Browsing by Author "Ashok K. Basak"

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[4+3]-Annulation of 3-Cyano-4-aryl-2-iminochromenes with 1,2-Diaminobenzene: An Access to Novel Chromenobenzodiazepines
(Georg Thieme Verlag, 2018) Mohini Mourya; Himanshu Sharma; Yogesh C. Joshi; Ashok K. Basak
3-Cyano-4-aryl-2-iminochromenes undergo [4+3]-annulation with 1,2-diaminobenzene under mild acidic conditions to generate novel chromenobenzodiazepines in good yields. The annulation reaction was also successful with 2-aminophenol and 2-aminothiophenol. The chromenobenzodiazepines could be conveniently reduced to the corresponding 4 H -chromenobenzodiazepines under mild acidic conditions. © 2018 SynOpen. All rights reserved.
Catalytic Cycloisomerization of Enyne Diesters Derived from Biaryls and Related Substrates
(John Wiley and Sons Inc, 2023) Manoj K. Saini; Karmdeo Prajapati; Rashi Sharma; Ashok K. Basak
Enyne diesters derived from biaryls and related substrates undergo intramolecular [2+2]-cycloaddition and concomitant cyclo-reversion when treated with 10 mol% In(OTf)3 in 1,2-dichloroethane. This enyne metathesis type cycloisomerization reaction enables the synthesis of phenanthrene, benzophenanthrene, benzotetraphene and other polycyclic (hetero)aromatic compounds bearing a synthetically useful diethyl β-(hetero)arylidene malonate moiety in a step- and atom-economic manner. In(OTf)3-catalyzed two independent intramolecular cycloisomerization on substrates 5 a and 5 b provides direct access to benzotetraphenes 6 a and 6 b containing bis(diethyl β-phenylmethylene malonate) moiety. Tetrasubstituted vinylidene malonate 4 a, substiututed by two different aromatic groups, is stereoselectively converted into E- and Z-isomer of triaryl group containing all carbon tetrasubstituted olefins (8 a, 8 a′ and 8 b) via Pd-catalyzed stereoselective decarboxylative arylation reaction. © 2023 Wiley-VCH GmbH.
Catalytic Cycloisomerization-Oxidative Cyclization Reaction Sequence of Enyne Diesters Derived from 2-Propargyloxyarylaldehydes
(American Chemical Society, 2024) Karmdeo Prajapati; Manoj Kumar Saini; Rajesh G. Gonnade; Ashok K. Basak
Enyne diesters derived from 2-propargyloxyarylaldehydes are converted into 2-oxopyranochromenes via In(OTf)3-catalyzed cycloisomerization and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated oxidative cyclization reaction sequence in one pot. The process possesses broad substrate scope and good functional group compatibility and generates various 4-(hetero)aryl-substituted 2-oxopyranochromenes in 32-79% yields (over two steps). 2-Oxopyranochromenes undergo selective decarboxylation under Krapcho conditions. When treated with aliphatic secondary amines in DMF, 2-oxopyranochromenes undergo decarboxylative amination at ambient temperature to generate 2-amino-substituted functionalized chromenes in good yields. © 2024 American Chemical Society.
Expanding the Scope of Chemoselective Reductive Cyclization: Synthesis of Unsubstituted 9H-Thioxanthenes and 10-Methyl 9,10-Dihydroacridines and Beyond
(John Wiley and Sons Inc, 2024) Shashi Kant Verma; Ashok K. Basak
Unsubstituted 9H-thioxanthenes are obtained from 2-(arylthio)benzaldehydes via In(OTf)3-catalyzed chemoselective reductive cyclization in diisopropyl ether at elevated temperature. The reaction possesses broad substrate scope, good functional group compatibility and proceeds via chemoselective reduction of thioxanthylium ion in the presence of a highly reactive arylaldehyde. Similarly, unsubstituted 10-methyl-9,10-dihydroacridines are also obtained via a Bronsted acid catalyzed chemoselective reductive cyclization of 2-(methylphenylamino)benzaldehydes in isopropanol (solvent). The reductive cyclization method can also be utilized for the synthesis of 9-trifluormethyl containing (thio)xanthenes. Mono-deuterated 9H-thioxanthene is obtained in moderate yield in deuterated solvent (THF-d8). © 2024 Wiley-VCH GmbH.
