Oxidized Juncuenin B Analogues with Increased Antiproliferative Activity on Human Adherent Cell Lines: Semisynthesis and
Biological Evaluation
Csaba Bús,
∥A ́ gnes Kulmány,
∥Norbert Kúsz, Tímea Gonda, István Zupkó, Attila Mándi, Tibor Kurtán, Barbara Tóth, Judit Hohmann, Attila Hunyadi,* and Andrea Vasas*
Cite This:J. Nat. Prod.2020, 83, 3250−3261 Read Online
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sı Supporting InformationABSTRACT: Phenanthrenes have become the subject of intensive research during the past decades because of their structural diversity and wide range of pharmacological activities. Earlier studies demonstrated that semisynthetic derivatization of these natural compounds could result in more active agents, and oxidative transformations are particularly promising in this regard. In our work, a natural phenanthrene, juncuenin B, was transformed by hypervalent iodine(III) reagents using a diversity-oriented approach.
Eleven racemic semisynthetic compounds were produced, the majority containing an alkyl substituted p-quinol ring. Purification of the compounds was carried out by chromatographic techniques, and their structures were elucidated by 1D and 2D NMR spectroscopic methods. Stereoisomers of the bioactive derivatives were separated
by chiral-phase HPLC and the absolute configurations of the active compounds, 2,6-dioxo-1,8a-dimethoxy-1,7-dimethyl-8-vinyl- 9,10-dihydrophenanthrenes (1a−d), and 8a-ethoxy-1,7-dimethyl-6-oxo-8-vinyl-9,10-dihydrophenanthrene-2-ols (7a,b) were determined by ECD measurements and TDDFT-ECD calculations. The antiproliferative activities of the compounds were tested on different (MCF-7, T47D, HeLa, SiHa, C33A, A2780) human gynecological cancer cell lines. Compounds1a−d,4a,6a, and7a possessed higher activity than juncuenin B on several tumor cell lines. The structure−activity relationship studies suggested that the p-quinol (2,5-cyclohexadien-4-hydroxy-1-one) moiety has a considerable effect on the antiproliferative properties, and substantial differences could be identified in the activities of the stereoisomers.
P
henanthrenes are a rather uncommon class of aromatic secondary metabolites and have a limited occurrence in nature. They are presumably formed by oxidative coupling of the aromatic rings of stilbene precursors.1Hitherto, more than 460 phenanthrenes, among them mono-, di-, and triphenan- threnes, were identified from different plant families, e.g., Orchidaceae, Juncaceae, Combretaceae, and Dioscoreaceae.2A large number of phenanthrenes with different substitution patterns have been reported with various pharmacological activities, such as antimicrobial, anti-inflammatory, cytotoxic, antiproliferative, spasmolytic, antiallergic, and anxiolytic effects.2,3 The most thoroughly investigated denbinobin, a phenanthrenequinone, showed promising antiproliferative activity.4−6 The mechanism of action of denbinobin involves caspase-dependent and caspase-independent apoptosis of colon cancer (COLO 205) cells.4 Moreover, by enhancing the synthesis of reactive oxygen species (ROS), denbinobin induces apoptosis in Jurkat leukemia and human lung adenocarcinoma (A549) cells.5,7A structure−activity relation- ship study showed that the quinone substructure is required for enhancement of mitogen-activated protein kinase (MAPK)-independent ROS production activity.5 Another phenanthre- nequinone, calanquinone A, differing from denbinobin by the presence of an additional methoxy group, and synthetic phenanthrenequinones also possessed cytotoxic activity on different cell lines (HepG2, Hep3B, Ca9-22, A549, MCF-7, MDA-MB-231, MRC-5). Structure−activity relationship stud- ies showed that intramolecular hydrogen bonding between the C-4 carbonyl and HO-5 in 3-methoxyphenanthrene-1,4-diones may be necessary for the cytotoxicity.6
The phenanthrene dehydroeffusol showed activity against two metastatic cancer cell lines, SGC-7901 (human gastric carcinoma) and AGS (human caucasian gastric adenocarcino- ma) in a dose-dependent manner (IC5035.9 and 32.9μM for
Received: May 5, 2020 Published: October 16, 2020
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SGC-7901 and AGS cells, respectively). Gastric cancer cell mediated vasculogenic mimicry on SGC-7901 cells were also inhibited by this compound using the tube formation assay.8 Dehydroeffusol effectively inhibited the gastric cancer cell invasion and migration without exerting significant acute toxicity.9Juncusol was observed to be cytotoxic on HeLa cells;
treatment for 24 h increased the cell population in the G2/M and sub-G1 phases.10 Juncuenin B possessed significant antiproliferative activity against different human tumor cell lines such as MDA-MB-231 (breast cancer), HeLa (cervical cancer), and A2780 (ovarian cancer) with IC50values 9.4, 2.9, and 7.3μM, respectively.10
The aim of this work was the semisynthetic modification of juncuenin B (1,7-dimethyl-8-vinyl-9,10-dihydrophenanthrene- 2,6-diol) to prepare a variety of structurally related compounds with higher antiproliferative activities. The experimental approach was inspired by a 3-fold of reasons. First, the free radical scavenging-related, oxidized metabolites of phenolic compounds represent a chemical space particularly rich in bioactive compounds.11 Second, we have recently demon- strated that the hypervalent iodine(III) reagent [bis- (trifluoroacetoxy)iodo]benzene (PIFA) may be efficiently used to obtain product mixtures rich in such oxidized scaffolds.12 Third, the p-quinol moiety, anticipated from a PIFA-catalyzed oxidation of p-phenols, is the pharmacophore of several potent inhibitors of ATR-dependent signaling,12,13a crucial mechanism of DNA damage response that is an emerging antitumor target and a subject of many ongoing clinical studies.14 Therefore, we followed a diversity-oriented semisynthetic approach using two hypervalent iodine reagents, PIFA, and (diacetoxyiodo)benzene (PIDA) to obtain oxidized derivatives of juncuenin B. Furthermore, various alcohols (MeOH, EtOH, andn-BuOH) were used as solvents to ensure formation of a set of differently substituted derivatives. Eleven chiral oxidation products (1−11), representing a wide structural diversity, were obtained in this way as racemates or stereoisomeric mixtures. Stereoisomers of the bioactive derivatives were resolved by chiral-phase HPLC and absolute configurations were assigned by ECD measurements and TDDFT-ECD calculations. The antiproliferative activity of the racemic compounds and separated optically active stereo- isomers was investigated on MCF-7, T47D, HeLa, SiHa, C33A, and A2780 cell lines, which enabled us to establish both structure−activity and stereochemistry−activity relationships.
