Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N‑Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation Yaru Gao,†,‡ Dehai Liu,‡ Zhenqian Fu,*,†,‡ and Wei Huang*,†,‡ †
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Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China ‡ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China S Supporting Information *
ABSTRACT: A novel strategy for the construction of 2,2diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral Nheterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.
O
Scheme 1. Examples of NHC-Catalyzed [3C + 3C] Annulation Reactions
ver the past decade, N-heterocyclic carbenes (NHCs) have been proven to be privileged organocatalysts and enabled the construction of a series of structurally diverse carbo- and heterocycles.1 In contrast to the well-studied NHCcatalyzed [3 + 3] annulation for the synthesis of six-membered heterocycles,2−4 the NHC-catalyzed [3C + 3C] annulation for the synthesis of six-membered carbocycles is far less studied.5 In 2014, the Chi group5a reported a pioneering work on an NHC-catalyzed oxidative formal [3C + 3C] cycloaddtion of βmethyl enals (or esters)5b with enones for the construction of benzene units (Scheme 1a). Subsequently, the group of Biju5c described an NHC-catalyzed formal [3C + 3C] annulation of α-arylidene pyrazolinones with enals for enantioselective synthesis of pyrazolone-fused spirocyclohexadienones (Scheme 1b). Despite this progress, further development of a novel NHC-catalyzed [3C + 3C] annulation reaction access to useful molecules is still highly desirable. Moreover, due to chiral spirocyclic scaffolds that are widespread in a large number of natural products,6 bioactive compounds,7 and organic optoelectronic materials,8 the synthesis of this class of privileged structures has received considerable attention.9 Especially, an NHC-catalyzed construction of these chiral spirocyclic compounds was identified as one of the most attractive strategies.10 As part of our ongoing interest in the NHC catalysis,11 we herein present a novel NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates 1 with 2ethylidene 1,3-indandione 212 for the synthesis of 2,2-diacyl spirocyclohexanones (Scheme 1c), which are often presented as core structural scaffolds in numerous polycyclic polyprenylated acylphloroglucinol natural products.13 Notably, compared to well-studied β-methyl enones as 4C synthons,14 2ethylidene 1,3-indandiones 2 are found as 3C synthons in this NHC-catalyzed reaction. © XXXX American Chemical Society
Easily prepared and purified potassium (E)-2-oxo-4-phenylbut-3-enoate (1a), which has been successfully applied in NHC-catalyzed reactions as an effective and versatile surrogate for α,β-unsaturated aldehyde,11b was initially selected in the Received: December 6, 2018
A
DOI: 10.1021/acs.orglett.8b03892 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 2. Substrate Scopea
reaction with 2-(1-phenylethylidene)-1H-indene-1,3(2H)dione 2a for optimizing conditions. The key results are summarized in Table 1. Delightingly, when triazolium N-Mes Table 1. Screening of Reaction Conditionsa
entry
NHC
base
LiCl (equiv)
solvent
yield (%)e
1 2 3 4 5 6 7b 8b 9b 10b 11c 12b 13b 14b 15b,d
A B A A A A A A A A A A A A
Et3N Et3N Cs2CO3 DIPEA K2CO3 NaOAc DIPEA DIPEA DIPEA DIPEA DIPEA DIPEA DIPEA DIPEA DIPEA
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0
THF THF THF THF THF THF THF DCM dioxane toluene dioxane dioxane dioxane dioxane dioxane
54 28 29 62 45 54 71 18 75 29 56 80 nd nd 36
a Reaction conditions: NHC precursor (15 mol %), 1a (1.5 equiv), 2a (0.1 mmol), base (1.5 equiv), LiCl (2.0 equiv), [O] (2.2 equiv), dioxane (1.0 mL), and 4 Å MS (10 mg), rt, 24 h. bDIPEA (2.0 equiv). c DIPEA (2.5 equiv). dCinnamaldehyde was used instead of 1a eYields (after SiO2 chromatography purification) were based on 2a. nd = not detected.
