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Hardy S, Martin SF. Multicomponent, Mannich-type assembly process for generating novel, biologically-active 2-arylpiperidines and derivatives. Tetrahedron 2014; 70:7142-7157. [PMID: 25267860 PMCID: PMC4175438 DOI: 10.1016/j.tet.2014.06.045] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
A multicomponent, Mannich-type assembly process commencing with commercially available bromobenzaldehydes was sequenced with [3+2] dipolar cycloaddition reactions involving nitrones and azomethine ylides to generate collections of fused, bicyclic scaffolds based on the 2-arylpiperidine subunit. Use of the 4-pentenoyl group, which served both as an activator in the Mannich-type reaction and a readily-cleaved amine protecting group, allowed sub-libraries to be prepared through piperidine N-functionalization and cross-coupling of the aryl bromide. A number of these derivatives displayed biological activities that had not previously been associated with this substructure. Methods were also developed that allowed rapid conversion of these scaffolds to novel, polycyclic dihydroquinazolin-2-ones, 2-imino-1,3-benzothiazinanes, dihydroisoquinolin-3-ones and bridged tetrahydroquinolines.
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Abstract
A concise total synthesis of the complex indole alkaloid (±)-actinophyllic acid was accomplished by a sequence of reactions requiring only 10 steps from readily-available, known starting materials. The approach featured a Lewis acid-catalyzed cascade of reactions involving stabilized carbocations that delivered the tetracyclic core of the natural product in a single chemical operation. Optimal conversion of this key intermediate into (±)-actinophyllic acid required judicious selection of a protecting group strategy.
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Myslinski JM, Clements JH, Martin SF. Protein-ligand interactions: probing the energetics of a putative cation-π interaction. Bioorg Med Chem Lett 2014; 24:3164-7. [PMID: 24856058 DOI: 10.1016/j.bmcl.2014.04.114] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2014] [Revised: 04/25/2014] [Accepted: 04/28/2014] [Indexed: 01/01/2023]
Abstract
In order to probe the energetics associated with a putative cation-π interaction, thermodynamic parameters are determined for complex formation between the Grb2 SH2 domain and tripeptide derivatives of RCO-pTyr-Ac6c-Asn wherein the R group is varied to include different alkyl, cycloalkyl, and aryl groups. Although an indole ring is reputed to have the strongest interaction with a guanidinium ion, binding free energies, ΔG°, for derivatives of RCO-pTyr-Ac6c-Asn bearing cyclohexyl and phenyl groups were slightly more favorable than their indolyl analog. Crystallographic analysis of two complexes reveals that test ligands bind in similar poses with the notable exception of the relative orientation and proximity of the phenyl and indolyl rings relative to an arginine residue of the domain. These spatial orientations are consistent with those observed in other cation-π interactions, but there is no net energetic benefit to such an interaction in this biological system. Accordingly, although cation-π interactions are well documented as important noncovalent forces in molecular recognition, the energetics of such interactions may be mitigated by other nonbonded interactions and solvation effects in protein-ligand associations.
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54
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Martin SF, Clements JH. Correlating structure and energetics in protein-ligand interactions: paradigms and paradoxes. Annu Rev Biochem 2013; 82:267-93. [PMID: 23746256 DOI: 10.1146/annurev-biochem-060410-105819] [Citation(s) in RCA: 58] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Predicting protein-binding affinities of small molecules, even closely related ones, is a formidable challenge in biomolecular recognition and medicinal chemistry. A thermodynamic approach to optimizing affinity in protein-ligand interactions requires knowledge and understanding of how altering the structure of a small molecule will be manifested in protein-binding enthalpy and entropy changes; however, there is a relative paucity of such detailed information. In this review, we examine two strategies commonly used to increase ligand potency. The first of these involves introducing a cyclic constraint to preorganize a small molecule in its biologically active conformation, and the second entails adding nonpolar groups to a molecule to increase the amount of hydrophobic surface that is buried upon binding. Both of these approaches are motivated by paradigms suggesting that protein-binding entropy changes should become more favorable, but paradoxes can emerge that defy conventional wisdom.