In order to gain insight into their tumor selectivity, the compounds have also been tested on the NIH/3T3 cell line.
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RESULTS AND DISCUSSIONEleven racemic and enantiomerically pure chiral semisynthetic derivatives (1−11) (Scheme 1 and Figure 1) have been prepared from juncuenin B, a naturally occurring vinyl- substituted achiral dihydrophenanthrene, in five transforma- tions (I−V) by hypervalent iodine(III) reagents using a diversity-oriented approach. PIFA and PIDA were used as
oxidants (PIFA in processes I−IV, PIDA in process V), under different conditions, in MeCN−MeOH (process I), MeOH (II), EtOH (III),n-BuOH (IV, V). PIFA and PIDA are well- known reagents for the oxidation of phenols leading to the formation of quinone-type products. This takes place through an aryloxyiodonium(III) intermediate forming a phenoxenium ion that undergoes a nucleophilic attack,15but PIFA and PIDA may also oxidize aromatic compounds via a single-electron transfer mechanism.16Following the oxidation, the mixture of products were subjected to solid-phase extraction on silica to remove the remaining oxidizing agent and the oxidation side- products. The purification process was followed by MPLC and HPLC.
During the reaction processes I−V, racemic mixtures of the compounds were formed, all of them bearing p- or o-quinol rings, substituted with methoxy-, ethoxy-, andn-butoxy groups based on the solvent used. In the case of reaction IV, only compound 10 could be isolated. It did not show structural analogy with the aforementioned compounds; therefore, the reaction was performed with PIDA under the same conditions (process V). This reaction yielded 11, analogous to the compounds derived from 4 and 7. As a result of chromato- graphic purifications, compounds with one stereogenic center were obtained as racemates (3, 4, 7−11), while compounds with two stereogenic centers were isolated as diastereomeric mixtures of two racemates (1a+1d, 1b+1c, 2a, 2b, 6a, 6b).
Separations of stereoisomers were performed using chiral- phase HPLC when the mixtures of stereoisomers or the racemates (1a−d, 4a,b, 7a,b) were found effective in the antiproliferative assay. This was done in order to compare the antiproliferative activity of the pure enantiomers and identify not only 2D structure−activity but also stereochemistry− activity relationships. The separation of the stereoisomers also enabled the characterization and determination of the absolute configurations of the bioactive derivatives by comparison of the experimental and TDDFT-calculated ECD spectra.
The structure determination was carried out by extensive spectroscopic analysis, using 1D and 2D NMR (1H−1H COSY, HSQC, and HMBC) spectroscopy and ECD data analysis.
In the NMR spectra of1a−1d, two sets of signals could be identified that were attributed to two racemic diastereomers1a +1dand1b+1c(ratio 1:1). The1H NMR spectrum of the1a− 1d mixture showed the presence of a pair of isomeric compounds (1a+1d and 1b+1c) as some signals were duplicated. Since these compounds are derived from juncuenin B, similar signals were found in the1H NMR spectrum: signals of twoo-coupled aromatic protons (δH6.24, d and 7.23, d/
7.25, d), an aromatic proton singlet (δH 6.47/6.49), two methyls (δH1.46, s and 2.08, s), a vinylic system atδH6.58, dd, 5.83, dd, and 5.69, dd (H-13, H2-14), and two methylene groups (δH2.63, m, 1.68, m/1.67, m andδH2.83, m, 2.57, dd).
Two methoxy groups (δH3.07, s and 2.92/2.93, s) were also Scheme 1. Transformations of Juncuenin B under Different Conditions
detected in the1H NMR spectrum (Table 1/1a+dand1b+c).
