An Endo-Selective Ionic Diels-Alder Reaction of a,b-Enone and a,b-Enal Acetals Catalyzed by Electrogenerated Acid

Inokuchi, T.; Tanigawa, S.; Torii, S. J. Org. Chem. 1990, 55, 3958.

Electrolysis of a,b-enone acetals or a,b-enal acetals and dienes in dichloromethane containing lithium perchlorate (LiClO₄) and tetrabutylammonium perchlorate (Bu₄NClO₄) as a source of electrogenerated acid (EG acid) at about -78 ｡C induced an ionic Diels-Alder reaction to afforf [4+2]adducts. Endo or exo selectivity of the adducts was affected significantly by the substituent pattern of the methyl group on dienophile olefins. The b-methyl group favored an endo-selectivity, while thea-methyl group promoted the exo-mode addition. The reaction of 3-penten-2-one ethylene acetal (1d) with cyclopentadiene (2) proceeded in a high endo-selectivity (endo:exo ratio = 71:1), which was 40 times as great as than of the corresponding unacetalized dienophile with 2 under thermal conditions. Reaction paths to rationalize these endo selectivities are proposed.