- Re-Engineering Organocatalysts for Asymmetric Friedel-Crafts Alkylation of Indoles through Computational Studies
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The discovery of efficient organocatalysts is generally carried out by thorough experimental screening of different candidates. We recently reported an efficient organocatalyst for iminium-ion-based asymmetric Diels-Alder reactions following a rational design approach. This result encouraged us to test this optimal catalyst in the mechanistically related Friedel-Crafts alkylation of indoles, but to our surprise, almost null enantioselectivity was observed. The results did not significantly improve with structurally related catalysts, and a totally unexpected facial selectivity inversion was also noticed. Using DFT calculations by modeling the competing transition structures with ONIOM, we could unravel the origins of those findings, further employed to predict the most efficient catalyst for this new transformation. The computational results were validated experimentally (up to 92:8 er), providing another successful example of a general strategy to accelerate catalyst development which still remains underexplored.
- Gerosa, Gabriela G.,Marcarino, Maribel O.,Spanevello, Rolando A.,Suárez, Alejandra G.,Sarotti, Ariel M.
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p. 9969 - 9978
(2020/09/03)
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- Utilization of fluoroform for difluoromethylation in continuous flow: A concise synthesis of α-difluoromethyl-amino acids
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Fluoroform (CHF3) can be considered as an ideal reagent for difluoromethylation reactions. However, due to the low reactivity of fluoroform, only very few applications have been reported so far. Herein we report a continuous flow difluoromethyl
- K?ckinger, Manuel,Ciaglia, Tanja,Bersier, Michael,Hanselmann, Paul,Gutmann, Bernhard,Kappe, C. Oliver
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supporting information
p. 108 - 112
(2018/01/12)
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- Synthesis and Deployment of an Elusive Fluorovinyl Cation Equivalent: Access to Quaternary α-(1′-Fluoro)vinyl Amino Acids as Potential PLP Enzyme Inactivators
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Developing specific chemical functionalities to deploy in biological environments for targeted enzyme inactivation lies at the heart of mechanism-based inhibitor development but also is central to other protein-tagging methods in modern chemical biology including activity-based protein profiling and proteolysis-targeting chimeras. We describe here a previously unknown class of potential PLP enzyme inactivators; namely, a family of quaternary, α-(1′-fluoro)vinyl amino acids, bearing the side chains of the cognate amino acids. These are obtained by the capture of suitably protected amino acid enolates with β,β-difluorovinyl phenyl sulfone, a new (1′-fluoro)vinyl cation equivalent, and an electrophile that previously eluded synthesis, capture and characterization. A significant variety of biologically relevant AA side chains are tolerated including those for alanine, valine, leucine, methionine, lysine, phenylalanine, tyrosine, and tryptophan. Following addition/elimination, the resulting transoid α-(1′-fluoro)-β-(phenylsulfonyl)vinyl AA-esters undergo smooth sulfone-stannane interchange to stereoselectively give the corresponding transoid α-(1′-fluoro)-β-(tributylstannyl)vinyl AA-esters. Protodestannylation and global deprotection then yield these sterically encumbered and densely functionalized quaternary amino acids. The α-(1′-fluoro)vinyl trigger, a potential allene-generating functionality originally proposed by Abeles, is now available in a quaternary AA context for the first time. In an initial test of this new inhibitor class, α-(1′-fluoro)vinyllysine is seen to act as a time-dependent, irreversible inactivator of lysine decarboxylase from Hafnia alvei. The enantiomers of the inhibitor could be resolved, and each is seen to give time-dependent inactivation with this enzyme. Kitz-Wilson analysis reveals similar inactivation parameters for the two antipodes, L-α-(1′-fluoro)vinyllysine (Ki = 630 ± 20 μM; t1/2 = 2.8 min) and D-α-(1′-fluoro)vinyllysine (Ki = 470 ± 30 μM; t1/2 = 3.6 min). The stage is now set for exploration of the efficacy of this trigger in other PLP-enzyme active sites.
- McCune, Christopher D.,Beio, Matthew L.,Sturdivant, Jill M.,De La Salud-Bea, Roberto,Darnell, Brendan M.,Berkowitz, David B.
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supporting information
p. 14077 - 14089
(2017/10/17)
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- Combinatorial aid for underprivileged scaffolds: Solution and solid-phase strategies for a rapid and efficient access to novel aza-diketopiperazines (aza-DKP)
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An efficient solution-phase synthesis of aza-diketopiperazines (aza-DKP, triazinediones) is reported. A structurally diverse collection of c-[aza-alkylGly-Pro] derivatives and yet unreported 2,4,5-trisubstituted-1,2,4- triazine-3,6-diones has been synthesized starting from Fmoc-l-Pro-OH and various Fmoc-l-amino acids. To extend the practical value of this class of dipeptidomimetics, a general solid-phase synthesis approach amenable to library production was developed on both Wang-PS and HMBA-PS resins. The final acidic treatment of the resins in TFA/water mixture at room temperature enabled the rapid and quantitative cyclization/release highly pure triazinediones. The conformational preferences and the spatial organization of the three substituents of a representative 2,4,5-trisubstituted-1,2,4-triazine-3,6-dione were investigated by X-ray diffraction and 1H NMR spectroscopy.
