An iron(II) complex with a hindered hydroxyethyl-pybox (he-pybox) ligand shows improved catalytic activity and enantioselectivity for asymmetric Mukaiyama-aldol reactions in aqueous media. This water-stable chiral Lewis acid promotes condensation of aromatic silyl enol ethers with a range of aldehydes with good yields, excellent syn-diastereoselectivity and up to 92% ee. The combination of the same ligand with ZnII salt is also demonstrated as a remarkably efficient and water-compatible chiral Lewis acid.
The asymmetric aldol-Tishchenko reaction of aromatic aldehydes with aliphatic and aromatic ketones has been developed as an efficient strategy for the synthesis of anti-1,3-diols in good yield with high diastereocontrol and good levels of enantioselectivity. This domino-type reaction is catalyzed by a chiral ytterbium complex that promotes both the aldol reaction through enolization of the carbonyl compound and the Evans-Tishchenko reduction of the aldol intermediate. The stereochemistry of the resulting diols is also investigated and finally proved by using CD techniques.
The asymmetric direct aldol condensation of aldehydes with ethyl- and propylketones is catalyzed by syn-alpha-aminoalcohol-Yb(OTf)3 complexes, yielding the anti-1,3-diol monoesters with high diastereocontrol and good enantioselectivity. Three adjacent stereogenic centers are created in a simultaneous aldol condensation and Evans-Tishchenko reduction in an acyclic system.
Alcohols Q 0230 Chiral Ytterbium Complex-Catalyzed Direct Asymmetric Aldol-Tishchenko Reaction: Synthesis of anti-1,3-Diols. -The asymmetric aldol-Tishchenko reaction of aromatic aldehydes with aliphatic and aromatic ketones is efficiently catalyzed by a new chiral Yb-amino alcohol complex. The reaction proceeds with generation of three new stereocenters in good yields and enantioselectivities. The reaction mechanism is discussed in detail. -(MLYNARSKI*, J.; RAKIEL, B.; STODULSKI, M.; SUSZCZYNSKA, A.; FRELEK, J.; Chem. Eur.
Alcohols Q 0230 Iron(II) and Zinc(II) Complexes with Designed pybox Ligand for AsymmetricAqueous Mukaiyama-Aldol Reactions. -The Fe and Zn catalysts efficiently promote the reaction of aliphatic and aromatic aldehydes. However, coupling of the pyridinealdehyde (VI) leads to a racemic product. -(JANKOWSKA, J.; PARADOWSKA, J.; RAKIEL, B.; MLYNARSKI*, J.; J. Org. Chem. 72 (2007) 6, 2228-2231; Inst. Org. Chem., Pol. Acad. Sci., PL-01-224 Warsaw, Pol.; Eng.) -Jannicke 30-054
Diastereoselective syntheses O 0031Direct Asymmetric Aldol-Tishchenko Reaction of Aliphatic Ketones Catalyzed by syn-Aminoalcohol-Yb(III) Complexes. -Optimized reaction conditions allow formation of anti-1,3-diol monoesters (III)/(IV) with high diastereocontrol and good enantioselectivity. Three adjacent stereogenic centers are created in a simultaneous aldol condensation and Evans-Tishchenko reduction in an acyclic system. -(MLYNARSKI*, J.; JANKOWSKA, J.; RAKIEL, B.; Chem. Commun. (Cambridge) 2005, 38, 4854-4856; Inst. Org. Chem., Pol. Acad. Sci., PL-01-224 Warsaw, Pol.; Eng.) -M. Paetzel
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