A series of [4 + 21 cycloadditions with 2,Sdimethylthiophene oxide (2), generated in situ by peracid oxidation of 2,Sdimethylthiophene (l), are described. In all cases the syn adduct (with respect to the sulfoxide oxygen) is formed exclusively. Recently the x-facial diastereoselectivity encountered in Diels-Alder reactions, where the addends possess two different reactive faces, has received increased attention. 1 In many of these cycloadditions, in which the diene contains an allylic stereogenic centre, the dienophile reacts preferentially with the more sterically hindered syn face. This is particularly true of C-5 substituted cyclopentadienes in which the allylic group contains an electronegative heteroatom such as oxygen, nitrogen, or chlorine. *J In contrast, a sulfur substituent reverses the facial selectivity and anti adducts resuk3
A number of vinyl sulfoxides have been examined as dienophiles4 and a stereogenic sulky1 group directed a pyrone cycloaddition to give a 88: 12 diastereomer ratio.5 Frequently, the interp retation of the results with acyclic systems is complicated by the necessity of determimn g the relative contribution and reactivity of various conformers in which the heteroatom is aligned "inside" as predicted by electronic factors or "outside" as favoured on steric grounds. Cyclic dienes avoid this complication and examples that are plane-nonsymmetric afford insight into the "addition directing" influence of diverse functional groups. Thiophene oxides provide a model to assess the relative importance of the competition between a lone pair and sulfur-oxygen bearing face. We wish to report that many diactivated, endo selective dienophiles, add preferentially to the syn oxygen face. Thiophene oxides thus represent latent butadienes that react with exclusive facial control.
In contrast, to the cycloadditions of thiophene-1.1~dioxides thiophene oxides have received much less