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
作者:Masakazu Ohkita、Osamu Nishizawa、Takashi Tsuji、Shinya Nishida
DOI:10.1021/jo00071a033
日期:1993.9
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 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-degrees-C in acetonitrile is estimated to be ca. 0.2%. 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 [1,51 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 toward this type of reaction.