Multinuclear enantiopure titanium self‐assembly complexes—synthesis,characterization and application to organic synthesis

Hexa‐ and nonanuclear titanium complexes were obtained by self‐assembly of titanium(IV)‐tert‐butoxide and D ‐mandelic acid. Suitable single crystals of these complexes were characterized by X‐ray structure analysis. When used with these complexes, aldol adducts were isolated with a high degree of regioselectivity in direct aldol additions of aromatic and aliphatic aldehydes to functionalized unsymmetrical ketones. High syn‐diastereoselectivities were obtained in aldol additions of enolizable aldehydes with hydroxyacetone and methoxyacetone. Copyright © 2007 John Wiley & Sons, Ltd.     

Unusual highly regioselective direct aldol additions with a moisture-resistant and highly efficient titanium catalyst

[reaction: see text] The extremely robust and water-stable tetranuclear complex Ti(4)(mu-BINOLato)(6)(mu(3)-OH)(4) was found to catalyze the direct aldol addition with high regioselectivities at the more steric alpha-encumbered side of unsymmetrical ketones. As few as 0.2 mol % loadings with this cluster were enough to afford complete conversions. The reaction proceeds very smoothly without a significant amount of byproducts. The formation of quaternary stereocenters is described.     

Tetranuclear BINOL-titanium complex in selective direct aldol additions

The extremely robust and water-stable tetranuclear complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1) catalyzes the direct aldol addition with high degrees of regioselectivity at the sterically more encumbered alpha-side of unsymmetrical ketones. The formation of quaternary stereocenters is described. Oxygen-containing ene components can also be used as starting material in these reactions. When used with aliphatic aldehydes, acetals 18 or acetals of aldol adducts 20 were observed. As few as 0.2 mol % loadings with this catalyst 1 were enough to complete the reactions. Mechanistical aspects of the reactions are discussed.     

Stereoselective amine-catalyzed carbohydrate chain elongation

Aldol additions of unprotected carbohydrates to 1.3-dicarbonyl compounds have been described. This transformation is based on a dual activation by tertiary amines and 2-hydroxypyridine.