Asymmetric Diels-Alder reactions of N-sulfinyl dienophiles using chiral Ti(IV)Lewis acids

Enantioselective hetero Diels-Alder (HDA) reactions of 1,3-cyclohexadiene with benzyl N-sulfinylcarbamate 1a and with N-sulfinyl-p-toluensulfonamide 1b promoted by chiral Ti(IV)-based Lewis acids are reported. The obtained yields and enantiomeric excess obtained are heavily dependant on the mode of catalyst preparation. Acceptable reproducibility was obtained with Ti(IV)-catalysts prepared from Me2TiCl2 and chiral diols. Testing of various diols gave the endo adduct in yields of up to 69% with 76% ee. The absolute configuration of the HDA products were established by chemical correlation and verified by X-ray crystallography.