Ru-Bing
Wang‡
a,
Shuang-Gang
Ma‡
*a,
Cooper S.
Jamieson‡
b,
Rong-Mei
Gao
c,
Yun-Bao
Liu
a,
Yong
Li
a,
Xiao-Jing
Wang
a,
Yu-Huan
Li
c,
Kendall N.
Houk
*b,
Jing
Qu
*a and
Shi-Shan
Yu
*a
aState Key Laboratory of Bioactive Substance and Function of Natural Medicines, Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, No.1 Xian Nong Tan Street, Beijing, 100050, People's Republic of China. E-mail: mashuanggang@imm.ac.cn; qujing@imm.ac.cn; yushishan@imm.ac.cn
bDepartment of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA. E-mail: houk@chem.ucla.edu
cInstitute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, No. 1 Tian Tan Xi Li, Beijing, 100050, People's Republic of China
First published on 10th April 2021
The construction of libraries of stereoisomers of natural products serves as an important approach to investigating the correlation between the stereostructure and biological activity. However, the total synthesis and isomerzation of polycyclic scaffolds with multiple chrial centers are rare. Spirooliganin (1), a new skeleton natural product isolated from the plant Illicium oligandrum, was structurally characterized by comprehensive analysis of NMR spectroscopic data and ECD which revealed an unprecedented 5–6–6–6–7 polycyclic framework with six chiral centers. Here we report a 17-step total synthesis to prepare a library of stereochemically diverse isomers of spirooliganin, including 16 diastereoisomers and 16 regioisomers. In addition to a regioselective hetero-Diels–Alder cycloaddition, the synthetic strategy involves a photo-induced stereoselective Diels–Alder reaction, which gives only the abnormal trans-fused product as rationalized by density functional theory calculations. Preliminary biological evaluation showed that spirooliganin and regioisomers 39 exhibited potent inhibition of Coxsackievirus B3. It also revealed the pharmacophore effect of the D-ring (16R,18R,24R, and 26R) for their antiviral activities.
Examples of the construction of stereoisomer libraries of natural products containing multiple chiral centers has been reported in the past few decades, such as annonaceous acetogenins, a class of fatty acid-derived natural products from members of the Annonaceae family,1–4 cladosporin, an isocoumarin-based metabolite from fungus,5 spirotryptostatin, a prenyl-containing alkaloid from Aspergillus fumigatus,6–8 and cocaine, a member of the tropane alkaloids isolated from the leaves of Erythroxylon coca.9 However, stereoisomeric variation in these studies was limited to scaffolds with long carbon-chain skeletons or polyketide macrolides10,11 that contain multiple oxygenated chiral centers, or molecules with two vicinal stereogenic centers.9,12–15 Due to the synthetic challenge, there exists no stereoisomer library constructed for natural products with a rigid, polycyclic fused ring scaffold and multiple resulting stereocenters. Thus, the stereostructure–activity relationship (SSAR) for many of these natural products is unknown.
Inspired by the potential antiviral activity of prenylated C6–C3 compounds,16–19 which are characteristic constituents of plants from the genus Illicium, we isolated and characterized spirooliganin (1) from the stem bark of I. oligandrum.20 The compound 1 features an unprecedented linear tetracycle (6–6–6–7, rings B, C, D, and E, Fig. 1). Remarkably, this molecule exhibits potent activity against Coxsackievirus B3 (CVB3), with an IC50 of 2.1 μM and a selectivity index (SI) value of 5.2. Here we report the construction of a library of stereoisomers of spirooliganin, including 16 diastereoisomers and 16 regioisomers, via a 17-step total synthesis. The synthetic strategy involves a regioselective hetero-Diels–Alder cycloaddition and an abnormal photochemical trans-selective Diels–Alder reaction, which has been rationalized by density functional theory calculations. The antiviral activity of the stereoisomer library was evaluated against CVB3, thus revealing the relationship between the stereostructure and antiviral activities of this analogue of natural products.
