Enantioselective Construction of Tetrahydroquinazoline Motifs via

Mar 20, 2018 - malononitriles.16b In the past five years, the cycloddition reactions .... 8-F (1l). 3. 3la. 95. >20:1. 88. 12. 6-Cl (1m). 2. 3ma. 75. ...
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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Enantioselective Construction of Tetrahydroquinazoline Motifs via Palladium-Catalyzed [4 + 2] Cycloaddition of Vinyl Benzoxazinones with Sulfamate-Derived Cyclic Imines Chang Wang, Yan Li, Yang Wu, Qijun Wang, Wangyu Shi, Chunhao Yuan, Leijie Zhou, Yumei Xiao, and Hongchao Guo* Department of Applied Chemistry, China Agricultural University, Beijing 100193, P. R. China S Supporting Information *

ABSTRACT: A palladium-catalyzed enantioselective [4 + 2] cycloaddition reaction of vinyl benzoxazinones with sulfamatederived cyclic imines is described, affording the tetrahydroquinazolines bearing several functional rings in high yields (up to 99% yield) with good to excellent diastereoselectivities and excellent enantioselectivities (up to 96% ee). This reaction represents the first Pd-catalyzed asymmetric decarboxylative cycloaddition of vinyl benzoxazinones with imines.

Q

Scheme 1. Asymmetric Synthesis of Tetrahydroquinazolines

uinazolines are privileged scaffolds featured in many natural and synthetic products. Many achiral and chiral quinazoline derivatives show a wide range of biological activities and pharmacological properties such as antibacterial, antidepressive, anti-inflammatory, and bronchodilator activities.1−6 Owing to the importance of the quinazoline derivatives, many synthetic methods to achiral and chiral quinazolines have been reported.7 However, examples on the enantioselective construction of chiral tetrahydroquinazoline compounds are extremely limited.8,9 In 2011, Gong reported a chiral phosphoric acid catalyzed redox reaction of o-aminobenzoketones with anilines, giving a series of tetrahydroquinazolines in high yields and good enantioselectivities (Scheme 1).8a Two years later, they demonstrated the combined catalysts of a gold complex and the chiral phosphoric acid catalyzed cascade hydroamination/redox reaction of ethynylphenyl-pyrrolidines with anisidines, affording the tetrahydroquinazoline derivatives with excellent enantioselectivity.8b Very recently, Xiao reported a practical one-pot Pdcatalyzed hydroamination/Ru-catalyzed photoredox reaction for preparation of chiral tetrahydroquinazolines.9 Although the above-mentioned methods have been established for the generation of chiral tetrahydroquinazoline motifs, developing their new asymmetric synthetic method is still highly desirable. The cycloaddition reactions utilizing palladium-stabilized zwitterions have proven to be powerful tools for the construction of heterocyclic compounds.10−16 Those palladium-stabilized zwitterions are commonly generated in situ using a low valent palladium catalyst and various allyl precursors such as allylsilanes,10 vinyl cyclopropanes,11 γ-methylidene-δ-valerolactones,12 vinylaziridines (or vinyloxazolidinones),13 vinyl epoxides (or vinylethylene carbonates),14 vinyl oxetanes,15 and vinyl benzoxazinones.16 Among these allyl precursors, vinyl benzoxazinones have been widely employed as C,N-1,4 synthons to achieve [4 + n] annulation reactions since Tunge first reported palladium-catalyzed [4 + 2] decarboxylative cycloadditions of 6vinyl-1,3-oxazinones and benzylidene malononitriles in 2006.16a © XXXX American Chemical Society

In 2008, Tunge also developed an asymmetric version of this cycloaddition between vinyl benzoxazinones and benzylidene malononitriles.16b In the past five years, the cycloddition reactions involving vinyl benzoxazinones have intensively been studied by Xiao, Glorius, Jorgensen, Lu, Shi, and Deng.16 Various substrates including activated alkenes,16e sulfur ylides,16d,g ketenes formed in stiu,16l methyleneindolinones,16k NHChomonenolates,16f,i iminium-ion activated α,β-unsaturated aldehydes,16h isatins,16j and mixed anhydrides formed in stiu16m have proven to be compatible reaction partners in the asymmetric Received: March 20, 2018