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55
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Myslinski JM, Clements JH, DeLorbe JE, Martin SF. Protein-Ligand Interactions: Thermodynamic Effects Associated with Increasing the Length of an Alkyl Chain. ACS Med Chem Lett 2013; 4. [PMID: 24349642 DOI: 10.1021/ml400211q] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Thermodynamic parameters were determined for complex formation between the Grb2 SH2 domain and tripeptides of the general form Ac-pTyr-Xaa-Asn in which the Xaa residue bears a linear alkyl chain varying in length from 1-5 carbon atoms. Binding affinity increases upon adding a methylene group to the Ala derivative, but further chain extension gives no extra enhancement in potency. The thermodynamic signatures of the ethyl and n-propyl derivatives are virtually identical as are those for the n-butyl and n-pentyl analogs. Crystallographic analysis of the complexes reveals a high degree of similarity in the structure of the domain and the bound ligands with the notable exception that there is a gauche interaction in the side chains in the bound conformations of ligands having n-propyl, n-butyl, and n-pentyl groups. However, eliminating this unfavorable interaction by introducing a Z-double bond into the side chain of the n-propyl analog does not result in an increase in affinity. Increases in the amount of nonpolar surface that is buried upon ligand binding correlate with favorable changes in ΔH°, but these are usually offset by corresponding unfavorable changes in -TΔS°; there is little correlation of ΔCp with changes in the amount of buried nonpolar surface.
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Shanahan CS, Fang C, Paull DH, Martin SF. Asymmetric Formal Total Synthesis of the Stemofoline Alkaloids: The Evolution, Development and Application of a Catalytic Dipolar Cycloaddition Cascade. Tetrahedron 2013; 69:7592-7607. [PMID: 24072939 DOI: 10.1016/j.tet.2013.03.104] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
A formal synthesis of didehydrostemofoline and isodidehydrostemofoline has been accomplished by preparing an intermediate in the Overman synthesis of these alkaloids from commercially available 2-deoxy-D-ribose. The work presented in this account chronicles the evolution of our explorations to identify the optimal steric and electronic control elements necessary to generate the tricyclic core structure of these alkaloids in a single operation from an acyclic precursor. The key step in the synthesis is a novel dipolar cycloaddition cascade sequence that is initiated by cyclization of a rhodium-derived carbene onto the nitrogen atom of a proximal imine group to generate an azomethine ylide that then undergoes spontaneous cyclization via dipolar cycloaddition. The synthesis features several other interesting reactions, including a Boord elimination to prepare a chiral allylic alcohol, a highly diastereoselective Hirama-Itô cyclization, and a useful modification of the Barton decarboxylation protocol.
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57
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Granger BA, Jewett IT, Butler JD, Hua B, Knezevic CE, Parkinson EI, Hergenrother PJ, Martin SF. Synthesis of (±)-Actinophyllic Acid and Analogs: Applications of Cascade Reactions and Diverted Total Synthesis. J Am Chem Soc 2013; 135:12984-6. [DOI: 10.1021/ja4070206] [Citation(s) in RCA: 62] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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58
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Bian Z, Marvin CC, Martin SF. Enantioselective total synthesis of (-)-citrinadin A and revision of its stereochemical structure. J Am Chem Soc 2013; 135:10886-9. [PMID: 23837457 DOI: 10.1021/ja405547f] [Citation(s) in RCA: 81] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The first enantioselective total synthesis of (-)-citrinadin A has been accomplished in 20 steps from commercially available materials via an approach that minimizes refunctionalization and protection/deprotection operations. The cornerstone of this synthesis features an asymmetric vinylogous Mannich addition of a dienolate to a chiral pyridinium salt to set the initial chiral center. A sequence of substrate-controlled reactions, including a highly stereoselective epoxidation/ring-opening sequence and an oxidative rearrangement of an indole to furnish a spirooxindole, are then used to establish the remaining stereocenters in the pentacyclic core of (-)-citrinadin A. The successful synthesis of citrinadin A led to a revision of the stereochemical structure of the core substructure of the citrinadins.