In(OTf)3 catalyzed reductive etherification of 2-aryloxybenzaldehydes and 2-(arylthio)benzaldehydes
(Elsevier Ltd, 2019) Anamika Prajapati; Mahendra Kumar; Ranjit Thakuria; Ashok K. Basak
2-Aryloxybenzaldehydes and 2-(arylthio)benzaldehydes undergo reductive etherification in presence of 5 mol% In(OTf)3 and stoichiometric amount of Et3SiH under solvent free conditions to generate novel symmetrical dibenzyl ethers and thioethers in excellent yields. In(OTf)3 is found to be superior in terms of catalytic activity over the other metal triflates tested for the reaction. Xanthenes and thioxanthenes, as anticipated, could not be obtained under these conditions. © 2019
K2CO3-Catalyzed Rapid Conversion of N-Sulfonylhydrazones to Sulfinates
(American Chemical Society, 2020) Harshita Singh Korawat; Ashok K. Basak
N-Sulfonylhydrazones derived from alkyl, aryl, and heteroaryl aldehydes and ketones undergo rapid conversion into the corresponding sulfinates when heated with 10 mol % K2CO3 in N,N-dimethylethylene urea (DMEU) at elevated temperature. The reaction conditions are amenable to several functional groups and suitable for gram-scale synthesis. Under these base-catalyzed conditions, N-tosylhydrazones derived from O-allylated and O-propargylated 2-hydroxyarylaldehydes do not undergo the well-established intramolecular [3 + 2]-cycloaddition reactions and generate corresponding sulfinates in good yields. The base-catalyzed transformation proceeds via crucial rapid intermolecular protonation of the diazo intermediate 11 to generate diazonium ion 12, which upon nucleophilic displacement by the sulfonyl ion 10 provides the desired sulfinate selectively. Copyright © 2020 American Chemical Society.
Lewis Acid Catalyzed Reductive Cyclization of 2-Aryloxybenzaldehydes and 2-(Arylthio)benzaldehydes to Unsubstituted 9H-Xanthenes and Thioxanthenes in Diisopropyl Ether
(Wiley-VCH Verlag, 2021) Shashi Kant Verma; Anamika Prajapati; Manoj Kumar Saini; Ashok K. Basak
Readily accessible 2-aryloxybenzaldehydes and 2-(arylthio)benzaldehydes undergo a sequence of reactions leading to a wide variety of unsubstituted 9H-xanthenes and thioxanthenes in high yields when heated with a Lewis acid in diisopropyl ether. This reductive cyclization method is compatible with several important functional groups. The method is also applicable for the selective reductive cyclization of the more electron-rich aryl ring of a 2,6-bis(aryloxy)benzaldehyde. The key feature of this transformation is the chemoselective reduction of a transient xanthylium ion in the presence of aldehydic group via intermolecular hydride transfer from diisopropyl ether (solvent). (Figure presented.). © 2020 Wiley-VCH GmbH
Pyridinium triflate catalyzed intramolecular alkyne-carbonyl metathesis reaction of O-propargylated 2-hydroxyarylaldehydes
(Elsevier Ltd, 2020) Manoj Kumar Saini; Harshita Singh Korawat; Shashi Kant Verma; Ashok K. Basak
3,5-Dibromopridinium trifluoromethanesulfonate catalyzes the intramolecular alkyne-carbonyl metathesis reaction of a variety of O-propargylated 2-hydroxyarylaldehydes and ketones bearing alkyl, aryl and heteroaryl substituted internal alkynes to provide various 3-(hetero)aroyl 2H-chromenes in high yields. © 2020 Elsevier Ltd
Shedding Light on Benzo[d]pyrrolo[2,1-b]thiazole-3-carbonitrile Sensitizer-Based TiO2 Photocatalysts for Enhanced Hydrogen Generation
(American Chemical Society, 2023) Nageshwarrao Chanda; Manoj Kumar Saini; Ashok K. Basak; Yarasi Soujanya; Sreedhar Bojja; Ujjwal Pal
In this work, we study a series of benzo[d]pyrrolo[2,1-b]thiazole-3-carbonitrile (BP) derivatives, which act as sensitizers for photocatalytic hydrogen generation applications. The metal-free BP organic sensitizers 1-(tert-butylamino)-2-(3,4,5-trimethoxyphenyl)benzo[d]pyrrolo[2,1-b]thiazole-3-carbonitrile, 2-(2-bromophenyl)-1-(tributylamino)benzo[d]pyrrolo[2,1-b]thiazole-3-carbonitrile, and 2-(2-bromophenyl)-1-(cyclohexylamino)benzo[d]pyrrolo[2,1-b]thiazole-3-carbonitrile, termed BP-1, BP-2, and BP-3, respectively, act as light-harvesting units when loaded onto n-type semiconductors. In this study, we paired these sensitizers with the n-type semiconductor platinized TiO2 to create the materials TiO2-BP-1, TiO2-BP-2, and TiO2-BP-3. TiO2-BP-1 showed superior hydrogen generation efficiency compared to TiO2-BP-2, TiO2-BP-3, and pristine platinized TiO2. The electron transfer dynamics that take place between the dyes and TiO2 were studied using photoemission spectroscopy. The light-harvesting capacity, theoretical highest occupied molecular orbital and lowest unoccupied molecular orbital levels, and redox potential behavior of the sensitizers were evaluated by completing systematic photophysical, electrochemical, and computational studies. For TiO2-BP-1, a significant hydrogen generation rate of 3.32 mmol g-1 h-1, with an apparent quantum yield (AQY) of 8.5% and a turnover number (TON) of 3344, was obtained at 420 nm, thus indicating the excellent light-harvesting capability of the BP-1 sensitizer. The increased photocatalytic activity of the devices when under simulated sunlight shows that BP derivatives have the potential to be useful photosensitizers in sustainable hydrogen generation applications. © 2023 American Chemical Society.
