- Strategy for contra-thermodynamic radical-chain epimerisation of 1,2-diols using polarity-reversal catalysis
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Polarity-reversal catalysis by thiols has been applied to provide an efficient method for the conversion of appropriate 1,2-diols into less or similarly stable diastereoisomers by epimerisation of their acetonides under radical-chain conditions. (C) 2000
- Dang,Roberts
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- Controlling Selectivity in Alkene Oxidation: Anion Driven Epoxidation or Dihydroxylation Catalysed by [Iron(III)(Pyridine-Containing Ligand)] Complexes
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A highly reactive and selective catalytic system comprising Fe(III) and macrocyclic pyridine-containing ligands (Pc-L) for alkene oxidation by using hydrogen peroxide is reported herein. Four new stable iron(III) complexes have been isolated and characterized. Importantly, depending on the anion of the iron(III) metal complex employed as catalyst, a completely reversed selectivity was observed. When X=OTf, a selective dihydroxylation reaction took place. On the other hand, employing X=Cl resulted in the epoxide as the major product. The reaction proved to be quite general, tolerating aromatic and aliphatic alkenes as well as internal or terminal double bonds and both epoxides and diol products were obtained in good yields with good to excellent selectivities (up to 93 % isolated yield and d.r.=99 : 1). The catalytic system proved its robustness by performing several catalytic cycles, without observing catalyst deactivation. The use of acetone as a solvent and hydrogen peroxide as terminal oxidant renders this catalytic system appealing.
- Tseberlidis, Giorgio,Demonti, Luca,Pirovano, Valentina,Scavini, Marco,Cappelli, Serena,Rizzato, Silvia,Vicente, Rubén,Caselli, Alessandro
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p. 4907 - 4915
(2019/08/30)
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- Gold-catalysed activation of epoxides: Application in the synthesis of bicyclic ketals
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Gold-catalysed generation of diol equivalents from epoxides and their intramolecular reaction with Ca≡C bonds to generate bicyclic ketals is presented. This reaction essentially involves the formation of an acetonide, which subsequently cyclises on the alkyne intramolecularly under gold catalysis conditions. This method could be extended to make optically pure bicyclic ketals. Deuterium incorporation experiments were carried out to ascertain the mechanism of the reaction. Sequential activation of epoxide and alkyne moieties by a gold catalyst in acetone as solvent has been achieved. This strategyhas been employed to synthesise bicyclic ketals from epoxy alkynes. Copyright
- Balamurugan, Rengarajan,Kothapalli, Raveendra Babu,Thota, Ganesh Kumar
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experimental part
p. 1557 - 1569
(2011/04/25)
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- Mild and efficient protection of diol and carbonyls as cyclic acetals catalysed by iron (III) chloride
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A friendly method for the protection of diols and carbonyls catalysed by hexahydrated iron (III) chloride has been developed. This method, which consists of the transformation of diols and carbonyls to cyclic acetals, functions in mild conditions and it is efficient for a wide range of diols.
- Karamé, Iyad,Alamé, Mohamad,Kanj, Ali,Baydoun, Ghinwa Nemra,Hazimeh, Hassan,El Masri, Mirvat,Christ, Lorraine
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experimental part
p. 525 - 529
(2012/05/19)
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- Fe(III) chloride catalyzed conversion of epoxides to acetonides
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A mild and efficient method for the preparation of acetonides from epoxides catalyzed by iron(III) chlorides has been developed.
- Saha, Sumit,Mandal, Samir Kumar,Roy, Subhas Chandra
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body text
p. 5928 - 5930
(2009/04/05)
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- A mild stereo- and enantiospecific conversion of 2,3-diaryl-substituted oxiranes into 2,2-dimethyl-1,3-dioxolanes by an acetone/amberlyst 15 system
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Amberlyst 15 is an efficient catalyst for the reaction of aryl-substituted oxiranes with acetone to prepare 2,2-dimethyl-1,3-dioxolanes in high yields. trans-2,3-Diaryloxiranes afford trans-acetonides enantiospecifically at room temperature, and the use of enantiopure 2,3-diaryloxiranes results in no loss of stereochemical integrity in the resulting trans-acetonides. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
- Solladie-Cavallo, Ariette,Choucair, Elias,Balaz, Milan,Lupattelli, Paolo,Bonini, Carlo,Di Blasio, Nadia
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p. 3007 - 3011
(2007/10/03)
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- Selective radical-chain epimerisation at electron-rich chiral tertiary C-H centres using thiols as protic polarity-reversal catalysts
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Radical-chain epimerisation at chiral tertiary CH centres adjacent to ethereal oxygen atoms can be brought about in the presence of thiols, the function of which is to act as protic polarity-reversal catalysts for hydrogen-atom transfer between pairs of nucleophilic α-alkoxyalkyl radicals. The viability of the method is demonstrated by epimerisation of a series of simple molecules that contain two chiral centres and then the procedure is applied to more complex carbohydrate-based systems, where it is possible to convert a readily available diastereoisomer into a rarer one in a straightforward manner. Of necessity, epimerisation always proceeds in the direction of thermodynamic equilibrium and, in general, the results obtained are in accord with the predictions of molecular mechanics calculations using the MMX force-field. When the required isomer is less stable than the starting diastereoisomer, thiol-catalysed epimerisation of a suitable derivative of the parent can provide a means to obtain the desired compound in satisfactory yield, after deprotection of the epimerised derivative. This strategy is demonstrated for the conversion of trans-cyclohexane-1,2-diol into the less stable cis-isomer and for related contra-thermodynamic isomerisation of some carbohydrates, as well as for the conversion of meso-1,2-diphenylethane-1,2-diol into the dl-form. Thiol-catalysed epimerisation at a CH centre adjacent to an ether-oxygen atom is much faster than at a similar centre adjacent to an amido-nitrogen atom, a result that can be understood in terms of the importance of polar effects on the rate of abstraction of hydrogen by electrophilic thiyl radicals.
- Dang,Roberts,Tocher
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p. 2452 - 2461
(2007/10/03)
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