The presence of two methylene signals (H2-9, H2-10) indicated this isomeric pair to be 9,10-dihydrophenanthrene derivatives. In the JMOD (J-modulated spin−echo experi- ment) spectrum, 20 carbon signals were displayed (Table 2/1a +d and 1b+c). In the 1H−1H COSY spectrum, correlations were observed betweenδH6.24, d andδH7.23/7.25, d (H-3−
H-4), betweenδH2.63, m, 1.68/1.67, m andδH2.83, m, 2.57, dd (H2-9−H2-10), and betweenδH5.83, brd/5.69, dd andδH
6.58, dd (H-13−H2-14) (Figure 2). The location of the methoxy groups was concluded from the HMBC spectra, as proton signal at δH3.07 (CH3O-1) showed correlations with δC83.8 (C-1), whereas the signal atδH2.92/2.93 (CH3O-8a) correlated withδC73.2 (C-8a) (Figure 2). Thus, the C-2 and C-6 hydroxy groups of juncuenin B were oxidized to carbonyl groups (δC 202.6 and 185.9, respectively) and methoxy- substituted dienone moieties were formed.
All of the above evidence confirmed the 2D structures of the 1a−1disomers (Figure 2).
The NMR spectra of the mixture of1b+1cwere similar to those of 1a+1d, only small differences were observed for the CH3O-1, C-10, and C-11 resonances of 1a+1d and 1b+1c, which suggested that these compounds are structurally related to each other. Some1H and13C NMR signals were duplicated in the spectra of 1b+1c, revealing the presence of a diastereomeric pair as for1a+1d.
After NMR studies the stereoisomers1a−1dwere separated on a Lux amylose-1 chiral-phase column [tR= 13.6 min (1a), 17.4 min (1b), 21.6 min (1c), and 23.6 min (1d)].
Experimental ECD spectra indicated mirror-image relation-
ships between1a/1d, and 1b/1c, respectively, verifying their enantiomeric relationships (Figure 3).
In order to elucidate the absolute configurations of 1a−d, the solution TDDFT-ECD method was applied to (1R,8aR)-1 and (1R,8aS)-1.17 The initial Merck Molecular Force Field (MMFF) conformational search resulted in nine conformers for each in a 21 kJ/mol energy window, respectively. These geometries were reoptimized at the ωB97X/TZVP PCM/
MeCN level18 resulting in three conformers for both stereoisomers (Figures S8 and S9, Supporting Information).
ECD spectra were computed at various levels and compared to the experimental spectra of 1a−1d. There were only minor differences in the experimental ECD spectra of 1a versus 1b and 1c versus 1d, and the computed ECDs are mainly influenced by one of the two chirality centers. Boltzmann- averaged ECD data of both (1R,8aR)-1 and (1R,8aS)-1 gave acceptable to good agreement with the experimental data of1a and1bwith the C-1 stereogenic center definitively influencing the ECD profile (Figure 4). Thus, the absolute configuration of this stereocenter could be determined as (1R) for thefirst- and second-eluted stereoisomers1aand1band (1S) for the third- and fourth-eluting1cand1d. The shape of the B3LYP/TZVP PCM/MeCN and PBE0/TZVP PCM/MeCN ECD spectra of (1R,8aR)-1 reproduced the experimental ECD curve of 1a better, while those of (1R,8aS)-1 were similar to the experimental spectrum of 1b (Figure 5). The better agree- ments allowed tentative assignment (8aR) for1aand (8aS) for 1b. DFT-NMR calculations of the B3LYP/6-31+G(d,p) conformers of (1R,8aR)-1 and (1R,8aS)-1 at the mPW1PW91/6-311+G(2d,p) level19 could not differentiate Figure 1.Structures of the prepared compounds1−11.
Table1.1 HNMR(500MHz)DataforCompounds1−11a δH(J,Hz) position1a+d1b+c2a2b34a+b56a6b7a+b891011 36.24,d (10.2)6.23,d (10.2)6.45,d(9.9)6.52,d (10.2)6.30,d (10.6)6.74,d(8.4)6.40,d(9.9)6.23,d(10.1)6.21,d(10.1)6.72,d(8.4)6.32,d (10.4)6.73,d(8.3)6.21,d (10.6)6.72,d(8.4) 47.23,d (10.2)/ 7.25,d (10.2)b