a
precursor A was employed with LiCl15,16 as an additive in the presence of Et3N in THF at room temperature, the desired [3 + 3] annulation product 3a was formed in 54% yield (entry 1). None of the [4 + 2] annulation product was found. The use of triazolium N-Ph precursor B instead of A decreased the product yield considerably. Several bases were next evaluated, and DIPEA proved to be the most effective choice (entries 3− 6). Increasing the amount of base loading to 2.0 equiv increased the yield of product 3a significantly (entry 7). Further solvent screening showed that dioxane was a better solvent (entries 7−10). Reducing the amount of additive LiCl increased the product yield considerably (entry 12). Notably, in the absence of the additive LiCl or the catalyst, none of desired product was found (entries 13 and 14). Replacement of 1a with cinnamaldehyde led to a considerable decrease in the product yield (entry 15). With optimized conditions in hand (Table 1, entry 12), we next evaluated the scope of the reaction for potassium 2-oxo-3enoates 1 by using 2a as a model substrate (Scheme 2). For salts 1 bearing both electron-rich and electron-deficient substituents on the γ-phenyl ring, the reactions proceeded smoothly to generate the corresponding products (3a−g) in good yields. γ-Styryl substituent 1h was also a suitable substrate for this transformation, delivering the product 3h
in 70% yield. Delightingly, salts 1 bearing γ-heteroaryl substituents (such as thiophen-2-yl, thiophen-3-yl, and pyridin-3-yl) reacted well to give the desired products 3i−3k in acceptable to excellent yields. Then, variation of the R2 group of 2-ethylidene 1,3-indandione 2 also showed a broad scope and led to similar results as for the salts 1. Notably, replacement of the aryl with a methyl group for R can provide the desired product 3r, albeit with a lower yield. Notably, in contrast to NHC-catalyzed α,β-unsaturated aldehyde reactions, clear TLC ensures easy separation of product for reactions involving salts 1. After successful establishment of this efficient strategy, a chiral NHC-catalyzed enantioselective [3C + 3C] annulation for the synthesis of chiral spirocyclic cyclohexanones was then investigated. In fact, it is very challenging to realize the asymmetric [3C + 3C] annulation due to the R group far away from the reaction site. Gratifyingly, the desired chiral product 3s was obtained in 90% yield with good enantioselectivity (81:19 er) when aminoindanol-derived triazolium precatalyst E was used (for details, see Supporting Information). Replacement of the methyl group with the ethyl or benzyl group did not improve enantioselectivity (Scheme 3). Absolute configuration of the asymmetric products 3 was confirmed via X-ray diffraction analysis of 3w.
Reaction conditions as in Table 1, entry 12. Yields (after SiO2 chromatography purification) were based on 2.
B
DOI: 10.1021/acs.orglett.8b03892 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 3. Substrate Scopea
the free carbene and form intermediate VII, which undergoes further oxidation finally to generate the product 3a. Subsequently, further synthetic utility of 2,2-diacyl cyclohexanones was performed as shown in Scheme 5. Ring opening Scheme 5. Synthetic Transformations
a
Reaction conditions as in Table S1, entry 10. Yields (after SiO2 chromatography purification) were based on 2.
The proposed pathway for NHC-catalyzed formal [3 + 3] annulation to assemble 2,2-diacyl spirocyclohexanones is illustrated in Scheme 4. The salt 1a was activated by LiCl
a
Reaction conditions: 3 (0.2 mmol), hydrazine hydrate (1.0 equiv), NaOAc (0.5 equiv), and DMF (2.0 mL), 100 °C, 8 h.
Scheme 4. Proposed Mechanism
of the resulting products 3 could be readily realized by treatment with hydrazine hydrate in DMF under basic conditions at 100 °C, leading to the formation of phthalazinones 4a−f in good to excellent yields. Notably, phthalazinones were found to possess a range of promising biological activities, such as antimicrobial activity, antitumor activity, and anticancer activity.17 They were used as PARP inhibitors and androgen receptor antagonists as well.18 In summary, an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandione has been demonstrated. A diverse set of 2,2-diacyl spirocyclohexanones 3 was generated in good to excellent yields. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral NHC was employed. Ring opening of the generated products 3 with hydrazine led to the formation of important bioactive phthalazinones in good to excellent yields. Other reaction models for salts 1 are currently under investigation in our laboratory.
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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.8b03892. Experimental procedures and spectral data for all new compounds (PDF)
via coordination to give more reactive intermediate I, which reacts with NHC to form homoenolate intermediate II after liberation of CO2. Subsequently, oxidation of homoenolate intermediate I results in the formation of α,β-unsaturated acyl triazolium intermediate III.1k Then, Michael addition of intermediate IV obtained from 2a via deprotonation, to α,βunsaturated acyl triazolium intermediate III, furnishes intermediate V. Intermediate V further undergoes proton transfer followed by intramolecular cyclization to regenerate
Accession Codes
CCDC 1858928 contains 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 C
DOI: 10.1021/acs.orglett.8b03892 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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[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 Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Zhenqian Fu: 0000-0002-3806-6684 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support by the National Natural Science Foundation of China (21602105), National Key R&D Program of China (2017YFA0204704), and Natural Science Foundation of Jiangsu Province (BK20171460).
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