Enantioselective Diels–Alder Reaction of 3-Nitrocoumarins Promoted by Chiral Organoammonium Salt Catalysts

Abstract

An enantioselective Diels–Alder reaction of 3-nitrocoumarins has been developed. A tryptophan-derived C ₁-symmetric organoammonium thiourea catalyst promoted the reaction of 3-nitrocoumarins with Danishefsky’s diene to give the corresponding adducts with good enantioselectivity (up to 94% ee). One of the resulting adducts was converted into a chiral carbocyclic quaternary β-amino alcohol.

Key words

nitrocoumarins - Diels–Alder reaction - asymmetric catalysis - organocatalysis - amino alcohols - Danishefsky’s diene

Publication History

Received: 06 August 2020

Accepted after revision: 02 September 2020

Article published online:
07 October 2020

• References and Notes

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• 16 For further examination of the activities of catalysts 5·HX, see the Supporting Information.
• 17 Asymmetric Diels–Alder Reaction of 1a with 2 (Table [1], Entry 8); Typical Procedure TFA (2.0 μL, 26 μmol) was added to a solution of 5e (16 mg, 33 μmol) in toluene (2 mL), and the mixture was stirred at rt for 10 min. To this solution was added 1a (25 mg, 0.13 mmol), and the mixture was cooled to –78 °C. Diene 2 (82 μL, 0.33 mmol) was added to the suspension, and the mixture was vigorously stirred at –78 °C for 24 h. The reaction was then quenched by the addition of 1 M aq HCl (5 mL), and the mixture was diluted with Et₂O. The aqueous layer was extracted with Et₂O (×2), and the organic layers were combined, dried (MgSO₄), filtered, and concentrated in vacuo. The residue was purified by column chromatography (silica gel) to give endo-3a (yield: 22 mg, 42%) and exo-3a (yield: 10 mg, 19%). (6aR,7S,10aR)-9-{[tert-Butyl(dimethyl)silyl]oxy}-7-methoxy-6a-nitro-6a,7,10,10a-tetrahydro-6H-benzo[c]chromen-6-one (endo-3a) Colorless oil; [α]_D ²⁴ +178.5 (c 0.65, CHCl₃) (76% ee). IR (film): 1782, 1666, 1564, 1369 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.33–7.27 (m, 2 H), 7.21–7.15 (m, 1 H), 7.09–7.04 (m, 1 H), 5.23 (ddd, J = 0.8, 1.6, 5.6 Hz, 1 H), 4.86 (d, J = 5.6 Hz, 1 H), 4.22 (dd, J = 7.6, 11.2 Hz, 1 H), 3.37 (s, 3 H), 2.70 (dd, J = 7.6, 18.4 Hz, 1 H), 2.05 (dddd, J = 0.8, 1.6, 11.2, 18.4 Hz, 1 H), 0.89 (s, 9 H), 0.20 (s, 3 H), 0.15 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 158.6, 152.1, 149.1, 129.3, 128.0, 125.8, 124.7, 117.5, 100.0, 91.0, 74.7, 57.1, 35.1, 34.3, 25.4 (3 C), 17.9, –4.5, –4.8. HRMS (FAB): m/z [M + H]⁺ calcd for C₂₀H₂₈NO₆Si: 406.1686; found: 406.1712. (6aR,7R,10aR)-9-{[tert-Butyl(dimethyl)silyl]oxy}-7-methoxy-6a-nitro-6a,7,10,10a-tetrahydro-6H-benzo[c]chromen-6-one (exo-3a) Colorless oil; [α]²² _D –17.9 (c 0.40, CHCl₃) (79% ee). IR (film): 1792, 1668, 1558, 1387 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.36–7.29 (m, 1 H), 7.22–7.13 (m, 2 H), 7.22–7.13 (m, 1 H), 5.02 (dt, J = 1.2, 3.2 Hz, 1 H), 4.87–4.84 (m, 1 H), 4.18 (t, J = 6.0 Hz, 1 H), 3.29 (s, 3 H), 2.49 (d, J = 6.0 Hz, 2 H), 0.91 (s, 9 H), 0.18 (s, 3 H), 0.16 (s, 3 H). ¹³C NMR (150 MHz, CDCl₃): δ = 157.6, 149.9, 149.5, 129.6, 127.9, 125.6, 121.0, 117.3, 101.2, 90.8, 78.9, 59.0, 42.4, 34.0, 25.4, 17.8, –4.6, –4.8. HRMS (FAB): m/z [M + H]⁺ calcd for C₂₀H₂₈NO₆Si: 406.1686; found: 406.1679.
• 18 3-Nitrocoumarin 1f was recovered (~70%), and no undesired byproducts were obtained.

Corey and co-workers proposed an interaction (π–π interaction or charge-transfer complex formation) between the indolyl group and acrolein derivatives in the transition state of a Diels–Alder reaction catalyzed by a tryptophan-derived chiral Lewis acid, see:
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• 21 Purchased from Aldrich (product no. 578002–5G).
• 22 CCDC 2016403 contains the supplementary crystallographic data for compound 12c. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.