Radical cyclization of unsaturated α-sulphonyl radicals. Preparation and stereochemistry of arylsulphonyl-tetrahydrofuran and -pyrrolidine

1-Bromo-1-p-tolylsulphonyl-2-allyloxyethane derivatives (2) reacted with tri-n-butyltin hydride in the presence of azobisisobutyronitrile (AIBN) in benzene at 70 °C for 7–13 h to afford 3-p-tolylsulphonyl-4-alkyltetrahydrofurans (5) in which the trans-isomer predominated. The substituents on the olefinic carbon have a market effect on the stereochemical course of the cyclization. 1-Bromo-1-p-tolylsulphonyl-2-allylaminoethane (4) gave the pyrrolidine derivative (6) more stereoselectively. These results are discussed in terms of the steric factors. Similarly, 1-bromo-1-p-tolylsulphonyl-2-prop-2-ynyloxyethane derivatives (3) produced 3-p-tolylsulphonyl-4-alkylidenetetrahydrofurans (9).