7.23,d (10.2)/ 7.25,d (10.2)b
6.61,d(9.9)6.89,d (10.2)9.06,d (10.6)7.34,d(8.4)6.66,d(9.9)7.21,d(10.1)7.21,d(10.1)7.3,d(8.4)8.18,d (10.4)7.45,d(8.3)8.71,d (10.6)7.30,d(8.4) 56.47,s/ 6.49,sb6.47,s/ 6.49,sb6.84,s6.45,s−6.58,s6.82,s6.43,s6.45,s6.54,s7.49,s6.42,s−6.55,s 91.68,m/ 1.67,m, 2.63,mb
1.68,m/ 1.67,m, 2.63,mb
1.92,m, 2.45,m1.43,m, 2.59,m8.19,d(8.8)1.76,ddd (14.4,11.3, 6.5),2.69,dd (14.4,5.7) 1.93,ddd (13.8, 10.0,4.3), 2.47,m 1.62,m,2.62, dd(14.0, 5.5) 1.66,m,2.63, dd(13.9, 5.9) 1.74,ddd (13.9,11.7, 6.5),2.68,dd (14.0,6.0) 8.15,d(8.8)2.71,mb7.62,d (8.3)1.74,ddd (14.1,11.6, 6.4),2.68,dd (14.1,5.8) 102.57,dd (20.2,5.9), 2.83,m
2.50,dd (20.3,6.0), 2.85,m 2.35,m, 2.83,m2.67,m, 2.89,m7.60,d(8.8)2.75,dd(17.3, 6.5),3.06,m2.39,m, 2.87,ddd (13.8,9.0, 4.3) 2.53,dd(14.4, 5.9),2.88, ddd(16.5, 10.6,5.5) 2.51,dd(20.4, 6.0),2.86, ddd(20.2, 10.7,6.0) 3.09,m,2.74, dd(17.3, 6.2)
7.61,d(8.8)2.84,mb7.66,d (8.3)2.73,dd(17.3, 6.4),3.09,m 111.46,s1.42,s1.97,s1.91,s1.58,s2.16,s1.96,s1.46,s1.43,s2.17,s1.56,s2.22,s1.52,s2.16,s 122.08,s2.08,s2.02,s2.00,s2.42,s2.09,s2.01,s2.07,s2.07,s2.09,s2.42,s1.49,s2.04,s2.08,s 136.58,dd (17.8, 12.1)
6.58,dd (17.8, 12.1) 6.50,dd (17.9, 12.0) 6.51,dd (17.9, 11.9) 6.98,dd (17.9, 11.4) 6.63,dd(17.9, 11.9)6.50,dd (17.8, 11.9) 6.61,dd(17.9, 11.9)6.61,dd(17.9, 12.0)6.65,dd(17.9, 11.9)7.02,dd (17.7, 11.4) 6.67,dd (17.8, 11.8) 6.62,dd (17.9, 11.8)
6.64,dd(17.9, 11.9) 145.69,dd (12.1,1.5), 5.83,dd (17.8,1.5)
5.69,dd (12.1,1.5), 5.83,dd (17.8,1.5) 5.63,dd (12.0,1.4), 5.78,dd (17.9,1.4) 5.67,dd (11.9,1.5), 5.70,dd (17.9,1.4) 5.38,dd (17.9,1.7), 5.82,dd (11.4,1.7) 5,66,dd(11.9, 1.7),5.85,dd (17.9,1.7) 5.62,dd (11.9,1.3), 5.80,dd (17.8,1.3) 5.68,dd(11.9, 1.3),5.87,dd (17.9,1.3) 5.68,dd(12.0, 1.5),5.89,dd (17.9,1.5) 5.65,dd(11.9, 1.5),5.89,dd (17.9,1.5) 5.41,dd (17.7.1.6), 5.84,dd (11.4,1.6) 5.55,dd (11.8,1.3), 5.87,dd (17.8,1.3) 5.45,dd (17.9, 1.0), 5.82,d (11.8)
5.65,dd(11.9, 1.7),5.89,dd (17.9,1.7) a MeasuredinCDCl3.b Interchangeable.1a+d:CH3O-1:3.07,s;CH3O-8a:2.92,s/2.93,s(interchangeablewith1b);1b+c:CH3O-1:3.12,s;CH3O-8a:2.92,s/2.93,s(interchangeablewith1a);2a: CH3O-4a:3.04,s;CH3O-8a:2.72,s;2b:CH3O-4a:3.10,s;CH3O-8a:3.14,s;3:CH3O-1:3.08,s;CH3O-5:3.79,s;4a+b:CH3O-8a:2.85,s;5:C2H5O-4a(interchangeablewith8a):2.76dq(7.0,14.1), 2.96,dq(7.0,14.1)(CH2),0.80,t(7.0)(CH3);C2H5O-8a(interchangeablewith4a):3.08dq(7.0,14.1),3.29,dq(7.0,14.1)(CH2),1.11,t(7.0)(CH3);6a:C2H5O-1(interchangeablewith8a):2.93, m,3.36dq(14.3,7.1)(CH2),1.21,t(7.0)(CH3);C2H5O-8a(interchangeablewith1):2.91,m,3.19dq(14.3,7.1)(CH2),1.01,t(7.0)(CH3);6b:C2H5O-1:3.10,dq(14.2,7.0),3.34,dq(14.3,7.1) (CH2),1.23,t(7.0)(CH3);C2H5O-8a:2.93,dq(14.2,7.0),3.17,dq(14.3,7.1)(CH2),0.97,t(7.0)(CH3);7a+b:C2H5O-8a:2.92,m,3.11,m(CH2),0.86,t(7.0)(CH3);8:C2H5O-1:3.00dq(14.2, 7.0),3.28dq(14.2,7.0)(CH2),1.20,t(7.0)(CH3);9:C2H5O-7:3.13,dq(13.9,7.1),3.26,dq(14.0,7.1)(CH2),1.17,t(7.0)(CH3);10:CH3-(CH2)3-O-1:3.22,m,2.92,m(1′-CH2),1.58,m(2′- CH2),1.28−1.40,m(3′-CH2),0.87,t(7.4)(4′-CH3);CH3-(CH2)3-O-5:3.14m,3.54,m(1″-CH2),1.47,m(2″-CH2),1.28−1.40,m(3″-CH2),0.81,t(7.5)/0.83,t(7.5)(4″-CH3)(interchangeable with4‴-CH3);3.10,m,3.45,m(1‴-CH2),1.47,m(2‴-CH2),1.28−1.40,m(3‴-CH2),0.81,t(7.5)/0.83,t(7.5)(4‴-CH3)(interchangeablewith4″-CH3);11:CH3-(CH2)3-O-8a:2.83,m,3.07,m (1′-CH2),1.13−1.23,m(2′-CH2),0.86−1.03,m(3′-CH2),0.62,t(7.4)(4′-CH3).
