Pseudo five component reaction towards densely functionalized spiro[indole-3,2′-pyrrole] by picric acid, an efficient syn-diastereoselective catalyst: insight into the diastereoselection on C(sp3)-C(sp3) axial conformation

Published in: Journal, Article, Research Support, Non-U.S. Gov’t, Volume : 17, Issue : 5, Pages : 1254-1265

DOI : 10.1039/c8ob02849d

Author : Sengupta, Ayon; Maity, Suvendu; Mondal, Animesh; Ghosh, Prasanta; Rudra, Sonali; Mukhopadhyay, Chhanda

Abstract : A new series of highly-functionalized spiro compounds of pyrrole were synthesized by a one pot, step-economic condensation of isatin, arylamine and β-keto ester catalyzed by wet picric acid. Initially, the reaction is proposed with an expectation of the formation of a multi-spiro heterocyclic framework of highly-substituted piperidine. However, the isomeric compound was characterized to be a five-membered pyrrole derivative with a diverse scope of variations having different types of substituents in the three components, resp. The possibility of formation of various diastereomers around the hindered single bond and the spiro carbon was limited, as only syn products I and II [R1 = t-Bu, Et, Bn; R2 = H, 4-Me, 4-MeO, etc.; R3 = H, Me; R4 = H, Me, Bn, allyl] were isolated in all the reactions performed under the standard conditions. Probably the reactions were mediated by the si-facial formation of the bonds in a picric acid stabilized charge transfer complex transition state. Also, the manner a mol. achieves the most stabilized energy minimized arrangement with all its substituents in space was studied by DFT calculations where I was more stable than II. The studies on the formation of I and II were carried out by variation of electronic and steric factors in each of the components of the reactions.