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Enantioselective Synthesis of Tetrahydropyridines/Piperidines via Stepwise [4 + 2]/[2 + 2] Cyclizations Zhen Wang,*,† Huacheng Xu,† Qin Su,† Ping Hu,† Pan-Lin Shao,† Yun He,† and Yixin Lu*,‡,§ †

School of Pharmaceutical Sciences and Innovative Drug Research Centre, Chongqing University, Chongqing 401331, P.R. China Department of Chemistry, National University of Singapore, Singapore 117543, Singapore § National University of Singapore (Suzhou) Research Institute, Suzhou 215123, P.R. China ‡

S Supporting Information *

ABSTRACT: A phosphine-catalyzed novel enantioselective [4 + 2]-annulation reaction between allene ketones and 1-azadienes has been developed, and tetrahydropyridines were obtained in good yields and with excellent enantioselectivities. Subsequent exposure of tetrahydropyridines to benzyne leads to a [2 + 2]-cyclization, creating optically enriched polycyclic piperidines with a quaternary stereogenic center and a cyclobutene moiety. The reported stepwise [4 + 2]/[2 + 2]-cycloadditions represent a new approach to access enantiomerically enriched nitrogen-containing six-membered ring systems.

S

offer a convenient route to access tetrahydropyridines, which may be subjected to further [2 + 2]-cycloaddition reaction10 with benzyne intermediates11 to construct polycyclic piperidine structures (Scheme 1). Herein, we document a novel

ince Lu’s seminal discovery of phosphine-catalyzed [3 + 2]annulation of allenoates with activated alkenes in 1995,1 phosphine-mediated cycloaddition reaction has progressed remarkably and has now become a common strategy for the construction of carbocyclics and heteroatom-containing ring structures.2 Besides the most widely studied [3 + 2]-cycloaddition reactions,3 various forms of [4 + 2]-annulations have also drawn more and more attention. In 2003, Kwon disclosed a novel [4 + 2]-annulation between α-substituted allenoates and imines, making use of α-substituted allenoates as a C-4 synthon for the ring formation.4a In the subsequent studies, Fu reported an enantioselective version of the above [4 + 2]-cyclization,4c and then Kwon made further advancement by developing a [4 + 2]annulation between α-substituted allenoates and activated alkenes for the creation of six-membered carbocyclic structures.4d Recently, Huang5a and Marinetti5b independently reported another interesting phosphine-catalyzed [4 + 2]-annulation employing γ-substituted allenes as a C-4 synthon. Very recently, we devised a phosphine-catalyzed [4 + 2]-annulation process to access 3,4-dihydropyran structural motifs by making use of αsubstituted allene ketones as a C-2 synthon.6 Tetrahydropyridines and piperidines are important structural motifs that are widely present in natural products and biologically significant molecules.7 Not surprisingly, their asymmetric synthesis has been intensively pursued.8 We therefore became interested in developing a novel [4 + 2]-annulation process for effective synthesis of this important class of compounds. We envisioned that the phosphine-catalyzed [4 + 2]-annulation employing 1-azadiene9 and α-substituted allene ketones might © 2017 American Chemical Society

Scheme 1. Synthesis of Tetrahydropyridines/Piperidine Derivatives via Phosphine-Catalyzed [4 + 2]-Annulation

phosphine-catalyzed [4 + 2]-annulation between allene ketones and 1-azadienes for the construction of tetrahydropyridines, followed by a sequential [2 + 2]-cyclization with benzyne intermediates,11 to furnish highly enantiomerically enriched polycyclic piperidines containing a quaternary stereocenter.12 We first evaluated efficiency of [4 + 2]-annulation between allene ketone 3a and 1-azadiene 2a catalyzed by amino acidderived bifunctional phosphines, 13 and the results are Received: April 24, 2017 Published: May 31, 2017 3111

DOI: 10.1021/acs.orglett.7b01221 Org. Lett. 2017, 19, 3111−3114

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Organic Letters

ee. The absolute configuration of 4a was determined to be (S) on the basis of X-ray crystallographic analysis.15 With the optimized reaction conditions in hand, we then established the scope of the above [4 + 2]-annulation reaction (Table 2). The variation of the α-substituents of the allene

summarized in Table 1. In the presence of L-Val-derived phosphine sulfonamide 1a, the [4 + 2]-cycloaddition took Table 1. Optimization of [4 + 2]-Cycloaddition of 1-Azadiene 2a and Allene Ketone 3aa

