Bipolenins K-N: New sesquiterpenoids from the fungal plant pathogen Bipolaris sorokiniana

Abstract

Chemical investigation of the barley and wheat fungal pathogen Bipolaris sorokiniana BRIP10943 yielded four new sativene-type sesquiterpenoid natural products, bipolenins K-N (1-4), together with seven related known analogues (5-11), and a sesterterpenoid (12). Their structures were determined by detailed analysis of spectroscopic data, supported by TDDFT calculations and comparison with previously reported analogues. These compounds were evaluated for their phytotoxic activity against wheat seedlings and wheat seed germination. The putative biosynthetic relationships between the isolated sesquiterpenoids were also explored.

Keywords: Bipolaris sorokiniana; phytotoxicity; sesquiterpenes; terpenes.

Figure 1

Structures of compounds 1–12 isolated from B. sorokiniana.

Figure 2

Key 2D NMR correlations of bipolenins K–N (1–4).

Figure 3

Key NOESY correlations of bipolenins K–N (1–4).

Figure 4

(a) Experimental ECD spectrum of 1 (MeOH) compared to TDDFT-calculated spectra (B3LYP-D3/def2-TZVPP) for the two possible enantiomers of 1, which were blue-shifted by 9 nm. (b) Comparison of experimental ECD spectra of 1–3 and 5 (MeOH).

Figure 5

Relationship of sesquiterpenoids isolated in this study. A) Different groups of sativene/longifolene-type sesquiterpenoid scaffolds; B) The branched pathways to sativene- and longifolene-type sesquiterpenoids. C) Detailed proposed pathways to the sativene-derived sesquiterpenoids from this study.