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

Emer, Enrico; Galletti, Paola; Giacomini, Daria

doi:10.1002/ejoc.200900181

Cited by 14 publications

(10 citation statements)

References 39 publications

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“…The peculiar behavior of the tropylium cation arises from a nonideal temperature effect, [22] a phenomenon that was recently described in organocatalytic reactions. [23] The proposed catalytic cycle for the reaction is depicted in Scheme 2. As a first step, we propose the formation of the enamine by reaction of the MacMillan catalyst as a salt with the corresponding aldehydes.…”

Section: Resultsmentioning

confidence: 99%

Organocatalytic Stereoselective α‐Alkylation of Aldehydes with Stable Carbocations

Benfatti

Benedetto

Cozzi

2010

Chemistry An Asian Journal

104

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The organocatalytic stereoselective alkylation of aldehydes is carried out with the four stable carbocations 1-4 in the presence of a catalytic amount (20 mol%) of MacMillan imidazolidinones 5-6. In all reactions, lutidine was used as a base. The alkylation reactions are investigated at different temperatures with linear and branched aldehydes. In the case of carbocation tropylium fluoroborate, an interesting reversal of alkylation product configuration was observed, which is driven by entropic effects in the reaction. The absolute configuration of the products obtained is determined by chemical correlation and found to be in general agreement with the model proposed by MacMillan to justify the stereoselectivity obtained in the reactions promoted by catalysts of type 5-6.

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Section: Resultsmentioning

confidence: 99%

Organocatalytic Stereoselective α‐Alkylation of Aldehydes with Stable Carbocations

Benfatti

Benedetto

Cozzi

2010

Chemistry An Asian Journal

104

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“…Moderate results were obtained by using 6-aminopenicillanic acid (6-APA, (327), 10 mol%) in the reaction between cyclohexanone and aromatic aldehydes yielding mainly the syn-isomer (43-86% yield, 2-14% de, and 18-23% ee) [385]. tert-Leucine (328, 10 mol%) was applied in the aldol reaction of cyclic ketones with aromatic aldehydes in neat conditions at 25 • C. While anti-derivatives for cyclohexanone and cyclopentanone were mainly obtained (51-94%, 0-80% de, and 83-98% ee), syn-isomers were achieved for larger cyclic rings (51-94%, 60-86% de, and 31-64% ee) [386].…”

Section: Ketones As Source Of Nucleophilementioning

confidence: 99%

Organocatalyzed Aldol Reactions

Guillena¹

2013

Modern Methods in Stereoselective Aldol Reactions

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The use of catalytic enantioselective methods to perform an aldol reaction provides a direct entry for the synthesis of chiral compounds and is probably the most attractive way to achieve this goal with high control of the chemo-, regio-, diastereo-, and enantioselectivity. The application of biochemical methods based on enzyme aldolases or antibodies, which are discussed in other chapters, avoid the use of preformed enolates or their equivalents for enhancing the global atom efficiency of the process [1] for which the narrow substrate scope is a major drawback. The discovery of methodologies to carry out the enantioselective direct aldol reaction [2] has eluded the production of stoichiometric by-products increasing the substrate scope. This task could be accomplished by using small molecules as catalysts, such as metal complexes (which are covered along this book) and organic molecules, also known as organocatalysts [3]. The growth of this last research area and the direct aldol reaction are closely linked [4], with the report on the use of (S)-proline as catalyst for the intermolecular aldol [5] definitely being the push for the development of the organocatalytic methods as a competitive methodology for the synthesis of chiral compounds.In a strict sense, an organocatalyzed reaction is a process in which all the involved reagents and catalysis are purely organic compounds of low-molecular weight, excluding boron-and silicon-containing derivatives. However, in some cases, the presence of a silyl group only plays a steric role and therefore can also be considered as an organocatalyzed process. Also, if the organocatalyst involved in a reaction is immobilized in a polymer, a dendrimer, or even an inorganic material that serves only as a support to facilitate its recovery [6], it could still be considered as an organocatalyst, although those types of derivatives have been scarcely used in the natural product synthesis and therefore will be excluded from this chapter. Therefore, a comprehensive overview of the direct aldol reaction [7], emphasizing the reactivity, scope, selectivity, and limitations of the methodology and giving several examples of their application to the synthesis of biologically active compounds, is discussed in this chapter.

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“…Good diastereoselectivity was obtained with M1 at 10 mol% of catalyst loading (entry 1, Table ); however, the conversion and enantioselectivity were lower than those found with 8 (entry 5, table 2). In addition, formation of the hemiacetal 13 was detected by 1 H‐NMR . Material M1 was then recovered by filtration and reused in a second run.…”

Section: Resultsmentioning

confidence: 99%

“…In addition, formation of the hemiacetal 13 was detected by 1 H-NMR. [25] Material M1 was then recovered by filtration and reused in a second run. The conversion and the enantioselectivity decreased and the amount of the by-product 13 increased (entry 2, Table 2).…”

Section: Resultsmentioning

confidence: 99%

Recyclable Silica‐Supported Proline Sulphonamide Organocatalysts for Asymmetric Direct Aldol Reaction.

et al. 2016

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We describe the preparation of several organosilicas based on proline sulphonamide scaffolds from two chiral monosilylated precursors following sol‐gel and grafting procedures. The materials were characterised by using 29Si solid‐state NMR spectroscopy, N2 sorption measurements, powder X‐ray diffractogram (p‐XRD) and elemental analysis. The catalytic performances and reusability of these hybrid silica materials have been evaluated in direct intermolecular asymmetric aldol reactions between ketones and aromatic aldehydes and in a Robinson annulation using simple and green conditions (water, room temperature). Good conversions and recyclability, and from moderate to good diastereo‐ and enantioselectivities have been achieved. The characteristics of the matrix, and structural features such as the nature and length of the linker, and the derivatisation site on the catalytic organic moiety have an influence on the activity and selectivity of the silica‐supported organocatalysts.

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

Cited by 14 publications

References 39 publications

Organocatalytic Stereoselective α‐Alkylation of Aldehydes with Stable Carbocations

Organocatalytic Stereoselective α‐Alkylation of Aldehydes with Stable Carbocations

Organocatalyzed Aldol Reactions

Recyclable Silica‐Supported Proline Sulphonamide Organocatalysts for Asymmetric Direct Aldol Reaction.

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