Two new polyketides from Nocardiopsis lucentensis DSM 44048

Abstract Two new polyketides, namely lucentides A (1) and B (2), together with 19-hydroxyprotylonolide (3) were isolated from Nocardiopsis lucentensis DSM 44048. Their structures were elucidated by analysis of their high-resolution mass spectrometry (HR-MS) and 1D, 2D nuclear magnetic resonance (NMR) spectroscopic data. The antibacterial activities of compounds 1–3 were evaluated. Graphical abstract

In a continuation of our search for bioactive small molecules from halophilic actinomycetes, Nocardiopsis lucentensis DSM 44048, which is purchased from the DSM Microbial Type Culture Collection (Yassin et al. 1993), was selected on the basis of cytotoxicity screenings against human tumour cell lines. It is a moderately halophilic actinomycete which shows optimal growth characteristics at 37 °C in pH 7.6 medium containing 10% (w/v) NaCl. our previously results suggested that new benzoxazole derivatives from DSM 44048 were responsible for cytotoxicity against HepG2 and HeLa (Sun et al. 2015) cell lines. Further investigation of extracts revealed the presence of additional new metabolites and now reported two new polyketides, lucentides A (1) and B (2), as well as the known 19-hydroxyprotylonolide (3) (Figure 1) (Tian et al. 2014).
Compound 2 was obtained as a colourless oil. The molecular formula was determined to be C 12 H 22 o 4 on the basis of the positive-ion-mode HReSIMS data (m/z 231.1592 [M + H] + ). The 1 H-and 13 C-NMR spectra along with HMQC experiments revealed the signals of four Me, three CH 2 , two CH groups and three quaternary C-atoms, including an ester C=o group (δ C 179.2 (C-1)), similar to those of 1. The same C12 unit was determined by the HMBC correlations from four Me to corresponding carbons. Whereas, the 5,6-dihydroxyl substitutions and a o-bearing quaternary C-atoms were confirmed by the down field shift of 79.4 (C-5), 74.6 (C-6) and 85.9 (C-4). The relative configuration of 2 was determined to be the same as that of 1 because of the same Noe correlations. Compound 2 was determined to be 5-(1,2-dihydroxy-2-methylpentyl)-3,5-dimethyldihydrofuran-2(3H)-one.
In addition to the two polyketides, compound 3 was isolated as a colourless oil. It was identified as 19-hydroxyprotylonolide ( Figure 1) by comparison of the MS and NMR data obtained with those reported in the literature (Sadakane et al. 1983;Yasumuro et al. 1994), and this kind of structures have been isolated from many other organisms, including Streptomyces fradiae KA-427. (omura et al. 1980) Streptomyces sp. strain KA-464 (Sadakane et al. 1983) and Micromonospora sp. YS-02930K1 (Yasumuro et al. 1994). DSM44048 now joins a growing list of producers of 3.
The antimicrobial activities of compounds 1-3 were performed by paper disc diffusion assay. The sterile filter paper disks impregnated with 50 μg of the compounds were placed on agar plates previously inoculated with Mycobacterium smegmatis mc 2 155, Bacillus subtilis ATCC 6051 and Candida albicans 5314. But all of them showed no evident antimicrobial activities against the tested strains.
The linear carbon scaffolds represented by 1 and 2 are rare among known natural products. We previously reported seven linear polyketides from Xylaria sp. NCY2 (Hu et al. 2010). Their structures could be classified to be a tetraketide. A proposed pathway for 1 and 2 could be from condensation of a propionate starter unit with three methyl malonate extender units, followed by varying modifications, such as hydroxylations at C-5 or C-6/C-7 and esterification at C-1. Recently, some new polyketides were discovered such as cytotoxic polyketides Plakortis simplex , amphirionin-4 from Amphidinium species (Minamida et al. 2014), actinopolysporins A-C from Actinopolyspora erythraea YIM 90600 (Zhao et al. 2011). Compounds of this class have characteristic linear polyketide structures not exceeding 600 molecular units with some oxygen functionalities (e.g. hydroxyl group, ketone, epoxide and/or tetrahydrofuran ring) and a few methyl and/or exomethylene branches. These features emphasise the plasticity and promiscuity of linear polyketide biosynthetic machinery which produce natural products of structural diversity.

General experimental procedures
The NMR spectra were recorded on a Bruker Avance DRX-400 NMR spectrometer operating at 400 MHz for 1 H and 100 MHz for 13 C in the indicated solvents. Chemical shifts are expressed in ppm (δ units) with TMS as the internal reference. HReSIMS was obtained on a LTQ Velos Pro HRMS instrument (Thermo Scientific). All solvents were analytical grade purchased from Qingdao Haiyang Chemical Co. Ltd.,Qingdao,P R China) and Sephadex LH-20 (25−100 μm; Pharmacia Biotek, Denmark) were used for column chromatography. Thin-layer chromatography was performed with glass precoated silica gel GF 254 plates (Qingdao Haiyang Chemical Co. Ltd.). Compounds were visualised under uV light and by spraying with H 2 So 4 /etoH (1:9, v/v) followed by heating.

Fermentation and extraction
The strain N. lucentensis DSM 44048 was inoculated on MP agar media (mannitol 2%, peptone 2%, sea salt 10%, agar 1.5%, pH 7.2) plates and cultured for 14 days at a total volume of 12 L at 38 °C. The fermented cultures were diced and then extracted three times overnight with etoAc-MeoH−AcoH (80:15:5, v/v/v) at room temperature. After the concentration under vacuum, the crude extract was partitioned between etoAc and H 2 o. etoAc-soluble partition was dissolved in 100 mL of 95% methanol and extracted five times with an equal volume of petroleum ether (Pe) to afford the defatted MeoH extract (3.9 g).

Isolation and purification of compounds 1-3
The MeoH extract was purified by column chromatography (CC) over Sephadex LH-20 eluted with MeoH to give fractions 1−8.

Supplementary material
Supplementary material including NMR and HReSI-MS spectra of compounds 1 and 2 are available online.

Disclosure statement
No potential conflict of interest was reported by the authors.