Synthesis Of polyaromatic heterocycles pyrrolo [1,2-a] indoles by Gold(I)-Catalyzed tandem Cyclization/C-H Activation/Cyclization.

The initial results on a novel procedure for the synthesis of pyrrolo[1,2-a]indoles via gold(I)-catalyzed tandem cyclization/C-H activation/cyclization is described. The procedure allowed access to a highly functionalized pyrrolo[1,2-a]indoles in a one-pot process starting from anilinodiynes.

The mechanism of this reaction by using gold(I) catalysis, interestingly proceeded via 5-endo-dig cyclization at only one of the alkyne of the diyne fragment, leading to the formation of 2-alkynylindole derivative. We are purposing a Csp³-H bond activation and generating gold(I)-carbene species with [1,5]-H migration and cyclization following by [1,2]-H shift.

The synthesis of starting material alkynes was developed. Initially, we started with the addition of Br₂ to 2 methylbut-3-yn-2-ol leading to the formation of the corresponding 4-bromo-2-methylbut-3-2-ol. In the next step we were used 4-bromo-2-methylbut-3-2-ol with phenylacetylene gave rise to 2-methyl-6-phenylhexa-3,5-diyn-2-ol. Among we used 2-methyl- 6-phenylhexa-3,5-diyn-2-ol reacted with KOH for evolving acetone and we obtained buta-1-3-diyn-1-ylbenzeze. Further, we treated 2-iodoaniline with benzhydryl bromide we get N-benzhydryl 2-iodoanilne. Then move to the final step for the formation of starting material by taking buta-1-3 -diyn-1-ylbenzeze with N-benzhydryl 2-iodoaniline and we get N-benzhydryl-2-(phenylbuta-1,3-diyn-1-yl).

Initial tandem gold(I)-catalyzed screening was carried out by using highly reactive cationic gold(I)-complex (Echavarren catalyst). Initially, we use between 5-20 mol% then we decided to 5 mol% gold(I) catalyst in DCM at room temperature.

The result we obtained 1-benzhydryl-2-(phenylethynyl)-1-H-indole moiety first cyclization was confirmed by using ¹H, ¹³C NMR spectroscopy, and X-ray. Interestingly we have developed intramolecular cyclization of indole moiety with high yield, short reaction time, and room temperature.