Table2.13 CNMR(125MHz)DataforCompounds1−11a δc,type position1a+d1b+c2a2b34a+b56a6b7a+b891011 183.8,C83.1,C133.0,C132.8,C83.5,C122.0,C132.5,C83.2,C82.4,C121.8,C82.5,C121.9,C81.9,C121.8,C 1a157.2,C157.0,C150.9,C154.4,C145.1,C133.9,C152.0,C157.9,C157.5,C139.1,C144.2,C141.0,C147.3,C139.2,C 2202.6,C202.4,C185.4,C184.7,C202.4,C155.8,CH185.3,C202.7,C202.5,C155.5,C202.9,C156.3,C201.4,C155.5,C 3125.9,CH125.9,CH131.9,CH131.8,CH124.1,CH113.8,CH131.2,CH125.8,CH125.8,CH113.7,CH124.6,CH114.2,CH125.1,CH113.6,CH 4139.7,CH139.7,CH144.8,CH146.5,CH143.6,CH124.6,CH145.6,CH139.6,CH139.6,CH124.5,CH139.8,CH125.7,CH142.7,CH124.6,CH 4a125.4,C125.4,C73.1,C77.3,C123.7,C125.2,C73.0,C125.0,C125.0,C125.5,C123.7,C126.0,C131.0,C125.5,C 5123.1,CH123.1,CH131.0,CH128.8,CH139.9,C122.2,CH130.6,CH122.6,CH122.6,CH121.8,CH103.3,CH117.3,CH94.6,C121.7,CH 5a151.9,C151.9,C159.6,C156.8,C122.9,C156.7,C160.5,C152.7,C152.9,C157.7,C130.7,C150.6,C136.1,C157.8,C 6185.9,C185.9,C186.2,C186.1,C147.8,C186.5,C186.3,C185.9,C186.0,C186.6,C154.0,C202.6,C194.4,C186.7,C 7135.8,C135.8,C134.1,C133.9,C125.2,C135.7,C133.8,C135.2,C135.3,C135.1,C124.4,C82.1,C130.4,C135.1,C 8149.6,C149.6,C150.2,C153.6,C134.1,C149.6,C150.7,C150.2,C150.4,C150.6,C138.2,C145.9,C147.9,C150.5,C 8a73.2,C73.2,C75.1,C76.7,C127.9,C73.8,C74.5,C72.7,C72.7,C73.4,C127.3,C131.6,C132.6,C73.2,C 933.1/34.0,CH2b33.1/34.0,CH2b34.3,CH241.1,CH2129.7,CH34.5,CH234.1,CH233.2,CH234.2,CH234.8,CH2129.0,CH25.8/26.6,CH2b129.0,CH34.8,CH2 1021.8,CH222.4,CH222.8,CH222.7,CH2121.3,CH23.9,CH222.9,CH221.9,CH222.5,CH223.9,CH2121.0,CH25.8/26.6,CH2b 127.0,CH23.9,CH2 1128.0,CH326.6,CH310.7,CH310.9,CH331.1,CH311.2,CH310.7,CH327.2,CH326.7,CH311.2,CH330.9,CH311.3,CH331.3,CH311.2,CH3 1212.3,CH312.3,CH312.0,CH312.3,CH313.9,CH312.3,CH312.0,CH312.2,CH312.3,CH312.3,CH313.6,CH328.2,CH313.0,CH312.3,CH3 13131.7,CH131.7,CH131.6,CH132.0,CH133.9,CH132.1,CH131.8,CH131.9,CH131.8,CH132.2,CH134.0,CH131.2,CH132.9,CH132.2,CH 14123.9,CH2123.9,CH2124.0,CH2124.0,CH2122.5,CH2123.3,CH2123.8,CH2123.7,CH2123.8,CH2123.3,CH2122.4,CH2121.7,CH2123.2,CH2123.3,CH2 a MeasuredinCDCl3.b Interchangeable.1a+d:CH3O-1:51.7;CH3O-8a:54.7;1b+c:CH3O-1:51.7;CH3O-8a:54.5;2a:CH3O-4a:51.1;CH3O-8a:52.5;2b:CH3O-4a:51.6;CH3O-8a:52.9;7:CH3O- 1:54.4;CH3O-5:61.1;4a+b:CH3O-8a:51.6;5:C2H5O-4a(interchangeablewith8a):60.2,CH2,14.7,CH3;C2H5O-8a(interchangeablewith4a):59.0,CH2,15.8,CH3;6a:C2H5O-1(interchangeable with8a):62.5,CH2,15.7,CH3;C2H5O-8a(interchangeablewith1):59.6,CH2,15.5,CH3;6b:C2H5O-1:62.2,CH2,15.8,CH3;C2H5O-8a:59.6,CH2,15.6,CH3;7a+b:C2H5O-8a:59.5,CH2,15.4CH3; 8:C2H5O-1:62.2,CH2,15.8,CH3;9:C2H5O-7:61.6,CH2,15.6,CH3;10:CH3-(CH2)3-O-7:66.3(1′-CH2),32.4(2′-CH2),19.2/19.3/19.4(3′-CH2)(interchangeablewith3″-CH2and3‴-CH2),14.1 (4′-CH3);CH3-(CH2)3-O-5:63.3(1″-CH2),31.8/31.9(2″-CH2)(interchangeablewith2‴-CH2),19.2/19.3/19.4(3″-CH2)(interchangeablewith3′-CH2and3‴-CH2),13.8/13.9(4″-CH2) (interchangeablewith4‴-CH3);63.6(1‴-CH2),31.8/31.9(2‴-CH2)(interchangeablewith2″-CH2),19.2/19.3/19.4(3‴-CH2)(interchangeablewith3′-CH2and3″-CH2),13.8/13.9(4‴-CH2) (interchangeablewith4″-CH3);11:CH3-(CH2)3-O-8a:63.5(1′-CH2),31.9(2′-CH2),19.1(3′-CH2),13.7(4′-CH3).
