@article {43352, title = {N - heterocyclic - carbene - catalyzed umpolung of imines}, journal = {Angewandte Chemie International Edition}, volume = {56}, year = {2017}, month = {MAR}, pages = {2730 - 2734}, type = {Article}, abstract = {

N-Heterocyclic carbene (NHC) catalysis has been widely used for the umpolung of aldehydes, and recently for the umpolung of Michael acceptors. Described herein is the umpolung of aldimines catalyzed by NHCs, and the reaction likely proceeds via aza-Breslow intermediates. The NHC-catalyzed intramolecular cyclization of aldimines bearing a Michael acceptor resulted in the formation of biologically important 2-(hetero)aryl indole 3-acetic-acid derivatives in moderate to good yields. The carbene generated from the bicyclic triazolium salt was found to be efficient for this transformation.

}, doi = {10.1002/anie.201611268}, author = {Patra, Atanu and Mukherjee, Subrata and Das, Tamal Kanti and Jain, Shailja and Gonnade, Rajesh Ghanshyam and Biju, Akkattu T.} } @article {45769, title = {N-Heterocyclic carbene-catalyzed aldol-lactonization of ketoacids via dynamic kinetic resolution}, journal = {ACS Catalysis}, volume = {7}, year = {2017}, month = {JUN}, pages = {3995-3999}, type = {Article}, abstract = {

N-Heteroryclic carbene (NHC)-catalyzed enantioselective aldol lactonization of acyclic ketoacids, proceeding via dynamic kinetic resolution, is presented. The carbene generated from the chiral anninoinclanol-derived triazolium salt in the presence of LiCl was the key for the success of this transformation. The reaction allowed the diastereoselective and enantioselective synthesis of cyclopentane-fused beta-lactones having three contiguous stereocenters. The reaction products are shown to undergo substrate-controlled beta-lactone opening in the presence of amines to afford succinimide derivatives with four contiguous stereocenters.

}, keywords = {aldol lactonization, beta-lactones, DKR strategies, ketoacids, N-heterocyclic carbenes, organocatalysis}, issn = {2155-5435}, doi = {10.1021/acscatal.7b00681}, author = {Mondal, Santigopal and Mukherjee, Subrata and Das, Tamal Kanti and Gonnade, Rajesh and Biju, Akkattu T.} } @article {46021, title = {N-Heterocyclic carbene-catalyzed umpolung of imines for the enantioselective synthesis of dihydroquinoxalines}, journal = {ACS Catalysis}, volume = {9}, year = {2019}, month = {MAY}, pages = {4065-4071}, type = {Article}, abstract = {

N-heterocyclic carbene (NHC) organocatalysis is widely employed for the umpolung of aldehydes and recently to the umpolung of Michael acceptors and aldimines. Described herein is the NHC-organocatalyzed umpolung of aldimines for the enantioselective synthesis of nitrogen heterocycles. The bisimines generated from the condensation of 1,2-phenylenediamines and salicylaldehydes undergo intramolecular cyclization in the presence of a chiral NHC catalyst, resulting in the formation of dihydroquinoxalines in moderate to good yields and er values. Detailed DFT studies shed light on the role of -OH groups in stabilizing the initially generated aza-Breslow intermediates via intramolecular hydrogen bonds. Preliminary photophysical studies on the synthesized dihydroquinoxalines revealed that these molecules can be used for the sensing of various acids and bases.

}, keywords = {dihydroquinoxalines, imines, N-heterocyclic carbenes, organocatalysis, umpolung}, issn = {2155-5435}, doi = {10.1021/acscatal.9b00737}, author = {Das, Tamal Kanti and Ghosh, Avik and Balanna, Kuruva and Behera, Pradipta and Gonnade, Rajesh G. and Marelli, Udaya Kiran and Das, Abhijit Kumar and Biju, Akkattu T.} } @article {46899, title = {N-heterocyclic carbene catalysis exploiting oxidative imine umpolung for the generation of imidoyl azoliums}, journal = {Journal of Organic Chemistry}, volume = {85}, year = {2020}, month = {APR }, pages = {5114-5121}, type = {Article}, abstract = {

Although NHC-catalyzed umpolung of imines are known, the related reactions under oxidative conditions are limited. Described herein is the two-step process involving the initial formation of aldimines from the corresponding aldehydes and 2-amino benzyl alcohols followed by NHC-catalyzed cyclization proceeding via the imidoyl azoliums under oxidative conditions. The reaction allowed the synthesis of trifluoromethylated 3,1-benzoxazines in good yields and broad scope. The role of NHC in the intramolecular cyclization and preliminary mechanistic experiments are also provided.

}, issn = {0022-3263}, doi = {10.1021/acs.joc.0c00360}, author = {Das, Tamal Kanti and Madica, Krishnaprasad and Krishnan, Jagadeesh and Marelli, Udaya Kiran and Biju, Akkattu T.} } @article {47144, title = {N-heterocyclic carbene (NHC)-catalyzed transformations for the synthesis of heterocycles}, journal = {Progress in Heterocyclic Chemistry}, volume = {31}, year = {2020}, month = {JAN}, pages = {1-82}, type = {Book Chapter}, abstract = {

N-heterocyclic carbene (NHC)-based organocatalysis has been widely used for unconventional access to various target molecules. The use of chiral NHCs in the process results in the enantioselective synthesis of various heterocycles. The underlying principle of many of the NHC-catalyzed reactions is the remarkable ability of NHCs to reverse the normal mode of reactivity of aldehydes (umpolung). NHCs can catalyze transformations proceeding via the umpolung or nonumpolung mode for the synthesis of various functionalized molecules. The recent advances in the application of NHCs for the synthesis of various heterocyclic compounds are highlighted in this chapter.

}, keywords = {aldehyde umpolung, Aldimine umpolung, benzoin reaction, Breslow intermediates, Enantioselective synthesis, Heterocyclic synthesis, N-Heterocyclic carbene, Stetter reaction}, doi = {10.1016/B978-0-12-819962-6.00001-4}, author = {Das, Tamal Kanti and Biju, Akkattu T.} }