Novel supported chiral bifunctional squaramides have been easily prepared starting from diamines derived from natural amino acids and commercially available aminoalkyl polystyrene resins. These squaramides behave as excellent stereoselective recoverable organocatalysts in different Michael additions, in neat conditions at room temperature. The reaction on 2-(2-nitrovinyl) phenol as electrophile lead, in excellent yields and enantioselectivities, to intermediates that can be easily transformed into 4H-chromenes, and spirochromanones.
Abstract.A series of supported bifunctional thioureas has been prepared, in one or two steps, from commercially available aminoalkyl polystyrene resins. They differ in the length of the tether attaching the thiourea to the polymer chain, and the nature of the amino thiourea component. All the materials are able to promote stereoselective aza-Henry reaction with very good stereoselection, and they can be recycled maintaining the catalytic activity.
Enantioenriched 3-amino-3-substituted oxindoles have been obtained by addition of different nucleophiles to N-Boc ketimines derived from isatin catalyzed by chiral bifunctional supported thioureas. The Mannich reaction occurs with excellent enantioselection, but poor diastereoselection when prochiral nucleophiles were used. The supported catalyst were recovered and reused for five times without loss of activity. The mixture of diastereoisomers formed when a prochiral structure was used as nucleophile was converted into a single enantioenriched pyrazolyl derivative.
Four novel chiral bifunctional thiosquaramides have been prepared from cyclopentyl dithiosquarates and diamines derived from natural l‐Valine and l‐tert‐Leucine. The novel thiosquaramides have been tested as organocatalyst in the nitro‐Michael addition of 3‐substituted oxindoles to different β‐aryl‐substituted nitroalkenes. The reaction occurred easily in high yields and excellent stereoselectivities, showing that the novel organocatalysts are much more effective than their thioureas and squaramides homologs.
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