Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Asymmetric Total Syntheses and Biological Studies of Tuberostemoamide and Sessilifoliamide A Yongsheng Hou,†,§ Tao Shi,†,§ Yuhang Yang,† Xiaohong Fan,† Jinhong Chen,† Fei Cao,‡ and Zhen Wang*,†,‡ †
School of Pharmacy, Lanzhou University, West Donggang Road No. 199, Lanzhou 730000, Gansu, China State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, China
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‡
S Supporting Information *
ABSTRACT: The first asymmetric total syntheses of tuberostemoamide, sessilifoliamide A, and their epimers have been accomplished via the common intermediate ethylstemoamide. The stereochemistry control relationship at C8/C9/C10 of ethylstemoamide is clearly revealed for the first time, and a subtle difference of substituent at the C10 position between stemoamide and ethylstemoamide (Me vs Et) drastically changes the stereoselectivity, which is significantly valuable for syntheses of ethylstemoamide structurally related Stemona alkaloids. Biological studies reveal that the activities of each epimer show a significant difference. 11,13-Bis-epi-sessilifoliamide A is expected to be a selective and reversible BChE inhibitor for the treatment of neurodegenerative diseases, and sessilifoliamide A may be a part of the antiinflammatory substances in Stemonaceae plants.
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disconnections and proposed the total synthesis strategy, as depicted in Scheme 1. We considered the initial disassembly of the CC bond in 1 through a dehydrogenation reaction to render 2. The oxaspirolactone moiety of 2 was envisioned to arise from a lactone-selective nucleophilic addition of an anion species to lactone 4 and subsequent lactonization. 4 could be
lants have been utilized as medicines in several countries for centuries,1−3 and total syntheses of plant natural products continue to be a powerful tool to provide sufficient materials for further understanding the functions of these compounds and drive new drug discovery.4−6 Stemonaceae plants have a long history of application in clinical use to treat bronchitis, pertussis, and tuberculosis, and as antihelmintics in China and Japan.7 Approximately 170 diverse alkaloids have been isolated to date.8 Nevertheless, only a small portion of Stemona alkaloids have been synthesized, among which only a few have been asymmetrically synthesized.7−9 Therefore, a single Stemona alkaloid is seldom developed for further clinical use, mainly due to the lack of comprehensive biological profiling, which is largely hampered by isolation and synthetic accessibility. Herein, two Stemona alkaloids, tuberostemoamide (1) and sessilifoliamide A (2), which were isolated by Lin10 and Takeya,11 respectively, were reported for their first asymmetric total syntheses and biological studies. From the structural perspective, these two naturally occurring molecules possess a similar 5/7/5 tricycle skeleton and continuous stereocenters as stemoamide (5) except the substituents of C10 are ethyl, so it is reasonable to propose that direct modification at the ester part of the durable functional group compound 4 (named as ethylstemoamide here) could be an ideal synthetic strategy to the abovementioned tetracyclic natural products. Inspired by the elegant work of Dai12 and Chida−Sato,13 as well as consideration of the structural relationship of stemoamide-group alkaloids (Figures S4−S5), we chose corresponding retrosynthetic © XXXX American Chemical Society
Scheme 1. Retrosynthetic Analysis of Tuberostemoamide (1) and Sessilifoliamide A (2)
Received: March 25, 2019
A
DOI: 10.1021/acs.orglett.9b01042 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 2. Synthetic Route to the Common Intermediate Ethylstemoamide 4a
a
Non-hydrogen atoms are shown as 30% ellipsoids.
further traced back to α,β-unsaturated ester 6 by SmI2-induced radical cyclization. 6 could be disassembled via deprotection, Wittig reaction, and oxidation of acetal 7. 7 would be divided in half via N-alkylation reaction to deliver the known Lpyroglutamic derivative 8 and iodide 9. If these selectivities and stereochemistry controls are successfully achieved, these alkaloids would be supplied from relatively simple 4 in a stepeconomical fashion. With the above analysis in mind, synthesis of 4 must be an urgent priority for the total syntheses of 1 and 2 (Scheme 2). Starting with L-pyroglutamic derivative 8 as the chiral pool, in the hope of accessing serviceable stereocontrol, N-alkylation reaction with iodide 9 in the presence of KHMDS and subsequent deprotection gave alcohol 7. Sequentially, alcohol 7 was subjected to the Swern oxidation conditions to deliver the corresponding aldehyde, to which was directly added reagent A after quenching with water, and then the (E)-isomer of α,βunsaturated ester 10 was obtained in 72% yield over two steps. After being subjected to the acidic conditions, 10 was deprotected to render aldehyde 6 in 90% yield. With 6 in hand, we then started our adventure with the initial aim to reach the 5/7/5 tricycle skeleton. On the basis of our previous work on the construction of 5/6 bicycle-containing γbutyrolactone, the tricycle skeleton in 4 was constructed using the powerful SmI2-induced conjugate addition of a ketyl radical to the α,β-unsaturated ester in aldehyde 6 (see Supporting Information for the proposed mechanism).14−16 By careful analysis of the surprising twin crystal and NMR data, the structure of product was assumed to be γ-butyrolactone 11 with unexpected stereochemistry at C8, C9, and C10. In order to obtain the expected stereochemistry in the key step, numerous efforts were devoted toward including using the (Z)isomer of 6, but no expected product was detected.
