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Zusammenfassung:
The concept of chelation controlled addition to chiral alkoxy aldehydes using Lewis acids and mild C-nucleophiles has been extended to include cyclopropylation. The mechanism of the TiCl4-mediated chelation controlled addition of enolsilanes and allylsilanes to chiral α-alkoxy aldehydes has been studied by rapid injection NMR techniques. Accordingly, an acyclic transition state is involved in which the silyl group does not migrate to the carbonyl oxygen atom. In contrast, LiClO4 is an effective Lewis acid (excess or catalytic amounts) which induces chelation controlled group transfer type of aldol additions
