Other Enantioselective Aza-Reactions

The stereochemical outcome can be explained by assuming a model in which the oxygen of the nitrone is coordinated to the boron of BLA, as shown in Fig. 21. The proposed transition state model, which shows re face approach of the indole nucleus to a C=N double bond, would be preferred. 

Scheeren and co-workers have found that the asymmetric 1,3-dipolar cycloaddition of nitrones with ketene acetals is strongly catalyzed by chiral oxazaborolidines derived from A-tosyl-L-a-amino acids (Eq. 83) [59a]. The 5,5-dialkoxyisoxazolidines are 

The enantioselectivity of the chiral oxaborolidine-catalyzed asymmetric 1,3-dipolar cycloaddition can be controlled by the a-side-chain substituent in this catalyst and the solvent (Tables 4 and 5) [59b]. A remarkable reversal of enantioselectivity is achieved with catalysts with aryl substituents in the a-side-chain and by adding ligand-like solvents. Both enantiomers of a chiral /i-amino ester have been prepared in two catalytic steps. 

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