Stereoselective Addition of Organotitanium Reagents to Carbonyl Compounds

Titanation of alkyllithium or ‐magnesium compounds using ClTi(OR) 3 results in reagents which show markedly increased diastereofacial selectivity (80–90%) in reactions with α‐chiral aldehydes or ketones. Titanation is also the method of choice in Grignard‐type additions to substituted cyclohexanones; CH 3 Ti(OCHMe 2 ) 3 ( 6a ) adds predominantly from the equatorial direction, while allyltitanium reagents 11b and 12 show axial preference. Crotyltitanium compounds react with carbonyl compounds to afford primarily adducts having anti ‐configuration, a process which is of particular value in case of ketones ( anti/syn ratios up to 99:1). Titanation of (trimethylsilyl)allyllithium ( 48 ) with Ti(OCHMe 2 ) 4 reverses regioselectivity in reactions with aldehydes and ketones, β‐hydroxy silanes 50 being the only observed products. These have anti ‐configuration and can be converted either into Z ‐ or E ‐dienes using the Peterson elimination under basic or acidic conditions, respectively.