Then a roundabout strategy was adopted to install the desired stereochemistry. We first embarked on the stereochemistry construction of C8 under control of C9a. 11 was treated with NBS in the presence of LiHMDS to introduce the bromide, which upon ensuing treatment with DBU in 80 °C gave γ-butenolide 12 (85% yield) with desired stereochemistry at C8 under thermodynamic control.17 When 12 was subjected to the known NaBH4/NiCl2·6H2O reduction conditions, which was usually used in the study of stemoamide (5),18−20 ethylstemoamide 4 and 9,10-bis-epi-ethylstemoamide 11a with a ratio of 1.5:1 were obtained, and this stereoselectivity was unexpectedly different from the major selectivity for the thermodynamically most stable stemoamide (5).17 The stereoselective difference of reduction reaction between stemoamide (5) and ethylstemoamide (4), which may be caused by the subtle change of alkyl substituent at the C10 position, sparked our great interest to explore the thermodynamic and kinetic issues in CC bond reduction. We subsequently examined two new reduction conditions (Table S2). When the Mg/MeOH system was used as the reducing agent, the ratio of 4 and 11a was increased with the decreasing temperature. The reaction could be well controlled at −40 °C, and the ratio could be as high as 10:1. In combination with the stereoselective experimental results with the mechanism in both NaBH4/NiCl2·6H2O and Mg/MeOH reduction, which included stereoselective hydrogen addition one by one to the CC bond, we came to the following conclusions: (1) the C9 position may be under not only thermodynamic but also kinetic control; (2) under the premise that the stereochemistry of C9 position was confirmed, the C10 position may be under thermodynamic control; only in this way could a single product with the thermodynamically most stable configuration at the C10 position be obtained (see Schemes S1−S2 for detailed analysis). B
DOI: 10.1021/acs.orglett.9b01042 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 3. Total Syntheses of Sessilifoliamide A (2) and Its Epimers from Ethylstemoamide 4a
a
Non-hydrogen atoms are shown as 30% ellipsoids.
afforded in 65% yield over three steps. With compounds 2 and 19 in hand, we explored the syntheses of their epimers at the C11 spirocenter under treatment with TFA and only 11,13-bisepi-sessilifoliamide A 20 in 61% yield was easily obtained. Surprisingly, only under treatment with the relative strong acid CSA among the selected acids (Table S3), 16 with unsatisfying yield (3%) was ultimately obtained and most of the starting material was recovered (25% brsm). The stereochemistry of 19 and 20 was confirmed by X-ray crystallography analysis. The 1 H, 13C NMR data and specific rotation of 19 and 20 are even more different from natural 2 compared to synthetic 2 (Tables S4, S5, and S6). At this point, synthetic 2 was the closest to the targeted sessilifoliamide A. With sessilifoliamide A and its epimers in hand, we were in a position to synthesize tuberostemoamide (1) (Scheme 4).
When 12 was subjected to Pd-catalyzed hydrogenation conditions, the CC bond was reduced under these harsh conditions. To our surprise, a mixture of 13 and 11a both with a cis ring fusion stereorelationship at C8 and C9 was afforded, which was mainly due to the completely facial selective cishydrogenation from the less hindered α-face (see Scheme S3 for detailed analysis).21 Notably, this is different from the case where the β-face is the least hindered when C10 is substituted by methyl.17 Further treatment of the mixture with K2CO3 in MeOH at room temperature for 24 h12,21 affored only 11a. The facile epimerization at C10 of 13 to 11a is in accordance with the fact that α-Me isomer is thermodynamically more stable than the β-Me isomer in the synthesis of 9,10-bis-epistemoamide.17,21 Having forged all of the requisite stereochemical information into 4, we entered the last stage of our total syntheses work: construction of the characteristic spirocenter in 1 and 2 (Scheme 3). The lactone-selective nucleophilic addition of anion species formed by treating iodide 14 with t-BuLi to the less hindered side of lactone carbonyl group in ethylstemoamide 4, followed by deprotection of the silicon group, led to diol 15. Notably, 15 would be easily decomposed under purification with silica gel; hence, the following lactonization turned out to be nontrivial because of the instability of 15. We directly chose the neutral TEMPO oxidation to achieve the key lactonization and synthesized 2 (72% over three steps) as the only product; this is the consequence of the stereochemistry at C11 site of 2 being influenced by the C10 position. The 13C NMR, X-ray crystallographic structure, and specific rotation [synthetic 2: [α]18.3D = −145.0 (c = 2.0 in CHCl3)] of the synthetic samples were matched with reported data [natural 2: [α]27D = −128.0 (c = 0.35 in CHCl3)], but the chemical shift at C5 in the 1H NMR shifted from 3.61 (natural) to 3.90 (synthetic).11 In consideration of the fact that the stereochemistry at C8/ C9/C9a are consistent in all 5/7/5 tricycle skeleton-containing Stemona alkaloids and C11 and C13 are two of the most easily epimerizable sites in natural 2, then we embarked on the syntheses of epimers of C11 and C13 sites to further verify the structure of synthetic 2. When the S configuration replaced the R configuration of iodide 14, 13-epi-sessilifoliamide A 19 was
Scheme 4. Syntheses of Tuberostemoamide (1) and 11-epiTuberostemoamide 21a
a
Non-hydrogen atoms are shown as 30% ellipsoids.
