A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams

Abstract

Open AccessCCS ChemistryCOMMUNICATION5 Aug 2022A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams Mao-Mao Zhang, Peng Chen, Wei Xiong, Xin-Shang Hui, Liang-Qiu Lu and Wen-Jing Xiao Mao-Mao Zhang CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 , Peng Chen CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 , Wei Xiong CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 , Xin-Shang Hui CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 , Liang-Qiu Lu *Corresponding author: E-mail Address: [email protected] CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000 and Wen-Jing Xiao CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079 State Key Laboratory of Applied Organic Chemistry, Department of Chemistry, Lanzhou University, Lanzhou 730000 https://doi.org/10.31635/ccschem.021.202101476 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Medium-sized nitrogen heterocycles, including eight-membered lactams, are important in synthetic and pharmaceutical chemistry because of their presence in a variety of natural products and drug molecules. Previous attempts at synthesis using a direct head-to-tail cyclization strategy suffered from competitive self-cyclization and oligomerization of dipolar species. Herein, we propose an alternative strategy, namely a dipolar cyclization/fragmentation strategy, to address these problems. The combination of a chiral Ir catalyst, Lewis acid, and base facilely produced chiral eight-membered lactams with high reaction efficiency and selectivity. Moreover, by combining this reaction with photoinduced ring contraction and esterification, a three-component reaction sequence was developed, further streamlining the synthesis of medium-sized chiral heterocycles. Download figure Download PowerPoint Introduction Medium-sized heterocycles are highly significant for the synthetic and pharmaceutical chemistry fields because they are abundant in a variety of natural products and drug molecules.1,2 Dipolar cycloadditions of metal-containing reactive dipoles (MCRDs, M = Rh, Au, Pd, etc.)3–7 have been established as one of the most efficient tools to construct these skeletons.8–20 In particular, recent studies have proven that Pd-catalyzed asymmetric processes involving π-allyl-Pd-type dipoles can produce a variety of medium-sized chiral heterocycles.21–37 Mechanistically, in situ- generated MCRDs (M = Pd) usually react with electrophilic dipolephiles to generate prolonged dipolar species; subsequently, intramolecular direct head-to-tail cyclizations produce the desired heterocycles (Figure 1, up). Despite impressive advances, this strategy has intrinsic limitations involving the competitive self-cyclization of MCRDs and the oligomerization of the prolonged dipolar species due to unfavorable entropy and enthalpy effects.38,39 Moreover, the more complex stereochemistry (e.g., ring conformation, related and absolute configuration) of the medium-sized rings increases the difficulty of selectively producing these compounds.2,7 To address these problems, we propose an alternative strategy (Figure 1, bottom): in situ-generated MCRDs (M = Ir)40–46 reacting with cyclic nucleophilic dipolephiles to construct normal-sized rings, which then undergo strain-release-driven fragmentations to form the medium-sized rings as the final products.47–49 Notably, both reaction steps are dynamically and thermodynamically favored, avoiding the aforementioned self-cyclization and oligomerization. Figure 1 | Strategies toward the enantioselective synthesis of medium-sized chiral rings with MCRDs. Download figure Download PowerPoint To substantiate the above proposal, we devised a catalytic asymmetric dipolar cyclization/fragmentation sequence through multiple Ir/Lewis acid/base catalysis (Figure 2).50–54 Starting from readily available aminophenyl vinyl alcohols and 2-acyl cyclobutanones, this protocol enables the asymmetric synthesis of eight-membered lactams. To the best of our knowledge, this kind of medium-sized heterocycle is a popular scaffold in many biologically active agents as exemplified in Figure 2a.55–60 Though these are impressive synthetic endeavors, the catalytic asymmetric synthesis starting from easily available feedstocks are still very rare.61–63 As depicted in Figure 2b, the detailed design plan consisted of two catalysis cycles. First, chiral Ir catalyst I coordinates with aminophenyl allyl alcohol 1a to produce π-allyl-Ir intermediate III with the help of a Lewis acid.64–66 Then, an asymmetric allylic alkylation of the enol intermediate IV, which is generated from 2-acyl cyclobutanone 2 and a Lewis acid, affords Ir-complex V. Subsequently, the release of intermediate 4 and the Ir catalyst I from V furnishes the Ir catalysis cycle (Figure 2b, left). Next, the treatment of 4 with base affords intermediate VI, which undergoes intramolecular cyclization to afford the hemiacetal anion VII (Figure 2b, right). Finally, the fragmentation of this species driven by strain release provides the desired eight-membered heterocycle 3.63,67–71 In the presence of a Lewis acid and base, π-allyl-Ir intermediate III possesses a dipolar reactivity, which promotes a reaction with the nucleophilic enol group first, followed by one with the electrophilic carbonyl groups.40–43 Figure 2 | (a) Selected examples of eight-membered lactams with biological activities. (b) Plan for the enantioselective synthesis of eight-membered lactams. Note: L*, chiral ligand; LA, Lewis acid; B, base; Ts, tosyl. Download figure Download PowerPoint Results and Discussion Condition optimization Initially, we examined the model reaction between aminophenyl allyl alcohol 1a and 2-ester cyclobutanone 2a using Carreira's ligand (R)-L, which is a privileged ligand in Ir-catalyzed asymmetric allylic alkylations.72 First, a series of Lewis acids were evaluated (see Supporting Information Table S2) with Cs2CO3 as the base. Zn(CF3SO3)2 demonstrated the best performance, affording the desired product in 64% yield, >99% ee, and 1.5:1 dr (Table 1, entry 1). The replacement of Zn(CF3SO3)2 with Brønsted acid CF3CO2H failed to produce product 3aa, which only resulted in the decomposition of the starting materials (Table 1, entry 2). Control experiments revealed that no conversion was observed in the absence of acid additive (Table 1, entry 3), and only a low yield was obtained without a base catalyst (Table 1, entry 4). Subsequently, the base effect was investigated to further improve the reaction efficiency and selectivity (see Supporting Information Table S3). The use of the inorganic base K2CO3 and organic bases N-methyl morpholine (NMM) and N,N-diisopropylethylamine (DIPEA) afforded slightly higher yields with similar enantio- and diastereoselectivities (Table 1, entries 5–7). Remarkably, the organic bases 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and 2-(tert-butyl)-1,1,3,3-tetramethylguanidine (TBTMG) significantly improved both the reaction efficiency and diastereoselectivity, affording the chiral lactam 3aa in >90% yield, with >99% ee and 9∶1 dr (Table 1, entries 8 and 9). Though substoichiometric base was enough to function as the catalyst in principle, a slightly reduced yield and dr value were observed when using 0.5 equiv of DBU (Table 1, entry 10). A possible reason is that part of the organic base would coordinate with Zn(OTf)2. Table 1 | Optimization of Reaction Conditionsa Entry Acid Base Yield (%) drb ee (%)c 1 Zn(CF3SO3)2 Cs2CO3 64 1.5∶1 >99 2d CF3CO2H Cs2CO3 0 ND ND 3 / Cs2CO3 0 ND ND 4 Zn(CF3SO3)2 / 9 ND ND 5 Zn(CF3SO3)2 K2CO3 74 1.5∶1 >99 6 Zn(CF3SO3)2 NMM 75 1.2∶1 >99 7 Zn(CF3SO3)2 DIPEA 80 1.6∶1 >99 8 Zn(CF3SO3)2 DBU 93 (87)e 9∶1 >99 9 Zn(CF3SO3)2 TBTMG 91 9∶1 >99 10f Zn(CF3SO3)2 DBU 86 6∶1 >99 Note: cod, 1,5-cyclooctadiene; rt, room temperature; DCE, 1,2-dichloroethane; ND, not determined. aUnless otherwise specified, reaction conditions were: 1a (0.10 mmol), 2a (0.20 mmol), [Ir(cod)Cl]2 (3 mol %), Carreira's ligand (R)-L (12 mol %), acid additive (0.2 equiv), DCE, rt, and 18 h; then, base (1.0 equiv) in toluene, rt, and 1 h. bDetermined by 1H NMR analysis of the reaction mixture using dodecane as an internal standard. cDetermined by chiral HPLC analysis. dUsing 0.5 equiv of Brønsted acids. eIsolated yield in parentheses. fUsing 0.5 equiv of DBU. Substrate scope With these optimal conditions in hand, we examined the generality of this sequential reaction. As summarized in Table 2, a variety of aminophenyl vinyl alcohols were effective in this transformation, affording the corresponding chiral eight-membered lactams in 71–91% yields, 97–>99% ee, and 7∶1–17∶1 dr (Table 2, 3ba– 3ja). The electronic characteristics of the substituents on the 4- or 5-positon of the benzene ring did not affect the enantioselectivities of the products, although they do have varied yields and dr values. Next, we probed the scope of the 2-acyl cyclobutanone components. This sequential reaction exhibited a good generality for the ester group (Table 2). A variety of 2-ester cyclobutanones bearing primary, secondary, and tertiary alkoxyl groups can successfully participate in this reaction, producing the desired products in 51–92% yields, >99% ee, and 4∶1–11∶1 dr (Table 2, 3ab– 3ap). Notably, a wide range of functional groups, such as CF3, ester, amide, halide, alkene, and alkyne, were compatible with the multiple catalysis system. In addition to esters, thioester and benzoyl groups were accessible with this transformation, delivering the corresponding medium-sized lactams successfully (Figure 3a, 3aq: 76% yield, >99% ee, and 3:1 dr; Figure 3b, 3ar: 54% yield, 98% ee, and >19:1 dr). In addition, the reaction performed on a 20-times scale produced the desired product with nearly the same result (Table 2, 3ab). The ring conformation and absolute configurations of products were established by single-crystal X-ray diffraction analysis. Lactam 3aa that was derived from 2-ester cyclobutanone 2a and lactam 3ar derived from 2-benzoyl cyclobutanone 2r possessed cis- and trans-configurations, respectively (Figure 3c). It is worth noting that treatment of 3aa with Cs2CO3 or DBU at 50 °C did not result in the