Efficient Syntheses of 2,5‐Dihydropyrroles, Pyrrolidin‐3‐ones, and Electron‐Rich Pyrroles from N‐Tosylimines and Lithiated Alkoxyallenes

Abstract

N‐Tosylimines and lithiated alkoxyallenes afforded the corresponding allenyl N‐tosylamines in good to excellent yields. The 5‐endo‐trig cyclizations of these intermediates to 2,5‐dihydropyrrole derivatives were achieved under strongly basic conditions with potassium tert‐butoxide or under milder conditions with either silver(I) or gold(I) catalysis. In general, gold chloride catalyzed reactions occurred with the lowest catalyst loadings and delivered the best yields. With phenyl‐substituted 2,5‐dihydropyrrole, we investigated typical reactions such as oxidative and reductive transformations. The acid‐promoted hydrolysis of the enol ether moieties of three 2,5‐dihydropyrroles furnished the corresponding pyrrolidin‐3‐ones, which were reduced to give 3‐hydroxypyrrolidine derivatives in good yields. The aromatization of 2,5‐dihydropyrroles to 3‐methoxypyrrole derivatives was accomplished by base treatment, which caused the elimination of p‐tolyl sulfinate. Electron‐rich pyrrole derivatives were synthesized in good yields even on a large scale. All of the experiments illustrate the efficacy and flexibility of the alkoxyallene approach to pyrrole derivatives.