Ruthenium(II)-Catalyzed Cyclization of Oxabenzonorbornenes with Propargylic Alcohols: Formation of Isochromenes

The ruthenium-catalyzed cyclization of a propargylic alcohol with an oxabenzonorbornene in methanol leads to an unanticipated isochromene framework. The catalytic cycle to form this product is believed to go through an oxidative cyclization of the two unsaturated partners with the ruthenium catalyst, followed by beta-hydride elimination, tautomerization and hydroruthenation. The ruthenacyclobutane thus obtained further undergoes [2+2] cycloreversion to form a ruthenium-carbene intermediate that atypically rearranges via a [1,3]-alkoxide shift and finally reductively eliminates to produce the desired compound. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).