Heterocycles by cycloaddition. Part 8. Preparation of [1]benzopyrano[4,3-b]pyrroles by intramolecular cycloaddition–extrusion of mesoionic oxazolium-5-olates

Abstract

The in situ Intramolecular cycloaddition–extrusion of the 2-[2-(prop-2-ynyloxy)phenyl]oxazolium-5-olates (4), prepared by cyclisation of the N-acylamino acids (3), gave the [1]benzopyrano[4,3-b]-pyrroles (6a–c) in good yields. Also the 2-acetylated product (6d) was formed from the α-unsubstituted acid (3c). These Intramolecular cycloadditions occurred preferentially even in the presence of an added dipolarophile.