the epimers due to the small chemical shift differences of the experimental values.20
The 1H NMR spectrum of compound 2a showed signals characteristic of juncuenin B analogues: signals of two o- coupled aromatic protons (δH 6.45, d and 6.61, d), one aromatic proton singlet (δH6.84), two methyl singlets (δH1.97 and 2.02), a vinyl group atδH6.50, dd, 5.78, dd and 5.63, dd, two methylene groups (δH1.92, m, 2.45, m, and δH2.35, m, 2.83, m), and two methoxy singlets (δH 2.72 and 3.04), respectively (Table 1). In the JMOD spectrum, signals of 20 carbon atoms were detected (Table 2). In the1H−1H COSY spectrum, correlations betweenδH6.45, d and 6.61, d (H-3− H-4), betweenδH2.35, m, 2.83, m, and δH1.92, m, 2.45, m (H-9−H-10), and betweenδH6.50, dd, and 5.78, dd, 5.63, dd (H-13−H2-14) were observed. The position of the methyl groups were determined based on the HMBC correlations of the methyl protons atδH1.97 and 2.02 with C-1 (δC133.0)
and C-7 (δC134.1), respectively. Methoxy groups attached at C-4a and C-8a were positioned by means of the HMBC interactions betweenδH3.04 (CH3O-4a) andδC73.1 (C-4a), and between δH 2.72 (CH3O-8a) and δC 75.1 (C-8a). The vinylic system was connected to C-8 via cross peaks between δH5.63, dd, 5.78, dd andδC150.2.
All NMR characteristics of compound 2b were similar to compound 2a, indicating that they are diastereomers. The main differences between2b and2awere found between the
13C NMR chemical shifts of C-4a and 8a and adjacent carbons, suggesting that2band2adiffer in the configurations of C-4a and 8a.
In the case of racemic compound 3, the signals of fouro- coupled aromatic protons [(δH6.30, d and 9.06, d (H-3−H-4), δH8.19, d and 7.60, d (H-9−H-10)], two methyls (δH1.58, s and 2.42, s), a vinyl group (δH6.98, dd, 5.82, dd and 5.38, dd, H-13 and H2-14), and two methoxy groups (δH3.08, s, and 3.79, s) were detected in the 1H NMR data (Table 1). The differences between juncuenin B and compound3involved the Figure 2.Diagnostic COSY (−) and HMBC (H→C) correlations of
compounds1a−d.
Figure 3.(a) Experimental ECD spectra of1a−1din MeCN and (b) experimental UV spectra of1a/1dand1b/1cin MeCN.
Figure 4.Experimental ECD spectrum of1ain MeCN (black line) compared with the calculated PBE0/TZVP PCM/MeCN spectrum of (1R,8aR)-1(purple line). Level of DFT optimization:ωB97X/TZVP PCM/MeCN. Bars represent the rotational strengths of conformer A.
Figure 5.Experimental ECD spectrum of1bin MeCN (black line) compared with the calculated B3LYP/TZVP PCM/MeCN spectrum of (1R,8aS)-1(red line). Level of DFT optimization:ωB97X/TZVP PCM/MeCN. Bars represent the rotational strengths of conformer A.
presence of an aromatic ring B, the oxidation of the phenolic group at C-2 to a carbonyl moiety (δC202.4), and the addition of methoxy groups at C-1 and C-5.
The1H NMR spectrum of the racemic mixture of4aand4b differ from juncuenin B in the presence of a methoxy group (δH2.85) (Table 1) and the carbonyl group at ring C. In the JMOD spectrum 19 carbon signals were observed (Table 2).
The methoxy group was located at C-8a, as the aforemen- tioned CH3O proton signal showed HMBC correlation with the carbon signal resonating at δC73.8. Following the NMR studies the enantiomers4a and 4b were separated on a Lux amylose-1 chiral-phase HPLC column [tR= 5.5 min (4a) and 10.3 min (4b)].
The 1D NMR data of compound5were found to be similar to those of2aand2b, except for the presence of two ethoxy groups in the molecule [δH2.96, dq/3.29, dq, 2.76 dq/3.08, dq (CH2) and 0.80, t/1.11, t (CH3); δC59.0/60.2 (CH2), 14.7/
15.8 (CH3)]. Relevant HMBC interactions suggested the side chains to be attached to the phenanthrene skeleton at C-4a and C-8a (Tables 1 and2).
