ADDITION OF A TIGLYLTITANOCENE TO CHIRAL ALDEHYDES - FACTORS INFLUENCING SIMPLE AND DIASTEREOFACIAL STEREOSELECTIVITIES

The eta(3)-tiglyltitanium(III) complex formed in situ from isoprene Ra s reacted with a series of chiral aldehydes, some of which bear alpha- and/or beta-benzyloxy groups, or a remote double bond, anti Diastereo meric homoallylic alcohols were formed stereospecifically starting fro m the alkoxy-free saturated aldehydes. In contrast, mixtures of anti a nd syn stereomers were obtained try using alkoxy- or epsilon-unsaturat ed aldehydes. The simple diastereoselectivities and diastereofacial se lectivities observed are rationalized in terms of the chelation Versus non-chelation-controlled process.