Hydrophobically Directed Aldol Reactions: Polystyrene‐Supported <scp>L</scp>‐Proline as a Recyclable Catalyst for Direct Asymmetric Aldol Reactions in the Presence of Water

Abstract6‐Aminopenicillanic acid (6‐APA) and two of its derivatives, 6‐APA benzyl ester (6‐APA‐OBn) and Penicillin G (PenG), have been evaluated as catalysts for use in direct cross‐aldol reactions in different solvents and mixtures. The effects of catalyst loading, reaction time, pH and temperature on the yield and stereoselectivity have been studied. 6‐APA proved to be an effective catalyst in terms of yield for the reaction between cyclohexanone and p‐nitrobenzaldehyde, especially in neat conditions. The stereoselectivity of the reaction was conditioned by the presence of two competing mechanisms: one an imine–enamine pathway and the other a Brønsted acid catalysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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“…Aldols 1a,b, 3-7a,b and enone 2 [17] are known products [18,19] and their NMR spectra agreed with reported data.…”

Section: Methodssupporting

confidence: 89%

Evaluation of 6‐APA as a New Organocatalyst for a Direct Cross‐Aldol Reaction

2009

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“…This polymeric organocatalyst (10 mol%) was used in the aldol reaction between cyclohexanone (2 equiv.) with different aromatic aldehydes in a small amount of water as solvent, affording mainly aldols anti-4 in good chemical yields, and excellent diastereo-and enantioselectivities (71-98%, 84-92% de, 90-98% ee) [61,62]. Using other cyclic ketones as nucleophiles, similar results were found, acetone giving low yields.…”

Section: Aqueous Aldol Reactions With Supported Organocatalystssupporting

confidence: 64%

Pursuing Chemical Efficiency by Using Supported Organocatalysts for Asymmetric Reactions under Aqueous Conditions

Guillena

Alonso²,

Baeza³

et al. 2015

COCAT

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Over the past decade, a great effort has been made by the chemical community to improve the efficiency and greenness of reactions. Merging the use of supported and recyclable organocatalysts and aqueous conditions to pursue this goal in the asymmetric synthesis of interesting enantioselective molecules lead to outstanding results in some cases. Progresses in this area also offer the possibility of having even more sustainable processes by implementing these reactions in the large scale production of chiral molecules using automated flow chemistry.

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“…In this context, we started a research project in order to find new organocatalysts, structurally simple, which are able to catalyze the asymmetric aldol reaction in water in low amounts (< 10 mol%) without the need of additives. In the last years our efforts have been devoted to the investigation of new recyclable proline-derived organocatalysts [18] and to the elucidation of some mechanistic aspects of pro- line-catalyzed reactions. [19] From a green chemistry point of view such studies are interesting because the direct aldol reaction is atomically economic, a low amount of an organocatalyst can be employed and the reaction medium is water.…”

Section: Introductionmentioning

confidence: 99%

New Simple Hydrophobic Proline Derivatives as Highly Active and Stereoselective Catalysts for the Direct Asymmetric Aldol Reaction in Aqueous Medium

et al. 2008

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New 4-substituted acyloxyproline derivatives with different hydrophobic properties of the acyl group were easily synthesized and used as catalysts in the direct asymmetric aldol reaction between cyclic ketones (cyclohexanone and cyclopentanone) and several substituted benzaldehydes. Reactions were carried out using water, this being the best reaction medium examined. Screening of these catalysts showed that compounds bearing the most hydrophobic acyl chains [4-phenylbutanoate and 4-(pyren-1-yl)butanoate] provided better results. The latter catalysts were successfully used in only 2 mol% at room temperature without additives to give aldol products in excellent stereoselectivities. These results demonstrate that derivatization of the proline moiety with the proper simple hydrophobic substituent in the 4-position can furnish highly active and stereoselective catalysts without the need of additional chiral backbones in the molecule. Finally, an explanation of the observed stereoselectivities in the presence of water is provided.

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Hydrophobically Directed Aldol Reactions: Polystyrene‐Supported L‐Proline as a Recyclable Catalyst for Direct Asymmetric Aldol Reactions in the Presence of Water

Cited by 156 publications

References 96 publications

Evaluation of 6‐APA as a New Organocatalyst for a Direct Cross‐Aldol Reaction

Evaluation of 6‐APA as a New Organocatalyst for a Direct Cross‐Aldol Reaction

Pursuing Chemical Efficiency by Using Supported Organocatalysts for Asymmetric Reactions under Aqueous Conditions

New Simple Hydrophobic Proline Derivatives as Highly Active and Stereoselective Catalysts for the Direct Asymmetric Aldol Reaction in Aqueous Medium

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