Regioselectivity of Enzymatic and Photochemical Single Electron Transfer Promoted Carbon−Carbon Bond Fragmentation Reactions of Tetrameric Lignin Model Compounds

Cho, Dae Won,Latham, John A.,Park, Hea Jung,Yoon, Ung Chan,Langan, Paul,Dunaway-Mariano, Debra,Mariano, Patrick S. American Chemical Society 2011 Journal of organic chemistry Vol.76 No.8

<P>New types of tetrameric lignin model compounds, which contain the common β-O-4 and β-1 structural subunits found in natural lignins, have been prepared and carbon−carbon bond fragmentation reactions of their cation radicals, formed by photochemical (9,10-dicyanoanthracene) and enzymatic (lignin peroxidase) SET-promoted methods, have been explored. The results show that cation radical intermediates generated from the tetrameric model compounds undergo highly regioselective C−C bond cleavage in their β-1 subunits. The outcomes of these processes suggest that, independent of positive charge and odd-electron distributions, cation radicals of lignins formed by SET to excited states of sensitizers or heme-iron centers in enzymes degrade selectively through bond cleavage reactions in β-1 vs β-O-4 moieties. In addition, the findings made in the enzymatic studies demonstrate that the sterically large tetrameric lignin model compounds undergo lignin peroxidase-catalyzed cleavage via a mechanism involving preliminary formation of an enzyme−substrate complex.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/joceah/2011/joceah.2011.76.issue-8/jo200253v/production/images/medium/jo-2011-00253v_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jo200253v'>ACS Electronic Supporting Info</A></P>

Effects of Alkoxy Groups on Arene Rings of Lignin β-O-4 Model Compounds on the Efficiencies of Single Electron Transfer-Promoted Photochemical and Enzymatic C–C Bond Cleavage Reactions

Lim, Suk Hyun,Nahm, Keepyung,Ra, Choon Sup,Cho, Dae Won,Yoon, Ung Chan,Latham, John A.,Dunaway-Mariano, Debra,Mariano, Patrick S. American Chemical Society 2013 Journal of organic chemistry Vol.78 No.18

<P>To gain information about how alkoxy substitution in arene rings of β-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1–C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric β-O-4 models undergo more rapid C1–C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1–C2 bond dissociation energies of β-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two β-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C–C bond cleavage was observed to occur in one case at the C1–C2 bond in the less alkoxy-substituted β-O-4 moiety. However, regioselective C1–C2 cleavage in the more alkoxy-substituted β-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of β-O-4 units would be more efficiently cleaved by SET mechanisms.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/joceah/2013/joceah.2013.78.issue-18/jo401680z/production/images/medium/jo-2013-01680z_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jo401680z'>ACS Electronic Supporting Info</A></P>

Observations Made in Exploring a Pyridinium Salt Photochemical Approach to the Synthesis of (+)-Lactacystin

Zhou, Jiwen,Gong, Maozhen,Mariano, Patrick S.,Yoon, Ung-Chan Korean Chemical Society 2008 Bulletin of the Korean Chemical Society Vol.29 No.1

The key step in a strategy for the synthesis of (+)-lactacystin involving photocyclization reaction of a cyclopenta-fused pyridinium salt has been probed by using a model substrate. Observations made in this effort led to the discovery of a highly unusual cascade process that leads to stereoselective formation of an interesting tricyclic carbamate. The results of this study are presented and discussed in the context of a (+)-lactacystin synthetic approach.

The Synthetic Potential of SET Photochemistry of Silicon-Substituted Polydonor-Linked Phthalimides

Yoon, Ung Chan,Mariano, Patrick S. Korean Chemical Society 2006 Bulletin of the Korean Chemical Society Vol.27 No.8

Our studies in the area of single electron transfer (SET) photochemistry have led to the discovery of efficient processes, in which regioselective formation of carbon-centered radicals takes place by nucleophile assisted desilylation of $\alpha$-trialkylsilyl substituted ether, thioether, amine and amide centered cation radicals. The rates of bimolecular desilylation of the intermediate cation radicals exceed those of other cation radical $\alpha$-fragmentation processes (e.g.,-deprotonation). This sereves as the basis for the design of highly regioselective, SET-induced photomacrocyclization reactions of polyether, polythioether, polyamide, and polypeptide linked phthalimides. Photocyclization reactions of trimethylsilyl-terminated substrates in these families are unique in that they produce polyfunctionalized macrocyclic substances in a highly efficient and regioselective manner. In addition, our studies in this area have led to important information about the factors that govern chemical and quantum efficiencies that should be applicable to a wide variety of redox processes promoted by SET from substrates containing more than one electron donor site.