A

DOI: 10.1021/acs.orglett.8b00905 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters decarboxylative [4 + n] cycloaddition for synthesis of diverse heterocyclic compounds. As part of our continued efforts on asymmetric cycloaddition,17 herein, we report a palladiumcatalyzed highly stereoselective [4 + 2] cycloaddition reaction of vinyl benzoxazinones with sulfamate-derived cyclic imines to access chiral tetrahydroquinazolines. To test the feasibility of the Pd-catalyzed [4 + 2] cycloaddition, we initially investigated the model reaction of 4-vinylbenzoxazinone 2a and sulfamate-derived cyclic imine 1a in dichloromethane at room temperature (Table 1). To our delight, under

After determining the optimized reaction conditions for the reaction, we investigated the scope of the sulfamate-derived cyclic imines 1 in the Pd-catalyzed asymmetric [4 + 2] cycloaddition for the construction of chiral tetrahydroquinazoline motifs. As summarized in Table 2, this decarboxTable 2. Scope of Sulfamate-Derived Cyclic Imines 1a

Table 1. Screening of the Reaction Conditionsa

entry

ligand

solvent

t/h

yield (%)b

drc

ee (%)d

1 2 3 4 5 6 7 8 9 10e

L1 L2 L3 L4 L2 L2 L2 L2 L2 L2

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 DCE CHCl3 THF toluene AcOEt toluene

5 1.5 1.5 1.5 2 2 2 2 2 48

87 95 77 98 91 99 85 95 87 72

5:1 16:1 >20:1 15:1 >20:1 19:1 14:1 >20:1 >20:1 >20:1

89 95 45 93 92 90 92 95 94 94

entry

R

t/h

3

yield (%)b

drc

ee (%)d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

6-Me (1b) 7-Me (1c) 7-i-Pr (1d) 6-t-Bu (1e) 7-t-Bu (1f) 8-t-Bu (1g) 6-OMe (1h) 7-OMe (1i) 8-OMe (1j) 7-F (1k) 8-F (1l) 6-Cl (1m) 7-Cl (1n) 8-Cl (1o) 6-Br (1p) 7-Br (1q)

2 2 4 3 3.5 5 2.5 2 3.5 2 3 2 2 2 5 4

3ba 3ca 3da 3ea 3fa 3ga 3ha 3ia 3ja 3ka 3la 3ma 3na 3oa 3pa 3qa

96 85 88 97 93 67 55 75 85 96 95 75 94 73 99 99

20:1 >20:1 16:1 14:1 15:1 16:1 19:1 >20:1 >20:1 >20:1 >20:1 19:1 >20:1 19:1 >20:1 >20:1

95 96 94 96 93 92 92 95 94 91 88 93 95 91 91 94

a

a Unless otherwise indicated, all reactions were carried out with 1 (0.1 mmol), 2a (0.15 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), and chiral ligand L2 (7.5 mol %) in 1 mL of solvent. bIsolated yield. cDetermined by 1H NMR analysis. dDetermined by HPLC analysis using a chiral stationary phase.

catalysis of a complex of Pd2(dba)3·CHCl3 (2.5 mol %) and axially chiral phosphoramidite ligand L1 (7.5 mol %), the [4 + 2] cycloaddition proceeded smoothly to deliver the desired cycloadduct in 87% yield with 89% ee, albeit in low diastereoselectivity (dr = 5:1) (entry 1). Then, several axially chiral phosphoramidite ligands were examined (entries 2−4). The ligand L2 was found to be the optimal ligand for the reaction in terms of diastereo- and enantioselectivity, affording the chiral tetrahydroquinazoline derivative 3aa in 95% yield with 16:1 dr and 95% ee (entry 2). With the use of chiral phosphoramidite L2 as the optimal chiral ligand, a quick screening of several solvents including dichloromethane, 1,2-didhloroethane (DCE), CHCl3, THF, toluene, and ethyl acetate (AcOEt) (entries 4−9) revealed that toluene is the optimal solvent, leading to the [4 + 2] cycloadduct 3aa in 95% yield with >20:1 dr and 95% ee (entry 8). Decreasing the catalyst loading seemed to have no remarkable influence on stereoselectivity, but sharply impaired the activity (entry 10). Finally, the optimal reaction conditions were determined as follows: with the use of Pd2(dba)3·CHCl3 (2.5 mol %) and chiral ligand L2 (7.5 mol %) as the catalyst in toluene at room temperature.

