Organocatalytic asymmetric Michael and Aza-Henry reactions for the synthesis of Nitrogen- & Oxygen-containing heterocyclic compounds

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The contents of this thesis entitled “Organocatalytic Asymmetric Michael and Aza-Henry Reactions for the Synthesis of Nitrogen- and Oxygen- Containing Heterocyclic Compounds” have been divided into five chapters based on the results of experimental works performed during the complete course of the PhD research period. Chapter 1 describes a general overview of organocatalysis with special emphasis on Michael and aza-Henry reactions. In the chapter 2, a series of highly enantioselective 3-acyloxypyrazoles are presented. Concerned method was established for the Michael/hemiketalization/retro-aldol reaction of α-nitroketones and unsaturated pyrazolones. A couple of simple derivatization of the products is also presented. Chapter 3 demonstrates an alternative appreach for the Michael/acetalization/acyl transfer reaction to provide enantiopure 2,4-disubstituted chromans with single diastereomeric ratio by using a wide range of 2-hydroxycinnamaldehydes instead of unsaturated pyrazolones. This protocol was again extended for the synthesis of the chiral 2,4-disubstituted chromans by valuable synthetic transformations such as reduction of nitro to amine and protection of this amine by Boc-anhydride and benzoyl anhydride, reduction of ester to alkane. Chapter 4 represents Michael-oxa-Michael reaction of deconjugated enones having αʹ-CH groups with electron poor oxadienes bearing cyano group. The following reaction was performed using quinine derived primary amine catalyst and benzoic acid as an additive for the synthesis of oxygen-containing heterocyclic compounds (3,4-dihydropyrans) with excellent enantioselectivities. Penultimately, chapter 5 depicts both nitrogen and oxygen-containing heterocyclic compounds (tetrahydropyrano[2,3-c]pyrazolones) which have been synthesized from alkylidene pyrazolones and cyclic ketones via domino Michael-hemiketalization reaction with high enantioselectivities and diastereoselectivities. Also, few synthetic transformations of the product including a spiro derivative formation have been demonstrated. Lastly, in chapter 6 aza-Henry reaction was illustrated to construct a series of tetrahydroquinolines where the nitroalkanes and aldehydes in the presence of quinine derived thiourea catalyst were employed to provide enantiomerically pure tetrahydroquinolines with moderate to good yields via intra molecular cyclization.
Supervisor: Subhas Chandra Pan