Chiral aldimines, stereoselective Mannich-type reactions

Loh et al. found a tritlic acid-catalyzed 2-oxonia Cope rearrangement, which was used in the stereocontrolled synthesis of linear 22i -homoallylic sterols (eq 48). Interestingly, poor stereoselectivity was observed when In(OTf)3 was employed as the catalyst for this reaction. Stereoselective Mannich-type reaction of chiral aldimines with 2-silyloxybutadienes in the presence of triflic acid gives the corresponding products with 70-92% de in 62-74% chemical yield, which are not obtained by general Lewis acid-promoted methods (eq 49). ... 

Ishihara developed a highly diastereo- and enantioselective direct Man-nich-type reaction of aldimines with 1,3-dicarbonyl compounds using chiral lithium binaphtholate salts as effective Lewis-acid-Bronsted-base catalysts (Scheme 2.5). ° The stereoselectivity of the Mannich products anti-S and syn-7 ) was reversed when the nucleophile was changed from acyclic 1,3-dicarbonyl compound 4 to cyclic compound 6. The molecular flexibility and acidity of the nucleophiles 4 and 6 would be the major factor in differentiating the two reaction pathways. 

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