP anion and the oxocarbenium ion chain end prevents chain-transfer events and enables a polymerization with living characteristics. Furthermore, terminating the polymerization with functional nucleophiles allows for chain-end functionalization in high yields.

2.
J Am Chem Soc ; 139(43): 15530-15538, 2017 11 01.
Artigo em Inglês | MEDLINE | ID: mdl-28985061

RESUMO

The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation-this species initiates the polymerization of the vinyl ether monomer-and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition-fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world applications.


Assuntos
Cátions/química , Polimerização/efeitos da radiação , Compostos de Vinila/química , Catálise/efeitos da radiação , Oxirredução/efeitos dos fármacos
3.
J Am Chem Soc ; 139(31): 10665-10668, 2017 08 09.
Artigo em Inglês | MEDLINE | ID: mdl-28745047

RESUMO

The ability to combine two polymerization mechanisms in a one-pot setup and switch the monomer selectivity via an external stimulus provides an excellent opportunity to control polymer sequence and structure. We report a strategy that enables monomer incorporation to be determined via the selection of the wavelength of light through selective activation of either cationic or radical processes. This method enables the synthesis of varying polymeric structures under identical solution conditions but with simple modulation of the external stimulus. Additionally, changes in the ratios of the two photocatalysts afford complementary chemical control over these reactions to design elaborated polymeric structures. Our strategy takes advantage of the unique regulation that can be accessed through light.

4.
Angew Chem Int Ed Engl ; 56(33): 9670-9679, 2017 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-28277625

RESUMO

During the last 40 years, researchers investigating photoinitiated cationic polymerizations have delivered tremendous success in both industrial and academic settings. A myriad of photoinitiating systems have been developed, thus allowing polymerization of a broad array of monomers (e.g., epoxides, vinyl ethers, alkenes, cyclic ethers, and lactones) under practical, inexpensive, and environmentally benign conditions. More recently, owing to progress in photoredox catalysis, photocontrolled cationic polymerization has emerged as a means to precisely regulate polymer chain growth. This Minireview provides a concise historical perspective on cationic polymerization induced by light and discusses the latest advances in both photoinitiated and photocontrolled processes. The latter are exciting new directions for the field that will likely impact industries ranging from micropatterning to the synthesis of complex biomaterials and sequence-controlled polymers.

5.
J Am Chem Soc ; 138(48): 15535-15538, 2016 12 07.
Artigo em Inglês | MEDLINE | ID: mdl-27934022

RESUMO

Photoinitiated cationic polymerizations are widely used in industrial processes; however, gaining photocontrol over chain growth would expand the utility of these methods and facilitate the design of novel complex architectures. We report herein a cationic polymerization regulated by visible light. This polymerization proceeds under mild conditions: a combination of a metal-free photocatalyst, a chain-transfer agent, and light irradiation enables the synthesis of various poly(vinyl ether)s with good control over molecular weight and dispersity as well as excellent chain-end fidelity. Significantly, photoreversible cation formation in this system enables efficient control over polymer chain growth with light.

7.
J Org Chem ; 80(14): 7019-32, 2015 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-26151079

RESUMO

Clopidogrel is a prodrug anticoagulant with active metabolites that irreversibly inhibit the platelet surface GPCR P2Y12 and thus inhibit platelet activation. However, gaining an understanding of patient response has been limited due to imprecise understanding of metabolite activity and stereochemistry, and a lack of acceptable analytes for quantifying in vivo metabolite formation. Methods for the production of all bioactive metabolites of clopidogrel, their stereochemical assignment, and the development of stable analytes via three conceptually orthogonal routes are disclosed.


