ASYMMETRIC DIELS-ALDER REACTION OF (S)-ALPHA-P-TOLYLSULFINYL ALPHA,BETA-UNSATURATED ESTERS - THE ROLE OF THE SULFINYL GROUP IN ASYMMETRIC DIELS-ALDER REACTIONS OF VINYLSULFOXIDES

The mechanistic models so far proposed to explain the behavior of viny lsulfoxides as dienophiles in asymmetric Diels-Alder reactions are rev ised on the basis of the results obtained in the reactions of (S)-benz yl 2-p-tolylsulfinyIacrylate 3 with cyclopentadiene, Dane's diene and furan under different conditions, as well as other results previously reported, concerning sulfinylacrylates, sulfinylmaleates and trialkoxy carbonyl sulfinyl ethenes. The moderate reactivity of sulfinylacrylate s and the scarce influence that the incorporation of additional alkoxy carbonyl groups exerts on their reactivity is explained by assuming a variable electronic influence of the sulfinyl group on the double bond , acting as withdrawing and donating electron group depending on the s ubstitution at the double bond. In the reactions with cyclopentadiene, both reactivity and stereoselectivity raised in the presence of ZnX2 as catalysts (ZnI2>ZnBr2>ZnCl2), but TiCl4 was found to be the most ef ficient catalyst allowing the reactions to take place at low temperatu res. With cyclopentadiene and furan, steric and electronic interaction s between the CH2 and O, respectively, and the substituent at sulfur o n the s-cis conformation of the vinylsulfoxide must be considered to p redict the favored stereochemical course. (C) 1997 Elsevier Science Lt d.