Domino routes to substituted benzoindolizines: tandem reorganization of 1,3-dipolar cycloadducts of nitrones with allenic esters/ketones and alternative cycloaddition-palladium catalyzed cyclization pathway |
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Authors: | Ashish Kapur Lakhwinder Singh Munusamy Elango Vivek Gupta Mohan Paul S Ishar |
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Institution: | a Bio-Organic and Photochemistry Laboratory, Department of Pharmaceutical Sciences, Guru Nanak Dev University, Amritsar 143 005, Punjab, India b Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, Tamil Nadu, India c P. G. Department of Physics, University of Jammu, Jammu Tawi 180 006, Jammu & Kashmir, India |
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Abstract: | Reactions of C-(4-oxo-4H1]benzopyran-3-yl)-N-phenyl nitrones (7) with allenic esters (8a-c) and allenic ketones (18a-d) furnish benzoindolizines (9a-k, 19a-d) in good yields. The formation of benzoindolizines is postulated to involve regioselective addition of 1,3-dipole to C2-C3 π bond of allenic esters/ketones followed by domino transformation of the cycloadducts, which involve an intramolecular aza Diels-Alder reaction in the intermediate C. DFT calculations of various parameters for diene and dienophile components in the proposed intermediate C have revealed that conformational constraints imposed by the alkyl groups (R=Me, Et) favor intramolecular aza-Diels-Alder cycloaddition. An alternative domino route to benzoindolizines (9a,d,g) involving sequential one-pot cycloaddition of azadienes (22a-c) with silyl-enol ether (23) followed by palladium(0)-catalyzed Heck coupling reaction has also been developed. Both these approaches represent novel domino routes for the synthesis of benzoindolizines. |
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Keywords: | Allenic esters Benzoindolizines Cycloaddition Domino routes Heck reaction |
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