Assuntos
Microssomos Hepáticos/metabolismo , Piperidinas/síntese química , Inibidores da Agregação de Plaquetas/síntese química , Inibidores da Agregação de Plaquetas/metabolismo , Pró-Fármacos/síntese química , Ticlopidina/análogos & derivados , Fenômenos Biológicos , Clopidogrel , Humanos , Microssomos Hepáticos/efeitos dos fármacos , Piperidinas/química , Inibidores da Agregação de Plaquetas/química , Pró-Fármacos/química , Estereoisomerismo , Ticlopidina/síntese química , Ticlopidina/química , Ticlopidina/metabolismo
8.
J Am Chem Soc ; 137(25): 8046-9, 2015 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-26088401

RESUMO

A solution to the classic unsolved problem of olefin hydromethylation is presented. This highly chemoselective method can tolerate labile and reactive chemical functionalities and uses a simple set of reagents. An array of olefins, including mono-, di-, and trisubstituted olefins, are all smoothly hydromethylated. This mild protocol can be used to simplify the synthesis of a specific target or to directly "edit" complex natural products and other advanced materials. The method is also amenable to the simple installation of radioactive and stable labeled methyl groups.


Assuntos
Alcenos/química , Metano/química , Catálise , Metilação , Modelos Moleculares , Estereoisomerismo
9.
J Am Chem Soc ; 137(25): 8078-85, 2015 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-26067765

RESUMO

The ubiquitous flavin-dependent monooxygenases commonly catalyze oxygenation reactions by means of a transient C4a-peroxyflavin. A recent study, however, suggested an unprecedented flavin-oxygenating species, proposed as the flavin-N5-oxide (Fl(N5[O])), as key to an oxidative Favorskii-type rearrangement in the biosynthesis of the bacterial polyketide antibiotic enterocin. This stable superoxidized flavin is covalently tethered to the enzyme EncM and converted into FADH2 (Fl(red)) during substrate turnover. Subsequent reaction of Fl(red) with molecular oxygen restores the postulated Fl(N5[O]) via an unknown pathway. Here, we provide direct evidence for the Fl(N5[O]) species via isotope labeling, proteolytic digestion, and high-resolution tandem mass spectrometry of EncM. We propose that formation of this species occurs by hydrogen-transfer from Fl(red) to molecular oxygen, allowing radical coupling of the formed protonated superoxide and anionic flavin semiquinone at N5, before elimination of water affords the Fl(N5[O]) cofactor. Further biochemical and spectroscopic investigations reveal important features of the Fl(N5[O]) species and the EncM catalytic mechanism. We speculate that flavin-N5-oxides may be intermediates or catalytically active species in other flavoproteins that form the anionic semiquinone and promote access of oxygen to N5.


Assuntos
Proteínas de Bactérias/metabolismo , Flavinas/metabolismo , Óxidos/metabolismo , Streptomyces/enzimologia , Flavinas/química , Nitrosaminas/metabolismo , Oxirredução , Óxidos/química , Transdução de Sinais , Streptomyces/química , Streptomyces/metabolismo , Especificidade por Substrato
10.
Acc Chem Res ; 48(3): 712-21, 2015 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-25702529

RESUMO

Collaboration between academia and industry is a growing phenomenon within the chemistry community. These sectors have long held strong ties since academia traditionally trains the future scientists of the corporate world, but the recent drastic decrease of public funding is motivating the academic world to seek more private grants. This concept of industrial "sponsoring" is not new, and in the past, some companies granted substantial amounts of money per annum to various academic institutions in exchange for prime access to all their scientific discoveries and inventions. However, academic and industrial interests were not always aligned, and therefore the investment has become increasingly difficult to justify from industry's point of view. With fluctuating macroeconomic factors, this type of unrestricted grant has become more rare and has been largely replaced by smaller and more focused partnerships. In our view, forging a partnership with industry can be a golden opportunity for both parties and can represent a true symbiosis. This type of project-specific collaboration is engendered by industry's desire to access very specific academic expertise that is required for the development of new technologies at the forefront of science. Since financial pressures do not allow companies to spend the time to acquire this expertise and even less to explore fundamental research, partnering with an academic laboratory whose research is related to the problem gives them a viable alternative. From an academic standpoint, it represents the perfect occasion to apply "pure science" research concepts to solve problems that benefit humanity. Moreover, it offers a unique opportunity for students to face challenges from the "real world" at an early stage of their career. Although not every problem in industry can be solved by research developments in academia, we argue that there is significant scientific overlap between these two seemingly disparate groups, thereby presenting an opportunity for a symbiosis. This type of partnership is challenging but can be a win-win situation if both parties agree on some general guidelines, including clearly defined goals and deliverables, biweekly meetings to track research progress, and quarterly or annual meetings to recognize overarching, common objectives. This Account summarizes our personal experience concerning collaborations with various industrial groups and the way it impacted the research programs for both sides in a symbiotic fashion.


