Highly diastereoselective intermolecular beta-addition of alkyl radicals to chiral 2-(arylsulfinyl)-2-cycloalkenones

The diastereoselectivity of intermolecular beta-addition of alkyl radicals to 2-(arylsulfinyl)-acycloalkenones depends largely upon the structure of the arylsulfinyl group. The reaction of a-cyclopentenones or 2-cyclohexenones having a sterically bulky arylsulfinyl group such as (3,5-di-tert-butyl-4-methoxyphenyl)-sulfinyl, (2,4,6-triisopropylphenyl)sulfinyl or (2,4,6-trimethylphenyl)-sulfinyl group gives 3-alkyl-2-(arylsulfinyl)-1-cyclopentanones or 3-alkyl-2-(arylsulfinyl)-1-cyclohexanones in excellent yields and with high diastereoselectivity. Both the X-ray crystallographic analysis and the NOE experiment in the H-1 NMR spectrum of (S)-2-[(2,4,6-triisopropylphenyl)-sulfinyl]- and (S)-2-[(2,4,6-trimethylphenyl)sulfinyl]-2-cyclopentenone reveal an effective shielding of one of the olefin faces at the beta-position by o-isopropyl and o-methyl groups. The addition of bidentate Lewis acids reverses the stereoselection through chelating the intermediates to give the addition products with high diastereoselectivity.