A stereochemically flexible approach to pyrrolidines based on 5-endo-trig iodocyclisations of homoallylic sulfonamides

5-endo-trig Iodocyclisations of the (E)-homoallylic sulfonamides 24 in the presence of potassium carbonate give excellent yields of trans-2,5-disubstituted-3-iodopyrrolidines 36. In the absence of base, these initial kinetic products undergo rapid isomerization to the corresponding cis-2,5-disubstituted pyrrolidines 37 by a ring opening-reclosure mechanism to these thermodynamic isomers. Octahydroindole derivatives 40 and 41 can be similarly obtained from the alk-2-enylcyclohexylsulfonamides 31 and 32. Attempted 5-endo-trig cyclisations of homoallylic carbamates were generally unsuccessful and resulted instead in relatively inefficient 6-exo cyclisations involving the carbamate carbonyl group. Deiodination provides both trans- and cis-2,5-disubstituted pyrrolidines [49 and 50]; the free amine derived from trans-2-butyl-5-pentylpyrrolidine 49d is a component of a fire ant defence pheromone. X-Ray crystallography was carried out for 37f, 36, and 37k.