Starting from 2 or 19, 1 was completed utilizing the mature method used in the synthesis of 12. In reverse, 1 could be reduced via Pd-catalyzed hydrogenation to give 2 as the major product (d.r. = 5.5:1). The configuration of 1 was determined by X-ray crystallographic analysis. The 1H NMR data and observed specific rotation [synthetic 1: [α]24.8D = −90.0 (c = 0.08 in MeOH)] of the synthetic sample were in agreement with those of the natural sample [natural 1: [α]D = −94.0 (c = 0.06) in MeOH)].10 Nevertheless, the 13C NMR data revealed C
DOI: 10.1021/acs.orglett.9b01042 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Accession Codes
the shift at C1 was 3.9 lower than that of natural 1. Again, using 20 as the reactant, we synthesized 11-epi-tuberostemoamide 21 in 73% yield and its structure was confirmed by X-ray crystallographic analysis. But there were more serious deviations of 13C NMR data between natural 1 and its C11 epimer 21 after unequivocal comparison (Table S7). By analogy to sessilifoliamide A, synthetic 1 is the most similar compound to the natural sample.22 On the basis of activities of Stemonaceae plants and the existing biological studies of some other Stemona alkaloids,7 we performed a biological evaluation of several compounds possessing a similar 5/7/5 tricycle-skeleton (compound 1, 2, 4, 12, 11a, 19, 20, 21) with stemoamide (5). 11,13-Bis-episessilifoliamide A 20 exhibited potential antiproliferative activity on the HepG2 cell line (IC50 = 99.02 ± 3.60 μM) and showed selective and reversible inhibitory activity on eqBChE (IC50 = 26.42 ± 3.43 μM, Table S9 and Figure S1), while the other seven compounds showed nearly no toxicity to all other tumor cell lines and normal cell line (all IC50 > 200 μM, Table S8) and showed no EeAChE inhibitory activity (all IC50 > 250 μM, Table S9). All tested compounds attenuated the production of LPS-induced pro-inflammatory mediators in RAW264.7 macrophage (Figure S2), and compound 2 inhibited the release of NO, TNF-α, and IL-6. These results suggested that the activities of each epimer show a significant difference. 11,13-Bis-epi-sessilifoliamide A 20 was expected to be a selective and reversible BChE inhibitor for treatment of neurodegenerative diseases, and sessilifoliamide A may be a part of the anti-inflammatory substances in Stemonaceae plants. In conclusion, we accomplished the first asymmetric total syntheses of tuberostemoamide (1), sessilifoliamide A (2), and their epimers, using ethylstemoamide 4 as the common intermediate. Notably, after adopting new reduction conditions, the stereochemistry control relationship at C8/C9/ C10 is clearly revealed for the first time and the kinetic and thermodynamic process of ethylstemoamide in CC bond reduction is different from the case of stemoamide. The first biological studies of the two Stemona alkaloids and four stereoisomers of them revealed that different epimers show a significant difference in biological activities. 11,13-Bis-episessilifoliamide A 20 was expected to be a selective and reversible BChE inhibitor for treatment of neurodegenerative diseases, and sessilifoliamide A may be a part of the antiinflammatory substances in Stemonaceae plants. By taking advantage of the versatility of the tricycle intermediate ethylstemoamide 4, total syntheses of related 5/7/5 tricyclecontaining alkaloids are currently underway, which, together with this work, would accelerate further biological and biosynthetic investigations of these fascinating natural products.
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CCDC 1874368−1874370, 1883782, 1883803−1883804, 1883806, and 1883814−1883815 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhen Wang: 0000-0003-4134-1779 Author Contributions §
Y.H. and T.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This manuscript is dedicated to Lanzhou University on the occasion of its 110th birthday. We are especially grateful to Prof. Wenhan Lin for providing the original 13C NMR spectrum of tuberostemoamide. We also gratefully acknowledge Prof. Chunan Fan, Prof. Xuegong She, and Prof. Quanyi Zhao for helpful discussions; Yongliang Shao and Rui Zhang for assistance with X-ray crystallographic analysis; and Hongyu Wang for assistance with specific rotation measurement. Financial support was provided by the Recruitment Program of Global Experts (1000 Talents Plan).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01042. Experimental procedures and compound characterization (PDF) D
DOI: 10.1021/acs.orglett.9b01042 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.9b01042 Org. Lett. XXXX, XXX, XXX−XXX