Table 2. Substrate Scope of [4 + 2]-Cycloaddition of Allene Ketones 3 and 1-Azadienes 2a

entry

catalyst

solvent

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1a 1b 1c 1d 1e 1f 1g 1h 1c 1c 1c 1c 1c 1c

toluene toluene toluene toluene toluene toluene toluene toluene xylene CHCl3 CH2Cl2 THF ether CH3CN

52 56 62 57 52 45 42 38 56 62 61 37 42 trace

92 93 94 79 91 78 86 (−) 95d 93 98 99 97 91 82

entry

R1

R2

4

yield (%)b

ee (%)c

1 2 3 4 5 6 7d 8e 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28f

C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 2-MeO−C6H4 3-MeO−C6H4 4-MeO−C6H4 2-Cl−C6H4 3-Cl−C6H4 4-Cl−C6H4 3-F−C6H4 4-F−C6H4 3-Me−C6H4 4-Me−C6H4 4-CF3−C6H4 4-CO2Me−C6H4 2,6-Cl2−C6H3 3,4-Cl2−C6H3 1-naphthyl 2-naphthyl 2-thienyl cyclohexyl

C6H5 2-Cl−C6H4 3-Cl−C6H4 4-Cl−C6H4 4-F−C6H4 4-Br−C6H4 4-Me−C6H4 4-MeO−C6H4 3,5-Cl2−C6H3 2-thienyl 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4 4-Cl−C6H4

4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q 4r 4s 4t 4u 4v 4w 4x 4y 4z 4ba 4bb 4bc

70 73 73 70 69 65 70 46 65 53 64 51 92 32 49 66 36 73 74 85 43 50 61 32 40 81 86 61

>99 93 99 99 >99 98 >99 >99 97 >99 98 98 98 >99 99 >99 >99 98 >99 98 99 >99 97 99 96 99 98 98

a

Reactions were carried out with the catalyst (0.01 mmol), 1-azadiene 2a (0.10 mmol), allene ketone 3a (0.15 mmol) in the solvent specified (1.0 mL) at room temperature for 24 h. bIsolated yield of 4a. c Determined by chiral HPLC analysis on a chiral stationary phase. d The ee value of the opposite configuration is defined as minus. a

Reactions were performed with 1c (0.02 mmol), 2 (0.20 mmol), and 3 (0.3 mmol) in CH2Cl2 (2.0 mL) at room temperature for 24 h. b Isolated yield. cDetermined by chiral HPLC analysis on a chiral stationary phase. dThe reaction time was 48 h. eThe catalyst loading was 20 mol % and the reaction time was 72 h. fThe scale of reaction was 0.1 mmol.

place smoothly to afford the desired annulation product in moderate yield and high enantioselectivity (entry 1). The hydrogen bond donating groups in the bifunctional phosphines were found to be crucial, which were consistent with our previous findings.13 Phosphine-amides were shown to be efficient catalysts, and L-Val-derived 1c with a 3,5-CF3-benzoyl group was most effective, furnishing the desired [4 + 2]-annulation product in 62% yield and 94% ee (entry 3). Phosphine 1d with a thiourea group was a less effective catalyst (entry 4). L-Thrderived 1e offered slightly inferior results (entry 5), as compared with analogous valine-based 1c. Dipeptide phosphines 1f and 1g containing an L-Thr-L-Thr motif led to the annulation products in poor yields and with decreased enantioselectivity (entries 6 and 7). Interestingly, O-TBDPS-D-Thr-L-tert-Leu-derived 1h afforded the cycloaddition product with opposite configuration; 95% ee was attainable, although the chemical yield was poor (entry 8). A quick solvent screening revealed that dichloromethane was the solvent of choice (entries 9−14). Under the optimized reaction conditions, 1c-catalyzed [4 + 2]-annulation in CH2Cl2 yielded the desired prouct in 61% yield and with 99%

ketones had little effects on the enantioselectivity of the reaction (Table 2, entry 1 versus Table 1, entry 11). Allene ketones with a range of aryl groups (R2) were shown to be suitable substrates, enantioselectivity of the reaction remained to be excellent regardless of electronic properties and substitution patterns of the aromatic groups (entries 2−9). Strong electron-donating group on the aryl seemed to lead to lower chemical yield (entry 8) and so was the presence of heterocyclic moiety in the allene ketone (entry 10). Moreover, the reaction also worked very well for 1-azadienes with different aryl/alkyl substituents; the enantioselectivities were extrememly high for all the examples examined, although the yields were poor occasionally (entries 11−28). However, the γ-substituted allenones and α-methylallenoates were found to not be suitable, unable to react with 13112

