Lewis acid-catalyzed crossed Aldol

The Mukaiyama Reaction —Lewis Acid-Catalyzed Crossed Aldol Reactions... 

Fig. 13). The cross-linked scandium-modified dendrimer was tested in a number of Lewis acid-catalyzed reactions, including Mukaiyama aldol additions to aldehydes and aldimines, Diels-Alder reactions, and Friedel-Crafts acylations. The dendritic catalyst was recovered by a simple filtration. The Mukaiyama aldol... 

Denmark utilized chiral base promoted hypervalent silicon Lewis acids for several highly enantioselective carbon-carbon bond forming reactions [92-98]. In these reactions, a stoichiometric quantity of silicon tetrachloride as achiral weak Lewis acid component and only catalytic amount of chiral Lewis base were used. The chiral Lewis acid species desired for the transformations was generated in situ. The phosphoramide 35 catalyzed the cross aldolization of aromatic aldehydes as well as aliphatic aldehydes with a silyl ketene acetal (Scheme 26) [93] with good yield and high enantioselectivity and diastereoselectivity. 

On the other hand, silver salts can act as a mild Lewis acid, promoting various reactions such as allylations, aldolizations, cycloadditions, and cyclizations. Silver salts can also be used as halide scavengers, acting as cocatalysts in cross-coupling reactions catalyzed by other metals, especially palladium. In the latter context, the exact role of silver salts is far from clear and may be more complex than just halide... 

LLC networks containing catalytic headgroups have also been shown to be useful for heterogeneous Lewis acid catalysis. The Sc(III)-exchanged cross-linked Hu phase of a taper-shaped sulfonate-functionalized LLC monomer has been shown to be able to catalyze the Mukaiyama aldol and Mannich reactions [115] with enhanced diastereoselectivity. This Sc(III)-functionalized Hu network affords condensation products with syn-to-anti diastereoselectivity ratios of 2-to-l, whereas Sc(III) catalysts in solution or supported on amorphous polymers show no reaction diastereoselectivity at all. 

Amino)cinnamoyl compounds 79 can be regarded as primary products they may arise from an aldol condensation catalyzed by base, add, or (most frequently) by Lewis acids [173]. Alternatively, quinoline formation may result from primary cross aldol addition, subsequent cydocondensation to an imine(4-hydroxy-3,4-dihydroquinoHne), and aromatization by H2O elimination [174]). 

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