Conformation study of the tricyclic sesquiterpene #beta#-panasinsene aided by ab initio calculations and simulated ~1H NMR parameters

摘要

Ab initio calculations of the tricyclic sesquiterpene ~.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 experimental1H 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 experimental1 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 9m = 39.5° and P = 3.8°, while the six-membered ring exists in a conformation close to the canonical chair, with angular variables 9 = 9.2° and P2 = 15.5°, and the total puckering amplitude Q = 54.5°. Application of this procedure to ~-panasinsen-5a-ol (2) and p..panasinsen-5-one (3) gave similar results.