Functional monomers and polymers, 129. Asymmetric robinson cyclization reaction catalyzed by polymer‐bound <scp>L</scp>‐proline

Kondo, Koichi; Yamano, Toru; Takemoto, Kiichi

doi:10.1002/macp.1985.021860906

Cited by 51 publications

(29 citation statements)

References 7 publications

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“…[85] Their behavior has already been discussed in the previous review. [4] More recent efforts involved the synthesis of prolinamide 28 ( Figure 11) obtained by condensation of proline with 4-methylbenzhydrylamino polystyrene (MBHA).…”

Section: Catalysts Derived From Amino Acidsmentioning

confidence: 93%

Immobilization of Organic Catalysts: When, Why, and How

2006

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This review article is divided in two parts. In the first part (Sections 2 and 3) selected examples of the publications that appeared in the literature in the period 2003–2005 in the field of immobilized organic catalysis are presented. When appropriate, the results of these publications are compared to those reported earlier and already discussed in a previous review article (see ref. 4). On the basis of this survey, in Section 4 some general considerations about when and why a supported version of an organic catalyst is worth developing are proposed. In Section 4 a list of suggestions about how the process of immobilization should be carried out is also included, taking into account several factors such as the properties of the catalyst, the nature of the support, and the mode of connection of the catalyst to the support.

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Section: Catalysts Derived From Amino Acidsmentioning

confidence: 93%

Immobilization of Organic Catalysts: When, Why, and How

2006

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“…[27] Already in 1985, a group of Japanese researchers reported the first type of polymer-supported proline, used for this very reaction (Scheme 1). [28] Although the catalytic efficiency and selectivity by today's standards of this supported proline must be perceived as mediocre, [28] it nevertheless bears a striking resemblance to the supports that about 20 years later would give the field of polymersupported proline its breakthrough. As for most reports nowadays, the Merrifield resin and the proline derivative trans-4-hydroxy--proline was the starting point (Scheme 1).…”

Section: Polymer-supported Proline Pioneering Effortsmentioning

confidence: 96%

“…Subsequent deprotection gave polymer-supported proline 13 (Scheme 1). [28] A close derivative containing a methylene spacer was also prepared. [28] Unfortunately, the catalyst was rather ineffective in the asymmetric Robinson cyclization, but the authors used optical rotation for determination of enantiomeric excess, a procedure prone to inaccuracies as small amounts of pollutants can give highly distorted results.…”

Section: Polymer-supported Proline Pioneering Effortsmentioning

confidence: 99%

Polymer‐Supported Chiral Organocatalysts: Synthetic Strategies for the Road Towards Affordable Polymeric Immobilization

2010

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In this microreview, we highlight the field of polymer‐supported organocatalysis, especially immobilized enamine and iminium organocatalysts. We try to formalize the overall synthetic strategies for polymeric immobilization as spanning the area of two overlapping regions, from a copolymer strategy favoured by low‐valued and small catalysts to a classical post‐modification strategy favoured by valuable and/or large catalysts. Organocatalysis is particularly interesting as it is probably best described as being located in the transitional region, and we will trace the historic and factual origins for the unfortunate predispositions towards post‐modification schemes. In addition, we try to identify affordable and useful syntheses of key organocatalyst immobilization intermediates, as well as polymer supports that are more compatible with a broader range of reaction solvent polarity, something of crucial importance in organocatalysis.

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“…An explanation for the observed differences may be that Agami and colleagues (21) had used optical rotation measurement for the ee determinations, whereas we had used a more accurate HPLC-based assay. Another strong piece of evidence for our one-proline mechanism came from studies with polymer-supported proline as the catalyst for asymmetric inter-and intramolecular aldolizations and for our previously discovered asymmetric Mannich reaction (10,(27)(28)(29). It was shown that rates and enantioselectivities of the supported catalysts are comparable with proline itself.…”

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confidence: 95%

New mechanistic studies on the proline-catalyzed aldol reaction

List

Hoang

Martin

2004

Proc. Natl. Acad. Sci. U.S.A.

324

144

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The mechanism of the proline-catalyzed aldol reaction has stimulated considerable debate, and despite limited experimental data, at least five different mechanisms have been proposed. Complementary to recent theoretical studies we have initiated an experimental program with the goal of clarifying some of the basic mechanistic questions concerning the proline-catalyzed aldol reaction. Here we summarize our discoveries in this area and provide further evidence for the involvement of enamine intermediates. D iscovered in the early 1970s, the Hajos-Parrish-EderSauer-Wiechert reaction (1, 2), a proline-catalyzed intramolecular aldol reaction, represents not only the first asymmetric aldol reaction invented by chemists but also the first highly enantioselective organocatalytic transformation [1(4) 3 2(5) 3 3(6)] (Eq. 1 of Scheme 1) (3-6). Inspired by Nature's phenomenal enzymes, which catalyze direct asymmetric aldolizations of unmodified carbonyl compounds (7, 8), we have recently extended the Hajos-Parrish-Eder-Sauer-Wiechert reaction to the first intermolecular variant (7 ϩ 8 3 9) (Eq. 2 of Scheme 1) (9), and to several other reactions including prolinecatalyzed asymmetric Mannich (10), Michael (11), ␣-amination (12), and intramolecular enolexo aldolization reactions (10 3 11) (13) (Eq. 3 of Scheme 1) (14-18).Similar to the aldolase enzymes, proline catalyzes direct asymmetric aldol reactions between two different carbonyl compounds to provide aldol products in excellent yields and enantioselectivities. Early on it has been speculated that in addition to operating on related substrates, both class I aldolases and proline may also share a similar enamine mechanism (19,20). However, there has been some debate over several mechanistic aspects of the reaction, and a number of alternative models have been proposed. For example, Hajos (1) suggested a mechanism that involves the ''activation'' of one of the enantiotopic acceptor carbonyl groups as a carbinol amine (A of Scheme 1). At least the stereochemistry of this model was questioned by Jung (19) soon after its initial proposal. An enamine mechanism was suggested by various groups already in the 1970s and 1980s (19-21). Nonlinearity studies by Agami and colleagues (21) have led to the proposal of a side-chain enamine mechanism that involves two proline molecules in the C-C-bondforming transition state, one engaged in enamine formation and the other as a proton transfer mediator (B of Scheme 1). Swaminathan et al. (22) favor a heterogeneous aldolization mechanism on the surface of crystalline proline (C of Scheme 1), despite the fact that many proline-catalyzed aldolizations are completely homogenous. Agami's widely accepted twoproline mechanism was recently challenged when we proposed a homogenous one-proline enamine mechanism for the intermolecular variant in which the various proton transfers are mediated by proline's carboxylic acid functionality (9). On the basis of density functional theory calculations, subsequently proposed a very similar mechanism for the in...

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Functional monomers and polymers, 129. Asymmetric robinson cyclization reaction catalyzed by polymer‐bound L‐proline

Cited by 51 publications

References 7 publications

Immobilization of Organic Catalysts: When, Why, and How

Immobilization of Organic Catalysts: When, Why, and How

Polymer‐Supported Chiral Organocatalysts: Synthetic Strategies for the Road Towards Affordable Polymeric Immobilization

New mechanistic studies on the proline-catalyzed aldol reaction

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