Highly Diastereoselective Intermolecular β-Addition of Alkyl Radicals to Chiral 2-(Arylsulfinyl)-2-cycloalkenones
摘要:
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.
An improvedprocedure of the Sharpless method for the preparation of chiral sulfinates by triphenylphosphine is described. A mixture of sulfonyl chlorides and diacetone-d-glucose or l-menthol in the presence of triethylamine was treated with triphenylphosphine in CH2Cl2 at 0°C to give the sulfinates in good yields.