Entwicklung einer Auswertungssoftware zur Anwendung residualer dipolarer Kopplungen in der organischen Strukturaufklärung

The hygrophorones, a class of cyclopentenones isolated from fruiting bodies of the genus Hygrophorus (basidiomycetes), show promising antifungal activity. While the constitution of 4,6diacetylhygrophorone A (3) and the relative configuration of the stereogenic centers in the cyclopentenone ring were elucidated using standard NMR and MS techniques, the relative configuration of the exocyclic stereogenic center could not be assigned. By introducing a sample of 3 into an alignment medium and measuring anisotropic NMR parameters, namely, residual dipolar couplings, we were able to unambiguously determine the relative configuration of all three stereogenic centers in 4,6-diacetylhygrophorone A simultaneously by fitting several structure proposals to the experimental data. ■ INTRODUCTION The fruiting bodies of basidiomycetes fungi represent a rich source of new natural products, some of them with bioactive properties. In 2004 it was recognized in field observations that fruiting bodies of the genus Hygrophorus are rarely attacked by parasitic fungi, and it was assumed that these mushrooms contain some biologically active compounds. Such compounds from natural sources could be very interesting in the pharmaceutical discovery process because they can be used as new lead structures in drug development. In the studies mentioned above, a series of 20 new 5(hydroxyalkyl)-2-cyclopentenone derivatives, grouped into families called hygrophorones A−G, were separated and isolated from different Hygrophorus species. The structures were elucidated by classical NMR and MS methods, and the results are shown in Figure 1. Chemically, hygrophorones are 2-cyclopentenones with hydroxy or acetoxy substituents at C4 and/or C5, and an odd-numbered 1′-oxidized alkyl chain is attached to C5 (the numbering scheme is shown in Figure 2). For the assignment of the relative configuration of the stereogenic centers within the ring, previous studies looked at long-range JH3−H4 and JH2−H4 coupling constants and compared them to the corresponding coupling constants of the known antibiotic compounds pentenomycin I and epipentenomycin I. Epipentenomycin I (1) shows a rather small JH3−H4 coupling constant of 2.1 Hz in comparison to pentenomycin I (2), with 2.7 Hz. For the JH2−H4 coupling constant the situation is vice versa, a small coupling constant of 1.2 Hz in 2 and a bigger one of 1.6 Hz in 1. Due to the similarity of the coupling constants of 4,6-diacetylhygrophorone A (3) (JH3−H4 = 2.1 Hz and JH2−H4 = 1.8 Hz) and epipentenomycin (1), it is believed that hygrophorones A may be 4,5-trans configured, while hygrophorones B are supposed to be 4,5-cis configured. This result was supported by nuclear Overhauser effect (NOE) contacts from the hydroxy group at C5 to the proton at C4 in the 4,5-trans-configured hygrophorones A, whereas no NOE contact could be observed in 4,5-cis-configured hygrophorones B. However, due to the quaternary carbon C5, there is no scalar coupling pathway relating the exocyclic stereogenic center on C6 to the stereogenic centers in the cyclopentenone ring. Also no characteristic NOE contacts could be observed for H6. The relative configuration of C6 is therefore still unknown and difficult to establish using classical NMR parameters. In this work we employed residual dipolar couplings (RDCs) as an anisotropic NMR parameter to determine the relative configuration of all three stereogenic centers of 4,6-diacetylhygrophorone A (3) simultaneously. A comparison with literature data as described above is not necessary, because, in contrast to the traditional NMR methods, dipolar couplings provide nonparametrized structural information of a nonlocal character. This is achieved by fitting the experimental RDCs to a global property, the alignment tensor. With the alignment tensor, distances and angles can be back-calculated and compared to structural proposals, thereby enabling the determination of the relative spatial arrangement of nuclei in a molecule, which are in turn interpreted in terms of the constitution, configuration, and preferred conformation (or Received: October 19, 2012 Published: May 9, 2013 Article