Lanthanoid triisopropoxides are active catalysts for aldol reactions. Aldehydes give the corresponding β-hydroxyaldehydes at low temperatures in good yields, whereas ketones are less reactive, but form condensation products at high temperatures. Exceptionally, γ- or δ-diketones easily undergo condensation to give five- and six-membered unsaturated ketones in high yields. The lanthanoid propoxides, catalyzing the Michael addition of ketones to α,β-unsaturated ketones, which give δ-diketones, are also good catalysts for the Robinson-type annelation. In these reactions, the catalytic activity of the lanthanum propoxide is higher than those of the heavy lanthanoid propoxides, and is almost comparable to that of sodium isopropoxide. Since aluminum triisopropoxide shows poor activity, the lanthanoid propoxides are considerably basic for trivalent metal alkoxides.

Aldol Reaction and Robinson-Type Anellation Catalyzed by Lanthanoid Triisopropoxides.-Lanthanoid triisopropoxides are active catalysts for aldol reactions. Among the lanthanoid catalysts tested, the La derivative exhibits the highest efficiency in the aldol reactions. It is superior to Al(OiPr)3 and even iPrONa. Aldehydes give the correspondig β-hydroxyaldehydes (cf. (II)/(III); (V)) in good yields under mild conditions. Monoketones prove to be less reactive but furnish condensation products (cf. (VII)), whilst diketones such as (VIII) easily undergo cyclizations. The lanthanoid triisopropoxides are also suitable catalysts for Robinson-type anellations (→(XII)). -(OKANO, T.; SATOU, Y.; TAMURA, M.; KIJI, J.; Bull. Chem.