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Lu NN, Yoo SJ, Li LJ, Zeng CC, Little RD. A comparative study of organic electron transfer redox mediators: electron transfer kinetics for triarylimidazole and triarylamine mediators in the oxidation of 4-methoxybenzyl alcohol. Electrochim Acta 2014. [DOI: 10.1016/j.electacta.2014.07.105] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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27
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Francke R, Little RD. Redox catalysis in organic electrosynthesis: basic principles and recent developments. Chem Soc Rev 2014; 43:2492-521. [DOI: 10.1039/c3cs60464k] [Citation(s) in RCA: 1071] [Impact Index Per Article: 107.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
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Francke R, Little RD. Optimizing electron transfer mediators based on arylimidazoles by ring fusion: synthesis, electrochemistry, and computational analysis of 2-aryl-1-methylphenanthro[9,10-d]imidazoles. J Am Chem Soc 2013; 136:427-35. [PMID: 24328337 DOI: 10.1021/ja410865z] [Citation(s) in RCA: 88] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
A significant improvement of the properties of redox catalysts based on the triarylimidazole framework can be achieved with a simple structural modification. By linking the ortho-carbons of the aromatics positioned at C-4 and C-5, a fused framework is generated, removing the distortion from planarity and enhancing the influence of the substituents on the redox properties. This modification leads not only to a much broader range of available redox potentials for the resulting phenanthro[9,10-d]imidazoles but also to improved stability of the corresponding radical cation. These concepts were verified with eight new phenanthro[9,10-d]imidazole derivatives, using cyclic voltammetry and DFT calculations. For this purpose, an optimized and general synthetic route to the desired compounds was developed. An excellent linear correlation of the calculated effective ionization potentials with the experimental oxidation potentials was obtained, allowing for an accurate prediction of oxidation potentials of derivatives yet to be synthesized. Moreover, high catalytic activity was found for electro-oxidative C-H activation reactions.
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29
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Li C, Zeng CC, Hu LM, Yang FL, Yoo SJ, Little RD. Electrochemically induced CH functionalization using bromide ion/2,2,6,6-tetramethylpiperidinyl-N-oxyl dual redox catalysts in a two-phase electrolytic system. Electrochim Acta 2013. [DOI: 10.1016/j.electacta.2013.10.093] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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30
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Li WC, Zeng CC, Hu LM, Tian HY, Little RD. Efficient Indirect Electrochemical Synthesis of 2-Substituted Benzoxazoles using Sodium Iodide as Mediator. Adv Synth Catal 2013. [DOI: 10.1002/adsc.201300502] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
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31
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Day DR, Jabaiah S, Jacobs RS, Little RD. Cyclodextrin formulation of the marine natural product pseudopterosin A uncovers optimal pharmacodynamics in proliferation studies of human umbilical vein endothelial cells. Mar Drugs 2013; 11:3258-71. [PMID: 24065164 PMCID: PMC3806459 DOI: 10.3390/md11093258] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2013] [Revised: 08/13/2013] [Accepted: 08/14/2013] [Indexed: 11/16/2022] Open
Abstract
Pseudopterosin A (PsA) treatment of growth factor depleted human umbilical vein endothelial cell (HUVEC) cultures formulated in hydroxypropyl-β-cyclodextrin (HPβCD) for 42 h unexpectedly produced a 25% increase in cell proliferation (EC50 = 1.34 × 10−8 M). Analysis of dose response curves revealed pseudo-first order saturation kinetics, and the uncoupling of cytotoxicity from cell proliferation, thereby resulting in a widening of the therapeutic index. The formulation of PsA into HPβCD produced a 200-fold increase in potency over a DMSO formulation; we propose this could result from a constrained presentation of PsA to the receptor, which would limit non-specific binding. These results support the hypothesis that the non-specific receptor binding of PsA when formulated in DMSO has ostensibly masked prior estimates of specific activity, potency, and mechanism. Collectively, these results suggest that the formulation of PsA and compounds of similar chemical properties in HPβCD could result in significant pharmacological findings that may otherwise be obscured when using solvents such as DMSO.
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32
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Zhang NT, Zeng CC, Lam CM, Gbur RK, Little RD. Triarylimidazole Redox Catalysts: Electrochemical Analysis and Empirical Correlations. J Org Chem 2012. [DOI: 10.1021/jo302309m] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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33
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Day DR, Little RD, Jacobs RS. Pseudopterosin A ‐ an Atypical Antagonist of Adenosine A2B Receptors that Accelerates Angiogenesis. FASEB J 2012. [DOI: 10.1096/fasebj.26.1_supplement.836.4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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34
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Gbur RK, Little RD. Unveiling the role of molecule-assisted homolysis: a mechanistic probe into the chemistry of a bicyclic peroxide. J Org Chem 2012; 77:2134-41. [PMID: 22356266 DOI: 10.1021/jo300297u] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Unlike the reaction of aryl-substituted diazenes, pyrolysis of alkyl-substituted diazenes in the presence of molecular oxygen generates an unexpectedly complex product mixture. Using deuterium labeling studies, in conjunction with quantum calculations, a reasonable mechanistic hypothesis for the decomposition of the resultant [3.3.0] peroxide, and subsequent formation of the keto-alcohol and Z-configured α,β-unsaturated keto-aldehyde, is proposed. Surprisingly, molecule-assisted homolysis plays a key role in this transformation, the details of which are discussed herein.
