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Au-Catalyzed 1,3-Acyloxy Migration/Cyclization Cascade: A Direct Strategy toward the Synthesis of Functionalized Abietane-Type Diterpenes

2020; Chinese Chemical Society; Volume: 3; Issue: 11 Linguagem: Inglês

10.31635/ccschem.020.202000582

2096-5745

Tian-Lu Zheng, Si-Zhan Liu, Chen-Yu Huo, Jing Li, Bowen Wang, Da‐Ping Jin, Fu Cheng, Xiaohong Chen, Xiaoming Zhang, Xue-Tao Xu, Shao‐Hua Wang,

Cancer Treatment and Pharmacology

Open AccessCCS ChemistryCOMMUNICATION1 Nov 2021Au-Catalyzed 1,3-Acyloxy Migration/Cyclization Cascade: A Direct Strategy toward the Synthesis of Functionalized Abietane-Type Diterpenes Tian-Lu Zheng, Si-Zhan Liu, Chen-Yu Huo, Jing Li, Bo-Wen Wang, Da-Ping Jin, Fu Cheng, Xiao-Ming Chen, Xiao-Ming Zhang, Xue-Tao Xu and Shao-Hua Wang Tian-Lu Zheng School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Si-Zhan Liu School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Chen-Yu Huo School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Jing Li School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Bo-Wen Wang School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Da-Ping Jin School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Fu Cheng School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Xiao-Ming Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Xiao-Ming Zhang School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 , Xue-Tao Xu School of Biotechnology and Health Science, Wuyi University, Jiangmen 529020 and Shao-Hua Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Pharmacy & State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 https://doi.org/10.31635/ccschem.020.202000582 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail A direct strategy toward the synthesis of functionalized abietane-type diterpenes and related polycyclic molecules was developed through an Au-catalyzed 1,3-acyloxy migration/cyclization/electrophilic aromatic substitution cascade. Unlike the known polyene-type cyclization strategies for the construction of abietane-type diterpene skeletons, propargylic ester groups were used for the cyclization process, which can readily lead to the key skeleton with C2 and C3 functionalization. As a demonstration of the potential application of this tandem reaction, a collective total synthesis of (±)-2-ketoferruginol, (±)-fleuryinol B, (±)-salviol, (±)-2β-acetoxyferruginol, and (±)-2β-acetoxysugiyl methyl ether was achieved. Among these molecules, (±)-fleuryinol B and (±)-2β-acetoxyferruginol were synthesized for the first time. Download figure Download PowerPoint Introduction Polyene-type cyclization, as a typical example of both domino and biomimetic transformation, has shown high efficiency in the construction of polycyclic ring systems with complexity and diversity from simple linear molecules.1–8 For this reason, such a transformation has also been used by many research groups as the key step in the synthesis of polycyclic natural products, showing its broad utility in organic synthesis. Among these developed strategies, the key point is how to introduce an appropriate functional group to initialize the subsequent cyclization process. Taking the most prevalent 6/6/6 tricyclic skeleton of diterpenes as an example, to date, a series of functional groups, such as C–C double9–32 or triple bonds,33–41 epoxides,42–50 allylic alcohols,51–57 and carbonyl-related groups,58–64 have been used as initiators for such cyclization (Scheme 1). In particular, the use of initiators that can simultaneously introduce a functional handle at the 3- or 4-position of the skeleton during cyclization will definitively improve the overall synthesis efficiency. However, to the best of our knowledge, there is still no report of an initiating functional group that can be used in such a tandem cyclization to readily install functional groups at the 2- and 3-positions,65,66 which is also a common feature of a series of abietane-type diterpenes (Figure 1).67–72 Although this type of skeleton with C2 functionalization can be obtained from corresponding compounds with C3 functionalization, it usually suffers from low synthetic efficiency as well as low regioselectivity.73 Therefore, it is worth exploring a new polyene-type cyclization mode with good synthetic efficiency and utility. Scheme 1 | Selected tandem cyclization strategies with C3 or C4 functionalization. Download figure Download PowerPoint Propargylic esters are versatile and readily accessible synthetic precursors and have found broad utility in organic synthesis.74–79 In particular, under the catalysis of π-acidic metals, propargylic esters can be converted to various reactive intermediates through a 1,2- or 1,3-acyloxy–migration process, during which the newly generated enol ester group provides an extra reactive site for subsequent transformation. Undoubtedly, if this special chemical property of propargylic ester can be rationally combined in a tandem transformation, then it will certainly lead to the