The marked difference in conformational behavior of the two diastereomers of 7-substituted-1,1-dichloro-7b-((Z)-8-chloro-6,7-dihydro-5H-benzo[7]annulen-9-yl)-1a,2,3,7b-tetrahydro-1H-cyclopropa[a]naphthalene, single crystal X-ray, 1H NMR and AM1 studies

Abstract

The 7-substituted of 1 and 2 were synthesized and conformational analysis carried out. While 7-substituted of 2 show two conformers in solution, 7-substituted of 1 show only one form in solution. AM1 semi-empirical molecular orbital calculations show that the conformation of cycloheptadiene ring in 1 and 2 is a twist boat form. In this conformation, the C-7 substituents can be oriented in pseudo equatorial (exo) and pseudo axial (endo) positions. The 3 J calculation with Haasnoot equation on optimized structure of 2 reproduces the observed 3 J coupling constants in exo and endo forms. Ring inversion of cycloheptadiene moiety in substituted 2 interconvert the e′–a′ (exo–endo) positions. The 3 J calculation on optimized structure of 1 shows that 7-substitution is in pseudo equatorial (exo) direction, as found in the crystal structure of 1a by single crystal X-ray crystallography. The barrier to ring inversion in 2a is determined by dynamic 1H NMR spectroscopy to be Δ G ≠ (335K) = 68.0 ± 0.5 kJ/mol.