ylative [4 + 2] cycloaddition could offer the desired products in moderate to excellent yields (55%−99%) with good to excellent diastereo- and enantioselectivities (88%−96% ee). The reaction was applicable to a variety of imines 1 bearing various substituents on the different positions of the benzene ring, regardless of whether an electron-donating or withdrawing group on the benzene ring could be tolerated. Although 6-MeOsubstituted cyclic imine 1h led to a moderate 55% yield, excellent diastereo- and enantioselectivity were still achieved in the reaction (entry 7). The absolute configuration of the [4 + 2] cycloadduct was unambiguously assigned as (8R, 13aR) through X-ray crystallographic analysis of the product 3oa (CCDC 1830007). Subsequent experiments were performed to evaluate the generality of the reaction with respect to the vinyl benzoxazinone 2. As indicated in Table 3, various vinyl benzoxazinones 2 performed the palladium-catalyzed [4 + 2] cycloaddition to give tetrahydroquinazolines in good to excellent yields with good to excellent diastereo- and enantioselectivities (89−95% ee). Vinyl benzoxazinones bearing an electron-withdrawing group displayed higher activity than the substrate having an electrondonating group at the same position of the benzene ring (entry 2 vs 4, 3 vs 5). Additionally, a slight variation of the N-protecting group on vinyl benzoxazinones was feasible. For example, the substrate bearing an N-benzenesulfonyl group produced the corresponding product in excellent yield with excellent stereoselectivity (entry 7).

Unless otherwise indicated, all reactions were carried out with 1a (0.1 mmol), 2a (0.15 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), and chiral ligand L2 (7.5 mol %) in 1 mL of solvent. bIsolated yield. cDetermined by 1H NMR analysis. dDetermined by HPLC analysis using a chiral stationary phase. e1 mol % Pd and 2.5 mol % ligand was used.

B

DOI: 10.1021/acs.orglett.8b00905 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 3. Scope of Vinyl Benzoxazinones 2a

entry

R/PG

t/h

3

yield (%)b

drc

ee (%)d

1 2 3 4 5 6 7

H/Ts (2a) 6-Me/Ts (2b) 7-Me/Ts (2c) 6-F/Ts (2d) 7-F/Ts (2e) 6-OMe/Ts (2f) H/PhSO2 (2g)

2 3 3 3 2 2 2

3aa 3ab 3ac 3ad 3ae 3af 3ag

95 84 88 93 91 97 98

>20:1 >20:1 >20:1 14:1 13:1 5:1 16:1

95 92 95 95 92 89 94

In conclusion, the Pd-catalyzed asymmetric [4 + 2] cycloaddition reaction of vinyl benzoxazinones with sulfamate-derived cyclic imines has been developed for the enantioselective construction of the tetrahydroquinazolines bearing several functional rings. This is the first time imines were applied in a Pd-catalyzed [4 + 2] cycloaddition reaction involving vinyl benzoxazinones. The reaction could be performed on gram scale, and further transformations of the cycloadducts provided other useful chiral tetraquinazoline derivatives.

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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.8b00905. Experimental procedure, characterization data, HPLC analysis data, NMR spectra, and X-ray crystallographic data (PDF)

a

Unless otherwise indicated, all reactions were carried out with 1a (0.1 mmol), 2 (0.15 mmol), Pd2(dba)3·CHCl3 (2.5 mol %), and chiral ligand L2 (7.5 mol %) in 1 mL of solvent. bIsolated yield. cDetermined by 1H NMR analysis. dDetermined by HPLC analysis using a chiral stationary phase.

Accession Codes

CCDC 1830007 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

As shown in Scheme 2, the reaction on gram scale (0.55 g of 1a) was also performed to give the desired product in 93% yield Scheme 2. Demonstration of Synthetic Utility

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongchao Guo: 0000-0002-7356-4283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21372256 and 21572264) and the Program for Changjiang Scholars and Innovative Research Team Project IRT1042.

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REFERENCES

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(1.30 g of 3aa) with >20:1 dr and 95% ee. In order to further investigate the potential of the asymmetric [4 + 2] cycloaddition reaction, subsequent transformations of the cycloadducts were investigated (Scheme 2). The chiral tetrahydroquinazoline product 3aa was subjected to hydrogenation by the catalysis of Pd/C, giving the product 4 in 98% yield with 94% ee. Treatment of the product 3aa with 9-BBN followed by H2O2 afforded the alcohol product 5 in 92% yield with 93% ee. Further oxidation of 5 with the use of PCC provided the aldehyde product 6 (83% yield, 93% ee). Additionally, bromination of the product 3ja by bromine in CCl4 produced the dibromo derivative 7ja in 90% yield with 1:1 dr (91%:86% ee). C

DOI: 10.1021/acs.orglett.8b00905 Org. Lett. XXXX, XXX, XXX−XXX

Letter

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DOI: 10.1021/acs.orglett.8b00905 Org. Lett. XXXX, XXX, XXX−XXX