Dipole-Forming Photochemical Group Transfer Reactions of Phthalimides and ${\alpha}-Ketoamides$

Yoon, Ung-Chan,Mariano Patrick S. Korean Society of Photoscience 2005 Journal of Photosciences Vol.12 No.3

Results of studies of SET-promoted dipole-forming photochemical group transfer reactions of phthalimide and ${\alpha}-ketoamide$ derivatives are discussed. Azomethine ylide forming photochemical reactions, which are initiated by intramolecular SET from tethered silylmethyl-, carboxymethyl-, and ${\beta}-hydroxyethyl$ containing electron donors to excited states of phthalimides, related maleimides, and conjugated imides, are presented first. Following this, investigations of regioselective 1,4-dipole forming photochemical reactions of N-trialkylsilylmethyl- and N-trialkylstannyl-${\alpha}$-ketoamides are described.

Organic Synthesis Based on Ruthenium Carbene Catalyzed Metathesis Reactions and Pyridinium Salt Photochemistry

조대원,Cho, Dae-Won,Mariano, Patrick S. Korean Chemical Society 2010 대한화학회지 Vol.54 No.3

이 총설 논문에서는 천연물 합성에서 새로운 합성전략으로 이용될 수 있는 세가지 합성방법에 대해 간단히 소개하고자 한다. 소개된 첫 번째 방법으로는 Grubbs에 의해 개발된 루테늄 카벤 촉매 고리 전이 복분해 반응을 이용한 합성방법으로 이 방법을 이용하여 알켄기로 치환된 사이클로알켄들을 열역학적으로 더 안정한 알켄기 치환 사이클로알켄으로 전환시킬 수 있어 새로운 유기합성과정으로 소개되었다. 두 번째 Grubbs에 의해 보고된 루테늄 카벤 촉매 다이엔아인 복분해 반응을 이용한 합성방법으로 이 합성방법을 이용한 과정들은 다이엔아인 화합물들을 접합 두고리 콘쥬게이트 다이엔 화합물들을 합성할 수 있게 한다. 마지막으로 새로 개발된 피리디늄 염의 광-전자고리화반응을 이용한 4-아미노사이클로팬텐-3,5-다이올 유도체 화합물들을 합성할 수 있는 방법을 소개하였다. 이 총설에서 루테늄 카벤 촉매 고리전이와 다이엔인 복분해 반응들을 함께 연결 이용하여 폴리하이드록시 인돌리지딘 알카로이드들과 레파디포르민과 실린드리신 알카로이드들을 피리듐 염 광화학 반응법을 통해 합성한 과정들을 상세히 소개하고 있다. In this account, three synthetic methodologies that serve as the basis for new strategies for the preparation of selected natural products are briefly introduced. One process, involving ruthenium carbene catalyzed ring rearrangement metathesis developed by Grubbs and his coworkers, transforms alkene-tethered cycloalkenes to thermodynamically more favored alkene-tethered cycloalkenes. Another ruthenium carbene promoted reaction, referred to as dienyne metathesis, was uncovered in early studies by Grubbs and his collaborators. This process converts dienynes to fused bicyclic conjugated dienes. Finally, a novel photo-electrocyclization reaction of pyridinium salts, which leads to the formation of 4-aminocyclopenten-3,5-diol derivatives, is discussed. Examples are provided to show the utility of these methodologies in natural product synthesis. Emphasis is given to studies in which pyridinium salt photochemistry is coupled with ring rearrangement and dienyne metathesis in routes for the synthesis of polyhydroxyalted indolizidine alkaloids and the construction of the tricyclic core of the lepadiformine and cylindricine alkaloids.

STRATEGIES FOR SPIROCYCLIC-AMINE SYNTHESIS BASED ON IMINIUM SALT SET-PHOTOCHEMISTRY

Kavash, Robert-W.,Mariano, Patrick-S. Korean Society of Photoscience 1996 Journal of Photosciences Vol.3 No.2

Preparative features of photoinduced single electron transfer (SET) reactions of selected N-silylmethallyl-iminium salts have been probed in the context of strategies for functionally complex spirocyclic amine synthesis.