The general synthetic strategy includes three main transformations: (1) use of a nonselective oxidation–cyclizaton to diversify the two chiral centers on the oxaspiro ring while the characteristic prenylated C6–C3 unit was retained, (2) use of a non-regioselective Diels–Alder reaction between 4,4-dimethylcyclohept-2-en-1-one and isoprene to give two kinds of cycloadditon product including regioisomers resulting from the opposite location of the exocyclic double bond and a gem-dimethyl group to maximize the isomers of 1, and (3) use of a regioselective hetero-Diels–Alder reaction of an o-quinone methide with himachalane cyclohexene to obtain the linearly fused 6–6–6–7-tetracycle, which matches the basic skeleton of 1 and produces diverse stereoisomers of tetracyclic intermediates. We firstly intended to install the basic skeleton of 1 using a convergent strategy21–23 of i and ii. However, this was not achieved, even after simplification of the starting material (iii to ii) (Fig. 2A). Therefore, we transferred to another linear synthetic strategy to achieve the synthesis of isomers of spirooliganin (Fig. 2B). Finally, library construction of stereo isomers of spirooliganin was realized as shown in Fig. 2C.
With the tetracyclic core in place, our attention was focused on the formation of the B ring by aromatization (Fig. 4). We expected to accomplish selective aromatization of the B ring in one step based on the slight activity difference between the α,β-unsaturated carbonyl and saturated carbonyl group. Many reported methods (Pd/C, I2/CH3OH, I2/DMSO/CH3NO2, DDQ, and NaH/PhSO2CH3) were attempted,27–29 such as (NaH/PhSO2CH3) reported by Xie et al. as a practical method for the synthesis of spirooliganones.27 However, these attempts gave at best a trace amount of the desired product. We speculated that this might be due to the presence of carbonyl on the seven-membered ring, which was accompanied by β-keto sulfoxide substitution when the reaction was heated to 100 °C using methyl benzenesulfinate for aromatization. We thus attempted an alternative strategy by protecting the carbonyl before aromatization. The reaction of rac-8a with glycol and trimethyloxymethane catalyzed by p-TsOH in benzene under violent reflux gave the corresponding protected racemates. The following aromatization of rac-9a not only converted the α,β-unsaturated ketone ring (B ring) into the corresponding phenol but also completely removed the protection of the carbonyl in two steps with 60% yield for rac-10a.
Fig. 4 Syntheses of spirooliganin diastereoisomers from rac-8a. The four diastereoisomers 19, 20, 21, and 22 formed via rac-17a1 were shown. |
With racemic phenols rac-10a in hand, the allylic and prenyl side chains of the B ring were introduced by a general alkylation/Claisen rearrangement reaction sequence. Treatment of rac-10a with allyl bromide and K2CO3 in refluxing acetone efficiently delivered allylic ether rac-11a in 95% yield, followed by a Wittig reaction using the ylide of methyltriphenylphosphonium bromide to give rac-12a in 90% yield. This intermediate was converted to o-allylic phenols rac-13a with high regioselectivity in 95% yield when heated in N,N-diethylaniline. Similarly, treatment of rac-13a with prenyl bromide and K2CO3 in refluxing acetone efficiently produced rac-14a, which was then heated in N,N-diethylaniline to give the p-prenyl phenols rac-15a with high regioselectivity in 75% yield over two steps. rac-15a was protected by tert-butyldiphenylsilyl chloride (TBDPSCl) to produce rac-16a in 96% yield, which was subjected to Sharpless dihydroxylation with AD-mix-β at 10–15 °C for 36 h, and two pairs of racemic diols, rac-17a1 (24% yield) and rac-17a2 (21% yield), were generated with poor stereoselectivity and 30% conversion. We attempted to prolong the reaction time and increase the reaction temperature, but tetrol derivatives were observed as the main product from HRMS data, which may be due to the oxidation of both the isopentenyl and allyl groups. The structures of the intermediates (rac-10a, rac-11a, rac-12a, rac-13a, rac-14, and rac-15a) were confirmed by a series of 1D and 2D-NMR spectroscopic analyses (Fig. S63–S66, S79–S82, S95–S98, S111–S114, S127–S130, and S143–S146†), and their stereochemistry was the same as that of rac-8a. The stereochemistry of two pairs of racemic diols (rac-17a1 and rac-17a2) was verified based on the absolute configuration of the final products 19–26 (Fig. S191–S230†).
Followed by the preparation of diols, a tandem oxidative dearomatization/cyclization reaction was adopted to construct the oxaspiro ring moiety. Treatment of rac-17a1 with TBAF in THF afforded the deprotected compounds, which was reacted with iodobenzene diacetate (PIDA) in hexafluoroisopropanol (HFIP) in the presence of K2CO3 to generate two pairs of scalemic enantiomers 19/20 in 20% yield and 21/22 in 22% yield after purification by silica column chromatography and preparative HPLC. The enantiomers were further separated by chiral HPLC to afford single diastereoisomers, including 19, the 26-epimer of spirooliganin.
