Structural optimization of thiourea-based bifunctional organocatalysts for the highly enantioselective dynamic kinetic resolution of azlactones
This article describes the synthesis of a library of structurally diverse bifunctional organocatalysts bearing both a quasi-Lewis acidic (thio)urea moiety and a Bronsted basic tertiary amine group. Sequential modification of the modular catalyst structure and subsequent screening of the compounds in the alcoholytic dynamic kinetic resolution (DKR) of azlactones revealed valuable structure-activity relationships. In particular, a "hit-structure" was identified which provides e.g. N-benzoyl-tert-leucine allyl ester in an excellent enantiomeric excess of 95%. The Royal Society of Chemistry 2006.
Berkessel, Albrecht,Mukherjee, Santanu,Mueller, Thomas N.,Cleemann, Felix,Roland, Katrin,Brandenburg, Marc,Neudoerfl, Joerg-M.,Lex, Johann
p. 4319 - 4330
(2008/09/18)
Nucleophilic substitutions using O-alkyl-N,N'-dialkylisoureas. Applications to ephedrines
Dialkylcarbodiimides in the presence of a Cu(I) catalyst react cleanly with the hydroxyl group of N-methylated (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine.These adducts react with nucleophiles like alkyl and aryl thiols as well as thioic acids and phthalimide to form the substitution products with overall retention of configuration.It is postulated that intramolecular paricipation of the amino group via an SN2 reaction leads to aziridium salts, which are subsequently opened by the nucleophiles via a second SN2 reaction.This synthetic approach is also useful for the inversion of simple secondary alcohols; on treatment with dicyclohexylcarbodiimide followed by benzothioic acid and treatment with LiAlH4, menthol was converted in good yield to neomenthane thiol.
Poelert, Martin A.,Hulshof, L. A.,Kellogg, Richard M.
p. 365 - 368
(2007/10/02)
Application of the Mitsunobu reaction to ephedrines and some related amino alcohols. Aspects of intramolecular participation of the amino group
Inversion of configuration at the benzylic hydroxyl group of (1S,2S)-pseudoephedrine (2) to afford (1R,2S)-ephedrine is known to be a difficult process.The Mitsunobu reactions of 1 and 2 might offer a route to achieve such inversions.In fact Mitsunobu reactions on 1 and 2 are known to proceed via aziridines formed on intramolecular SN2 substitution by the amine functionality.The Mitsunobu reactions of N-methylated and N-benzylated ephedrines have been found to proceed via the corresponding aziridinium ions.These aziridinium ions can be opened (SN2 substitution) by nucleophiles like phthalimide and thiols.Intramolecular participation in 2 can be avoided by use of the tert-butyloxycarbonyl-(BOC) or benzyloxycarbonyl- (CBZ) protected derivatives.Mitsunobu reactions on these derivatives lead to inversion of configuration at the benzylic hydroxyl center.In contrast the BOC and CBZ derivatives of 1 are deprotected under Mitsunobu conditions.The Mitsunobu reactions of threo (1S,2S)-2-amino-1,3-propanediol have also been examined.An attempt to achieve protection by reaction with dimethylformamide dimethyl acetal led to the more substituted 2-oxazoline as established by X-ray crystallography.The desired inversion of configuration of the benzylic hydroxylic group was eventually achieved by protection of the amino substituent as the phthalimide and protection of the primary hydroxyl group as the tosylate.
Poelert, Martin A.,Hulshof, L. A.,Kellogg, Richard M.
p. 355 - 364
(2007/10/02)
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