Organocatalytic γ′[C(sp3)–H] Functionalization of Ynones: An Unusual Approach for the Cyclopentannulation of Benzothiophenes

Abstract

An efficient organocatalytic approach for the cyclopenta[b]annulation of benzothiophenes via γ′[C(sp3)–H] functionalization of ynones is described. Nucleophilic addition of an organophosphine to the designed ynones generates heteroaryl-based ortho-quinodimethanes (oQDMs), which undergo carbocyclization to provide a variety of cyclopenta-fused benzothiophenes. This approach also constitutes an unusual organophosphine-catalyzed intramolecular hydroalkylation of ynones.

Conceptual advances in nucleophilic organophosphine-promoted transformations

“…7 In general, phosphines add onto the electron-deficient carbon–carbon multiple bonds to form zwitterionic species, which can be trapped by various electrophiles or pronucleophiles to accomplish the desired transformation, Scheme 1. Some of the well-known applications of phosphines involving an initial nucleophilic addition to activated olefins include the Rauhut–Currier reaction, 8 Morita–Baylis–Hillman reaction, 9 redox transformations, 10 aldol-type reactions, 11 hydroalkylation and hydroacylation reactions, 12 etc. Some of the attractive features associated with phosphines are (i) tunable nucleophilicity and manoeuvrable stereochemical outcome depending upon the electronic and steric nature of the groups present on the phosphine, (ii) their ability to promote transformations via the P III –P V redox cycle, and (iii) being economical and readily available.…”