Abstract-A novel supported amino alcohol linker was synthesized and utilized for attachment of picolinic acid derivatives onto different supports. When the resin bound molecule was further activated, the PyOX-moiety could be constructed reliably in enantiopure form. Furthermore, an efficient Pd-catalyzed modification of a picolinic acid derivative is presented. q
Preparation of novel chiral gold nanoparticles (AuNP) is described. The major advantage of nanoparticles is to combine the easy handling of solid-supported ligands and the efficiency of soluble ligands, as the molecular size of the support approaches the minimum. This is even of industrial interest, because this kind of technology offers a way of avoiding leaching of the catalyst metal in the reaction and separation of the entire catalyst using simple filtration. In this paper, we describe the attachment of a chiral soluble PyOX-ligand on a nano support via simple ligand exchange. As a result, we achieved a core smaller than ever before in experimental chemistry (1.2 ± 0.2 nm) and promising enantioselectivity in a preliminary test reaction.
Abstract-A short and convenient synthesis of a carboxy functionalized PyOX-core is presented. The carboxy functionality offers a wide variety of possibilities for further modification. In this paper, the core is functionalized with a mercapto tail. Ó 2004 Elsevier Ltd. All rights reserved.In the late 1980s, non-C 2 -symmetric oxazoline ligands and especially the 2-(2 0 -pyridyl)oxazolines (PyOX) were shown to be excellent ligands in asymmetric synthesis. Chiral PyOX-ligands have been used in, e.g. hydrosilylation 1 and Michael reactions. 2 In the late 1990s, C 1 -symmetric PyOX was also found to be an outstanding ligand in Pdcatalyzed allylic alkylation reactions, 3 being superior to the C 2 -symmetric ligands (e.g. PyBOX) due to itÕs ability to form two different palladium complexes. 3a The metal complex forming ability of the PyOX-core also confers biological activity, e.g. as iron chelators. 4 We present herein a new and convenient method to construct PyOX-ligands substituted with functionalities suitable for further conversion, e.g. to nanomaterials. Our approach is based on amido alcohol formation, mesylation and base-assisted cyclization. To our knowledge, this general method has only been tried by Meyers, 5 using long reaction times. In our hands, the cyclization in this one-pot protocol required very long reaction times and prolonged heating to complete, which led to dark coloured reaction mixtures, as also observed by Wuts. 6 PyOX-ligands have usually been prepared by longer synthetic routes from 2-pyridyl nitriles by heating with an amino alcohol in a solvent with a metal salt catalyst like ZnCl 2 , 7a CuCl 2 7b or Cd(OAc) 2 . 7c Another common route to the PyOX-core involves imidate formation 8 and further reaction with the desired amino alcohol.Amido alcohol 5 was constructed from L L-phenylalaninol 4 and pyridine-2,5-dicarboxylic acid 1 as follows. Exhaustive esterification of 1, followed by selective hydrolysis of the more electrophilic 9 ester at the 2-position of the diester 2 gave the monoacid 3. The acid was then converted to the corresponding acid chloride and reacted with amino alcohol 4. In the case of 2-pyridyl acids, the coupling is very selective using equimolar amounts of amino alcohol and acid and no ester byproducts were observed after recrystallization. This was, however, not the case when the corresponding benzoic acids were used. 10 (Scheme 1) Cyclization of 5 was performed in two steps for three reasons: ease of purification, reaction efficiency and ease of reaction monitoring. The similar polarity of amido alcohol 5 and oxazoline 7 makes monitoring on TLC very difficult. The formation and disappearance of mesylate 6, however, were easily followed by TLC. Mesylation of 5 proceeded very fast using DMAP as catalyst at room temperature, total conversion was always reached within 15 min. The mesylate 6 is stable to aqueous extractions and silica and it was isolated by a simple extraction and recrystallization. It was converted to the PyOX-adduct 7 using DBU as the base. 6 No byprodu...
Abstract-A short and convenient synthesis of a carboxy functionalized PyOX-core is presented. The carboxy functionality offers a wide variety of possibilities for further modification. In this paper, the core is functionalized with a mercapto tail. Ó 2004 Elsevier Ltd. All rights reserved.In the late 1980s, non-C 2 -symmetric oxazoline ligands and especially the 2-(2 0 -pyridyl)oxazolines (PyOX) were shown to be excellent ligands in asymmetric synthesis. Chiral PyOX-ligands have been used in, e.g. hydrosilylation 1 and Michael reactions. 2 In the late 1990s, C 1 -symmetric PyOX was also found to be an outstanding ligand in Pdcatalyzed allylic alkylation reactions, 3 being superior to the C 2 -symmetric ligands (e.g. PyBOX) due to itÕs ability to form two different palladium complexes. 3a The metal complex forming ability of the PyOX-core also confers biological activity, e.g. as iron chelators. 4 We present herein a new and convenient method to construct PyOX-ligands substituted with functionalities suitable for further conversion, e.g. to nanomaterials. Our approach is based on amido alcohol formation, mesylation and base-assisted cyclization. To our knowledge, this general method has only been tried by Meyers, 5 using long reaction times. In our hands, the cyclization in this one-pot protocol required very long reaction times and prolonged heating to complete, which led to dark coloured reaction mixtures, as also observed by Wuts. 6 PyOX-ligands have usually been prepared by longer synthetic routes from 2-pyridyl nitriles by heating with an amino alcohol in a solvent with a metal salt catalyst like ZnCl 2 , 7a CuCl 2 7b or Cd(OAc) 2 . 7c Another common route to the PyOX-core involves imidate formation 8 and further reaction with the desired amino alcohol.Amido alcohol 5 was constructed from L L-phenylalaninol 4 and pyridine-2,5-dicarboxylic acid 1 as follows. Exhaustive esterification of 1, followed by selective hydrolysis of the more electrophilic 9 ester at the 2-position of the diester 2 gave the monoacid 3. The acid was then converted to the corresponding acid chloride and reacted with amino alcohol 4. In the case of 2-pyridyl acids, the coupling is very selective using equimolar amounts of amino alcohol and acid and no ester byproducts were observed after recrystallization. This was, however, not the case when the corresponding benzoic acids were used. 10 (Scheme 1) Cyclization of 5 was performed in two steps for three reasons: ease of purification, reaction efficiency and ease of reaction monitoring. The similar polarity of amido alcohol 5 and oxazoline 7 makes monitoring on TLC very difficult. The formation and disappearance of mesylate 6, however, were easily followed by TLC. Mesylation of 5 proceeded very fast using DMAP as catalyst at room temperature, total conversion was always reached within 15 min. The mesylate 6 is stable to aqueous extractions and silica and it was isolated by a simple extraction and recrystallization. It was converted to the PyOX-adduct 7 using DBU as the base. 6 No byprodu...
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