A One-Step Synthesis of 2-Norbornanone Ethylene Acetals from 2-Cyclopenten-1-one Ethylene Acetals and Dienophiles via Cycloaddition of in Situ Generated 2-(2-Hydroxyethoxy)cyclopenta-1,3-dienes and Intramolecular Reacetalization

Article abstract of DOI:10.1021/jo00071a033

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 62-100percent yields.The additions reactions are highly stereo- and regioselective.Of the several solvents examined, acetonitrile is the most satisfactory.In CHCl3 or CCl4, 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 deg C in acetonitrile is estimated to be ca. 0.2percent.The reactions of the 2-, 3-, and 5-methyl-substituted derivatives 2-4 with dienophiles similarly led to the production of the corresponding 2-norbornanone acetals through the additions of the dienophiles to the 1,3-cyclopentadien-2-yl ether intermediates 6-8 selectively derived from 2-4 via 1,2-elimination.The formation of isomeric adducts resulting either from 1,4-elimination in 1-4 or from <1,5> hydrogen migration in the enol ether intermediates is not detected.The addition of 2-chloroacrylonitrile, a ketene equivalent, to 1-4 followed by alkaline hydrolysis provides singly acetalized 2,5-norbornadiones in two steps in good yields. 2-Cyclohexen-1-one ethylene acetal (44) also undergoes the addition of dienophiles in the <2+4> manner directly to give bicyclo<2.2.2>octan-2-one ethylene acetals, but is substantially less reactive than 1 towards this type of reaction.