- Bonnet, Dominique,Margathe, Jean-Francois,Radford, Sally,Pflimlin, Elsa,Riche, Stephanie,Doman, Pete,Hibert, Marcel,Ganesan
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scheme or table
p. 323 - 334
(2012/07/13)
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- Facile synthesis of β-amino disulfides, cystines, and their direct incorporation into peptides
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Herein, we report a simple and efficient methodology for the synthesis of β-amino disulfides by regioselective ring opening of sulfamidates with benzyltriethylammonium tetrathiomolybdate [BnNEt3] 2MoS4. Stability and reactivity of different protecting groups under the reaction conditions have been discussed. This methodology has also been extended to serine and threonine derived sulfamidates to furnish cystine and 3,3′-dimethyl cystine derivatives. Georg Thieme Verlag.
- Nasir Baig,Kanimozhi, Catherine K.,Sudhir, V. Sai,Chandrasekaran, Srinivasan
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scheme or table
p. 1227 - 1232
(2009/09/06)
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- β-lactam-containing cyclopeptide analogs
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Cyclic peptide analogs containing a β-lactam moiety were prepared. Reacting Fmoc-protected amino acid-derived diazo ketones 1, 2 with benzylidene-protected amino esters 3, 4 in a photochemically induced Staudinger-type reaction, trans-substituted β-lactams 5a/b and 6a/b were formed in 35-70% yield (dr 60:40-70:30). N-Terminal chain elongation to the respective acyclic precursors 14, 17a/b and 19a/b was achieved using conventional peptide synthesis (i.e. the pentafluorophenyl ester protocol). After saponification, activation as pentafluorophenyl esters and subsequent cleavage of the N-terminal Boc-protecting group, the pre-strained (3R,4S)-configured isomers could be cyclized without the need for high dilution or prolonged reaction times. Contrary to this, the (3S,4R)-configured isomers did not cyclize but gave polymeric material. The conformational stability of the cyclic peptidomimetics 16, 20, and 21 which were obtained in high yield, was elucidated by means of NMR spectroscopy. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.
- Maier, Thomas C.,Podlech, Joachim
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p. 4379 - 4386
(2007/10/03)
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- A facile stereospecific synthesis of α-hydrazino esters
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A convenient route to make α-hydrazino esters from their corresponding α-amino esters is reported. A key step is selective nitrosamine reduction using activated Zn, conc. HCl, and methanol at low temperatures giving nearly quantitative yields of the pure α-hydrazino esters.
- Oguz, Umut,Guilbeau, Garett G.,McLaughlin, Mark L.
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p. 2873 - 2875
(2007/10/03)
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- New approach for the general synthesis of oxotetrahydroindoles via intramolecular cycloadditions of azomethine ylides with tethered alkynes
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A new method for the synthesis of oxotetrahydroindoles has been achieved employing an intramolecular dipolar cycloaddition approach involving mesoionic species (munchnones) with electron-deficient alkynes. The methodology is quite general and convergent as shown by the synthesis of a variety of tri- and tetrasubstituted oxotetrahydroindoles 18, 21, 24, 27, 30, and 34. LiI-based ester cleavage in the presence of an electrophilic acetylenic ketone was critical for formation of the requisite cycloaddition substrates. The cycloaddition is virtually unaffected by the presence of gem-dimethyl groups in the side chain (cf. 38). The presence of a substituted benzyl or a phenethyl moiety on nitrogen, a polarized acetylene, and an appropriate tether between dipole and dipolarophile are essential for obtaining optimal efficiency.
- Nayyar, Naresh K.,Hutchison, Darrell R.,Martinelli, Michael J.
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p. 982 - 991
(2007/10/03)
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- 2-Thiazolyl α-amino ketones: A new class of reactive intermediates for the stereocontrolled synthesis of unusual amino acids
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The thiazole-based one carbon homologation of four α-amino acids (L-phenylalanine, L-leucine, L-threonine, and L-serine) to the corresponding α-hydroxy β-amino aldehydes and acids in both configurations at C(α), is described. The methodology involves the following key operations: (i) the conversion of an α-amino ester to a 2-thiazolyl α-amino ketone; (ii) the stereocontrolled reduction of a ketone carbonyl to either syn or anti α,β-amino alcohols; (iii) the aldehyde release from the thiazole ring; (iv) the oxidation of the aldehyde to a carboxylic acid. The methodology was only partially applied to L-phenylglycine because of some limitations in operation (i).
- Dondoni,Perrone
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p. 1162 - 1176
(2007/10/02)
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