Browsing by Author "Pragya Pali"

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Copper(II)-Catalyzed [3+2] Annulation of Thioamides with AIBN: Facile Access to Highly Functionalized Thiazolidin-4-ones
(Georg Thieme Verlag, 2022) Pragya Pali; Dhananjay Yadav; Gaurav Shukla; Maya Shankar Singh
An efficient and versatile copper-catalyzed intermolecular radical [3+2] annulation of thioamides with azobisisobutyronitrile (AIBN) is described. This two-component copper(II)-catalyzed transformation is achieved in one pot via cascade formation of C-S/C-N bonds through cyclization of an in situ generated N,S-acetal intermediate derived from a?-ketothioamide. This operationally simple method allows direct access to synthetically demanding thiazolidin-4-ones in good to excellent yields containing diverse functional groups of different electronic and steric nature. The readily available reaction partners, the avoidance of expensive/toxic reagents and a gram-scale synthesis are additional attributes of this strategy. AIBN plays a dual role as a radical initiator and an unusual source of a two-carbon coupling partner. Notably, the products possess Z stereochemistry with regard to the exocyclic C=C double bond at position 2 of the thiazolidine ring. © 2021. Thieme. All rights reserved.
Electrochemical Synthesis of 1,2,3-Thiadiazoles from α-Phenylhydrazones
(American Chemical Society, 2021) Gaurav Shukla; Priya Saha; Pragya Pali; Keshav Raghuvanshi; Maya Shankar Singh
We have developed an electrochemical approach for the synthesis of fully substituted 1,2,3-thiadiazoles from α-phenylhydrazones at room temperature, which is very challenging and complementary to the conventional thermal reactions. The key step involves anodic oxidation of phenylhydrazone derivatives at a constant current followed by N,S-heterocyclization. The protocol is remarkable in that it is free of a base and free of an external oxidant and can be converted to a gram scale for postsynthetic drug development with functional thiadiazoles. Most importantly, the electrochemical transformation reflected efficient electro-oxidation with an operationally friendly easy procedure with ample functional molecules. Cyclic voltammograms support the mechanism of this electro-oxidative cyclization process. © 2021 American Chemical Society.
Metal-Free One-Pot Annulative Coupling of 2-Hydroxybenzaldehydes with β-Ketothioamides: Access to Diverse 2-Arylimino-2 H-Chromenes
(American Chemical Society, 2022) Pragya Pali; Dhananjay Yadav; Subash C. Sahoo; Maya Shankar Singh
A concise and practical one-pot sustainable approach for expedient synthesis of 2-arylimino-2H-chromenes by two-component cascade [4 + 2] annulative coupling of easily available 2-hydroxybenzaldehydes with β-ketothioamides has been developed in good yields for the first time. Remarkably, metal- and additive-free conditions, use of simple K2CO3as a mild base, open atmosphere, exclusive regioselectivity, step/atom economy, nonhazardous reagents, and easy purification are added characteristics to the strategy. This annulative protocol will not only provide an efficient method to access diverse chromene scaffolds, but also enrich the research domain of β-ketothioamides. © 2022 American Chemical Society. All rights reserved.
One-Pot Access to Tetrasubstituted 2-Aminothiophenes via Regio- and Chemoselective Domino Reactions of Dithioesters with Fumaronitrile at Room Temperature
(John Wiley and Sons Inc, 2023) Anup Kumar Yadav; Vipin Kumar; Pragya Pali; Subhasish Ray; Abhineet Verma; Maya Shankar Singh
Herein, we report a one-pot viable protocol to synthesize tetrasubstituted 2-aminothiophenes engaging readily accessible α-enolic dithioesters and abundant fumaronitrile under transition metal-free conditions at room temperature in open air. The reaction proceeds via successive Michael-type addition/intramolecular cyclization/isomerization cascades. The added features are benign conditions, exclusive regio- and chemoselectivity, excellent atom-/step-economy, easy purification, and tolerance of wide range of functional groups of a diverse electronic and steric nature. This protocol not only provided a robust and modular approach to various 2-aminothiophenes in moderate to excellent yields, but also demonstrated the potential of dithioesters and fumaronitrile in the challenging intermolecular cross-coupling reactions widening the arsenal of synthetic methods. © 2023 Wiley-VCH GmbH.
Photo-oxidative Ruthenium(II)-Catalyzed Formal [3 + 2] Heterocyclization of Thioamides to Thiadiazoles
(American Chemical Society, 2021) Pragya Pali; Gaurav Shukla; Priya Saha; Maya Shankar Singh
An operationally simple and sustainable one-pot photo-oxidative formal [3 + 2] heterocyclization of β-ketothioamides with aryldiazonium salts catalyzed by Ru(bpy)3Cl2 has been realized to provide 2,4-disubstituted 5-imino-1,2,3-thiadiazoles in good to high yields under mild reaction conditions for the first time. The reaction proceeded via an α-phenylhydrazone adduct of thioamides leading to 1,2,3-thiadiazoles via N-S bond formation at room temperature. Notably, the products possess Z-stereochemistry with regard to the exocyclic C═N double bond at the 5-position of the ring. ©
Radical-Cascade Avenue to Access 1,2-Dithioles Employing Dithioesters and Edman’s Reagent
(American Chemical Society, 2023) Pragya Pali; Maya Shankar Singh
An operationally simple and efficient domino etiquette has been developed for the facile construction of 1,2-dithioles employing easily accessible dithioesters as a three-atom CCS synthon and aryl isothiocyanates as a two-atom CS unit in the absence of any catalyst and additive at room temperature under open air. The reaction proceeded efficiently affording the desired 1,2-dithioles in good yields having various functional groups of a diverse electronic and steric nature. This approach avoids possible toxicity and tiresome workup conditions and features easy to handle, cheap, and readily accessible reagents, O2 as a green oxidant, and gram-scale ability. Notably, the final S-S bond formation and cascade ring construction follow a radical pathway, which has been recognized via a radical trapping experiment with BHT during the course of the reaction. Notably, the exocyclic C═N bond at position 3 of 1,2-dithiole possesses Z stereochemistry. © 2023 American Chemical Society.
Visible-Light Photocatalysis of Eosin Y: HAT and Complementing MS-CPET Strategy to Trifluoromethylation of β-Ketodithioesters with Langlois' Reagent
(American Chemical Society, 2020) Sonam Soni; Pragya Pali; Monish Arbaz Ansari; Maya Shankar Singh
A metal- A nd oxidant-free photoinduced strategy for thioxo sulfur-selective trifluoromethylation of β-ketodithioesters at room temperature is reported. Excellent Z/E-stereoselectivity has been achieved with cheap and viable Langlois' reagent (CF3SO2Na, sodium triflinate) in the presence of eosin Y, which acts as a hydrogen atom transfer (HAT) catalyst. The reaction proceeds via disulfide intermediate disulfanediylbis(3-(alkylthio)-1-phenylprop-2-en-1-one) (a dimer of β-ketodithioester) followed by complementing proton-coupled electron transfer-mediated reverse HAT cycle of eosin Y. This operationally simple and efficient protocol allows direct access to triflinated α-oxoketene dithioacetals in good to excellent yields bearing diverse synthetically useful functional groups of different electronic and steric nature. Copyright © 2020 American Chemical Society.