Synthesis of Nucleosides and Related Compounds. XXVI. The Difference in Effects between High Pressure and LiClO₄ for the Diels-Alder Reaction of Cyclopentadiene with Methylenemalonates or O-Acetylisonitrosomalonates

Abstract

Diels-Alder reactions of cyclopentadiene with methylenemalonates or O-acetylisonitrosomalonates in the presence of LiClO₄ as the catalyst were examined and the results were compared with those obtained under high pressure without the catalyst. Diels-Alder reaction of cyclopentadiene with dimethyl acetoxymethylenemalonate (1) in the presence of LiClO₄ afforded the [4+2] adduct (2) as a mixture of endo- and exo-isomers, whose ratio was 2.0, irrespective of the solvent or the concentration of LiClO₄. Asymmetric Diels-Alder reaction of cyclopentadiene with di-l-methyl acetoxymethylenemalonate (3) was accelerated remarkably by LiClO₄. The configurations of both endo and exo adducts corresponded to the natural form (D-form). These results suggested strongly that LiClO₄ would behave as a bidentate Lewis acid catalyst just like titanium tetrachloride. the hetero Diels-Alder reaction of cyclopentadiene with O-acetylisonitrosomalonate in LiClO₄-ether produced the adduct (5) in a higher yield than the reaction performed under high pressure.