We also prepared the other 12 diastereoisomers (23–34) of spirooliganin from rac-17a2 and rac-8b, as well as 16 regioisomers (35–50) from rac-8c and rac-8d (Fig. 5). The synthetic route was same as that used for diastereoisomer synthesis from rac-8a. The structures of the final products, including the absolute configurations, were unambiguously characterized by comprehensive NMR spectroscopic analysis, a series of NOE experiments, ECD analysis, and Mosher's method (Fig. S2, S3, and S191–S369, Tables S12–S20†). Moreover, the X-ray crystal structure of a pair of enantiomers, 31 and 32, obtained from methanol containing trace H2O also confirmed the fascinating highly linearly fused tetracyclic (6–6–6–7) skeleton bearing a 1-oxaspiro [4.5]deca-6,9-dien-8-one moiety and six dense chiral centers (Fig. 6).
We located the transition states for the cycloadditions of both the cis- and trans-cycloheptenones (Fig. 7B). The cis-cycloheptenone can react via endo or exo transition states to form two regiosomeric cis-fused cycloheptenone adducts. The lowest energy transition states for the regioisomeric cycloadditions are TS-1 and TS-2. These transition states are endo and have free energies of activation of 37.1 and 37.2 kcal mol−1, respectively. There is a minute (0.1 kcal mol−1) preference for C-1 of isoprene to react at the β carbon of the α,β-unsaturated ketone, in agreement with the frontier molecular orbital prediction. The barriers for these cycloadditions are very high, and under mild reaction conditions would not be observed.
Isomerization of cis-cycloheptenone to trans-cycloheptenone is uphill thermodynamically by 31.9 kcal mol−1. The trans-cycloheptenone can, similarly, undergo cycloadditions to form two regiosomeric trans-fused cycloheptenone adducts. In this case, the lowest energy transition states TS-3 and TS-4 are exo with respect to the cycloheptenone ketone and have low barriers of 18.1 and 18.3 kcal mol−1 from the trans-cycloheptenone, respectively. The transition states are slightly more asynchronous than those from the cis-cycloheptenone, but still clearly concerted. Now, there is a small preference for the opposite regioselectivity compared to the cis-enone reaction. The barriers for these cycloadditions of trans-cycloheptenone are nearly 20 kcal mol−1 lower than that of cis-cycloheptenone.
These calculations rationalize why the cis-fused adduct was not observed and showed the origins of stereochemistry and regioselectivity. In short, the cis-cycloheptenone is unreactive in cycloadditions, but the trans-cycloheptenone is highly reactive and leads by a concerted cycloaddition to the trans-fused product.
Groups | Compounds | CEsb | Configuration | TC50c (μM) | IC50 (μM) | SId | |
---|---|---|---|---|---|---|---|
A ring | D ring | ||||||
a TC50 and IC50 data represent the mean values for three independent determinations. b Cotton effects at λmax 255 nm. c Cytotoxic concentration required to inhibit Vero cell growth by 50%. d The SI value equaled TC50/IC50. e The SI could not be determined under the test conditions. f Positive control. g nM. | |||||||
1 | (−) | 4R,11R | 16R,18R,24R,26S | 11.1 ± 1.5 | 2.1 ± 1.1 | 5.2 | |
A | 19 | (−) | 4R,11R | 16R,18R,24R,26R | 57.7 ± 5.4 | >33.3 | —e |
20 | (+) | 4S,11S | 16S,18S,24S,26S | 57.7 ± 2.8 | >33.3 | —e | |
21 | (+) | 4S,11R | 16R,18R,24R,26R | 57.7 ± 9.1 | 33.3 ± 4.2 | 1.7 | |
22 | (−) | 4R,11S | 16S,18S,24S,26S | 57.7 ± 5.3 | >33.3 | —e | |
23 | (+) | 4S,11S | 16R,18R,24R,26R | 48.1 ± 5.2 | 11.1 ± 2.5 | 4.3 | |
24 | (−) | 4R,11R | 16S,18S,24S,26S | 48.1 ± 6.3 | >11.1 | —e | |
25 | (−) | 4R,11S | 16R,18R,24R,26R | 57.7 ± 7.0 | 33.3 ± 2.2 | 1.7 | |
26 | (+) | 4S,11R | 16S,18S,24S,26S | 57.7 ± 6.6 | >33.3 | —e | |
B | 27 | (+) | 4S,11R | 16S,18R,24R,26S | >100.0 | >33.3 | —e |
28 | (−) | 4R,11S | 16R,18S,24S,26R | >100.0 | >33.3 | —e | |
29 | (−) | 4R,11R | 16S,18R,24R,26S | 48.1 ± 5.2 | >11.1 | —e | |
30 | (+) | 4S,11S | 16R,18S,24S,26R | 23.1 ± 4.2 | >11.1 | —e | |
31 | (−) | 4R,11S | 16S,18R,24R,26S | 57.7 ± 1.0 | 22.6 ± 3.3 | 2.6 | |
32 | (+) | 4S,11R | 16R,18S,24S,26R | 100.0 ± 8.8 | 33.3 ± 2.9 | 3.0 | |
33 | (+) | 4S,11S | 16S,18R,24R,26S | 48.1 ± 4.4 | >11.1 | —e | |
34 | (−) | 4R,11R | 16R,18S,24S,26R | 57.7 ± 2.4 | >33.3 | —e | |
C | 35 | (−) | 4R,11R | 16R,18R,24R,26R | 23.1 ± 4.1 | 11.11 ± 1.35 | 2.1 |
36 | (+) | 4S,11S | 16S,18S,24S,26S | 33.3 ± 3.2 | >11.11 | —e | |
37 | (+) | 4S,11R | 16R,18R,24R,26R | 19.3 ± 3.7 | 11.1 ± 2.3 | 1.7 | |
38 | (−) | 4R,11S | 16S,18S,24S,26S | 19.3 ± 2.4 | >11.1 | —e | |
39 | (+) | 4S,11S | 16R,18R,24R,26R | 16.0 ± 2.2 | 3.7 ± 1.2 | 4.3 | |
40 | (−) | 4R,11R | 16S,18S,24S,26S | 16.0 ± 1.8 | >3.7 | —e | |
41 | (−) | 4R,11S | 16R,18R,24R,26R | 19.3 ± 4.1 | 8.6 ± 1.3 | 2.2 | |
42 | (+) | 4S,11R | 16S,18S,24S,26S | 48.1 ± 6.2 | 8.6 ± 1.5 | 5.6 | |
D | 43 | (+) | 4S,11R | 16S,18R,24R,26S | >100.0 | >33.3 | —e |
44 | (−) | 4R,11S | 16R,18S,24S,26R | >100.0 | >33.3 | —e | |
45 | (−) | 4R,11R | 16S,18R,24R,26S | 57.7 ± 6.9 | >33.3 | —e | |
46 | (+) | 4S,11S | 16R,18S,24S,26R | 57.7 ± 5.2 | >33.3 | —e | |
47 | (−) | 4R,11S | 16S,18R,24R,26S | 48.1 ± 4.1 | 11.11 | 4.3 | |
48 | (+) | 4S,11R | 16R,18S,24S,26R | 57.7 ± 4.4 | >33.3 | —e | |
49 | (+) | 4S,11S | 16S,18R,24R,26S | 69.3 ± 8.0 | >33.3 | —e | |
50 | (−) | 4R,11R | 16R,18S,24S,26R | 69.3 ± 3.3 | 33.3 | 2.1 | |
Ribavirinf | — | 8196.7 ± 45.6 | 1577.5 ± 84.4 | 5.2 | |||
Pleconarilf | — | 40.5 ± 2.6 | 2.9 ± 0.5g | 14045.1 |
For the SSAR study, we applied an effective total synthesis route that maximizes isomer formations to construct a library of stereochemically diverse isomers of spirooliganin. The 17-step total synthesis started from readily available materials and included five main transformations: (1) an intramolecular ring-closure reaction to furnish 2-cycloheptenone and a photo-induced Diels–Alder cycloaddition to assemble the trans-fused 6–7 ring system; (2) a three-component regioselective hetero-Diels–Alder cycloaddition reaction to build the 6–6–6–7 tetracyclic core; (3) aromatization of the α,β-unsaturated six-membered ketone ring; (4) a series of substitution reactions and aromatic Claisen rearrangements to introduce the side chain; and (5) tandem oxidative dearomatization/cyclization to produce the 5–6 oxaspiro moiety. X-ray crystallography studies confirmed the structures of the intermediates and one final diastereoisomer product. The photochemical Diels–Alder reaction was shown to be an unusual trans-selective cycloadditon, which broke the common Diels–Alder reaction rule and a 100% trans-fused product was obtained. This has been completely evidenced, by density functional theory calculations, to be due to the unreactivity of the cis-cycloheptenone in cycloadditon.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 2009862, 2009865, 2009868, 2009871 and 2009873. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc01277k |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2021 |