Substrate‐Controlled Michael Additions of Titanium Enolates from Chiral α‐Benzyloxy Ketones to Conjugated Nitroalkenes

Lewis acid mediated substrate-controlled reactions of the titanium(IV) enolates of chiral α-benzyloxy ketones with conjugated nitroalkenes give 2,4-anti-4,5-syn Michael adducts in good yields and diastereomeric ratios. The supplementary Lewis acid plays a key role in the outcome of these transformations, probably as a consequence of the formation of dimetallic enolates that increase the reactivity of the enolate and direct the approach of the nitroalkene. Importantly, the most appropriate Lewis acid depends on the electrophilic partner: TiCl4 is the most suitable Lewis acid for β-aryl-nitroalkenes, whereas the best results for β-alkyl-nitroalkenes are obtained with SnCl4. Finally, the nitro group of the resultant compounds can be converted into the corresponding amine, oxime, and nitrile groups under mild conditions, which permits the synthesis of a variety of enantiomerically pure derivatives in excellent yields.