Conformational study of the tricyclic sesquiterpene β‐panasinsene aided by ab initio calculations and simulated ¹H NMR parameters†

Ab initio calculations of the tricyclic sesquiterpene β‐panasinsene ( 1 ) provided theoretical H—C—C—H dihedral angles that allowed us to predict the corresponding 1 H– 1 H vicinal coupling constant values, using a generalized Karplus‐type equation. These values, together with the experimental 1 H chemical shifts extracted from an HMQC spectrum, were used as the initial input data for the full spectral simulation of 1 , which provided an excellent correlation with the experimental 1 H NMR spectrum after a few iterations. The complete assignment of the 1 H NMR parameters allowed us to determine the conformation of 1 in solution. The four‐membered ring exists in a more flattened conformation than cyclobutane, with torsion angles of ∼12°. The five‐membered ring possesses a half‐chair conformation with pseudorotational parameters θ m = 39.5° and P = 3.8°, while the six‐membered ring exists in a conformation close to the canonical chair, with angular variables θ = 9.2° and P 2 = 15.5°, and the total puckering amplitude Q = 54.5°. Application of this procedure to β‐panasinsen‐5α‐ol ( 2 ) and β‐panasinsen‐5‐one ( 3 ) gave similar results.