reversal of the related configuration, and the decomposition of 3aa was observed. The cyclization reaction of 2-benzoyl cyclobutanone 2r using organic base DBU and TBTMG also proceeded smoothly at 40 °C, albeit with slightly low yields (see the effects of temperature and base in Supporting Information Table S4). Table 2 | Substrate Scope for the Two-Component Reactiona Note: NHP, N-hydroxyphthalimide. aAll reactions were performed on a 0.1 mmol scale under the conditions referred to in Table 1, entry 8; isolated yields, ee, and dr values were determined by chiral HPLC analysis of the purified product and 1H NMR analysis of the reaction mixture. bUsing TBTMG as the base. Figure 3 | (a and b) The catalytic dipolar cyclization/fragmentation reaction of 2-thioester cyclobutanone 2p and 2-benzoyl cyclobutanone 2r. (c) The determination of the absolute configurations of representative products. Download figure Download PowerPoint Here, given our previous experiences merging the photo-Wolff rearrangement73,74 with the Pd-catalyzed asymmetric dipolar cycloadditions,35–37,75,76 we further implemented a three-component reaction sequence by introducing the photoinduced ring contraction and esterification (see Supporting Information Table S1 and Figure S1).77,78 Starting from diazoketone 5, alcohols or thiols 6, and aminophenyl vinyl alcohols 1, the sequential reaction proceeded smoothly to afford the corresponding lactams in high yields and selectivities (Table 3, 65–91% yields, up to >99% ee, and >19∶1 dr). Tolerance of the variation of electrical properties and functional groups was observed under the sequential UV light irradiation and multiple catalysis. This procedure significantly improved the synthetic efficiency of complex molecules. Table 3 | Selected Results for the Three-Component Reactiona aAll reactions were performed on a 0.1 mmol scale under the conditions referred to in Table 1, entry 9 with in situ-generated 2-oxocyclobutane-1-carboxylates (see details in Supporting Information); isolated yields, ee, and dr values were determined by chiral HPLC analysis of the purified product and 1H NMR analysis of the reaction mixture. bUsing DBU as the base. Demonstrations of synthetic utility Next, we performed a few synthetic transformations of chiral lactam 3ab to demonstrate the utility of this methodology. The treatment of 3ab with reducing reagents SmI2 or LiAlH4 facilely removed the protecting group Ts (Figure 4a, 7) or converted the ester group to the hydroxyl group (Figure 4b, 8). Moreover, a Ru-catalyzed cross metathesis and a Pd-catalyzed Heck reaction were performed to smoothly provide the chiral lactam products bearing an ethyl acrylate (Figure 4c, 9) or a styrene group (Figure 4d, 10). These two transformations significantly compensated for the shortfall of this methodology, which requires the use of a terminal allyl alcohol. Figure 4 | (a–d) Synthetic transformations of product 3ab. Download figure Download PowerPoint Conclusion We have developed an asymmetric dipolar cyclization/fragmentation sequence via multiple Ir/Zn/base catalysis. Starting from readily available aminophenyl vinyl alcohols and 2-acyl cyclobutanones, this methodology enables the enantio- and diastereoselective synthesis of chiral eight-membered lactams with wide substrate scope and good functional-group tolerance. Moreover, the combination of photoinduced ring contraction and esterification with this reaction sequence further streamlines the asymmetric synthesis of chiral eight-membered lactams from easily available feedstocks. Chiefly, this work demonstrates a catalytic asymmetric dipolar cyclization/fragmentation strategy that provides a new avenue for the synthesis of medium-sized chiral heterocycles. Supporting Information Supporting Information is available and includes the details for condition optimization, experimental procedures, characterization data, X-ray crystallographic structure of products, and 1H NMR, 13C NMR, high-resolution mass spectrometry (HRMS), and high-performance liquid chromatography (HPLC) for products. Conflict of Interest There is no conflict of interest to report. Funding Information This research was made possible as a result of a generous grant from the National Natural Science Foundation of China (nos. 21822103, 21772052, 21772053, 21820102003, and 91956201), the Program of Introducing Talents of Discipline to Universities of China (111 Program, B17019), and the Natural Science Foundation of Hubei Province (no. 2017AHB047) Acknowledgments The authors are grateful to the International Joint Research Center for Intelligent Biosensing Technology and Health for support and appreciate Prof. Jia-Rong Chen and Prof. Ying Cheng for stimulating discussions of this work. References 1. Cossy J.; Arseniyadis S.; Meyer C.Metathesis in Natural Product Synthesis: Strategies, Substrates and Catalysts; Willey-VCH: Weinheim, 2010. Google Scholar 2. Reyes R. L.; Iwai T.; Sawamura M.Construction of Medium-Sized Rings by Gold Catalysis.Chem. Rev.2020, 121, 8926–8947. Google Scholar 3. 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