Conformational analysis of 8-oxabicyclo[3.2.1]oct-6-en-3-one derivatives by NMR and theoretical calculations

This work describes the conformational analysis of alcohols 2α,4α-dimethyl-6,7-exo-isopropylidenedioxy-8-oxabicyclo[3.2.1]octan-3α-ol (5) and -3β-ol (6) and of alcohols 1,2α,4α,5-tetramethyl-6,7-exo-isopropylidenedioxy-8-oxabicyclo[3.2.1]octan-3α-ol (7) and -3β-ol (8). Based on NMR data and AM1, HF/6-31G* and DFT/B3LYP/6-31G* geometry optimization, the pyran ring of compounds (5) to (7) is found to have a chair conformation. In the case of (8), NMR data indicate the presence of a boat conformer (8boat). However, the chair conformer (8chair) is predicted to have smaller relative energy as given by calculations employing different basis sets for either calculations on isolated molecules or including the solvent used in the NMR analyses (CDCl3). In contradistinction to thermodynamic results, DFT/B3LYP/6-31G* calculations of hydrogen (δH) and carbon (δC) chemical shifts of (8boat) were concordant with NMR experimental data of (8). Therefore, the chemical shift calculations agree better with the experimental results than does the quantum chemical calculations leading to the conformational analysis of (8).