New Diterpenes and Diterpene Glycosides with Antibacterial Activity from Soft Coral Lemnalia bournei

Abstract

Five new biflorane-type diterpenoids, biofloranates E-I (1-5), and two new bicyclic diterpene glycosides, lemnaboursides H-I (6-7), along with the known lemnabourside, were isolated from the South China Sea soft coral Lemnalia bournei. Their chemical structures and stereochemistry were determined based on extensive spectroscopic methods, including time-dependent density functional theory (TDDFT) ECD calculations, as well as a comparison of them with the reported values. The antibacterial activities of the isolated compounds were evaluated against five pathogenic bacteria, and all of these diterpenes and diterpene glycosides showed antibacterial activities against Staphylococcus aureus and Bacillus subtilis, with MICs ranging from 4 to 64 µg/mL. In addition, these compounds did not exhibit noticeable cytotoxicities on A549, Hela, and HepG2 cancer cell lines, at 20 μM.

Keywords: Lemnalia bournei; antibacterial activity; diterpene glycosides; diterpenes; soft coral.

Figure 1

Chemical structures of compounds 1–8.

Figure 2

¹H-¹H COSY and key HMBC correlations of compounds 1–7.

Figure 3

Key NOE correlations of compounds 1–7.

Figure 4

(a) Experimental ECD spectra of compounds 1–3 and 5 and calculated ECD spectrum of compound 1. (b) Experimental and calculated ECD spectra of compound 4. (c) Experimental ECD spectra of compounds 6–8.