Ene Reaction Of Singlet Oxygen Research Articles

Bioinspired Network Analysis Enabled Divergent Syntheses and Structure Revision of Pentacyclic Cytochalasans.

We accomplished the divergent total syntheses of ten pentacyclic cytochalasans (aspergillin PZ, trichodermone, trichoderones, flavipesines, and flavichalasines) from a common precursor aspochalasin D and revised the structures of trichoderone B, spicochalasin A, flavichalasine C, aspergilluchalasin based on structure network analysis of the cytochalasans biosynthetic pathways and DFT calculations. The key steps of the syntheses include transannular alkene/epoxyalkene and carbonyl-ene cyclizations to establish the C/D ring of pentacyclic aspochalasans. Our bioinspired approach to these pentacyclic cytochalasans validate the proposed biosynthetic speculation from a chemical view and provide a platform for the synthesis of more than 400 valuable cytochalasans bearing different macrocycles and amino-acid residues.

Influence of Functional Groups on the Ene Reaction of Singlet Oxygen with 1,4-Cyclohexadienes.

The photooxygenation of 1,4-cyclohexadienes has been studied with a special focus on regio- and stereoselectivities. In all examples, only the methyl-substituted double bond undergoes an ene reaction with singlet oxygen, to afford hydroperoxides in moderate to good yields. We explain the high regioselectivities by a "large-group effect" of the adjacent quaternary stereocenter. Nitriles decrease the reactivity of singlet oxygen, presumably by quenching, but can stabilize proposed per-epoxide intermediates by polar interactions resulting in different stereoselectivities. Spiro lactams and lactones show an interesting effect on regio- and stereoselectivities of the ene reactions. Thus, singlet oxygen attacks the double bond preferentially anti to the carbonyl group, affording only one regioisomeric hydroperoxide. If the reaction occurs from the opposite face, the other regioisomer is exclusively formed by severe electrostatic repulsion in a perepoxide intermediate. We explain this unusual behavior by the fixed geometry of spiro compounds and call it a "spiro effect" in singlet oxygen ene reactions.

Photooxidation of 2-(tert-Butyl)-3-Methyl-2,3,5,6,7,8-Hexahydroquinazolin-4(1H)-one, an Example of Singlet Oxygen ene Reaction.

Singlet oxygen ene reactions produce 2-(tert-butyl)-4a-hydroperoxy-3-methyl-2,4a, 5,6,7,8-hexahydroquinazolin-4(3H)-one quantitatively during diffusion crystallization of 2-(tert-butyl)-3-methyl-2,3,5,6,7,8-hexahydroquinazolin-4(1H)-one in n-hexane/CH2Cl2 solvent mixture. To confirm this photo-oxidation, a 1H-NMR study in CDCl3 was performed with exposure to ambient conditions (light and oxygen), with neither additional reactants nor catalysts. A theoretical study at the B3LyP/6311++G** level using the QST2 method of locating transition states suggests a two-step mechanism where the intermediate, which unexpectedly did not come from the peroxide intermediate, has a low activation energy.

Isolation and Biomimetic Oxidation of Prenylbruceol A, an Anticipated Meroterpenoid Natural Product from Philotheca myoporoides

Reinvestigation of the coumarin meroterpenoids of Philotheca myoporoides using pressurized hot water extraction (PHWE) procedures led to the isolation of prenylbruceol A, a proposed biosynthetic precursor of seven previously reported bruceol derivatives, prenylbruceols B-H. Protobruceol-I, ostruthin, dipetalactone, and a new dihydrocoumarin natural product were isolated alongside prenylbruceol A. A biomimetic singlet oxygen ene reaction of prenylbruceol A allowed the semisynthesis of prenylbruceols B, C, and D.

Evaluation of Cytotoxic Potentials of Some Isoindole-1, 3-Dione Derivatives on HeLa, C6 and A549 Cancer Cell Lines.