Assuntos
Química Orgânica/organização & administração , Comportamento Cooperativo , Indústrias/organização & administração , Pesquisa/organização & administração , Humanos
11.
Angew Chem Int Ed Engl ; 53(45): 12091-6, 2014 Nov 03.
Artigo em Inglês | MEDLINE | ID: mdl-25244630

RESUMO

Physicochemical properties constitute a key factor for the success of a drug candidate. Whereas many strategies to improve the physicochemical properties of small heterocycle-type leads exist, complex hydrocarbon skeletons are more challenging to derivatize because of the absence of functional groups. A variety of C-H oxidation methods have been explored on the betulin skeleton to improve the solubility of this very bioactive, yet poorly water-soluble, natural product. Capitalizing on the innate reactivity of the molecule, as well as the few molecular handles present on the core, allowed oxidations at different positions across the pentacyclic structure. Enzymatic oxidations afforded several orthogonal oxidations to chemical methods. Solubility measurements showed an enhancement for many of the synthesized compounds.


Assuntos
Carbono/química , Hidrogênio/química , Produtos Biológicos/química , Oxirredução , Solubilidade
12.
Nature ; 503(7477): 552-556, 2013 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-24162851

RESUMO

Flavoproteins catalyse a diversity of fundamental redox reactions and are one of the most studied enzyme families. As monooxygenases, they are universally thought to control oxygenation by means of a peroxyflavin species that transfers a single atom of molecular oxygen to an organic substrate. Here we report that the bacterial flavoenzyme EncM catalyses the peroxyflavin-independent oxygenation-dehydrogenation dual oxidation of a highly reactive poly(ß-carbonyl). The crystal structure of EncM with bound substrate mimics and isotope labelling studies reveal previously unknown flavin redox biochemistry. We show that EncM maintains an unexpected stable flavin-oxygenating species, proposed to be a flavin-N5-oxide, to promote substrate oxidation and trigger a rare Favorskii-type rearrangement that is central to the biosynthesis of the antibiotic enterocin. This work provides new insight into the fine-tuning of the flavin cofactor in offsetting the innate reactivity of a polyketide substrate to direct its efficient electrocyclization.


Assuntos
Proteínas de Bactérias/metabolismo , Flavinas/metabolismo , Flavoproteínas/metabolismo , Oxigenases de Função Mista/metabolismo , Streptomyces/enzimologia , Antibacterianos/biossíntese , Proteínas de Bactérias/química , Biocatálise , Hidrocarbonetos Aromáticos com Pontes/metabolismo , Cristalografia por Raios X , Ciclização , Flavoproteínas/química , Marcação por Isótopo , Oxigenases de Função Mista/química , Modelos Químicos , Modelos Moleculares , Oxirredução , Policetídeos/metabolismo , Conformação Proteica , Streptomyces/metabolismo , Especificidade por Substrato
13.
J Am Chem Soc ; 134(5): 2547-50, 2012 Feb 08.
Artigo em Inglês | MEDLINE | ID: mdl-22276612

RESUMO

Intermolecular Ritter-type C-H amination of unactivated sp(3) carbons has been developed. This new reaction proceeds under mild conditions using readily available reagents and an inexpensive source of nitrogen (acetonitrile). A broad scope of substrates can be aminated with this method since many functional groups are tolerated. This reaction also allows for the direct, innate C-H amination of a variety of hydrocarbons such as cyclohexane without the need of prefunctionalization or installation of a directing group.


Assuntos
Álcoois/química , Amidas/síntese química , Aminas/síntese química , Hidrocarbonetos/química , Cetonas/química , Amidas/química , Aminação , Aminas/química , Estrutura Molecular , Estereoisomerismo
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA