molecules Article Dimacrolide Sesquiterpene Pyridine Alkaloids from the Stems of Tripterygium regelii Dongsheng Fan, Guo-Yuan Zhu, Ting Li, Zhi-Hong Jiang * and Li-Ping Bai * State Key Laboratory of Quality Research in Chinese Medicine, Macau Institute for Applied Research in Medicine and Health, Macau University of Science and Technology, Taipa, Macau SAR, China; [email protected] (D.F.); [email protected] (G.-Y.Z.); [email protected] (T.L.) * Correspondence: [email protected] (Z.-H.J.); [email protected] (L.-P.B.); Tel.: +853-8897-2777 (Z.-H.J.); +853-8897-2403 (L.-P.B.) Academic Editor: Derek J. McPhee Received: 20 July 2016; Accepted: 3 August 2016; Published: 29 August 2016 Abstract: Two new dimacrolide sesquiterpene pyridine alkaloids (DMSPAs), dimacroregelines A (1) and B (2), were isolated from the stems of Tripterygium regelii. The structures of both compounds were characterized by extensive 1D and 2D NMR spectroscopic analyses, as well as HRESIMS data. Compounds 1 and 2 are two rare DMSPAs possessing unique 2-(30-carboxybutyl)-3-furanoic acid units forming the second macrocyclic ring, representing the first example of DMSPAs bearing an extra furan ring in their second macrocyclic ring system. Compound 2 showed inhibitory effects on the proliferation of human rheumatoid arthritis synovial fibroblast cell (MH7A) at a concentration of 20 µM. Keywords: Tripterygium regelii; dimacrolide sesquiterpene pyridine alkaloids; anti-inflammation 1. Introduction Celastraceae is a large family comprising about 97 genera and 1194 species, which are distributed mainly in the tropics and subtropics. Among them, 14 genera with 192 species are native to China [1]. Some plants from several genera including Tripterygium, Celastrus, Euonymus, and Maytenus, are used as folk medicines or traditional Chinese medicines [1,2]. Plants of this family are the richest source of diverse dihydro-β-agarofuran sesquiterpenoids, which have been considered as characteristic metabolites and chemotaxonomic markers of this family [3,4]. This class of sesquiterpenes has attracted researchers’ interest because of a variety of promising bioactivities, such as anti-inflammatory, immunosuppressive, multidrug resistance reversal, cytotoxic, antitumor and anti-HIV activities, etc. [3,4]. Dimacrolide sesquiterpene pyridine alkaloids (DMSPAs) are a structurally unique class of dihydro-β-agarofuran sesquiterpenoids. They were characterized by a polyhydroxylated dihydroagarofuran core and two dicarboxylic acid derivatives linked via four ester bonds forming two macrocyclic systems. DMSPAs are a rare class of compounds naturally occurring in a limited number of plants, such as cathedulin E3 [5–8], cathedulin E4 [5–8], cathedulin-K19 [9], and cathedulin-K20 [9] from Catha edulis (Forsk), triptonine A [10,11] and triptonine B [10,11] from Tripterygium hypoglaucum, and tripterygiumine A [12] from Tripterygium wilfordii. Moreover, triptonine B has been reported to inhibit HIV replication with EC50 value < 0.10 µg/mL in H9 lymphocytes with a significant therapeutic index (TI) value (TI > 1000) [11]. Tripterygium regelii is distributed throughout northeast China, Korea and Japan [13], and has been used as a folk medicine in China to treat rheumatoid arthritis, jaundice, swelling, etc. [14]. A few earlier phytochemical studies on this plant showed the presence of diterpenoids [15], triterpenoids [16–20] and alkaloids [21,22]. Recently, twelve new dihydro-β-agarofuran sesquiterpenoids and three new Molecules 2016, 21, 1146; doi:10.3390/molecules21091146 www.mdpi.com/journal/molecules Molecules 2016, 21, 1146 2 of 8 Molecules 2016, 21, 1146 2 of 8 triterpenoids have been isolated and identified from the stems of T. regelii in our laboratory [23,24]. new triterpenoids have been isolated and identified from the stems of T. regelii in our laboratory During[23,24]. our During ongoing our search ongoing for secondarysearch for metabolitessecondary metabolites from T. regelii from, this T. phytochemicalregelii, this phytochemical investigation wasinvestigation extended, resulting was extended, in the resulting isolation in of the two isolation new DMSPAs of two new1 and DMSPAs2 (Figure 1 and1). Herein,2 (Figure we 1). report Herein, the isolationwe report and the structural isolation elucidation and structural of both elucidation compounds, of both as compounds, well as their as inhibitory well as their effect inhibitory on proliferation effect ofon human proliferation rheumatoid of human arthritis rheumatoid synovial arthritis fibroblast synovial (MH7A) fibroblast cells. (MH7A) cells. FigureFigure 1. 1.The The chemical chemical structuresstructures of compounds 11, ,22 andand triptonine triptonine A. A. 2. Results and Discussion 2. Results and Discussion Compound 1 was obtained as a white amorphous powder. The molecular formula of C38H45NO16 Compound 1 was obtained as a white amorphous powder. The molecular formula of C38H45NO16 was deduced from an [M + H]+ ion peak at m/z 772.2820 (calcd. for C38H46NO16, 772.2811) in the was deduced from an [M + H]+ ion peak at m/z 772.2820 (calcd. for C H NO , 772.2811) in the HRESIMS. Its UV spectrum showed absorption bands at λmax 231 and 253 nm,38 46indicating16 the presence λ HRESIMS.of aromatic Its UVrings. spectrum In the 1H-NMR showed absorptionspectroscopic bands data atof 1max (Table231 and1), resonances 253 nm, indicating for six oxygenated the presence 1 ofmethines aromatic [ rings.δH 6.61 In(1H, the s, H-6),H-NMR 5.50 spectroscopic(1H, dd, J = 6.0, data3.6 Hz, of H-8),1 (Table 4.751 ),(1H, resonances d, J = 3.0 Hz, for H-3), six oxygenated 4.42 (1H, methinesd, J = 6.0[ Hz,δH 6.61H-9),(1H, 4.19 (1H, s, H-6), d, J = 5.50 3.6 Hz, (1H, H-1) dd, andJ = 3.84 6.0, (1H, 3.6 dd, Hz, J = H-8), 3.6, 3.0 4.75 Hz,(1H, H-2)], d, oneJ = methine 3.0 Hz, [ H-3),δH 4.422.62 (1H, (1H, d, d,J J= = 6.03.6 Hz,Hz, H-9),H-7)], 4.19two oxygenated (1H, d, J = 3.6methylenes Hz, H-1) [δ andH 5.96 3.84 and (1H, 3.84 dd,(eachJ =1H, 3.6, d, 3.0J = 11.4 Hz, Hz, H-2)], oneH methine2-13); 5.62[ δandH 2.62 4.76 (1H (each, d, 1H,J = 3.6d, J Hz, =14.4 H-7)], Hz, twoH2-15)], oxygenated one hydroxyl methylenes proton [[4.64δH 5.96 (1H, and s), 3.84OH-4], (each two 1H, d, tertiaryJ = 11.4 methyl Hz,H2 -13);groups 5.62 [δH and 1.61 4.76 and (each 1.58 (each 1H, d,3H,J =14.4 s, H3-12 Hz, and H2 H-15)],3-14)] one and hydroxyl an acetyl protongroup [ [4.64δH 2.12 (1H, s),(3H, OH-4], s, OAc-6)] two tertiary indicated methyl the pres groupsence [ofδH a1.61 polyoxygenated and 1.58 (each dihydro- 3H, s,β H-agarofuran3-12 and H sesquiterpene3-14)] and an unit acetyl group[23,25–28].[δH 2.12 An(3H, evoninic s, OAc-6)] acid moiety indicated [25–29] the presencewas deduced of a polyoxygenatedby the signals for dihydro- a 2,3-disubstitutedβ-agarofuran sesquiterpenepyridine [δH 8.66 unit (1H, [23, dd,25– 28J =]. 4.8, An 1.8 evoninic Hz, H-6′), acid 8.16 moiety(1H, dd, [J25 = –7.8,29] 1.8 was Hz, deduced H-4′) and by 7.39 the (1H, signals dd, J = for 0 0 a 2,3-disubstituted7.8, 4.8 Hz, H-5′)], pyridine two methines [δH 8.66 [δH (1H,4.62 (1H, dd, J qd,= 4.8, J = 1.86.6, Hz,1.2 Hz, H-6 H-7), 8.16′) and (1H, 2.44 dd, (1H,J = br 7.8, q, 1.8J = 6.6 Hz, Hz, H-4 ) H-8′)], and two secondary methyl groups0 [δH 1.36 and 1.13 (each 3H, d, J = 6.6 Hz, H3-9′ and H3-100 ′)]. and 7.39 (1H, dd, J = 7.8, 4.8 Hz, H-5 )], two methines [δH 4.62 (1H, qd, J = 6.6, 1.2 Hz, H-7 ) and The remaining signals for a 2,3-disubstituted0 furan [δH 7.42 (1H, d, J = 1.8 Hz, H-8″), 6.73 (1H, d, J = 2.44 (1H, br q, J = 6.6 Hz, H-8 )], and two secondary methyl groups [δH 1.36 and 1.13 (each 3H, d, 1.8 Hz, H-7″)],0 a methine [δ0H 2.50 (1H, m, H-2″)], two mehylenes [δH 3.77 (1H, m, H-4″a) and 2.96 (1H, J = 6.6 Hz,H3-9 and H3-10 )]. The remaining signals for a 2,3-disubstituted furan [δH 7.42 (1H, d, ddd, J = 14.4, 7.2, 4.2 Hz, H-4″b); 2.06 and 1.91 (each 1H, m, H2-3″)], and one secondary methyl group J = 1.8 Hz, H-8”), 6.73 (1H, d, J = 1.8 Hz, H-7”)], a methine [δH 2.50 (1H, m, H-2”)], two mehylenes [δH 1.14 (3H, d, J = 7.2 Hz, H-10″)] were attributed to a 2-(3′-carboxybutyl)-3-furanoic acid unit in the [δH 3.77 (1H, m, H-4”a) and 2.96 (1H, ddd, J = 14.4, 7.2, 4.2 Hz, H-4”b); 2.06 and 1.91 (each 1H, m, 1H-NMR spectrum. The 13C-NMR spectroscopic data (Table 1) along with DEPT and HSQC spectra H -3”)], and one secondary methyl group [δ 1.14 (3H, d, J = 7.2 Hz, H-10”)] were attributed to 2showed the presence of the units mentioned aboveH in 1. These characteristic NMR data suggested 1 a 2-(30-carboxybutyl)-3-furanoic acid unit in the 1H-NMR spectrum. The 13C-NMR spectroscopic data to be a dimacrolide sesquiterpene pyridine alkaloid [5–12].