Zinc Triflate as Lewis Acid in Nucleophilic Addition to Cyclic N-Acyliminium Ions

Abstract

Zinc triflate-mediated nucleophilic addition of allytri­methylsilane, silyl enol ethers and terminal acetylenes to cyclic N-acyliminium ions at room temperature in CH₂Cl₂ is described. The corresponding α-substituted heterocycles were obtained in moderate to good yields. The versatility of this reagent was demonstrated in the one-pot generation of the N-acyliminium ion and the zinc alkynylide species, followed by their coupling reaction to afford propargylic adducts in moderate yields.

Key words

zinc triflate - nucleophilic addition - N-acyliminium ions

References

• For recent reviews on the chemistry of N-acyliminiun ions, see:
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13

The allylation reactions and the addition of silyl enol ethers to N-acyl iminium ion precursors using Zn(OTf)₂ gave comparable yields to those obtained when BF₃·OEt₂ was employed.

15

For clarity, only the structures of the major diastereoisomers erythro-20 and erythro-21 are depicted in Table [3] . Data for erythro-20, see ref. 2b.
Data for erythro-21: ¹H NMR (CDCl₃, 298 K): δ = 1.13 (d, J = 6.95 Hz, 3 H), 1.30-1.70 (s, 6 H); 1.50 (s, 9 H), 2.65-2.79 (s, 1 H), 4.02-4.23 (m, 2 H), 4.69-4.82 (s, 1 H), 7.50-7.60 (m, 3 H), 7.99 (d, J = 6.95 Hz, 2 H). ¹³C NMR (CDCl₃, 298 K): δ = 15.4, 19.6, 25.4, 27.6, 28.5, 38.8, 39.3, 53.0, 79.8, 128.4, 129.0, 133.5, 137.1, 155.6, 203.5. IR (KBr, film): 2974, 2933, 1685, 1415, 1365, 1170, 1147, 968 cm^-1. Anal. Calcd for C₁₉H₂₇NO₃: C, 71.92; H, 8.51; N, 4.41. Found: C, 71.59; H, 8.24; N, 4.37.

17

Representative Procedure.
To a suspension of Zn(OTf)₂ (0.24 mmol) in dry CH₂Cl₂ (1 mL) at r.t. was added a substrate 3b (0.20 mmol), diluted in dry CH₂Cl₂ (1 mL). After 10 min, allyltrimetylsilane (4, 0.40 mmol) was added. The mixture was stirred 3 h at r.t. and quenched with sat. NaHCO₃ (2 mL), extracted with CH₂Cl₂ (2 × 5 mL), dried with anhyd Na₂SO₄. After filtration, the solvent was evaporated under reduced pressure and the residue was chromatographed on silica gel (5% MeOH in CHCl₃) to afford 5 in 78% yield.
Data for compound 5: ¹H NMR (300 MHz, CDCl₃): δ = 1.74 (m, 1 H), 2.14 (m, 2 H), 2.34 (m, 3 H), 3.68 (m, 2 H), 4.60 (d, J = 15.8 Hz, 1 H), 5.10 (s, 1 H), 5.12 (d, J = 15.8 Hz, 1 H), 5.58 (s, 1 H), 5.65 (m, 1 H), 5.75 (s, 1 H). ¹³C NMR (300 MHz, CDCl₃): δ = 23.4, 29.8, 37.2, 48.2, 56.2, 118.9, 119.1, 128.2, 132.5, 175.2. IR (KBr, film): 3076, 2976, 2920, 1695, 1639, 1426, 1254, 1113, 915 cm^-1. HMRS (EI): m/z calcd for C₁₀H₁₄NOBr: 245.0239; found: 245.0242.