A one-step synthesis of 2-norbornanone ethylene acetals from 2-cyclopenten-1-one ethylene acetals and dienophiles via [2 + 4] cycloaddition of in situ generated 2-(2-hydroxyethoxy)cyclopenta-1,3-dienes and intramolecular reacetalization

2-(2-Hydroxyethoxy)cyclopenta-1,3-diene (5) which is generated reversibly from 2-cyclopenten-1-one ethylene acetal (1) under mild, neutral conditions can be intercepted with a variety of dienophiles ultimately to give 2-norbornanone ethylene acetals in 62-100% yields. The addition reactions are highly stereo- and regioselective. Of the several solvents examined, acetonitrile is the most satisfactory. In CHCl 3 or CCl 4 , decomposition of 1 is induced, leading to diminished yields of the adducts. The intermediate 5 is detected by UV spectroscopy, and its content relative to 1 at a stationary state at 70 o C in acetonitrile is estimated to be ca. 0.2%