CYCLOADDITION OF TUNGSTEN-ETA(1)-3-FURYL COMPOUNDS WITH ALKENES AND ALKYNES - SYNTHESES AND RING-CLEAVAGE OF TUNGSTEN-ETA(1)-OXABICYCLOHEPTENE AND TUNGSTEN-ETA(1)-OXABICYCLOHEPTADIENE COMPOUNDS

Cycloaddition of tungsten-eta(1)-3-furyl compounds with a variety of o rganic alkenes and alkynes proceeds smmothly at ambient conditions, yi elding tungsten-eta(1)-oxabicyclo[2.2. 1]heptene and -eta(1)-oxabicycl o[2.2.1]heptadiene compounds in high regioselectivities and stereosele ctivities. The cycloaddition is proposed to proceed via a mechanism in volving a zwitterion intermediate. Cleavage of oxygen bridges of eta(1 )-oxabicyclo[2.2.1]heptenes and eta(1)-oxabicyclo[2.2.1]heptadienes oc curs on treatment of CF3SO3H and Lewis acid, leading to atypical carbo n-carbon bond scission and deoxygenation reactions to give highly subs tituted benzene and furan derivatives, respectively. The role of the t unsten fragment in these demetalation reactions is discussed in detail .