Cycloaddition reactions of 1-phenylseleno-2-(p-toluenesulfonyl)ethyne

1-Phenylseleno-2-(p-toluenesulfonyl)ethyne (1) is an effective dienophile and dipolarophile. It underwent facile Diels-Alder reactions with a variety of dienes to afford vicinal sulfone- and selenide-functionalized 1,4-cyclohexadienes. Unexpected regiochemistry that is the opposite of what is obtained with simple acetylenic sulfones was observed with several unsymmetrical dienes containing methyl or methoxy substituents at the 1- or 2-position. Acetylene 1 reacted with (trimethylsilyl)methyl azide, diazomethane, and 2,4,6-trimethylbenzonitrile N-oxide via 1,3-dipolar cycloadditions to afford the corresponding triazole, 1,2-diazole, and isoxazole products. It also underwent an ene reaction with beta-pinene that showed anomalous regiochemistry compared to other acetylenic sulfones. The Diels-Alder cycloadducts obtained from the reaction of 1 with 2,3-dimethyl-1,3,butadiene and 1,3-cyclohexadiene were readily converted into the corresponding beta-keto sulfones and ketones, thus rendering 1 as the synthetic equivalent of p-toluenesulfonylketene and ketene, respectively. Base-catalyzed elimination of TsOH from the Diels-Alder cycloadduct obtained with 2,3-dimethyl-1,3-butadiene afforded the corresponding aryl phenyl selenide, while the adduct from piperylene underwent oxidation to its selenoxide, followed by a Pummerer-type reaction to produce 2-(phenylseleno)-3-(p-toluenesulfonyl)toluene. The reaction of the bicyclic Diels-Alder product obtained from 1,3-cyclohexadiene with MeCu(SePh)Li resulted in substitution of the phenylseleno moiety by a methyl group, whereas similar treatment, of the monocyclic adduct derived from piperylene effected elimination of PhSeH and aromatization.