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Synlett 2024; 35(09): 997-1000
DOI: 10.1055/a-2210-0973
DOI: 10.1055/a-2210-0973
cluster
Chemical Synthesis and Catalysis in Germany
Enantioselective (3+2)-Annulation of β-Keto Esters with Azoalkenes towards Bicyclic Dihydropyrroles via Cooperative Palladium and Brønsted Acid Catalysis
Financial support of this work through the Deutsche Forschungsgemeinschaft (DFG) (SCHN 441/14-1) is gratefully acknowledged.
Abstract
A cooperative catalytic process through palladium and Brønsted acid activation is developed for the conjugate addition of cyclic β-keto esters to azoalkenes directly followed by hemiaminal formation upon cyclization. This transformation is enabled by utilizing chiral Pd-aqua complexes as combined Brønsted acid–base catalysts. Thus, bicyclic and highly functionalized dihydropyrroles with two contiguous quaternary stereogenic centers are formed in excellent yields as single diastereomers and with exceptional enantioselectivity.
Key words
asymmetric catalysis - azoalkenes - (3+2) cycloaddition - cooperative catalysis - heterocycles - Pd-aqua complexes - Pd-enolatesSupporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/a-2210-0973.
- Supporting Information
Publication History
Received: 05 October 2023
Accepted after revision: 13 November 2023
Accepted Manuscript online:
13 November 2023
Article published online:
14 December 2023
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- 13 Annulation; General Procedure Azoalkene 5 (0.20 mmol), β-keto ester 3 (0.30 mmol, 1.50 equiv), and [(R)-(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl]-diaquo-palladium(II) bis(triflate) (1e) (0.01 mmol, 5 mol%) were added to a screw-cap vial and dissolved in dry ethyl acetate (2.0 mL) and then stirred until complete consumption of the starting material (monitored by TLC). Purification by flash column chromatography over SiO2 resulted in the corresponding products 8. 3a-(tert-Butyl) 3-Ethyl 6a-Hydroxy-2-methyl-1-(phenylamino)-4,5,6,6a-tetrahydrocyclopenta[b]pyrrole-3,3a(1H)-dicarboxylate (8a) Yield: 79.7 mg (99%); beige solid; [α]D 23 –46 (c 1.00, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.24–7.18 (m, 2 H), 6.92–6.78 (m, 3 H), 6.15 (s, 1 H), 4.22 (dq, J = 10.8, 7.1 Hz, 1 H), 4.06 (dq, J = 10.8, 7.1 Hz, 1 H), 3.79 (s, 1 H), 2.75 (s, 1 H), 2.28 (s, 3 H), 2.04–1.87 (m, 2 H), 1.80–1.61 (m, 3 H), 1.49 (s, 9 H), 1.26 (t, J = 7.1 Hz, 3 H). 13C NMR (100.6 MHz, CDCl3): δ = 165.9, 162.0, 148.9, 129.5, 120.6, 112.6, 104.8, 82.1, 64.3, 59.2, 35.6, 28.2, 23.1, 14.6, 12.4. IR (KBr): 3421, 3327, 2977, 5930, 1721, 1667, 1602, 1497, 1454, 1377, 1369, 1326, 1317, 1289, 1245, 1186, 1165, 1100, 1065, 993, 921, 886, 846, 753, 695, 666, 506 cm–1. HRMS (ESI): m/z [M + H]+ calcd for C22H31N2O5: 403.223; found: 403.226. HPLC: Chiralpak IA (n-hexane/2-propanol = 90:10, flow rate = 1.0 mL min–1, λ = 280 nm); t R = 10.05 min (major), t R = 28.73 (minor); e.r. 99:1.
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