Diastereo- and Enantioselective Syntheses of Trisubstituted Benzopyrans by Cascade Reactions Catalyzed by Monomeric and Polymeric Recoverable Bifunctional Thioureas and Squaramides

Abstract

4-Vinylphenyl-substituted squaramides have been tested as organocatalysts for the diastereo- and enantioselective synthesis of trisubstituted benzopyrans via an oxa-Michael intramolecular nitro-Michael cascade reaction. Both the enantio- and diastereoselection were good to moderate, depending on the nature of the chiral scaffold in the catalyst. The diastereoselection is better for the most active catalyst because the final products epimerize at C-3 along the time. Supported squaramide sq- 9 prepared by copolymerization of sq- 4 with styrene and divinylbenzene is also effective in promoting the cascade reaction, and it is recoverable and reusable for five cycles maintaining the activity.

The chemistry of 2-hydroxy-β-nitrostyrenes: versatile intermediates in synthetic organic chemistry

“…They confirmed that nitroolefins with electron-withdrawing groups are more reactive than those with electron-donating groups or simple nitrostyrene, resulting in a faster reaction rate and better stereoselectivities. 56 Xu et al also confirm that by using squaramide-cinchona bifunctional organocatalyst Cat-18, the products 153 can be obtained in 57-82% isolated yields, up to >20 : 1 dr, and 86->99% ee (Scheme 47). 57…”

1,2-trans-Diaminocyclohexane (DACH) in Asymmetric Catalysis: Nearing Fifty Years of Faithful Service and Counting

“…Andrés and Pedrosa 811 reported an oxa-Michael intramolecular nitro-Michael cascade reaction of 2-hydroxy-β-nitrostyrenes 763 with a series of β-substituted or unsubstituted β-nitrostyrenes 764 using 5 mol% of 4-vinylphenyl-substituted squaramide organocatalyst Cat. 150 to access various optically active trisubstituted chromanes 765 and 766 in excellent yields but with moderate diastereo- and enantioselectivities (Scheme 256 ).…”

Synthesis of Enantioenriched 3‐Amino‐3‐Substituted Oxindoles by Stereoselective Mannich Reaction Catalyzed by Supported Bifunctional Thioureas.

“…Our research interest in developing easily recoverable supported bifunctional chiral thioureas [17] and squaramides [18] as organocatalysts led us to undertake a study on the ability of these catalysts to promote the synthesis of enantioenriched 3-aminooxindoles. We report now our results on the reaction of N-tertbutoxycarbonyl ketimines derived from isatin with different nucleophiles catalyzed by monomeric and polymer-supported thioureas.…”