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59
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Granger BA, Wang Z, Kaneda K, Fang Z, Martin SF. Multicomponent assembly processes for the synthesis of diverse yohimbine and corynanthe alkaloid analogues. ACS COMBINATORIAL SCIENCE 2013; 15:379-86. [PMID: 23697376 DOI: 10.1021/co400055b] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
A strategy involving a Mannich-type multicomponent assembly process followed by a 1,3-dipolar cycloaddition has been developed for the rapid and efficient construction of parent heterocyclic scaffolds bearing indole and isoxazolidine rings. These key intermediates were then readily elaborated using well-established protocols for refunctionalization and cross-coupling to access a diverse 180-member library of novel pentacyclic and tetracyclic compounds related to the Yohimbine and Corynanthe alkaloids. Several other new multicomponent assembly processes were developed to access dihydro-β-carboline-fused benzodiazepines, pyrimidinediones, and rutaecarpine derivatives.
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60
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Fu TH, McElroy WT, Shamszad M, Heidebrecht RW, Gulledge B, Martin SF. Studies toward welwitindolinones: formal syntheses of N-methylwelwitindolinone C isothiocyanate and related natural products. Tetrahedron 2013; 69:5588-5603. [PMID: 23976796 PMCID: PMC3748834 DOI: 10.1016/j.tet.2013.03.010] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The formal syntheses of N-methylwelwitindolinone C isothiocyanate (4) and several other welwitindolinones 5-8 were achieved by the independent synthesis of 79. The synthesis featured a Lewis acid-mediated coupling between a heteroaryl carbinol and bis-TMS enol ether, an intramolecular enolate arylation, and an unprecedented intramolecular allylic alkylation of a γ-acyloxyenone.
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61
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Fang C, Paull DH, Hethcox JC, Shugrue CR, Martin SF. Enantioselective iodolactonization of disubstituted olefinic acids using a bifunctional catalyst. Org Lett 2013; 15:972. [PMID: 23356976 DOI: 10.1021/ol400125b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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62
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Hethcox JC, Shanahan CS, Martin SF. Total synthesis of (-)-dihydroprotolichesterenic acid via diastereoselective conjugate addition to chiral fumarates. Tetrahedron Lett 2013; 54:2074-2076. [PMID: 23539490 DOI: 10.1016/j.tetlet.2013.02.014] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
A diastereoselective conjugate addition of a variety of monoorganocuprates, Li[RCuI], to chiral fumarates to provide funtionalized succinates has been developed. The utility of this reaction is demonstrated in a concise total synthesis of (-)-dihydroprotolichesterenic acid that required only four steps and proceeded in an overall 31% yield.
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63
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Martin SF. Strategies for the Synthesis of Alkaloids and Novel Nitrogen Heterocycles. ADVANCES IN HETEROCYCLIC CHEMISTRY 2013. [DOI: 10.1016/b978-0-12-408100-0.00003-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/04/2023]
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64
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Fang C, Paull DH, Hethcox JC, Shugrue CR, Martin SF. Enantioselective iodolactonization of disubstituted olefinic acids using a bifunctional catalyst. Org Lett 2012; 14:6290-3. [PMID: 23199100 PMCID: PMC3528805 DOI: 10.1021/ol3030555] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The enantioselective iodolactonizations of a series of diversely substituted olefinic carboxylic acids are promoted by a BINOL-derived, bifunctional catalyst. Reactions involving 5-alkyl- and 5-aryl-4(Z)-pentenoic acids and 6-alkyl- and 6-aryl-5(Z)-hexenoic acids provide the corresponding γ- and δ-lactones having stereogenic C-I bonds in excellent yields and >97:3 er. Significantly, this represents the first organocatalyst that promotes both bromo- and iodolactonization with high enantioselectivities. The potential of this catalyst to induce kinetic resolutions of racemic unsaturated acids is also demonstrated.
