A ONE-STEP SYNTHESIS OF 2-NORBORNANONE ETHYLENE ACETALS FROM 2-CYCLOPENTEN-1-ONE ETHYLENE ACETALS AND DIENOPHILES VIA [2-SITU GENERATED - 2-(2-HYDROXYETHOXY)CYCLOPENTA-1,3-DIENES AND INTRAMOLECULAR REACETALIZATION(4] CYCLOADDITION OF IN)

2-(2-Hydroxyethoxy)cyclopenta-1,3-diene (5) which is generated reversi bly from 2-cyclopenten-1-one ethylene acetal (1) under mild, neutral c onditions can be intercepted with a variety of dienophiles ultimately to give 2-norbornanone ethylene acetals in 62-100% yields. The additio n reactions are highly stereo- and regioselective. Of the several solv ents examined, acetonitrile is the most satisfactory. In CHCl3 or CCl4 , decomposition of 1 is induced, leading to diminished yields of the a dducts. The intermediate 5 is detected by UV spectroscopy, and its con tent relative to 1 at a stationary state at 70-degrees-C in acetonitri le is estimated to be ca. 0.2%. The reactions of the 2-, 3-, and 5-met hyl-substituted derivatives 2-4 with dienophiles similarly led to the production of the corresponding 2-norbornanone acetals through the add itions of the dienophiles to the 1,3-cyclopentadien-2-yl ether interme diates 6-8 selectively derived from 2-4 via 1,2-elimination. The forma tion of isomeric adducts resulting either from 1,4-elimination in 1-4 or from [1,51 hydrogen migration in the enol ether intermediates is no t 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 ethyl ene 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 reacti on.