Kinetic and thermodynamic control in the formation of stereoisomeric 1:1 (4π+ 2π) thermal cycloadducts of furans with hexachloronorbornadienes

Abstract

The thermally, relatively stable, main product of the reaction of furan with dienophile 5a has been found to belong unambiguously to the endo–exo series of stereoisomeric adducts analogous to aldrin (endo–exo-3,4,5,6,11,11-hexachlorotetracyclo[6.2.1.1^3.6.0^2.7]dodeca-4,9-diene) 8. Also, the endo–endo isomeric adduct 16 has been found to comprise a minor component of the total products. In similar reactions of 2-methyl-, 2-ethyl- and 2,5-dimethyl-furan with 5a, it is shown that the respective endo–endo adducts 21, 23 and 25 are important reaction products and that the thermally unstable endo–endo adduct 25 predominates (6:1) over its endo–exo isomer 26 in the early phases of reaction, its abundance falling with heating time. The reactions of especially the alkylated furans with 5a provide useful sources of compounds (‘oxaisodrins’) having the skeletal features of isodrin endo–endo-3,4,5,6,11,11-hexachlorotetracyclo[6.2.1.1^3.6.0^2.7]dodeca-4,9-diene 1 otherwise not easily accessible.