A Zinc-Mediated Deprotective Annulation Approach to New Polycyclic Heterocycles.

A straightforward approach to new polycyclic heterocycles, 1-benzo[4,5]imidazo[1,2-][1,3]oxazin-1-ones, is presented. It is based on the ZnCl-promoted deprotective 6-- heterocyclization of -Boc-2-alkynylbenzimidazoles under mild conditions (CHCl, 40 °C for 3 h). The zinc center plays a dual role, as it promotes Boc deprotection (with formation of the -butyl carbocation, which can be trapped by substrates bearing a nucleophilic group) and activates the triple bond toward intramolecular nucleophilic attack by the carbamate group.