Concave reagents. 12. Polymerfixation of Concave Pyridines

Lüning, Ulrich; Gerst, Matthias

doi:10.1002/prac.19923340803

Cited by 11 publications

(8 citation statements)

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“…Therefore, a concave pyridine 1 has already been attached to a Merrifield polymer resin [11]. But the selectivity of a concave pyridine 1 containing a dioxaoctane chain in the base catalyzed addition of alcohols to ketenes is small.…”

Section: Methodsmentioning

confidence: 99%

“…By nucleophilic aromatic substitution, a 2-benzyloxy-ethoxy substituent can be introduced. Oxidation with SeO 2 gives 4-(2-benzyloxyethoxy)-pyridine-2,6-dicarbaldehyde (5) [11].…”

Section: Methodsmentioning

confidence: 99%

“…Analogously to the synthesis of other 4-substituted concave pyridines [7], 2 was built up by a two step bismacrocyclization starting from the pyridine-2,6-dicarbaldehyde 5 [11]. In general, 4-substituted dialdehydes are accessible in a few synthetic steps from chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic acid) via the key compound 4-chloropyridine-2,6-bismethanol (3) [7b].…”

Section: Methodsmentioning

confidence: 99%

“…The mass balance was > 97%, the yield of 13a and 13b was > 92% with a selectivity 13a/13b of 10. 13.05 g (67.9 mmol) of 1,11-diamino-3,6,9-trioxaundecane [7a] (6) was added to a solution of 19.37 g (67.9 mmol) of 4-(2-benzyloxyethoxy)-pyridine-2,6-dicarbaldehyde [11] (5) and 7.53 g (67.9 mmol) of dry CaCl 2 in 1.8 l of dry methanol. Under N 2 , the solution was heated to reflux until the reaction was completed [ca.…”

Section: Methodsmentioning

confidence: 99%

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Concave reagents. 31. A Merrifield bound concave pyridine for the selective acylation of polyols

Lüning¹,

Hacker

1999

J. prakt. Chem.

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The enhancement of selectivity is a major goal in the development of new reagents and catalysts, and continuously new answers are found for questions of chemo-, regio-, diastereoand enantioselectivity. But in many cases, the price for increased selectivities is a costly synthesis of the reagent or catalyst.While catalysts leave a reaction sequence unaltered reagents are consumed. However, many reagents can also be used over and over again, if a method has been established which "recharges" the reagent. For example a protonation or deprotonation can reestablish an acid or a base, respectively, or redox-active reagents can be "recharged" after the reaction by the use of an appropriate oxidizing or reducing process. Therefore, the following considerations are valid for both classes, for catalysts and for reagents which can be regenerated, although only one of the classes may be mentioned.A multistep synthesis of a new catalyst (or reagent) will only pay off if it can be recovered and purified easily. These requirements are fulfilled by polymer bound catalysts.Since the pioneering work of Merrifield [1], polymeric resins play an important role in organic synthesis [2]. Their use as a platform for the synthesis of larger molecules has enabled the chemist to construct automatic machines for the synthesis of oligopeptides or oligonucleotides [3]. In these reactions, the growing oligomer is covalently bound to the polymer, and the reactions are carried out by alternating turns of dipping the resin into the reagent solutions and washing. The same principle is used in combinatorial chemistry which uses several different reagents in parallel reactions to generate libraries of oligomers [4]. In the final reaction step of these reactions, the products must be cleaved off the polymer.A second use of polymers in organic chemistry exchanges the positions of substrate and reagent: the reagent is bound to a polymer, the substrate is dissolved. After the reaction, product and reagent can again be easily separated by filtration, with the reagent remaining on the polymer. If the reagent is a catalyst or can easily be regenerated it can thus be used over Abstract. The concave pyridine 2a has been synthesized in 61% yield in the two macrocyclization steps. After deprotection to give 2b, the concave pyridine has been attached to a Merrifield resin, and the resulting polymer 10 containing 0.3 mmol 2/g has been used as a selective acylation catalyst for the addition of propane-1,2-diol (11) and the glucose derivative 14a to diphenylketene (12) to form selectively 2-hyand over again, and the synthetic effort in synthesizing the catalyst or reagent pays off. Therefore, many reagents and catalysts have been attached to polymers [5]. In addition with polymer bound catalysts, continuous flow set-ups can easily be realized, which will be shown here for a concave reagent.Concave reagents [6] possess a lamp-shade like geometry with a reactive group (light bulb) on the inside of the molecule. As with enzymes where the high selectivity is largely c...

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Section: Methodsmentioning

confidence: 99%

“…By nucleophilic aromatic substitution, a 2-benzyloxy-ethoxy substituent can be introduced. Oxidation with SeO 2 gives 4-(2-benzyloxyethoxy)-pyridine-2,6-dicarbaldehyde (5) [11].…”

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Concave reagents. 31. A Merrifield bound concave pyridine for the selective acylation of polyols

Lüning¹,

Hacker

1999

J. prakt. Chem.

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“…In a two step bis-macrocyclization also concave pyridines with a spacer in 4-position of the pyridine ring can be synthesized. Such a synthesis has been demonstrated already for a smaller bimacrocycle (lacking one ethylene oxide unit in the polyether chain of 1b) [7] and can be copied for the construction of the concave pyridine 1b [8]. After deprotection of the benzyl protected OH group in 1b by palladium catalyzed hydrogenolysis [8], the OH group of the product 1c can be substituted by iodine giving the key intermediate 1d.…”

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confidence: 98%

Concave reagents 29 [1] Dendrimer fixed concave pyridine

Lüning

Marquardt

1999

J. prakt. Chem.

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Dendrimers fill the gap between polymers and "normal" organic molecules because they combine the large molecular weight of polymers with monodispersity [2]. Hence the construction of appropriately substituted dendritic molecules should solve some of the problems which are encountered when reagents or catalysts [3] are attached to polymers in order to allow an easier recovery.A suitable dendrimer catalyst will therefore combine the following properties: (i) Dendrimers are monodispers. Hence the selectivity will be well defined in contrast to polymer bound catalysts where varying selectivities may be observed due to the polydispersity. (ii) A dendrimer can be constructed in such a way that all catalytic groups possess the same environment which will also give a uniform selectivity, in contrast to crosslinked polymers in which the environment of the catalytic centers may vary. (iii) Dendrimers are usually soluble. Thus no problem will occur at the boundary of phases. (iv) In comparison to standard catalysts, dendrimer bound catalysts possess a higher molecular weight which allows separation by nanofiltration. (v) Linear polymers can "sneak" through small holes of a nanofiltration membrane, a process called reptation. This will be avoided if the molecules are branched like dendrimers. (vi) The solubility and the monodispersity allow the standard analytic procedures of organic chemistry like NMR spectroscopy although the molecular weights are rather high.In this work we describe the exploitation of these advantages of dendrimer bound catalysts for concave reagents [4]. As catalyst we chose the concave pyridine 1a which has shown remarkable selectivities in the base catalyzed addition of ketenes to alcohols and polyols, e.g. monosaccharides [5]. In order to attach a molecule like 1a to a polymer or a dendrimer, the oxygen atom in 4-position of the pyridine ring must be substituted by a spacer which contains a functionality allowing a further connection. The resulting molecules possess up to twelve concave catalytic pyridine centers on the outside of the molecules, and can be used as selective catalysts in the base catalyzed addition of alcohols to diphenylketene.

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