Aldol-Type Additions of Titanated Hydrazones

21 203 was prepared by quenching the lithium enolate (95 5 trans/cis mixture) with 3 in THF. Aldol addition was performed at —78 C (0.5 h), conversion to 205 being 90% (80% isolated by distillation). 

Reactions of titanated hydrazones with aldehydes occur cleanly at —20 °C (Equation 69). It is not clear whether the observed erythro-selectivity (Table 8) depends upon the geometry of the double bond, since attempts to prepare Z configurated analogs were not rewarding115). The lithiated precursors themselves are unsuitable for selective additions. Titanation not only increases stereo-differentiation, but also chemoselectivity 115). The assumption of a pericyclic transition state means that chair orientations 213 and 214 must be considered, the latter being of higher energy. However, boat transition states also explain the results U5). 

The effect of varying R1/R2, the ligands at titanium and the substituents at the terminal nitrogen atom should allow for more insight. In this connection it is interesting to note that the titanated form of cyclohexanone dimethylhydrazone reacts with benzaldehyde 98% erythro-selectively115). 

In case of triisopropoxide derivatives, some degree of ligand redistribution occurs, so that more than one titanium species is present. 

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