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; CH3Ti(OCHMe2)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(OCHMe2)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.