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Dive into the research topics where Jose S. Madalengoitia is active.

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Featured researches published by Jose S. Madalengoitia.


Tetrahedron Letters | 1996

Conformational stability of proline oligomers

Rui Zhang; Jose S. Madalengoitia

Abstract Reexamination of the solution conformation of a series of N-Boc O-benzyl protected proline oligomers (dimer-hexamer) by 500 MHz NMR reveals that these oligomers adopt the poly-L-proline type II conformation in solution and do not require a critical chain length for conformational stability as has been previously reported.


Tetrahedron Letters | 2000

Lewis acid mediated diastereoselective and enantioselective cyclopropanation of Michael acceptors with sulfur ylides

Ahmed Mamai; Jose S. Madalengoitia

The reaction of N-enoyloxazolidinones with diphenylsulfonium isoproylide in the presence of Lewis acids is explored. The diastereoselectivity of the reaction of N-enoyloxazolidinones 1a and 1b with diphenylsulfonium isopropylide could be mediated by the addition of some Lewis acids. The reaction of N-enoyloxazolidinone 4 with diphenylsulfonium isoproylide was also explored with chiral Lewis acids. When this reaction was run with bis(oxazoline) ligand 6 and a number of Lewis acids, product was obtained in as high as 95% ee. A loss of stereoselectivity was observed with less than stoichiometric amounts of Lewis acid.


Organic Letters | 2009

Facile 1,3-diaza-Claisen Rearrangements of Tertiary Allylic Amines Bearing an Electron-Deficient Alkene

Rachel M. Aranha; Amy M. Bowser; Jose S. Madalengoitia

Tertiary allylic amines with an electron-deficient alkene react with isocyanates and isothiocyanates to give highly substituted ureas and thioureas arising from formal 1,3-diaza-Claisen rearrangements. Isocyanates and isothiocyanates with strong electron-withdrawing groups are more reactive. Similarly, the data suggest that a stronger electron-withdrawing substituent on the alkene favors a faster reaction, but this may be offset by sterics in the cyclic transition state.


Tetrahedron Letters | 1999

Stereoselective alkylations of a bicyclic lactam derived from pyroglutamic acid

Rui Zhang; Floyd Brownewell; Jose S. Madalengoitia

Abstract Improvements in the stereoselective alkylation of a bicyclic lactam derived from pyroglutamic acid are described. This methodology is used to synthesize a conformationally constrained homo-glutamic acid analog.


Journal of Organic Chemistry | 2013

Structure-Reactivity Relationships of Zwitterionic 1,3-diaza-Claisen Rearrangements

Rachel Aranha Potter; Amy M. Bowser; Yanbo Yang; Jose S. Madalengoitia; Joseph W. Ziller

Bridged bicyclic tertiary allylic amines aza-norbornene 1 and isoquinuclidene 2 add to isocyanates, isothiocyanates, and in situ-generated carbodiimides to form zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements to afford highly substituted ureas, thioureas, and guanidines, respectively. Aza-norbornene 1 is significantly more reactive toward 1,3-diaza-Claisen rearrangements than isoquinuclidene 2. This reactivity difference is most likely due to the inherent ring strain in the aza-bicyclo[2.2.1]heptene ring system of aza-norbornene 1. The most apparent reactivity trend of the heterocumulenes is that the most electron-deficient heterocumulenes are more reactive toward 1,3-diaza-Claisen rearrangements. The introduction of a new stereocenter α- to the nucleophilic nitrogen in aza-norbornene 1 and isoquinuclidine 2 decreases the reactivity toward 1,3-diaza-Claisen rearrangements, while the exodiastereomers 3b and 4b are less reactive than the corresponding endodiastereomers 3a and 4a. Isocyanates that bear an electron-withdrawing group react with allylic amines 1-3b to afford mixtures of ureas and isoureas; however, with excess isocyanate and heat, thermodynamic equilibration is possible affording ureas. Inspired by this observation, a one-pot reaction of isocyanates with amines 1, 2, and 3b followed by BF3·OEt2-catalyzed isomerization of the urea/isourea mixture was developed that affords the corresponding ureas in excellent yields.


Tetrahedron Letters | 2000

Optimization of variables for screening solid-supported metal complexes as oxidation catalysts

Amarnath Natarajan; Jose S. Madalengoitia

Abstract Evaluation of polymer-supported metal complexes as oxidation catalysts revealed that product distribution is not dependent on metal stoichiometry, or resin but is dependent on the oxidant, oxidant stoichiometry and the metal incubation times. The model systems investigated under the optimized conditions afforded highly reproducible results, indicating that this methodology can be used to reliably screen libraries of metal complexes.


Tetrahedron Letters | 2000

Molecular diversity approach to the synthesis of peptide-derived ruthenium complexes and their evaluation as oxidation catalysts

Amarnath Natarajan; Jose S. Madalengoitia

Abstract This paper reports the synthesis of a small library of peptide-derived ruthenium complexes as potential oxidation catalysts. Evaluation of the solid-supported complexes identified several potentially promising epoxidation catalysts. Liberation of the promising catalysts from the solid-support resulted in catalysts with improved activity.


Organic Letters | 2017

Intramolecular Formation of Zwitterionic Intermediates in 1,3-Diaza-Claisen Rearrangements

Joel D. Walker; Rebecca B. Watson; Stevenson Flemer; Yanbo Yang; Jose S. Madalengoitia

Isothioureas tethered to bridged-bicyclic tertiary allylic amines can be converted to carbodiimides through reaction with Hg(II) salts. Intramolecular cyclization of the tethered tertiary allylic amines to the carbodiimides afford zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements, affording highly substituted tricyclic guanidines.


Journal of Peptide Science | 2012

On-resin synthesis of novel arginine-isostere peptides bearing substituted amidine headgroups

Stevenson Flemer; Jose S. Madalengoitia

A methodology is presented for the facile synthesis of Arg‐containing peptides modified at the guanidine headgroup as substituted amidine cores. This process allows for the iterative construction of these Arg isosteres while the peptide is being built out on the solid support, providing a high potential for diversity in substitution pattern in the resulting peptide. A series of N‐Pmc‐substituted thioamides were condensed with deprotected δ‐N Orn‐bearing peptides while attached to the solid support using Mukaiyamas reagent as coupling reagent, yielding isosteric Arg‐containing analogs. Peptides were cleaved using trimethylsilyl trifluoromethanesulfonate/TFA and analyzed in their crude form in order to illustrate the amenability of this process toward production of peptide isolates in high crude purity. Arg‐containing peptides having a single Arg isostere were utilized to show the general utility of this approach as well as a multiple‐Arg‐containing construct, illustrating the amenability of this method toward stepwise construction of differently substituted amidine headgroups within the same peptide. Copyright


Journal of Organic Chemistry | 1999

CYCLOPROPANATION REACTIONS OF PYROGLUTAMIC ACID-DERIVED SYNTHONS WITH AKYLIDENE TRANSFER REAGENTS

Rui Zhang; and Ahmed Mamai; Jose S. Madalengoitia

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Rui Zhang

University of Vermont

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Amarnath Natarajan

University of Nebraska Medical Center

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