Stereochemistry of the 2 : 1 [4 + 2] adducts of tetrachlorocyclopentadienone acetals with cyclohepta-1,3,5-triene, tropone, cyclohexa-1,4-diene, and p-benzoquinone

Abstract

The 2 : 1 [4 + 2] adducts of tetrachloropentadienone acetals with cycloheptatriene and cyclohexa-1,4-diene have endo,syn,endo-structures [(III) and (XI)], whereas those with tropone and p-benzoquinone possess endo,anti,endo-geometry [(IV) and (XIII)]. The 2 : 1 adducts derived from cycloheptatriene and cyclohexa-1,4-diene have ‘fixed’ conformations, the central ring systems being sterically incapable of conformational inversion. Variable-temperature n.m.r. studies indicate that conformational rigidity also exists in the 2 : 1 adduct of tropone, although the cyclohept-4-enone ring could in principle undergo a degenerate conformational change [(IVa)⇌(IVb)].