The study of the reactions of (S)-2-p-tolylsulfinyl butenolide (2) with cyclic and acyclic dienes is reported. The reactivity and stereoselectivity of this dienophile is compared with those of (S)-2-p-tolylsulfinyl cyclopentenone (3) in order to obtain information about the stereochemical models so far proposed. (C) 1997 Elsevier Science Ltd.