Asymmetric Diels-Alder Reaction Catalyzed by a Chiral Titanium Reagent

A highly enantioselective Diels-Alder reaction has been developed by employing a chiral titanium reagent generated in situ from dichlorodiisopropoxytitanium and the chiral diol 1d, which is easily derived from tartaric acid. With a catalytic amount of the titanium reagent, various acyloxazolidinone derivatives of α, β-unsaturated carboxylic acids react smoothly with dienes in the presence of 4A molecular sieves to give the corresponding optically active cycloadducts. Examination of the solvents revealed that the enantioselectivity and the reactivity of this reaction are widely dependent on the acceptor and donor properties of the solvents. By utilizing mesitylene, CFCl₃, or a mixed solvent of toluene and petroleum ether (or hexane), high enantioselectivity is achieved, and various synthetically useful chiral intermediates are obtained by a simple reaction procedure.