The p-addition of alkyl radicals to chiral 2-(arylsulfinyl)-2- cyclopentenones 1. gives 3-alkyl-2-(arylsulfinyl)- 1 -cyclopentanones 2 with stereoselectivities depending on the bulkiness of alkyl radicals as well as the aryl sulfoxides. The X-ray analysis of the single crystal as well as the NOE experiment of 2,4,6-triisopropylphenyl sulfoxides U showed the effective shielding at the P position by the triisopropylphenyl group. The reaction of a diastereomeric mixture of 4-methyl-2-sulfinyl-cyclopentenones and a gives the kinetically resolved addition product. Radical reaction of acyclic sulfinylpentenones gives abnormal Pummerer-type reaction products. A CH2C12 solution of a-sulfinylcyclopentenones L was treated with trialkylboranes (7) in the presence of TiC12(0i-Pr)2 (8) or without Lewis acid at 0°C. The results are summarized in Table 1. The reaction of (R)-2-( (3,5-di-tert-butyl-4-methoxyphenyl)sulfinyl)-2-cyclopentenone