Shedding light on small molecule coumarin dyes for efficient photocatalytic hydrogen evolution
(Elsevier Ltd, 2023) Nageshwarrao Chanda; Spandana Gonuguntla; Shashi Kant Verma; Ashok K. Basak; Yarasi Soujanya; Mohsen Ahmadipour; Sreedhar Bojja; Ujjwal Pal
The development of simple coumarin-based organic dyes loaded over TiO2 is found to possess great potential for efficient photocatalytic hydrogen generation. Herein, we studied a series of coumarin-based organic dyes, namely, 4-(naphthalene-1-yl)-2-oxo-2H-benzo[h]chromene-3-carbonitrile loaded over Pt–TiO2 is referred to as TC-2. The electronic as well as structural properties of the dyes were visualized experimentally and theoretically with the modification of the structural chromophoric groups over the meso-position of the coumarin dyes and revealed that the meso-substitution of the naphthalene ring over the TC-2 resulted in the decrease in electron-hole recombination dynamics. TC-2 photocatalyst depicted enhanced activity of (H2 rate-5.54 mmolg−1h−1) under neutral pH (AQY-14.5%, TON-5970), which is majorly obtained due to the increased conjugation of the naphthalene ring which thereby increases the light-sorption capacity of the materials in comparison to that of the others, synthesized materials in presence of TEOA. The extended π-π∗ conjugation in the substituent modification and the cyano groups over the simple coumarin groups leads to enhanced activity. This work paves the way towards sustainable development of dye-sensitized photocatalysts for green fuel generation. © 2022 Hydrogen Energy Publications LLC
Stereoselective synthesis of 9-vinyl substituted unsymmetrical xanthenes and thioxanthenes
(Elsevier Ltd, 2020) Anamika Prajapati; Mahendra Kumar; Ranjit Thakuria; Ashok K. Basak
Activated 2°-allylic alcohols derived from 2-aryloxybenzaldehydes and 2-(arythio)benzaldehydes undergo intramolecular Friedel-Crafts alkylation reaction when heated with catalytic amount of a Lewis acid in 1,2-dichloroethane to provide highly E-selective 9-vinyl substituted unsymmetrical novel xanthenes and thioxanthenes in good yields. © 2020 Elsevier Ltd
Synthesis of Azonia Aromatic Heterocycles Bearing 6-6-6-5-6 Pentacyclic Core via Intramolecular [4 + 2]-Cycloaddition and Oxidative Aromatization Reaction Sequence in One Pot
(American Chemical Society, 2024) Manoj Kumar Saini; Karmdeo Prajapati; Ashok K. Basak
Cationic aza-heterocycle-fused compounds have gained wide applications in materials science, biological applications, and synthetic organic chemistry. In this report, synthesis of benzothiazolochromenopyridinium tetrafluoroborates, a novel molecular scaffold, bearing 6-6-6-5-6 pentacyclic core is described that proceeds via (i) piperidine-catalyzed Knoevenagel condensation between 2-propargyloxyarylaldehydes bearing internal alkynes and 2-benzothiazoleacetonitrile, (ii) intramolecular formal [4 + 2]-cycloaddition, and (iii) crucial molecular oxygen-mediated oxidative aromatization reaction sequence in one pot. These quaternary pyridinium salts are obtained at ambient temperature in good to high yields. © 2023 American Chemical Society
Wittig Ylide Mediated Decomposition of N -Sulfonylhydrazones to Sulfinates
(American Chemical Society, 2018) Deepika Choudhary; Vineeta Khatri; Ashok K. Basak
N-Sulfonylhydrazones generate sulfinates selectively when treated with a stabilized Wittig ylide in a polar aprotic solvent at elevated temperature. The transition metal and base free decomposition method is applicable to N-sulfonylhydrazones generated from a number of aromatic and heteroaromatic aldehydes and ketones. In the case of N-tosylhydrazones derived from O-allyl and O-propargyl salicylaldehydes, selective formation of sulfinate occurs over intramolecular [3 + 2]-cycloaddition reaction. © 2018 American Chemical Society.