High Stereocontrol in the Preparation of Silyl-Protected γ-Substituted Enoldiazoacetates

Abstract

A robust and efficient synthesis of triisopropylsilyl (TIPS)-protected γ-substituted enoldiazoacetates with excellent Z stereocontrol by using lithium bis(trimethylsilyl)azanide (LiHMDS) as a base and TIPSOTf as a silyl transfer reagent is reported. Despite their increased size compared to previously tert-butyldimethylsilyl (TBS)-protected γ-unsubstituted enoldiazoacetates, a high product yield with exceptional stereocontrol has been achieved in copper-catalyzed [3+3] cycloaddition reaction with nitrones by using a chiral indeno bisoxazoline ligand.

Key words

enoldiazoacetates - Z selectivity - cycloaddition - copper catalysis - nitrones - bisoxazolines

• References and Notes


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• 9 General Procedure for the Preparation of (Z)-Enoldiazoacetates 3: A 10 mL oven-dried vial equipped with a magnetic stirring bar was charged with diazoacetate 2 (0.50 mmol) and the system was filled with nitrogen. THF (2.0 mL) was then added, and the reaction solution was cooled to –78 °C (dry ice/acetone bath). LiHMDS (0.60 mL, 1.0 M in the hexanes) was introduced dropwise in 2 min, followed by the addition of TIPSOTf (168.5 mg, 0.55 mmol) at –78 °C. The resulting solution was stirred at –78 °C until the reaction was complete (monitored by TLC, about 5–15 min). THF was then removed under reduced pressure, and the residue was purified by column chromatography on silica gel which was pretreated with triethylamine (5 vol.%)/hexanes (eluent: pure hexanes) to give the desired products 3 in high yields and excellent stereoselectivity. Methyl (Z)-2-Diazo-3-[(triisopropylsilyl)oxy]hex-3-enoate (3a): Orange oil; Yield: 138 mg (88%). ¹H NMR (500 MHz, CDCl₃): δ = 5.07 (t, J = 7.2 Hz, 1 H), 3.80 (s, 3 H), 2.15–2.22 (comp, 2 H), 1.24–1.16 (comp, 3 H), 1.12 (d, J = 6.6 Hz, 18 H), 1.01 (t, J = 7.4 Hz, 3 H), ¹³C NMR (125 MHz, CDCl₃): δ = 165.8, 133.4, 116.3, 52.1, 19.7, 18.0, 14.1, 13.5; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₃₁N₂O₃Si: 327.2098; found: 327.2093. Methyl (Z)-2-Diazo-3-[(triisopropylsilyl)oxy]pent-3-enoate (3b): Orange oil; Yield: 111 mg (71%). ¹H NMR (500 MHz, CDCl₃): δ = 5.13 (q, J = 7.0 Hz, 1 H), 3.77 (s, 3 H), 1.70 (d, J = 7.0 Hz, 3 H), 1.20–1.13 (comp, 3 H), 1.10 (d, J = 6.7 Hz, 18 H); ¹³C NMR (125 MHz, CDCl₃): δ = 165.8, 134.7, 108.7, 52.1, 18.0, 13.5, 11.9; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₅H₂₉N₂O₃Si: 313.1942; found: 313.1935. Methyl (Z)-2-Diazo-5-phenyl-3-[(triisopropylsilyl)oxy]pent-3-enoate (3c): Orange oil, Yield 165 mg (85%). ¹H NMR (500 MHz, CDCl₃): δ = 7.29 (t, J = 7.5 Hz, 2 H), 7.23 (d, J = 7.1 Hz, 2 H), 7.19 (t, J = 7.2 Hz, 1 H), 5.31 (t, J = 7.4 Hz, 1 H), 3.78 (s, 3 H), 3.54 (d, J = 7.4 Hz, 2 H), 1.26–1.18 (comp, 3 H), 1.13 (d, J = 6.9 Hz, 18 H); ¹³C NMR (125 MHz, CDCl₃) δ = 165.6, 141.0, 134.6, 128.5, 128.5, 126.1, 112.6, 52.1, 32.5, 18.0, 13.5; HRMS (ESI): m/z [M + H]⁺ calcd for C₂₁H₃₃N₂O₃Si: 389.2255; found: 389.2251. For more examples, see the Supporting Information.
• 10 Jing C, Cheng Q.-Q, Deng Y, Arman H, Doyle MP. Org. Lett. 2016; 18: 4550