Intramolecular cyclization of bis(2,4,6-trialkylphenyl) ketenes to isochromenes

Abstract

The crowded ketenes, (2,4,6-R₃C₆H₂)₂CCO, R = Prⁱ, Me, undergo intramolecular cyclization of the ketene oxygen on an o-alkyl group to give substituted isochromenes. The reaction is two orders of magnitude faster at 170 °C for R = Prⁱ than for R = Me. A stepwise reaction involving a 1,5-sigmatropic rearrangement to a conjugated tetraenal which does not accumulate and rapidly cyclizes to the isochromene is suggested.