Four new pyrrole alkaloids from the rhizomes of Amomum koenigii
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Four new pyrrole alkaloids from the rhizomes of Amomum koenigii Hong Yin1 · Chao Guo2 · Jin‑Ming Gao1 Received: 17 May 2020 / Accepted: 15 August 2020 © The Japanese Society of Pharmacognosy 2020
Abstract Four new pyrrole alkaloids, named amokoens A–D (1–4), together with three known compounds (5–7) were isolated from the rhizomes of Amomum koenigii. Their structures and absolute configurations were established by spectroscopic data, including 1D and 2D NMR, and the optical rotation calculations. All the isolates were evaluated for their effects on nitric oxide (NO) production in lipopolysaccharide-induced RAW264.7 macrophages. Compounds 1–7 inhibited NO production ranging from 27.1 to 82.4% at a concentration of 100 μM, and compounds 5 and 6 showed efficacious inhibitory activities with IC50 values of 42.2 and 69.3 µM, respectively. Keywords Amomum koenigii · Pyrrole alkaloid · OR calculations · NO inhibitory activity
Introduction
Results and discussion
Amomum koenigii J. F. Gmelin, belonging to the Zingiberaceae family, is widely distributed in Guangxi and Yunnan provinces of China, Thailand, and India [1]. It is not only an edible vegetable but also used in folk medicine to treat stomach diseases [2]. The previous investigation indicated that A. koenigii contains flavonols and eicosenones [3]. In our recent studies on medicinal plants of the Amomum genus, some unusual norditerpenoids [4], diterpenoids [4] and sesquiterpenoids [5] were identified. In our continuing endeavor to search for bioactive constituents from the Amomum genus, four new pyrrole alkaloids, named amokoens A–D (1–4), together with three known compounds (5–7) were isolated from the rhizomes of A. koenigii (Fig. 1). Herein, their isolation, structure elucidation and NO inhibitory activities are discussed.
Compound 1 was isolated as yellowish oil. Its molecular formula was deduced as C12H17NO4 on the basis of HRESIMS at m/z 240.1238 [M + H]+ (calcd for C12H18NO4, 240.1230). The UV absorption at 293 nm was characteristic of pyrrole2-aldehyde [6]. In the 1H spectrum, the chemical shifts and coupling constants of the two proton signals at δH 7.10 (1H, d, J = 4.0 Hz, H-3) and 6.33 (1H, d, J = 4.0 Hz, H-4) indicated the presence of a 2,5 disubstituted pyrrole ring [6]. All the proton and carbon signals of compound 1 (Table 1) were assigned by HMQC and HMBC spectra. Analysis and comparison of NMR data suggested that the structure of compound 1 was similar to morrole B [7], except for the presence of a methoxy group (δH 3.70/δC 52.8) and an ethoxy group [δH 3.52/δC 66.5 (–OCH2CH3) and δH 1.20/δC 15.3 (–OCH2CH3)]. The positions of the methoxy and ethoxy groups were confirmed to C-1′ and C-6 by HMBC correlations of the methoxy protons (δH 3.70, –OCH3) with C-1′ (δC 172.4) and H-6 (δH 4.61, 4.57) with the methylene carbon of the ethoxy group (δC 66.5, –OCH2CH3), respectively (Fig. 2). To further determine the absolute configuration of C-2′ for compound 1, the optical rotation calculations were performed at the B3LYP/6-31G(d,p) level with dens
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