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Zeng CC, Zhang NT, Lam CM, Little RD. Novel Triarylimidazole Redox Catalysts: Synthesis, Electrochemical Properties, and Applicability to Electrooxidative C–H Activation. Org Lett 2012; 14:1314-7. [DOI: 10.1021/ol300195c] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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36
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Bai YX, Ping DW, Little RD, Tian HY, Hu LM, Zeng CC. Electrochemical oxidation of catechols in the presence of ketene N,O-acetals: indole formation versus α-arylation. Tetrahedron 2011. [DOI: 10.1016/j.tet.2011.09.126] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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37
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Zhong W, Little RD. Exploration and determination of the redox properties of the pseudopterosin class of marine natural products. Tetrahedron 2009. [DOI: 10.1016/j.tet.2009.09.021] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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38
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Park YS, Little RD. Rearrangements of electrochemically generated cation radicals: The quest for regiochemical control and an application to total synthesis. Electrochim Acta 2009. [DOI: 10.1016/j.electacta.2009.01.020] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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39
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Zhong W, Moya C, Jacobs RS, Little RD. Synthesis and an evaluation of the bioactivity of the C-glycoside of pseudopterosin A methyl ether. J Org Chem 2008; 73:7011-6. [PMID: 18710290 DOI: 10.1021/jo801432t] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The Suzuki-Miyaura cross-coupling protocol was applied to the synthesis of 1a, the C-glycoside analogue of PsA methyl ether. This marks the first construction of a C-glycoside for this class of marine natural products, thereby offering an opportunity to compare its bioactivity to the natural substances. Its activity profile resembled that of PsA (1) and PsA O-methyl ether (1b) when assayed for its anti-inflammatory activity and its ability to inhibit phagocytosis. We conclude that the intact structure is present when a pseudopterosin expresses its anti-inflammatory and phagocytosis inhibitory properties and that they are, therefore, not likely to be prodrugs. Results show that 1a is an effective binding agent toward the A2A and A3 adenosine receptors, displaying IC50 values of 20 and 10 microM, respectively.
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Park YS, Little RD. Redox Electron-Transfer Reactions: Electrochemically Mediated Rearrangement, Mechanism, and a Total Synthesis of Daucene. J Org Chem 2008; 73:6807-15. [DOI: 10.1021/jo801199s] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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41
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Hoarau C, Day D, Moya C, Wu G, Hackim A, Jacobs RS, Little RD. iso-PsE, a new pseudopterosin. Tetrahedron Lett 2008. [DOI: 10.1016/j.tetlet.2008.05.106] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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42
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Nishiguchi GA, Graham J, Bouraoui A, Jacobs RS, Little RD. 7,11-epi-thyrsiferol: completion of its synthesis, evaluation of its antimitotic properties, and the further development of an SAR model. J Org Chem 2007; 71:5936-41. [PMID: 16872175 DOI: 10.1021/jo060519z] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We (a) describe the completion of a total synthesis of 7,11-epi-thyrsiferol (4), (b) compare the antimitotic activities of thyrsiferol (2), Delta15,28-dehydrothyrsiferol (3), and 7,11-epi-thyrsiferol (4), (c) evaluate the synergistic behavior of the title compound and colchicine to inhibit cell proliferation, and (d) describe the results of conformational searches that provide additional insight concerning the SAR profile of the thyrsiferol family of natural products.
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Park YS, Wang SC, Tantillo DJ, Little RD. A Highly Selective Rearrangement of a Housane-Derived Cation Radical: An Electrochemically Mediated Transformation. J Org Chem 2007; 72:4351-7. [PMID: 17497802 DOI: 10.1021/jo070190x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14*+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained sigma framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.