Norcantharimides are known as norcantharidine derivatives and contain an isoindole skeleton structure. Isoindole derivatives have positive effect on inflammatory pathologies including cancers. Considering this information, firstly, isoindole derivatives containing different functional groups 4-13 have been synthesized from 2-alkyl/aryl-3a, 4,7,7a-tetrahydro-1H-isoindole-1, 3(2H)-dione. For the synthesis of all compounds, 2-alkyl/aryl-3a, 4,7,7a-tetrahydro-1H-isoindole- 1,3(2H)-dione was used as the starting compound. The syntheses were based on two main reactions: Ene-reaction of singlet oxygen and epoxidation. Secondly, their anticancer activities were evaluated against HeLa, C6 and A549 cancer cell lines by the BrdU assay. Anticancer activities of synthesized compounds (4-13) and 5-FU (5-Florouracil) against HeLa, C6 and A549 cells were investigated at four concentrations (100, 50, 25 and 5 μM). IC50 values of compounds 4-13 were calculated for all cancer cell lines. The investigated compounds showed anticancer activity against the cancer cell lines depending on doses. Compound 7 containing azide and silyl ether exhibited higher inhibitory activity than the other compounds and 5-FU against A549 cancer cell lines (IC50 =19.41± 0.01 μM). Compounds 9 and 11 were determined to exhibit cell-selective activity against HeLa cancer cell lines. Compound 11 had higher activity than the positive control at 100 μM concentrations against C6 cancer cell lines. According to the results observed, isoindole derivatives 7, 9, and 11 might be good potential anticancer agents for the treatment of cervical and glioma cancer due to their antiproliferative properties, having less cytotoxic effects on healthy cells. In addition, compound 7 could be used in in vivo studies of all three-cancer cell lines (C6, HeLa, and A549).

Total Synthesis of Rhodonoids A, B, E, and F, Enabled by Singlet Oxygen Ene Reactions.

Singlet oxygen is a versatile reagent for the selective oxidation of organic compounds under mild reaction conditions. It is frequently invoked in biosynthetic pathways, so it is especially suitable for application in the biomimetic synthesis of natural products. Herein, we show that use of the singlet oxygen ene reaction, combined with [2 + 2] cycloadditions, leads to concise, divergent, and redox-economic total syntheses of several polycyclic members of the rhodonoid family of meroterpenoids.

Biomimetic Synthetic Studies on the Bruceol Family of Meroterpenoid Natural Products.

A biomimetic approach to total synthesis can offer several benefits, including the development of cascade reactions for the rapid generation of molecular complexity, and guidance in the structure revision of old natural products and the anticipation of new ones. Herein, we describe how a biomimetic synthesis of bruceol, a pentacyclic meroterpenoid, led to the anticipation, isolation, and synthesis of isobruceol. The key step in the synthesis of both bruceol and isobruceol was an intramolecular hetero-Diels-Alder reaction of an o-quinone methide that was formed by dearomatization of an electron-rich chromene. The synthesis of an elusive biosynthetic intermediate also allowed a concise synthesis of eriobrucinol via a photochemical [2 + 2] cycloaddition. Furthermore, some speculation on the biosynthesis of prenylated bruceol derivatives inspired the development of a Claisen/Cope/Diels-Alder cascade reaction. We also report the generation of halogenated bruceol derivatives and the synthesis of several protobruceol natural products using singlet oxygen ene reactions.

Two Useful Directing Modes in Singlet Oxygen Reactivity: Electrostatic Effects in the Ene Reaction with Allylic Alcoholates and a Chemoselectivity Change with α‐Alkoxy Michael Esters

AbstractThe front cover artwork is provided by Axel Griesbeck and Diana But at the University of Cologne (Germany). The image shows a cartoon of the hydrogen‐transfer process initiating the singlet oxygen ene reaction with several strong regio‐ and stereodirecting effects following the rule of thumb: If you give singlet oxygen your little finger it will soon have your whole hand (as adapted from a quote by Sigmund Freud). Read the full text of the Article at10.1002/cptc.201800133.

