Browsing by Author "Kumar, Indresh"

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1,3-Carbon D–A strategy for [3 + 2] cycloadditions/annulations with imines: synthesis of functionalized pyrrolidines and related alkaloids
(RSC, 2014) Kumar, Indresh
Cycloaddition/annulation reactions remain the most attractive methods for the synthesis of five membered heterocyclic ring systems. Among the three possible strategies for [3 + 2] cycloaddition, this review focuses on 1,3-carbon donor–acceptor (C3, D–A) cycloaddition/annulation reactions with imines to synthesize pyrrolidines. The formal [3 + 2] cycloaddition, which includes the in situ 1,3-carbon D–A precursor generation through metal catalysis, Lewis acid catalysis and organocatalysis approaches is highlighted. The scope and limitations of this strategy along with its applications in the synthesis of natural product alkaloids reported during the last decade are outlined.
2-Iodoxybenzoic Acid (IBX): A Versatile Reagent
(Thieme, 2005) Kumar, Indresh
The importance of hypervalent iodine reagents in organic chemistry has been demonstrated in recent years, and they have been found to have several desirable properties: they are mild, selective, efficient and eco-friendly. [1] 2-Iodoxybenzoic acid (IBX) has been developed as a powerful reagent for several organic transformations, and a recent surge in interest was driven by the publication of an improved method for its synthesis. [2a] IBX is a powerful single-electron transfer oxidant that readily accepts a new heteroatom-based ligand, and has been applied successfully for the construction of novel heterocycles.
4-Methoxy-N-(4-nitrobenzyl)aniline
(ACTA, 2012) Kumar, Indresh
In the title compound, C(14)H(14)N(2)O(3), the nitro group is nearly coplanar with the benzene ring to which it is bonded [dihedral angle = 1.70 (2)°], and this ring is para-substituted by the amino-methyl-ene group. The dihedral angle between the benzene rings is 57.8 (1)°. The crystal structure is stabilized by N-H⋯O and C-H⋯O hydrogen bonds and weak C-H⋯π inter-actions are also observed.
Assembly of Gold Nanoparticles on Single-Walled Carbon Nanotubes by Using Click Chemistry
(Ingenta, 2009-05) Kumar, Indresh
Azide moiety-functionalized gold nanoparticles were conjugated with acetylene functionalized single-walled carbon nanotubes by employing copper-catalyzed Huisgen's [3+2] dipolar cycloaddition 'click chemistry' reaction. Evidences of conjunction were observed by TEM, EDX and Raman images.
Asymmetric synthesis of a 3,4-substituted pyrrolidine by l-proline catalyzed direct enolexo aldolization
(Elsiever, 2007-06-11) Kumar, Indresh
The asymmetric synthesis of a 3,4-substituted N-tosyl pyrrolidine has been achieved by using l-proline catalyzed 5-enolexo aldolization with high enantio and diastereoselectivity.
Asymmetric Synthesis of Bridged N-Heterocycles with Tertiary Carbon Center through Barbas Dienamine-Catalysis: Scope and Applications
(ACS, 2021-11) Kumar, Indresh; Kumar, Dalip
A direct aza-Diels–Alder reaction between 2-aryl-3H-indolin-3-ones and cyclic-enones has been developed to access chiral indolin-3-one fused polycyclic bridged compounds. This method proceeds via proline-catalyzed Barbas-dienamine intermediate formation from various cyclic-enones such as 2-cyclopenten-1-one, 2-cyclohexene-1-one, and 2-cycloheptene-1-one, followed by a reaction with 2-aryl-3H-indol-3-ones. Several indolin-3-ones fusing [2.2.2], [2.2.1], and [3.2.1] skeletons decorated with a tertiary carbon chiral center have been prepared. Computational studies (DFT) supported the observed stereoselectivity in the method. The synthesized compounds have shown exciting photophysical activities and selective sensing of Pd2+ and Fe3+ ions through the fluorescence quenching “switch-off” mode.
Asymmetric trienamine catalysis: new opportunities in amine catalysis
(RSC, 2013) Kumar, Indresh
Amine catalysis, through HOMO-activating enamine and LUMO-activating iminium-ion formation, is receiving increasing attention among other organocatalytic strategies, for the activation of unmodified carbonyl compounds. Particularly, the HOMO-raising activation concept has been applied to the greatest number of asymmetric transformations through enamine, dienamine, and SOMO-activation strategies. Recently, trienamine catalysis, an extension of amine catalysis, has emerged as a powerful tool for synthetic chemists with a novel activation strategy for polyenals/polyenones. In this review article, we discuss the initial developments of trienamine catalysis for highly asymmetric Diels–Alder reactions with different dienophiles and emerging opportunities for other types of cycloadditions and cascade reactions.
