Efficient, Regioselective Access to Bicyclic Imidazo[1,2-x]- Heterocycles via Gold- and Base-Promoted Cyclization of 1-Alkynylimidazoles

摘要

Reactions of 1-alkynylimidazoles involving the formation of their 2-lithio derivatives followed by addition of aldehydes or ketones are presented. The method gives access to 1-alkynyl-2-(hydroxymethyl)imidazoles which undergo 6-endo-dig or 5-exo-dig cyclization under AuCl3- or base-catalyzed conditions to yield imidazo[1,2-c]- oxazoles and imidazo[2,1-c][1,4]oxazine heterocycles. Under transition metal catalysis, the reaction occurs in a regiospecific manner, leading exclusively to the product of 6-endo-dig attack, whereas under basic conditions, the reaction takes place in a regioselective manner giving preferentially the product from 5-exo-dig attack.