Metal-Free Intramolecular Carbonyl-Olefin Metathesis of ortho-Prenylaryl Ketones

Abstract

On treatment with boron trifluoride etherate as a Lewis acid, acetophenone and benzophenone derivatives bearing a prenyl (or a related) side chain in ortho-position were shown to undergo intramolecular carbonyl-olefin metathesis, in the absence of any transition-metal catalyst. The cationic cyclization process is supposed to proceed via an oxetane intermediate, which fragments to give the cyclization product (indene or 1,2-dihydronaphthalene) and a ketone (acetone) as a stoichiometric by-product. Several substrates were shown to afford the metathesis products with up to 93% yield.

olefin metathesis - cyclization - Lewis acids - carbenium ions - oxetanes - natural products

References and Notes

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It proved to be important to quench the reaction mixture by addition of aq NaHCO₃ prior to extractive workup. Otherwise significant amounts of a by-product (a dimer of 17 as indicated by GC-MS) were formed probably through a proton-mediated process.

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<A NAME="RB14911ST-17">17</A>

General Procedure for BF ₃ ˙Et ₂ O-Induced Ring-Closing Metathesis: In an argon-flushed Schlenk tube ortho-prenyl acetophenone (13a; 250 mg, 1.32 mmol) dissolved in anhyd CH₂Cl₂ (26 mL) was cooled to -40 ˚C. Dropwise addition of BF₃˙OEt₂ (0.25 mL, 1.98 mmol) afforded a yellow solution, which was stirred for 1 h at -40 ˚C. After addition of sat. aq NaHCO₃ (20 mL) at 0 ˚C the mixture was extracted with CH₂Cl₂ (3 × 40 mL). The combined organic layers were dried (MgSO₄), filtered and concentrated under reduced pressure. The residue was purified by chromatography on silica(cyclohexane) to provide 3-methyl-1H-indene (17; 150 mg, 1.15 mmol, 87%) as a colorless oil; R _f 0.65 (pentane). ^¹H NMR (300 MHz, CDCl₃): δ = 2.18 (dd, 3 H), 3.33 (m, 2 H), 6.21 (d, 1 H) 7.20 (dt, 1 H), 7.33 (q, 2 H), 7.46 (d, 1 H). ^¹³C NMR (75 MHz, CDCl₃): = 13.0, 37.6, 118.8, 123.6, 124.4, 126.0, 128.7, 139.9, 144.3, 146.1. IR-ATR:(film) = 3013 (w), 2931 (w), 1681 (m), 1460 (m), 1433 (m), 1379 (m), 1015 (m), 1001 (m), 943 (m), 760 (s), 717 (s) cm^-¹. HRMS (EI, 70 eV): m/z calcd for C₁₀H₁₀: 130.0782; found: 130.079.