Access to π-Extended Heterocycles Containing Pyrrolo-Coumarin Cores Involving −COCH<sub>3</sub> as a Traceless Directing Group and Materializing Two Successive sp<sup>2</sup>C–H/sp<sup>3</sup>N–H and sp<sup>2</sup>C–H/sp<sup>2</sup>N–H Activations
Dwaipayan Das, Asish R. Das
Abstract
An efficient protocol has been developed for the preparation of π-extended N-heterocycles involving a Rh(III)-catalyzed C–H activation reaction starting from 3-acetamidocoumarins and internal alkynes. The isolation of the intermediate pyrrolo-coumarin suggests that the −COCH3 group in acetamidocoumarins performs the role of a traceless directing group. Besides, the use of commercially available [Cp*RhCl2]2 adds more importance as no additional modification of the catalyst is required. A two-step protocol bearing intermediate pyrrolo-coumarin can be further functionalized to highly decorated heterocyclic moieties materializing sp2 C–H and sp2 N–H coupling. Moreover, one of the pyrrolo-coumarin compounds (3da) is capable of differentiating between Cr(III) and Cr(VI) ions as revealed via fluorescence spectroscopy. In addition, intermediate pyrrolo-coumarin is further functionalized to spirocyclic N-heterocycles.