Conformational study of the tricyclic sesquiterpene beta-panasinsene aidedby ab initio calculations and simulated H-1 NMR parameters

Ab initio calculations of the tricyclic sesquiterpene beta -panasinsene (1) provided theoretical H-C-C-H dihedral angles that allowed us to predict th e corresponding H-1-H-1 vicinal coupling constant values, using a generaliz ed Karplus-type equation. These values, together with the experimental H-1 chemical shifts extracted from an HMQC spectrum, were used as the initial i nput data for the full spectral simulation of 1, which provided an excellen t correlation with the experimental H-1 NMR spectrum after a few iterations . The complete assignment of the H-1 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 similar to 12 degrees. The five-membered ring possesses a half-chair conformation wit h pseudorotational parameters theta (m) = 39.5 degrees and P = 3.8 degrees, while the six-membered ring exists in a conformation close to the canonica l chair, with angular variables theta = 9.2 degrees and P-2 = 15.5 degrees, and the total puckering amplitude Q = 54.5 degrees. Application of this pr ocedure to beta -panasinsen-5 alpha -ol (2) and beta -panasinsen-5-one (3) gave similar results. Copyright (C) 2001 John Wiley & Sons, Ltd.