Catalyst-free direct regiospecific multicomponent synthesis of C3-functionalized pyrroles
(RSC, 2022) Kumar, Indresh
An operationally simple catalyst-free protocol for the direct regiospecific synthesis of β-(C3)-substituted pyrroles has been developed. The enamine intermediate, in situ generated from succinaldehyde and a primary amine, was trapped with activated carbonyls before the Paal–Knorr reaction in a direct multicomponent “just-mix” fashion to furnish pyrroles with overall good yields. Several C3-substituted N-alkyl/aryl/H pyrroles have been produced under open-flask conditions with high atom economy and avoiding protection–deprotection chemistry.
Cycloaddition Reactions: A Controlled Approach for Carbon Nanotube Functionalization
(Wiley, 2011-08-31) Kumar, Indresh
The functionalization of carbon nanotubes (CNTs) by means of cycloaddition reactions plays an important role in achieving mechanical, electrical, and biological functions and enhancing their dispersion in different matrixes. Through these reactions, an enormous variety of molecules can be coupled onto CNTs in a very controlled manner and they can be exploited to prepare CNT-based hybrid materials for a wide range of applications.
Direct Amine-Catalyzed Enantioselective Synthesis of Pentacyclic Dibenzo[b,f][1,4]oxazepine/Thiazepine-Fused Isoquinuclidines along with DFT Calculations
(ACS, 2020-10) Kumar, Dalip; Kumar, Indresh
A direct protocol for the asymmetric synthesis of dibenzoxazepine/thiazepine-fused [2.2.2] isoquinuclidines is developed. The reaction proceeds through a proline-catalyzed direct Mannich reaction followed by an intramolecular aza-Michael cascade sequence between 2-cyclohexene-1-one and various tricyclic imines, like dibenzoxazepines/thiazepines, as an overall [4 + 2] aza-Diels–Alder reaction. A series of pentacyclic isoquinuclidines have been prepared, with complete endo-selectivity, in good to high yields and excellent enantioselectivity (>99:1). Density functional theory (DFT) calculations further support the observed high stereochemical outcome of the reaction.
Direct Amine-Catalyzed Enantioselective Synthesis of Pentacyclic Dibenzo[b,f][1,4]oxazepine/Thiazepine-Fused Isoquinuclidines along with DFT Calculations
(ACS, 2020) Kumar, Indresh; Kumar, Dalip
A direct protocol for the asymmetric synthesis of dibenzoxazepine/thiazepine-fused [2.2.2] isoquinuclidines is developed. The reaction proceeds through a proline-catalyzed direct Mannich reaction followed by an intramolecular aza-Michael cascade sequence between 2-cyclohexene-1-one and various tricyclic imines, like dibenzoxazepines/thiazepines, as an overall [4 + 2] aza-Diels–Alder reaction. A series of pentacyclic isoquinuclidines have been prepared, with complete endo-selectivity, in good to high yields and excellent enantioselectivity (>99:1). Density functional theory (DFT) calculations further support the observed high stereochemical outcome of the reaction.
Direct Amine-Catalyzed Enantioselective Synthesis of Pentacyclic Dibenzo[b,f][1,4]oxazepine/Thiazepine-Fused Isoquinuclidines along with DFT Calculations
(ACS, 2020-10-08) Kumar, Dalip; Kumar, Indresh
A direct protocol for the asymmetric synthesis of dibenzoxazepine/thiazepine-fused [2.2.2] isoquinuclidines is developed. The reaction proceeds through a proline-catalyzed direct Mannich reaction followed by an intramolecular aza-Michael cascade sequence between 2-cyclohexene-1-one and various tricyclic imines, like dibenzoxazepines/thiazepines, as an overall [4 + 2] aza-Diels–Alder reaction. A series of pentacyclic isoquinuclidines have been prepared, with complete endo-selectivity, in good to high yields and excellent enantioselectivity (>99:1). Density functional theory (DFT) calculations further support the observed high stereochemical outcome of the reaction.
Direct catalytic synthesis of densely substituted 3-formylpyrroles from imines and 1,4-ketoaldehydes
(RSC, 2014) Kumar, Indresh
A sustainable method for the direct access to highly substituted 3-formylpyrroles from 1,4-ketoaldehydes and imine via formal [3 + 2] cycloaddition is reported. This reaction involves a one-pot amine catalyzed chemoselective Mannich-cyclization-aerobic oxidation sequence with good to high yields. Further application of the gram scale reaction as well as synthesis of fully substituted 3-formylpyrrole is also shown.
