Synthesis and Tandem Diels-Alder Reactivity of 1,3 Dimethoxy-5,6,7,8-tetramethylidene-bicyclo[2.2.2]oct-2-ene

Abstract

The synthesis of 1,3-dimethoxy-5,6,7,8-tetramethylidenebicyclo[2.2.2]oct-2-ene (13) is presented. 13 added one equivalent of ethylenetetracarbonitrile (TCNE) to give the corresponding monoadduct 18 (k₁^II = 204 × 10^-3 dm³mol^-1s^-1 at 25 °C, ΔH₁^‡ = 10.7 ± 0.5 kcal mol^-1, ΔS₁^‡= −26 ± 2 cal K^-1mol^-1). 18 added another equivalent of TCNE and afforded the bisadduct 19 (k₂^II = 0.6 × 10^-1 dm³mol^-1s^-1 at 25 °C, ΔH₂^‡  = 14 ± 1 kcal mol^-1, ΔS₂^‡=  = −26 ± 4 cal K^-1mol^-1). The tandem Diels-Alder reactivity of 13 given by the rate constant ratio k₁^II/k₂^II = 340 (at 25 °C) is compared with that reported for 2,3,5,6-tetramethylidenebicyclo[2.2.2]octane (3), 5,6,7,8-tetramethylidenebicyclo[2.2.2]oct-2-ene (4), and [2.2.2]hericene (5).