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65
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Fang C, Shanahan CS, Paull DH, Martin SF. Enantioselective formal total syntheses of didehydrostemofoline and isodidehydrostemofoline through a catalytic dipolar cycloaddition cascade. Angew Chem Int Ed Engl 2012; 51:10596-9. [PMID: 22987285 PMCID: PMC3541778 DOI: 10.1002/anie.201205274] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2012] [Indexed: 11/09/2022]
Abstract
Sweet to the core: Enantioselective formal total syntheses of the title compounds were accomplished in 24 steps from 2-deoxy-D-ribose. The synthesis features a novel cascade of reactions culminating in an intramolecular dipolar cycloaddition to form the tricyclic core of the stemofoline alkaloids from an acyclic diazo imine intermediate.
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66
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Fang C, Shanahan CS, Paull DH, Martin SF. Enantioselective Formal Total Syntheses of Didehydrostemofoline and Isodidehydrostemofoline through a Catalytic Dipolar Cycloaddition Cascade. Angew Chem Int Ed Engl 2012. [DOI: 10.1002/ange.201205274] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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67
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Nichols AL, Zhang P, Martin SF. Concise Approach to 1,4-Dioxygenated Xanthones via Novel Application of the Moore Rearrangement. Tetrahedron 2012; 68:7591-7597. [PMID: 22962501 DOI: 10.1016/j.tet.2012.05.094] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
The rapid synthesis of 1,4-dioxygenated xanthones and related natural products employing the Moore rearrangement as a key transformation has been developed. The approach features an acetylide stitching step to unite a substituted squaric acid with a protected hydroxy benzaldehyde derivative to provide a key intermediate that undergoes facile Moore rearrangement to deliver a hydroxymethyl aryl quinone. Subsequent oxidation, hydroxy group deprotection and cyclization then affords highly functionalized xanthones. The utility of the approach was demonstrated by its application to a concise and efficient synthesis of the naturally-occurring xanthone 1. The structure of a natural product that had been named dulcisxanthone C was also corrected to that of the xanthone 1.
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68
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Sahn JJ, Martin SF. Expedient synthesis of norbenzomorphan library via multicomponent assembly process coupled with ring-closing reactions. ACS COMBINATORIAL SCIENCE 2012; 14:496-502. [PMID: 22857149 DOI: 10.1021/co300068a] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A 124-member norbenzomorphan library has been prepared utilizing a novel multicomponent assembly process (MCAP) followed by a variety of ring-closing reactions to generate norbenzomorphan scaffolds that were readily derivatized via a series of aryl halide cross-coupling and nitrogen functionalization reactions. Biological screening has revealed some novel activities that have not been previously associated with this class of compounds.
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69
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Orr STM, Tian J, Niggemann M, Martin SF. Novel Approach to the Lundurine Alkaloids: Synthesis of the Tetracyclic Core. Org Lett 2012; 14:4701. [DOI: 10.1021/ol302236g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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70
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Fu TH, McElroy WT, Shamszad M, Martin SF. Formal syntheses of naturally occurring welwitindolinones. Org Lett 2012; 14:3834-7. [PMID: 22830424 DOI: 10.1021/ol301424h] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The formal syntheses of N-methylwelwitindolinone C isothiocyanate, N-methylwelwitindolinone C isonitrile, N-methylwelwitindolinone D isonitrile, 3-hydroxy-N-methylwelwitindolinone C isothiocyanate, and 3-hydroxy-N-methylwelwitindolinone C isonitrile are reported. The synthesis features several novel processes, including a Lewis acid mediated coupling between a benzylic-type heteroaromatic alcohol and a highly functionalized silyl ketene acetal, an intramolecular enolate arylation, and a regioselective, Pd(0)-catalyzed π-allylic cyclization of a γ-benzoyloxy enone moiety that is revealed by unmasking a furan ring.