Compounds 6a and 6b were obtained as racemic diastereomers. The difference between compounds 1a−dand compounds 6a and 6b involved the replacement of the C-1 and C-8a methoxy groups with ethoxy substituents (Tables 1 and2).
Compounds7aand7bdiffer from compounds4aand4bin the presence of an ethoxy group [δH2.92, m, 3.11, m (CH2), 0.86, t (CH3);δC59.5 (CH2), 15.4 (CH3)] at C-8a instead of a methoxy group (Tables 1 and 2). Compounds 7a and 7b, similar to1a−dand4−6, also contain a carbonyl moiety at C- 6 (δC186.6). Following the NMR analysis, the enantiomers7a and7b were separated by HPLC on a Lux amylose-1 chiral- phase column [tR = 4.7 min (7a) and 9.1 min (7b)] and mirror-image ECD spectra were recorded (Figure 6).
For the configurational assignments of7aand7b, the above- mentioned TDDFT-ECD protocol was applied to (R)-7 and also on the truncated methoxy model compound (R)-4.
DFT reoptimization of the initial six MMFF conformers of both (R)-4 and (R)-7 yielded six conformers above 1%
population indicating that the ethoxy group does not have considerable freedom (Figure S64, Supporting Information).
The Boltzmann-weighted ECD spectra of both sets of
conformers reproduced well the experimental ECD spectrum of the first-eluting enantiomer 7a (Figure 7and Figure S37,
Supporting Information). Consequently, the absolute config- urations of7a and4a, which were separated under the same conditions, could be elucidated as (R) and that of the second- eluting7band4bas (S).
Interestingly, racemic compound8is the only one in which ring C is unchanged compared to the starting material, juncuenin B. An ethoxy group [δH3.00, dq, 3.28, dq (CH2), 1.20, t (CH3);δC62.2 (CH2), 15.8 (CH3)] is attached at C-1, and the phenolic group at C-2 of juncuenin B was oxidized to a carbonyl moiety (δC 202.9). Moreover, 8 is an unsaturated compound containing a Δ9,10 double bond (δH 8.15, d and 7.61, d, H-9−H-10).
Compound9is a unique derivative because an ethoxy group [δH3.13, dq, 3.26, dq (CH2), 1.17, t (CH3);δC61.6 (CH2), 15.6 (CH3)] is attached to the phenanthrene skeleton at C-7, as suggested by spectroscopic data evidence. The HMBC correlation of H3-12 (δH1.49) with the oxygen-bearing C-7 at δC82.1, as well as the relevant cross peaks H-5/C-7, H-13/C- 7, H-5/C-8a, and H-13/C-8a indicated the presence of aΔ8,8a double bond instead of a Δ7,8 olefinic bond, to support the above conclusion.
In the case of compound10, it was apparent from the 1D NMR spectra that threen-butoxy groups are connected to the phenanthrene skeleton. Both the phenolic substituents of juncuenin B were oxidized to carbonyl groups, and a Δ9,10 double bond was also formed. The location of the methyl, vinyl, andn-butoxy groups were determined by their diagnostic HMBC correlations. Three-bond heteronuclear interactions of C-1 (δC81.9) with the oxymethylene protons atδH2.92, m, and of the deshielded C-5 (δC 94.6) with two additional oxymethylenes at δH 3.14, m, and 3.54, m confirmed the positions of then-butoxy groups at C-1 and C-5, respectively.
The 1H NMR spectrum of the racemic compound 11 displayed signals attributable to ann-butoxy moiety [δH2.83, m, 3.07, m (CH2), 1.13−1.23, m (CH2) 0.86−1.03, m (CH2), and 0.62, t (CH3); δC 63.5 (CH2), 31.9 (CH2), 19.1 (CH2), and 13.7 (CH3)]. Since the oxidative transformation was Figure 6.Experimental ECD spectra of7aand7bin MeCN.
Figure 7.Experimental ECD spectrum of7ain MeCN (black line) compared with the calculated CAM-B3LYP/TZVP PCM/MeCN spectrum of (R)-7a(olive line). Level of DFT optimization:ωB97X/
TZVP PCM/MeCN. Bars represent the rotational strengths of conformer A.
performed in a reaction medium containing n-BuOH as solvent, and the aromatic A-ring and HO-2 remained intact during the reaction, it was anticipated that then-butoxy chain is situated at C-8a. This presumption was corroborated by a strong HMBC cross peak between the oxymethylene protons and C-8a.
The antiproliferative assays of the products were carried out by a standard MTT method on human breast (MCF-7, T47D), cervical (HeLa, SiHa, C33A), and ovarian (A2780) cancer cells, and NIH/3T3 mouse embryonic fibroblast cell lines. In thefirst step, the diastereomeric and racemic mixtures 1−11and the starting compound, juncuenin B, were tested to obtain information about their antiproliferative activity. Among the tested samples, compounds1a−d,4a+4b,6a,7a+7b, and 11 exhibited significant tumor cell proliferation inhibitory activities with IC50values close to that of juncuenin B (Table 3, and Tables S2 and S3,Supporting Information). The IC50 value of juncuenin B on MCF-7 cell line (11.7 μM) was determined for thefirst time. Similar to the structural points of view, it is evident that compounds without ap-quinol ring (3, 8,9, and10) and compounds with twop-quinol moieties (2a, 2b, 5) did not exert antiproliferative activity. Compounds substituted at C-4a with an alkoxy group were also inactive.