DOI: 10.1021/acs.orglett.7b01221 Org. Lett. 2017, 19, 3111−3114

Letter

Organic Letters azadiene to yield the corresponding [4 + 2] cycloaddition products.15 We next tested the feasibility of further manipulating the tetrahydropyridine products derived from the [4 + 2]-cycloaddition via a subsequent [2 + 2]-cyclization. Toward this end, tetrahydropyridine 4a was treated with benzyne, generated from 2-(trimethylsilyl)aryl triflate in the presence of KF and 18-crown6, to afford [2 + 2]-cycloaddition product 6a with virtually perfect enantioselectivity (Scheme 2).10 Taken together,

Scheme 4. Scale-up Experiments and Hydrogenation of 4b

Scheme 2. Asymmetric Synthesis of Chiral Polycyclic Piperidines 6a through [2 + 2] Cyclization between 4a and 5a

structurally unique polycyclic piperidines containing a cyclobutene moiety and a quaternary stereogenic center could be efficiently prepared in highly enantioselectively manner from 1azadienes and allenes via a stepwise [4 + 2]/[2 + 2] reaction sequence. The general applicability of [2 + 2]-cyclization between the [4 + 2]-annulation products 4 to aryne 5 was subsequently investigated. As shown in Scheme 3, the in situ generated

diastereomers could be further separated by the column (see the Supporting Information for details). In summary, we have successfully developed a novel [4 + 2]annulation reaction between allene ketones and 1-azadienes for highly enantioselective synthesis of chiral tetrahydropyridines. We have also applied the [2 + 2]-cycloaddition to arynes to further derivatize tetrahydropyridines into optically enriched polycyclic piperidines with a quaternary stereogenic center and a cyclobutene moiety. Notably, employment of 1-azadienes provides a unique opportunity for phosphine-catalyzed asymmetric synthesis of nitrogen-containing six-membered ring system. The [4 + 2]/[2 + 2]-reaction sequence reported herein represents a new synthetic approach to access chiral tetrahydropyridines and polycyclic piperidines. We are currently evaluating the biological profiles of the novel structural motifs synthesized in this report and shall disclose our findings in due course.

Scheme 3. [2 + 2]-Cycloaddition between Tetrahydropyridines and Arynes

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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.7b01221. Crystallographic data for 4a (CIF) Crystallographic data for 6b (CIF) Synthesis procedure, analytical details, and copies of NMR spectra and HPLC data of all the compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]

aryne reacted with tetrahydropyridines 4 smoothly to afford polycyclic piperidines 6 in good chemical yields and nearly perfect enantioselectivities. The absolute configurations of piperidines 6 were assigned on the basis of X-ray crystallographic analysis of 6b.14 The practical aspects of this novel [4 + 2]/[2 + 2]-stepwise synthetic sequence were next evaluated. Both tetrahydropyridine 4n and polycyclic piperidine 6b were prepared at a large scale, with enantioselectivities of the reactions well maintained (Scheme 4A,B). Furthermore, tetrahydropyridine 4b could be hydrogenated in the presence of Pd/C, furnishing piperidine 7b in a highly diastereoselective and enantio-retentative manner (Scheme 4C). The ketone group in 7b was reduced in good yield by using L-selectride, and

ORCID

Zhen Wang: 0000-0002-1165-2552 Yun He: 0000-0002-5322-7300 Yixin Lu: 0000-0002-5730-166X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. acknowledges the National Natural Science Foundation of China (Nos. 21602023, 21572027 and 21402150) for financial 3113

DOI: 10.1021/acs.orglett.7b01221 Org. Lett. 2017, 19, 3111−3114

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support; Y.L. thanks the National University of Singapore (R143-000-599-112), and the National Natural Science Foundation of China (21672158) for generous financial support.

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DOI: 10.1021/acs.orglett.7b01221 Org. Lett. 2017, 19, 3111−3114