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71
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Paull DH, Fang C, Donald JR, Pansick AD, Martin SF. Bifunctional catalyst promotes highly enantioselective bromolactonizations to generate stereogenic C-Br bonds. J Am Chem Soc 2012; 134:11128-31. [PMID: 22726214 PMCID: PMC3397382 DOI: 10.1021/ja305117m] [Citation(s) in RCA: 155] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
A novel bifunctional catalyst derived from BINOL has been developed that promotes the highly enantioselective bromolactonizations of a number of structurally distinct unsaturated acids. Like some known catalysts, this catalyst promotes highly enantioselective bromolactonizations of 4- and 5-aryl-4-pentenoic acids, but it also catalyzes the highly enantioselective bromolactonizations of 5-alkyl-4(Z)-pentenoic acids. These reactions represent the first catalytic bromolactonizations of alkyl-substituted olefinic acids that proceed via 5-exo mode cyclizations to give lactones in which new carbon-bromine bonds are formed at a stereogenic center with high enantioselectivity. We also disclose the first catalytic desymmetrization of a prochiral dienoic acid by enantioselective bromolactonization.
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72
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Fu TH, Bonaparte A, Martin SF. Corrigendum to “Synthesis of β-heteroaryl propionates via trapping of carbocations with π-nucleophiles” [Tetrahedron Lett. 50 (2009) 3253–3257]. Tetrahedron Lett 2012. [DOI: 10.1016/j.tetlet.2012.04.102] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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73
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Wang Z, Kaneda K, Fang Z, Martin SF. Corrigendum to “Diversity oriented synthesis: concise entry to novel derivatives of Yohimbine and Corynanthe alkaloids” [Tetrahedron Lett. 53 (2011) 477–479]. Tetrahedron Lett 2012. [DOI: 10.1016/j.tetlet.2012.01.100] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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74
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Donald JR, Wood RR, Martin SF. Application of a sequential multicomponent assembly process/huisgen cycloaddition strategy to the preparation of libraries of 1,2,3-triazole-fused 1,4-benzodiazepines. ACS COMBINATORIAL SCIENCE 2012; 14:135-43. [PMID: 22273436 DOI: 10.1021/co2002087] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A strategy featuring a multicomponent assembly process followed by an intramolecular azide-alkyne dipolar (Huisgen) cycloaddition was implemented for the facile synthesis of three different 1,2,3-triazolo-1,4-benzodiazepine scaffolds. A diverse library of 170 compounds derived from these scaffolds was then created through N-functionalizations, palladium-catalyzed cross-coupling reactions, and several applications of α-aminonitrile chemistry.
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75
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Shi Y, Zhu CZ, Martin SF, Ren P. Probing the effect of conformational constraint on phosphorylated ligand binding to an SH2 domain using polarizable force field simulations. J Phys Chem B 2012; 116:1716-27. [PMID: 22214214 PMCID: PMC3277292 DOI: 10.1021/jp210265d] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Preorganizing a ligand in the conformation it adopts upon binding to a protein has long been considered to be an effective way to improve affinity by making the binding entropy more favorable. However, recent thermodynamic studies of a series of complexes of the Grb2 SH2 domain with peptide analogues having constrained and flexible replacements for a phosphotyrosine residue revealed that less favorable binding entropies may result from constraining ligands in their biologically active conformations. Toward probing the origin of this unexpected finding, we examined the complexes of four phosphotyrosine-derived analogues with the Grb2 SH2 domain using molecular dynamics simulations with a polarizable force field. Significantly, the computed values for the relative binding free energies, entropies, and enthalpies of two pairs of constrained and unconstrained ligands reproduced the trends that were determined experimentally, although the relative differences were overestimated. These calculations also revealed that a large fraction of the ligands lacking the constraining element exist in solution as compact, macrocyclic-like structures that are stabilized by interactions between the phosphate groups and the amide moieties of the C-terminal pY+2 residues. In contrast, the three-membered ring in the constrained ligands prevents the formation of such macrocyclic structures, leading instead to globally extended, less ordered conformations. Quasiharmonic analysis of these conformational ensembles suggests that the unconstrained ligands possess significantly lower entropies in solution, a finding that is consistent with the experimental observation that the binding entropies for the unconstrained ligands are more favorable than for their constrained counterparts. This study suggests that introducing local constraints in flexible molecules may have unexpected consequences, and a detailed understanding of the conformational preferences of ligands in their unbound states is a critical prerequisite to correlating changes in their chemical structure with protein binding entropies and enthalpies.
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