Silyl Ethers as Latent Pronucleophiles in Enantioselective Lewis Base Catalyzed Synthesis of Allylic Ethers from Allylic Fluorides

Abstract

Allylic ethers are a common occurrence in natural products, and are often used as intermediates in target-oriented synthesis. Their synthesis often relies on the use of transition-metal catalysts. Here, we report an organocatalytic method for the allylation of O-centered nucleophiles, the Lewis base catalyzed allylation of silyl ethers with allylic fluorides. The method relies on the concept of latent pronucleophiles in Lewis base catalysis to overcome common limitations in substrate scope, even permitting the allylation of sterically congested O-pronucleophiles. When chiral Lewis base catalysts are used, the allyl ethers are produced in an enantioenriched form through kinetic resolution of fluorides, where the stereoselectivity is determined by the chiral catalyst.

Key words

Lewis base catalysis - allylation - allyl ethers - silyl ethers - stereoselectivity

Publication History

Received: 05 September 2024

Accepted after revision: 22 October 2024

Accepted Manuscript online:
22 October 2024

Article published online:
05 November 2024

• References and Notes

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• 20a Allylic Substitution; General ProcedureThe appropriate latent pronucleophile 2 (1.3 equiv) and DABCO (10 mol%) were dissolved in anhyd CH₂Cl₂ (0.25 M). A solution of the appropriate MBH fluoride 1 (1.0 equiv) in anhyd CH₂Cl₂ (0.25 M) was added, and the resulting mixture was stirred at r.t. under an inert atmosphere until the reaction was complete (TLC). The mixture was then concentrated, and the residue was purified by column chromatography (silica gel, EtOAc–PE).Methyl 2-[(2-Naphthylmethoxy)(phenyl)methyl]acrylate (3a)Colorless solid; yield: 29.3 mg (0.088 mmol, 87%). IR (ATR):3012, 2889, 1256, 1432, 1157, 1099, 1029, 725, 642 cm^–1. ¹H NMR (250 MHz, CDCl₃): δ = 7.78–7.92 (m, 4 H), 7.28–7.55, (m, 8 H), 6.43 (s, 1 H), 6.12 (t, J = 1.37 Hz, 1 H), 5.45 (s, 1 H), 4.69 (s, 2 H), 3.73 (s, 3 H). ¹³C NMR (151 MHz, CDCl₃): δ = 166.33, 141.30, 139.60, 135.65, 133.30, 133.02, 128.44, 128.12, 128.03, 127.92, 127.82, 127.72, 126.44, 126.09, 125.87, 125.85, 125.35, 78.62, 70.96, 51.84. HRMS (ESI): m/z [M + Na]⁺ calcd for C₂₂H₂₀NaO₃: 355.1310; found: 355.1301.
• 21 Enantioselective Allylic Substitution; General ProcedureThe appropriate MBH fluoride 1 (1 equiv) was added to a solution of the appropriate latent pronucleophile 2 (2 equiv) and (DHQD)₂PHAL (10–20 mol%) in anhyd 1,4-dioxane (0.2 M), and the mixture was stirred under argon at r.t. until the reaction was complete (TLC). The crude mixture was directly purified by column chromatography (silica gel, EtOAc–PE).Methyl 2-[[(4-Cyanobenzyl)oxy](phenyl)methyl]acrylate (3i′)Colorless oil; yield: 22.4 mg (0.073 mmol, 73%, er 96:4). HPLC [Phenomenex Lux Cellulose-1, hexane–EtOH (95:5), 0.7 mL/min, 25 °C, λ = 220 nm]: t _maj = 13.45 min; t _min = 16.87 min.