Direct catalytic synthesis of β-(C3)-substituted pyrroles: a complementary addition to the Paal–Knorr reaction
(RSC, 2021) Kumar, Indresh
The synthesis of β-(C3)-functionalized pyrroles is a challenging task and requires a multistep protocol. An operationally simple direct catalytic synthesis of β-substituted pyrroles has been developed. This one-pot multicomponent method combined aqueous succinaldehyde as 1,4-dicarbonyl, primary amines, and isatins to access hydroxyl-oxindole β-tethered pyrroles. Direct synthesis of the β-substituted free NH-pyrrole is the central intensity of this work. DFT-calculations and preliminary mechanism investigation support the possible reaction pathway.
Direct catalytic synthesis of β-(C3)-substituted pyrroles: a complementary addition to the Paal–Knorr reaction
(RSC, 2021) Kumar, Indresh
The synthesis of β-(C3)-functionalized pyrroles is a challenging task and requires a multistep protocol. An operationally simple direct catalytic synthesis of β-substituted pyrroles has been developed. This one-pot multicomponent method combined aqueous succinaldehyde as 1,4-dicarbonyl, primary amines, and isatins to access hydroxyl-oxindole β-tethered pyrroles. Direct synthesis of the β-substituted free NH-pyrrole is the central intensity of this work. DFT-calculations and preliminary mechanism investigation support the possible reaction pathway.
Direct multicomponent synthesis of C3-arylated pyrroles under catalyst-free conditions
(Thieme, 2025) Kumar, Indresh
An operationally simple catalyst-free protocol for the direct regiospecific synthesis of C3-arylated/alkenylated pyrroles has been developed. The enamine-intermediate, in situ generated from succinaldehyde and a primary amine, was trapped with activated carbonyls before the Paal–Knorr reaction in a direct multicomponent ‘just-mix’ protocol to furnish pyrroles in good yields. Several C3-substituted N-alkylpyrroles have been prepared under open-flask conditions, avoiding protection-deprotection chemistry.
Dynamic Equilibrium: Is it an important concept in chemical biology?
(IISC, 2011) Kumar, Indresh
Dynamic equilibrium is one of the important aspects of study, because its existence can be observed in the infinitesimally small cells within the body to the huge adversities of the nature. In organic chemistry, a single compound having dynamic equilibrium states gives a driving force to create diversity-oriented synthesis of library of products from the same single compound. In chemical biology, the protein/nucleic acids are in constant equilibrium with their changing conformations/folds, which is responsible for the biological activity with a very small level of energy barrier for the conformational/folds inter-conversion. In medicinal/ pharmaceutical chemistry, a drug having dynamic equilibrium states plays an important role in the delivery of the drug on the active site across the cell membrane in a dynamic fashion and also acts as self-protection for the active drug molecule. Herein, we present a brief account on the existence and application of dynamic equilibrium states in chemical and biological chemistry as well as its existence in other inorganic complexes. Information regarding the existence of exact 1:1 ratio of the two dynamic equilibrium forms of chemical entities in the chemical reaction from organic, inorganic and biological perspectives have been discussed. We believe that this review is the first of its kind to discuss the importance of dynamic equilibrium states in chemical and biological systems, addressing the question to the scientific community as and the importance of the concept for further study.
Efficient Synthesis of Fused 1,2,3-Triazolo-δ-lactams Using Huisgen [3 + 2] Dipolar Cycloaddition “Click-chemistry” in Water
(CSJ, 2007) Kumar, Indresh
A general approach for the quick synthesis of various 1,2,3-triazolo-δ-lactams has been described, which involves the Huisgen [3 + 2] dipolar cycloaddition of azides derived from different amino acids with dimethyl acetylenedicarboxylate in water followed by cyclization.
Electrochemical Oxidative Addition of Nucleophiles on 2-Arylindoles: Synthesis of C2-Heteroquaternary Indolin-3-ones
(ACS, 2022-11) Kumar, Indresh
An electrochemical method has been developed to synthesize 2,2-disubstituted indolin-3-ones under mild conditions. A series of nucleophiles have been added to the 2-arylindole-3-ones, generated in situ under metal-free electrochemical oxidative dearomatization of 2-arylindoles, to afford 2,2-disubstituted 3-carbonyl indoles with heteroquaternary centers in 57–79% yields.
Electrochemical Oxidative Coupling Between Benzylic C(sp3)–H and N–H of Secondary Amines: Rapid Synthesis of α-Amino α-Aryl Esters
(ACS, 2021-06) Kumar, Indresh
An intermolecular electrochemical coupling between the benzylic C(sp3)–H bond and various secondary amines is reported. The electronic behavior of two electronically rich units viz the α-position of α-aryl acetates and amines was engineered electrochemically, thus facilitating their reactivity for the direct access of α-amino esters. A series of acyclic/cyclic secondary amines and α-aryl acetates were tested to furnish the corresponding α-amino esters with high yields (up to 92%) under mild conditions.