Two Useful Directing Modes in Singlet Oxygen Reactivity: Electrostatic Effects in the Ene Reaction with Allylic Alcoholates and a Chemoselectivity Change with α‐Alkoxy Michael Esters

AbstractThe singlet oxygen reaction with trisubstituted E‐allylic alcohols resulted in the unselective formation of regioisomeric secondary and tertiary allylic hydroperoxides. Increasing the steric demand at the allylic hydroxymethyl group leads to a moderate large‐group effect that can be strongly enhanced by deprotonation of a dimethylated allylic alcohol and even more pronounced by cation complexation. The presence of alkoxide groups did additionally alter the diastereoselectivity of the singlet oxygen ene reactions with chiral allylic alcohols. The molecular combination of Michael ester and enol ether in α‐alkoxy ester substrates made possible the design of chemoselective substrates for singlet oxygen ene versus [2+2] cycloaddition, e. g. a 3,3‐dimethylated Michael ester as a selective substrate for the ene reaction and an adamantyl analogue as a selective 2+2 substrate. The Z/E‐monomethylated substrates as probes for cis regioselectivity revealed that there is no site preference for Z‐alkoxy hydrogen transfer on the way to the corresponding ene product.

Total Syntheses of Highly Oxidized ent-Kaurenoids Pharicin A, Pharicinin B, 7-O-Acetylpseurata C, and Pseurata C: A [5+2] Cascade Approach

The unprecedented oxidative dearomatization-induced [5+2] cycloaddition/pinacol-type 1,2-acyl migration cascade efficiently generates a quaternary carbon center and assembles the highly oxygenated bicyclo[3.2.1]octane framework of ent-kaurene diterpenoids. By incorporation of the subsequent retro-aldol/aldol process and singlet oxygen ene reaction, this concise and convergent approach has enabled the first asymmetric total syntheses of pharicin A, pharicinin B, 7-O-acetylpseurata C, and pseurata C.

Strong Asymmetry in the Perepoxide Bifurcation Mechanism: The Large‐Group Effect in the Singlet Oxygen Ene Reaction with Allylic Alcohols

AbstractThe singlet oxygen reactions of sterically crowded allylic alcohols result in the preferential formation of γ‐hydroperoxy alcohols with regioselectivities up to 98:2. The kinetic cis effect still prevails in the first, rate‐determining step of this two‐step non‐intermediate ene mechanism (kE‐5/kZ‐5=1.9). The primary allylic alcohol 3 showed no regioselectivity at all, increasing the steric demands at the α carbon and directing the singlet oxygen addition towards the γ carbon up to 98:2 regioselectivity. DFT calculations rationalized these results and show the decisive role of the symmetry‐breaking bifurcation following the early transition states. The use of the products as substrates for the synthesis of five‐ and seven‐membered ring peroxides, namely 4‐methylene‐1,2‐dioxolanes and 6‐methylene‐1,2,4‐trioxepanes, by rare earth metal complexes as Lewis acids is also demonstrated.

Computed Regioselectivity and Conjectured Biological Activity of Ene Reactions of Singlet Oxygen with the Natural Product Hyperforin.

Hyperforin is a constituent of St. John's wort and coexists with the singlet oxygen sensitizer hypericin. Density functional theory, molecular mechanics and Connolly surface calculations show that accessibility in the singlet oxygen "ene" reaction favors the hyperforin "southwest" and "southeast" prenyl (2-methyl-2-butenyl) groups over the northern prenyl groups. While the southern part of hyperforin is initially more susceptible to oxidation, up to 4 "ene" reactions of singlet oxygen can take place. Computational results assist in predicting the fate of adjacent hydroperoxides in hyperforin, where the loss of hydrogen atoms may lead to the formation of a hydrotrioxide and a carbonyl instead of a Russell reaction.

ChemInform Abstract: Model Studies on Peroxidic Glutathione Transferase (GST) Inhibitors: C5‐Methylated 1,2,4‐Trioxanes with C6‐Acrylate Side Chains.

AbstractThe diastereoselective porphyrin‐sensitized singlet oxygen ene reaction of allylic alcohol (I) gives the key precursor (II), which can be easily converted into the title trioxane derivatives.

