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

✍ Scribed by César A. Flores-Sandoval; Carlos M. Cerda-García-Rojas; Pedro Joseph-Nathan

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 166 KB
• Volume: 39
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.810

✦ Synopsis

Abstract

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. Copyright © 2001 John Wiley & Sons, Ltd.