NMR spectral assignment of 2α- and 3β-methylhopanes and evidence for boat conformation in D ring of 17α(H),21α(H)-hopanes

• Published in: Magnetic resonance in chemistry 2010-12, Vol.48 (12), p.951-954
• Main Authors: Kildahl-Andersen, Geir, Nytoft, Hans Peter, Johansen, Jon Eigill
• Format: Article
• Language: English
• Subjects: 13C NMR; 1H NMR; biomarker; boat conformation; Magnetic Resonance Spectroscopy; Methane - chemistry; methyl hopane; Molecular Structure; revised assignment; Thermodynamics; triterpane; Triterpenes - chemistry
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.2688

The full 1H and 13C NMR chemical shift assignment of 2α‐methyl‐17α(H),21β(H)‐hopane is presented. This compound is formed in mature sediments from biogenic sources of 2β‐methyl‐17β(H),21β(H)‐hopanoids, which include several cyanobacteria. In addition, full 1H and 13C NMR chemical shift data of all four 17,21 isomers of 3β‐methylhopane have been assigned. The thermodynamically most stable 3β‐configuration corresponds to that found in bacterial sources. The data presented here suggest minor corrections to the 13C chemical assignments reported earlier for 17α(H)‐hopanes. Moreover, spectral evidence indicates an unexpected ring‐D boat conformation of 17α(H),21α(H)‐hopanes, which may serve to explain the steric strain reported for this isomer. Copyright © 2010 John Wiley & Sons, Ltd. The full 1H and 13C NMR chemical shift assignment of 2α‐methyl‐17α(H),21β(H)‐hopane and all four 17,21 isomers of 3β‐methylhopane has been carried out. Spectral evidence indicate an unexpected boat conformation in the D ring of 17α(H),21α(H)‐hopanes.