Acetals comparison with imine formation

The electron-rich nature of silyl ketene acetals enables their facile coupling with imines in the presence of a titanium salt [101,102]. Interesting results are obtained from comparison of the efficiency of a titanium salts of the type 71X4. Switching the halogen X from F to Cl to Br to I, uniformly increases the product yield and diaster-eoselectivity of the TiX4-catalyzed reaction between a ketene silyl acetal and an imine (Eq. 28) [103]. The finding illustrated by Eq. (28) was further applied to diastereose-lective carbon-carbon bond formation as shown in Eq. (29) [103]. More examples of the titanium-mediated reaction of ketene silyl acetals and imines or their derivatives, for example (N,6>)-acetals, are summarized in Table 3. 

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