Stereocontrolled Synthesis of Triazacyclopenta[cd]pentalenes by Intramolecular 1,3-dipolar Cycloaddition Reactions of Azomethine Imines

The construction of triazacyclopenta[cd]pentalene diesters 6 by the reaction of dihydropyrrole alpha-ketoesters 3 with acylated hydrazines was evaluated further as a potential central strategic step in the total syntheses of complex guanidine alkaloids such as palau'amine and styloguanidine. Successful cyclocondensations were realized with acid-stable 2,2,2-trichloroethyl carbazate and thiosemicarbazide, but not tert-butyl carbazate. The substituent on the pyrrolidine nitrogen can be alkoxycarbonyl, sulfonyl, or an N-alkyl-2-acylpyrrole group. Siloxy substitution at C1 of the alpha-ketoester side chain is also tolerated.