Cooperative Chiral Guanidine/AgPF₆ Catalyzed Asymmetric Isocyanoacetate Aldol Reaction with Isatins

Dedicated to Professor Peter Vollhardt on his 69th birthday

Abstract

A cooperative chiral guanidine/Ag(I) catalyst system was developed for the asymmetric isocyanoacetate aldol reaction with isatins. The corresponding chiral spirooxindole oxazolines were obtained in good yields (up to 99%), moderate diastereoselectivities (up to 88:12 dr) and good enantioselectivities (up to 90% ee).

Key words

aldol reaction - cooperative catalysis - isatins - isocyanoacetates - guanidine

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• 12 General procedure for cooperative chiral guanidine/AgPF₆ catalyzed asymmetric isocyanoacetate aldol reaction with isatins: To a stirred mixture of isocyanoacetate 1 (0.12 mmol), 2-MeOC₆H₄CO₂H (10 mol%), 4a (10 mol%), and AgPF₆ (11 mol%) in THF (0.5 mL) was added isatin 2 (0.1 mmol) and THF (0.5 mL) at 0 °C. The reaction was then allowed to proceed at 0 °C for 4–6 h. Upon completion, the reaction mixture was purified by flash chromatography on silica gel to afford the desired product. Analytical data of some typical compounds: 3aa: White solid; yield: 30.8 mg (72%); 72:28 dr; 85%/0% ee; [α]^11.8 _D = −28.1 (c = 0.22, CH₂Cl₂); HPLC (Daicel Chiralcel ADH; n-hexane/i-PrOH, 70/30; 1.0 mL/min; 254 nm): t_R  = 23.95 (major), 19.55 (minor) min; ¹H NMR (400 MHz, CDCl₃): δ = 7.49–7.38 (m, 2 H), 7.27–7.09 (m, 10 H), 7.08–7.01 (m, 2 H), 6.87 (d, J = 7.9 Hz, 1 H), 4.94 (d, J = 12.3 Hz, 1 H), 4.85 (d, J = 12.3 Hz, 1 H), 3.11 (d, J = 13.0 Hz, 1 H), 2.99 (s, 3 H), 2.88 (d, J = 13.0 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 173.51, 170.07, 154.26, 145.10, 134.91, 131.38, 130.29, 128.36, 128.33, 128.21, 128.09, 127.16, 126.80, 123.37, 122.65, 109.24, 99.98, 87.31, 84.88, 67.28, 45.17, 26.25. HRMS (ESI): m/z calcd for [C₂₆H₂₂N₂O₄+H⁺]: 427.1658; found: 427.1654. 3da: White solid; yield: 30.6 mg (78%); 76:24 dr; 90%/2% ee; [α]^13.6 _D = −6.5 (c = 0.38, CH₂Cl₂); HPLC (Daicel Chiralcel ADH; n-hexane/i-PrOH, 70:30; 1.0 mL/min; 254 nm): t_R  = 11.91 (major), 18.92 (minor) min; ¹H NMR (400 MHz, CDCl₃): δ = 7.39 (td, J = 7.8, 1.0 Hz, 1 H), 7.35–7.27 (m, 6 H), 7.6 (s, 1 H), 7.12–7.07 (m, 1 H), 6.77 (d, J = 7.8 Hz, 1 H), 5.18 (d, J = 12.3 Hz, 1 H), 5.08 (d, J = 12.3 Hz, 1 H), 2.87 (s, 3 H), 1.84–1.73 (m, 1 H), 1.64 (d, J = 6.0 Hz, 2 H), 1.01 (d, J = 6.6 Hz, 3 H), 0.74 (d, J = 6.6 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 173.57, 170.85, 153.86, 145.05, 135.04, 131.10, 128.47, 128.44, 128.30, 126.92, 123.37, 122.49, 108.99, 87.43, 83.25, 67.32, 47.48, 26.01, 25.20, 24.31, 23.73. HRMS (ESI): m/z calcd for [C₂₃H₂₄N₂O₄+H⁺]: 393.1814; found: 393.1809.