Synthesis of 3-acyl-4-alkenylpyrrolidines by platinum-catalyzed hydrative cyclization of allenynes

A series of nitrogen-tethered allenynes ('5-aza-1,2-dien-7-ynes') 1 were transformed to the corresponding 3-acyl-4-alkenylpyrrolidines 3 when treated with a catalytic amount of PtCl2 in MeOH at 70 degrees. Initial Pt-promoted cyclization forms a nonclassical carbocationic intermediate. In contrast to the cycloisomerization in toluene, which produced the bicyclic cyclobutenes 2, the intermediate is intercepted by addition of an oxygen nucleophile to achieve the formal hydrative cyclization.