Studies Leading to the Development of a Single-Electron Transfer (SET) Photochemical Strategy for Syntheses of Macrocyclic Polyethers, Polythioethers, and Polyamides

Cho, Dae Won,Yoon, Ung Chan,Mariano, Patrick S. American Chemical Society 2011 Accounts of chemical research Vol.44 No.3

<P>Organic photochemists began to recognize in the 1970sthat a new mechanistic pathway involving excited-state single-electron transfer (SET) could be used to drive unique photochemical reactions. Arnold’s seminal studies demonstrated that SET photochemical reactions proceed by way of ion radical intermediates, the properties of which govern the nature of the ensuing reaction pathways. Thus, in contrast to classical photochemical reactions, SET-promoted excited-state processes are controlled by the nature and rates of secondary reactions of intermediate ion radicals. In this Account, we discuss our work in harnessing SET pathways for photochemical synthesis, focusing on the successful production of macrocyclic polyethers, polythioethers, and polyamides.</P><P>One major thrust of our studies in SET photochemistry has been to develop new, efficient reactions that can be used for the preparation of important natural and non-natural substances. Our efforts with α-silyl donor-tethered phthalimides and naphthalimides have led to the discovery of efficient photochemical processes in which excited-state SET is followed by regioselective formation of carbon-centered radicals. The radical formation takes place through nucleophile-assisted desilylation of intermediate α-silyl-substituted ether-, thioether-, amine-, and amide-centered cation radicals.</P><P>Early laser flash photolysis studies demonstrated that the rates of methanol- and water-promoted bimolecular desilylations of cation radicals (derived from α-silyl electron donors) exceeded the rates of other cation radical α-fragmentation processes, such as α-deprotonation. In addition, mechanistic analyses of a variety of SET-promoted photocyclization reactions of α-silyl polydonor-linked phthalimides and naphthalimides showed that the chemical and quantum efficiencies of the processes are highly dependent on the lengths and types of the chains connecting the imide acceptor and α-silyl electron donor centers. We also observed that reaction efficiencies are controlled by the rates of desilylation at the α-silyl donor cation radical moieties in intermediate zwitterionic biradicals that are formed by either direct excited-state intramolecular SET or by SET between the donor sites in the intervening chains.</P><P>It is important to note that knowledge about how these factors govern product yields, regiochemical selectivities, and quantum efficiencies was crucial for the design of synthetically useful photochemical reactions of linked polydonor−acceptor substrates. The fruits of these insights are exemplified by synthetic applications in the concise preparation of cyclic peptide mimics, crown ethers and their lariat- and bis-analogs, and substances that serve as fluorescence sensors for important heavy metal cations.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2011/achre4.2011.44.issue-3/ar100125j/production/images/medium/ar-2010-00125j_0003.gif'></P>

Novel Naphthalene Based Lariat-Type Crown Ethers Using Direct Single Electron Transfer Photochemical Strategy

박혜정,성남경,김수한,안소현,윤웅찬,조대원,Patrick S. Mariano 대한화학회 2013 Bulletin of the Korean Chemical Society Vol.34 No.12

This study explored a direct SET-photochemical strategy to construct a new family of thioene conjugatednaphthalamide fluorophore based lariat-crown ethers which show strong binding properties towards heavy metal ions. Irradiations of designed nitrogen branched (trimethylsilyl)methylthio-terminated polyethylenoxytethered naphthalimides in acidic methanol solutions have led to highly efficient photocyclization reactions to generate naphthalamide based lariat type thiadiazacrown ethers directly in chemo- and regio-selective manners which undergo very facile secondary dehydration reactions during separation processes to produce their corresponding amidoenethio ether cyclic products tethered with electron donating diethyleneoxy- and diethyenethio-side arm chains. Fluorescence and metal cation binding properties of the lariat type enamidothio products were examined. The photocyclized amidoenethio products, thioene conjugated naphthalamide fluorophore containing lariat-thiadiazacrowns exhibited strong fluorescence emissions in region of 330-450 nm along with intramolecular exciplex emissions in region of 450-560 nm with their maxima at 508 nm. Divalent cation Hg2+ and Pb2+ showed strong binding to sulfur atom(s) in side arm chain and atoms in enethiadiazacrown ether rings which led to significant enhancement of fluorescence from its chromophore singlet excited state and concomitant quenching of exciplex emission. The dual fluorescence emission responses towards divalent cations might provide a new guide for design and development of fluorescence sensors for detecting those metals.

Observations Made in Exploring a Pyridinium Salt Photochemical Approach to the Synthesis of (+)-Lactacystin

Jiwen Zhou,윤웅찬,Maozhen Gong,Patrick S. Mariano* 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.1

The key step in a strategy for the synthesis of (+)-lactacystin involving photocyclization reaction of a cyclopenta-fused pyridinium salt has been probed by using a model substrate. Observations made in this effort led to the discovery of a highly unusual cascade process that leads to stereoselective formation of an interesting tricyclic carbamate. The results of this study are presented and discussed in the context of a (+)-lactacystin synthetic approach.