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Hong J, Yee R, Little RD. Progress toward the synthesis of the bicyclo (6.3.0) framework using TMM diyls. ARKIVOC 2006. [DOI: 10.3998/ark.5550190.0008.719] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022] Open
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45
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Yee R, Mallory J, Parrish J, Carroll GL, Little RD. The influence of electrogenerated Sm(II), electrogenerated Yb(II), and magnesium ions produced at a sacrificial magnesium anode, upon the diastereoselectivity of electroreductive cyclization reactions. J Electroanal Chem (Lausanne) 2006. [DOI: 10.1016/j.jelechem.2006.02.005] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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46
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Miranda JA, Wade CJ, Little RD. Indirect electroreductive cyclization and electrohydrocyclization using catalytic reduced nickel(II) salen. J Org Chem 2006; 70:8017-26. [PMID: 16277323 DOI: 10.1021/jo051148+] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
[Chemical reaction: See text] We describe efforts to achieve the electroreductive cyclization (ERC) and the electrohydrocyclization (EHC) reactions using catalytic nickel(II) salen as a mediator. While nickel(II) salen proved effective, the analogous cobalt complex as well as nickel(II) cyclam were not. The transformations were achieved in yields ranging from 60 to 94% using either a mercury pool or an environmentally preferable reticulated vitreous carbon (RVC) cathode. These examples represent the first instances wherein a nickel salen complex has been used in this manner. Clear differences between the voltammetric behavior of the ERC and EHC substrates were observed. The bisenoate 14, for example, displays a substantially larger catalytic current. When the structurally modified mediator 31 was used, the electron-transfer pathway shuts down. Instead, the reduced form of 31 behaves as an electrogenerated base, leading to the formation of the intramolecular Michael adduct 23. Presumably, the methyl groups of the modified ligand diminish the ability of the reduced form of the complex to serve as a nucleophile but not as a base. Aldehyde 23 was also characterized as a side product of the nickel(II) salen mediated electroreductive cyclization of 11. Given that it is absent from nonmediated processes, its formation is linked to the presence of the mediator. To account for the results, we favor the existence of a mechanistic continuum involving an equilibrium between nickel(II) salen (15) and two reduced forms, one being the metal-centered species 16, the other being a ligand-centered species 17. We postulate that one form may be more prominently involved with the chemistry than another, depending upon the electronic properties/requirements of the substrate, and suggest that the equilibrium will shift to accommodate the need. Thus, for a hard electrophile like an alkyl halide, the properties of 16 ought to dominate, whereas 17 ought to predominate as the reactive species accounting for the chemistry described herein since it properly matches a soft ligand-centered nucleophile with a soft electron deficient alkene.
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A. Miranda J, Daniel Little R. Investigation of Vinylcyclopropane Monoradical Cyclization-Fragmentation as a Possible Route towards Eight-Membered Rings. HETEROCYCLES 2006. [DOI: 10.3987/com-06-s(w)52] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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48
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Nishiguchi GA, Little RD. From C-Glycosides to Pyranopyrans: An Approach to Thyrsiferol Using Titanium(III)-Promoted Redox Couplings. J Org Chem 2005; 70:5249-56. [PMID: 15960529 DOI: 10.1021/jo050465d] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
An approach to the pyranopyran ring system that is found in many natural products, including thyrsiferol, is described. The route entails the assembly of an alpha,beta-unsaturated ketone (11) from geraniol and dihydropyran (23) from acetyl acetaldehyde dimethyl acetal (19) and their titanium(III)-promoted coupling to afford a respectable 60% yield of keto alcohol 26. The sigma-bond formed in this process corresponds to the pro-C(9)-C(10) bond of thyrsiferol (4). Attempts to invert the stereochemistry at the pro-C(11) center were thwarted by the congestion imparted by the presence of the vicinal TBS-ether. Consequently, cyclization of the coupling adduct under conditions developed by Olah and Prakash and co-workers led to the cis-fused pyranopyran 27. X-ray analysis of this crystalline material confirmed each of the stereochemical assignments. After much effort, it was determined that the hydroxyl group at C(12) could be removed by treating the derived methyl xanthate with a tri-n-butylphosphine-borane complex under radical-forming conditions. The reaction sequence worked well, despite the hindered working environment and the presence of a potentially labile C-Br bond.
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49
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50
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Gerken JB, Wang SC, Preciado AB, Park YS, Nishiguchi G, Tantillo DJ, Little RD. Remote Substituent Effects upon the Rearrangements of Housane Cation Radicals. J Org Chem 2005; 70:4598-608. [PMID: 15932295 DOI: 10.1021/jo050087c] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The rearrangement of a series of housane-derived cation radicals was investigated. Surprisingly, 2-aryl-substituted systems rearranged regioselectively and in a process whose selectivity proved to be independent of the electronic character of para substituents. The major reaction pathway is suggested to be the one that allows maximum delocalization, and allows it to be maintained for as long as possible. Bridging is invoked to account for the regio- and stereoselectivity. When a nonbridging trimethylsilylmethyl substituent is appended to C2, the regioselectivity is eroded entirely. B3LYP/6-31G(d) calculations corroborate the notion that bridging plays a role. While bridging ought to stabilize an intermediate by allowing delocalization of the charge/spin, there should be an accompanying entropic penalty. To determine the relative importance of enthalpic and entropic factors in determining the product selectivity, the rearrangement of the p-methoxyphenyl-substituted housane was investigated as a function of temperature. Enthalpic factors dominated over the entire temperature range that was explored. Overall, the results indicate that it is possible to influence the direction of migration in housane-derived cation radical rearrangements even when the regiochemical control unit is not directly appended to the migration terminus. This finding suggests that there may be other substituents that can be placed at C2 that could do the same, perhaps more efficiently.
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