development of new synthetic methodologies with good transformation efficiency. Recently, in connection with our interests in the development of propargylic ester or allene participated tandem reactions and gold(I) catalysis,80–86 two Au-catalyzed cyclization/semipinacol cascades have been developed to provide two new strategies for the construction of cyclohexene-type skeletons containing a quaternary carbon center.87,88 Based on these results, we envisaged that it was highly likely that a propargylic ester can be used as the initiator to trigger a polyene-type cyclization that involved a 1,3-acyloxy migration/cyclization/electrophilic aromatic substitution cascade, providing a new direct synthetic strategy for abietane-type diterpenes and related natural products (Scheme 2). In particular, in the presence of a corresponding enol ester group, selective functionalization of the abietane-type diterpene skeleton can be realized. In this work, we present the development of the aforementioned tandem transformation and its application in the collective total syntheses for (±)-2-ketoferruginol, (±)-fleuryinol B, (±)-salviol, (±)-2β-acetoxyferruginol, and (±)-2β-acetoxysugiyl methyl ether, among which (±)-fleuryinol B and (±)-2β-acetoxyferruginol were synthesized for the first time. Scheme 2 | Our design of the Au-catalyzed polyene-type tandem cyclization reaction. Download figure Download PowerPoint Results and Discussion According to our original assumption, compound 1a was selected as the model substrate for investigating optimal reaction conditions (see Supporting Information). Fortunately, the use of previously optimized conditions for an acyloxy shift/cyclization/semipinacol rearrangement cascade can afford the expected product 2a as a single isomer in 70% yield (Table 1, entry 1).87 As the counterion effect is often observed with Au-catalyzed transformations, some other silver salts were screened, but did not lead to better results (Table 1, entries 2–8). Thus, using the combination of Au(PPh3)Cl and AgNTf2 as the catalyst, the tandem transformation was then tested in different solvents, among which two higher yields of 80% and 78% were obtained in toluene and CH2Cl2, respectively. Following the above results, we further improved the reaction yield to 90% by lowering the reaction temperature to −40 °C in toluene (Table 1, entry 17). Moreover, only 1a was recovered in the absence of Au(PPh3)Cl (Table 1, entry 18), and the use of Au(PPh3)NTf2 as a catalyst gave 2a in 88% yield (Table 1, entry 19), which confirmed that the reaction was an Au(I)-catalyzed transformation. Therefore, subsequent investigation of this reaction was performed in toluene at −40 °C using Au(PPh3)Cl and AgNTf2 as the catalyst. Figure 1 | Selected abietane-type diterpenes with C2 or C3 functionalization. Download figure Download PowerPoint Table 1 | Reaction Condition Optimizationa Entry Silver salt Solvent time/hour Temperature (°C) Yield (%)b 1 AgNTf2 (CH2Cl)2 8 −30 70 2 AgSbF6 (CH2Cl)2 8 −30 58 3 AgOTf (CH2Cl)2 8 −30 60 4 Ag2CO3 (CH2Cl)2 15 −30 15 5 AgOAc (CH2Cl)2 15 −30 25 6 AgNO3 (CH2Cl)2 15 −30 30 7 AgClO4 (CH2Cl)2 8 −30 65 8 Ag3PO4 (CH2Cl)2 8 −30 30 9 AgNTf2 Toluene 8 −30 80 10 AgNTf2 CH2Cl2 8 −30 78 11 AgNTf2 CH3CN 8 −30 Trace 12 AgNTf2 MeOH 8 −30 32 13 AgNTf2 Dioxane 8 −30 52 14 AgNTf2 CHCl3 8 −30 57 15 AgNTf2 Tetrahydrofuran (THF) 8 −30 Trace 16 AgNTf2 CH2Cl2 8 −40 80 17 AgNTf2 Toluene 8 −40 90 18c AgNTf2 Toluene 8 −40 Ndd 19 Au(PPh3)NTf2 Toluene 8 −40 88 aUnless specified, all reactions were carried out with 1a (0.1 mmol, 1 equiv), Au(PPh3)Cl (10 mol %), Ag salt (10 mol %), and solvent (2 mL) in a reaction tube. bIsolated yield of product 2a. cAu(PPh3)Cl was not added. dNot detected. Based on the above optimized reaction conditions (Table 1, entry 17), the substrate scope for this tandem reaction was then investigated, and the results are shown in Table 2. Among the tested substrates, most were converted to the desired products in good to excellent yields as a single diastereoisomer, whose relative configuration was confirmed by X-ray crystal analysis of compound 2a.a In the case of the R1 group, apart from the acetyl group, the use of three other acyl groups, that is, pivloyl, benzoyl, and cyclohexanecarbonyl, all afforded the expected products in excellent yields. Next, with R1 as the acetyl group, a variety of substrates with different substituents on the phenyl ring were tested under optimized conditions. When R2 was a single substituent, the steric hindrance effect was clearly observed. In particular, with R2 as a relatively bulky phenyl group, its presence at the ortho-position led to product 2h in much lower yield than that of 2i and 2j. It should be noted that substrate 1l with an isopropyl group on the phenyl ring was also applicable to this reaction affording product 2l with a typical abietane type of diterpene skeleton in 86% yield, which paved the way for the development of a new synthetic strategy for related bioactive natural products and their derivatives. Furthermore, the