NHC–gold(i) catalysed [4 + 2] cycloaddition–acyclic addition of dialkyl substituted propargylic esters with 1,3-diphenylisobenzofuran: synthesis of novel benzo[c]fluorenols and substituted dienes

Abstract

A gold carbene complex [IPrAuCl/AgSbF₆] catalysed novel cycloaddition of propargylic esters with 1,3-diphenylisobenzofuran is reported. A [4 + 2] cycloaddition followed by sequential aromatic allylation leading to pentannulation with the expulsion of benzoic acid, then 1,2-phenyl migration coupled with ring opening and aromatisation leading to a new class of benzofluorenols, is discovered. This process involves facile multiple C–C bond formation. An accompanying second pathway involving the attack of benzoyloxy anion on the central allenic carbon affording substituted dienes is also observed. Key products are characterised by X-ray structure determination.