Compounds 1a−d (IC50s 11.6 μM on MCF-7, 15.0 μM on T47D, 15.6μM on HeLa, 20.0μM on SiHa, 6.7μM on C33A, and 7.7μM on A2780, respectively),4a+4b(IC50s 6.5μM on MCF-7, 1.9μM on HeLa, 7.0μM on C33A, and 9.1μM on A2780, respectively), and 7a+7b (IC50s 6.2 μM on MCF-7, 16.9μM on T47D, 5.4μM on HeLa, 6.1μM on C33A and 5.9 μM on A2780, respectively) with the highest activity were selected for further studies.
In the second step, the enantiomerically pure compounds (1a, 1b, 1c, 1d, 4a, 4b, and 7b) were evaluated for their antiproliferative activity against the same cell lines (Table 4).
Among these compounds, the highest activities were recorded for1c, 4a, and7a, reaching or exceeding that of the positive control cisplatin against HeLa and/or T47D cell lines. (R)-7a was found to be the most promising compound with
substantial antiproliferative effects against all tested cell lines except for SiHa, but it was inactive against the nontumoral NIH/3T3 cells.
Compounds4a,4b,7a,7b, and11contain a nonoxidized A ring and a C-ring comprising a p-quinol alkyl ether moiety.
Moreover,4aand4bare substituted with a methoxy-,7a, and 7bwith an ethoxy-, and 11with an n-butoxy group at C-8a, respectively. In the case of these compounds, the effect of the length of the ether chain at C-8a is undefined: on the T47D, HeLa, C33A, and A2780 cell lines, compound 7a with an ethoxy group, showed the highest inhibition, while its enantiomer (7b) had no activity. On the HeLa cell line, the antiproliferative activity was inversely proportional to the chain length of the alkoxy groups (IC50 values 11 > 7a > 4a).
Compounds 4aand 7a showed an antiproliferative effect on HeLa, C33A, MCF-7, and A2780 cells, which was comparable to or stronger than that of juncuenin B, while their stereoisomers 4b and 7b had no relevant influence on the cell proliferation, indicating that (8aR) is the preferred absolute configuration of these compounds. Regarding the pure enantiomers1a−d, the (1S,8aR)-configuration (1c) was the most beneficial for the antiproliferative effect. The activities of (1S,8aR)-1c, (R)-4a, and (R)-7a suggest the preference of the (R) configuration for the antiproliferative activity. Since1a is less active than1c, the (1S) configuration seems to be also essential in the case of quinoidal compounds.
It is worth mentioning that several of the semisynthetic compounds demonstrated an altered cell line specificity as compared to their parent compound, juncuenin B. For example, while juncuenin B exerted approximately 2.5-fold stronger activity on MCF-7 than on T47D cells, a similar rate of selectivity in the opposite direction was observed for the enantiopure compound7a. While both of these cell lines are estrogen receptorαpositive and therefore estrogen-dependent, they show critical differences in their bioenergetic properties,21 as well as in apoptotic events including the activation of different caspases and mitochondrial changes.22This selectivity inversion suggests that compounds obtained from juncuenin B Table 3. IC50Values (μM, Mean±SEM) of the Diastereomeric and/or Racemic Mixturesa
calculated IC50values (μM±SEM)
compound MCF-7 HeLa C33A A2780 NIH/3T3
juncuenin B − 2.9±0.5b − 7.3±1.3b >30
1a−1d − − 6.7±0.5 7.7±1.0 17.7±0.7
4a+4b 6.5±1.1 1.9±0.4 7.0±1.8 9.1±0.5 14.9±3.7
7a+7b 6.2±0.2 5.4±0.1 6.1±0.4 5.9±1.1 >30
cisplatin 5.8 − 1.8 1.3b 2.7
aCompounds2a,2b,3,5,6b,8,9, and10were found to be inactive or moderately active (Table S1,Supporting Information). Determinations were performed using an MTT assay by treating the cells with compounds (0.1−30μM) for 72 h. Data are based on two independent experiments.
bIC50value from literature.10
Table 4. IC50Values of the Enantiopure Compounds 1a−1d, 4a, 4b, 7a, and 7ba
Calculated IC50values (μM±SEM)
compound MCF-7 T47D HeLa C33A A2780 NIH/3T3
(1R,8aS)-1b − − − − 9.0±1.0 >30
(1S,8aR)-1c − − − 5.3±0.9 7.7±0.6 21.6±0.4
(8aR)-4a 6.2±1.6 − 0.9±0.4 5.3±0.6 8.7±0.8 25.8±4.6
(8aR)-7a − 8.8±1.1 3.7±0.9 4.9±0.3 2.8±0.3 >30
cisplatin 5.8 9.8b −b 1.8 1.3b 2.7
aDeterminations were performed using an MTT assay by treating the cells with compounds (0.1−30μM) for 72 h. Data are based on two independent experiments.bIC50value from literature.10