reaction was applicable for substrates with two or three substituents on the phenyl ring, and all of them could be converted to corresponding products 2n– 2s in good to excellent yields. In particular, the presence of a bromo atom in product 2q provided an additional reaction site for subsequent derivatization. Based on the above results, substrate 1t with a thiophene ring was also applied to this reaction and transformed to desired product 2t in 55% yield. In addition, the potential practicality of this reaction was also demonstrated by a gram-scale reaction of substrate 1a, which gave product 2a in 83% yield. Table 2 | Substrate Scope Investigation of the Tandem Reactiona aUnless specified, all reactions were carried out with 1 (0.1 mmol, 1 equiv), Au(PPh3)Cl (10 mol %), AgNTf2 (10 mol %), and toluene (2 mL) in a reaction tube at −40 °C for 8 h. bOne gram of 1a was used. To present the potential utility of this tandem reaction in the synthesis of the corresponding natural products, five typical abietane-type diterpenes, (±)-2-ketoferruginol, (±)-fleuryinol B, (±)-salviol, (±)-2β-acetoxyferruginol, and (±)-2β-acetoxysugiyl methyl ether, were selected as target molecules using compound 2r as the starting material (see Supporting Information).89,90 In the presence of K2CO3, hydrolyzation of 2r gave the key intermediate 3 in 95% yield. Demethylation of 3 with BBr3 gave (±)-2-ketoferruginol in 85% yield, which was converted to (±)-fleuryinol B and (±)-salviol with NaBH4 and CeCl3·7H2O in 62% and 20% yields, respectively. Meanwhile, reduction of intermediate 3 with NaBH4 also afforded compound 4 as the sole product. Next, protection of the hydroxyl group of 4 produced compound 5, which was transformed to (±)-2β-acetoxyferruginol by BBr3 at 79% yield over two steps. Moreover, diketone intermediate 6 was obtained through the oxidation of 3 by CrO3. Subsequently, a regioselective reduction of the carbonyl group at the 2-position with relatively bulky l-selectride followed by acyl protection gave the fifth natural product (±)-2β-acetoxysugiyl methyl ether in 52% yield over two steps. The synthetic efficiencies for (±)-2-ketoferruginol, (±)-salvioland, and (±)-2β-acetoxysugiyl methyl ether were improved from (12 steps, 1.5% overall yield), (13 steps, 0.05% overall yield), and (14 steps, 1.2% overall yield) to (7 steps, 21% overall yield), (8 steps, 4% overall yield), and (9 steps, 9% overall yield), respectively (Scheme 3).89,90 Scheme 3 | Synthetic utility of the tandem reaction. Download figure Download PowerPoint Conclusions Using propargylic ester as the initiating functional group, we successfully realized an Au(I)-catalyzed 1,3-acyloxy shift/cyclization/electrophilic aromatic substitution cascade, affording a direct synthetic strategy for the construction of a functionalized octahydrophenanthrene skeleton. This method not only further enriches the content of polyene-type cyclization patterns, but also provides an efficient synthetic strategy for the synthesis of functionalized abietane-type diterpenes as well as related polycyclic molecules, which was supported by the collective total syntheses of (±)-2-ketoferruginol, (±)-fleuryinol B, (±)-salviol, (±)-2β-acetoxyferruginol, and (±)-2β-acetoxysugiyl methyl ether. Currently, application of this tandem reaction for the synthesis of more complicated polycyclic natural products as well as their derivatives for corresponding functional studies is ongoing in the same group. Footnotes a CCDC number 2033151. Supporting Information Supporting Information is available and includes experimental procedures, characterization data, 1HNMR and 13C NMR spectra for new compounds, and CIF file for 2a. Conflict of Interest There is no conflict of interest to report. Funding Information This work is supported by the National Natural Science Foundation of China (nos. 21472077 and 21772071) and Department of Education of Guangdong Province (nos. 2017KTSCX185, 2017KSYS010, 2016KCXTD005, and 2019KZDXM035). Acknowledgments The authors acknowledge Professor Daniel J. Watts for his helpful discussions regarding the structures of 2-ketoferruginol and 2β-acetoxysugiyl methyl ether, and Professor Shuji Usui for sharing his research results. References 1. Yoder R. A.; Johnston J. N.A Case Study in Biomimetic Total Synthesis: Polyolefin Carbocyclizations to Terpenes and Steroids. Chem. Rev.2005, 105, 4730–4756. Google Scholar 2. Snyder S. A.; Levinson A. M.3.05 Polyene Cyclizations. 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A1981, 44, 353–370. Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 3Issue 11Page: 2795-2802Supporting Information Copyright & Permissions© 2020 Chinese Chemical SocietyKeywordspropargylic esterditerpenetandem reactionpolyene cyclizationAu catalysisAcknowledgmentsThe authors acknowledge Professor Daniel J. Watts for his helpful discussions regarding the structures of 2-ketoferruginol and 2β-acetoxysugiyl methyl ether, and Professor Shuji Usui for sharing his research results. Downloaded 1,051 times Loading ...