Effect of catalyst and solvent on the stereochemistry of Diels-Alder reactions between cyclopentadiene and 3-Phenylsulfinylprop-2-enoic acids and methyl esters

Abstract

In Diels-Alder reactions between cyclopentadiene and the isomeric 3-phenylsulfinylprop-2-enoic acids and methyl esters, greatest stereoselectivity (94% d.e.) results when methyl (2)-3-phenylsulfinylprop-2-enoate (7) and benzene solvent are used. The major diastereoisomer, methyl (2-endo,3-endo,S-syn)-3-phenylsulfinylnorborn-5-ene-2-carboxylate (8), also predominates when silica gel or aluminiumchloride catalysts are used, but is formed in lesser amount to the anti-epimer (9) in the presence of stannic chloride. Stereoselectivity of the reaction is decreased as reaction rate accelerates. Mixturesof all four possible diastereoisomers are formed in reactions with (E)-3-phenylsulfinylprop-2-enoic acid (15) and its methyl ester (16), and the ratios of products vary with catalyst and solvent. The stereochemistry at sulfur in these adducts has been correlated unambiguously by chemical and spectroscopic methods.