Model Studies on Peroxidic Glutathione Transferase (GST) Inhibitors: C5‐Methylated 1,2,4‐Trioxanes with C6‐Acrylate Side Chains

AbstractThe 1,2‐hydroxy hydroperoxide 7 was obtained with high threo diastereoselectivity from the ester‐conjugated allylic alcohol 6 through a porphyrin‐sensitized singlet oxygen ene reaction. Compound 7 was used as substrate for peroxy(trans)acetalization with aldehydes, orthoesters, and acetals, catalyzed by boron and indium Lewis acids, as well as by PPTS as a Brønsted acid catalyst (for trioxane 10). The 1,2,4‐trioxane 8 was formed with high diasteroselectivity with all‐equatorially positioned substitutents at C‐3, C‐5, and C‐6. Additionally, perorthoester 9 was obtained by indium(III)‐catalyzed condensation of an orthoester with the hydroperoxide 7. The new compounds 8 and 9 exhibited moderate inhibition of human placental glutathione transferase (GST), comparable to that shown by the 5‐unsubstituted model trioxane 11.

Tetraphenylporphyrin‐Catalyzed Tandem Photooxygenation of Polyenes and 1,4‐Dienes: Multiple and Diverse Oxyfunctionalizations

AbstractTandem singlet oxygen reactions photocatalyzed by meso‐tetraphenylporphyrin were performed with substrates designed for primary α‐gem‐selectivity or high vinylogous gem‐selectivity. The 1,3,5‐triene ester 1 was designed as a model compound for the natural compound crocin and resulted in a highly regioselective singlet oxygen ene reaction with ε‐hydrogen activation (93:7 ene vs. [4+2] cycloaddition). The initially formed hydroperoxide 2a adds another singlet oxygen molecule in a slower reaction step to furnish the endoperoxide 3a. From the hydroperoxide stage, oxygen transfer gives the epoxide 4, reduction leads to the allylic alcohol 2b and subsequent photooxygenation (to 3b) followed by reduction results in the furan derivative 5. The allylic alcohol could also be reacted with other dienophiles as shown for the PTAD adduct 6. From the 1,4‐diene 7, a gem‐regioselective singlet oxygen ene reaction generates a mixture of the hydroperoxides 8a and 9a. Product 8a adds another singlet oxygen molecule to give the endoperoxide 10a. Catalyst activity was demonstrated by analysis of the changes in the electronic absorption properties.magnified image

ChemInform Abstract: Recent Developments in Solvent and Salt Effects on Ene Reactions of Singlet Oxygen

ChemInform Abstract: Recent Developments in Solvent and Salt Effects on Ene Reactions of Singlet Oxygen

Intermediates in the Formation and Thermolysis of Peroxides from Oxidations with Singlet Oxygen

Herein we describe the recent mechanistic understandings of the singlet oxygen ene reaction to give hydroperoxides and the [4+2] cycloaddition affording endoperoxides. Both experimental findings and theoretical work conclude in the formation of intermediates structurally similar to perepoxides during the ene reaction. Such intermediates mainly control the regio- and stereoselectivities of this reaction class. For the [4+2] cycloaddition, both a synchronous concerted reaction (benzene, naphthalenes) and a stepwise reaction with a non-symmetric zwitterionic intermediate (larger acenes) have been found. The thermolysis of endoperoxides derived from acenes proceeds stepwise for anthracenes, but in a concerted manner for less stable adducts such as naphthalene.

Recent Developments in Solvent and Salt Effects on Ene Reactions of Singlet Oxygen

Recent Developments in Solvent and Salt Effects on Ene Reactions of Singlet Oxygen

Comparison of the singlet oxygen ene reactions of cyclic versus acyclic β,γ-unsaturated ketones: an experimental and computational study

The photooxygenation of two β,γ-unsaturated ketones was studied by experimental and computational methods: 5-methyl-hex-4-en-2-one (1) and cyclohex-3-en-1-one (6) as model compounds for acyclic versus cyclic deconjugated enones. The open-chain substrate delivered a 1:1 mixture of regioisomers 2a,b following the established cis-selectivity model whereas the cyclic substrate reacts with 1O2 to give preferentially the conjugated product 7. This effect is in agreement with the mechanistic two-stage no-intermediate model and on a computational level corresponds to a regioselectivity control following the steepest decent pathway from the corresponding transition stages in a valley ridge potential energy surface region.

ChemInform Abstract: Computing Free‐Energy Profiles Using Multidimensional Potentials of Mean Force and Polynomial Quadrature Methods

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