Ru-Catalyzed Enantioselective Dipolar Cycloadditions of Ethyl Diazopyruvate

Abstract

The decomposition of ethyl diazopyruvate (1) with [RuCl₂(pybox)] catalysts in the presence of enol ethers affords ­dihydrofurans derived from formal 1,3-cycloaddition. Enantio­selectivities of up to 74% have been achieved.

Key words

asymmetric catalysis - cycloaddition - diazo compounds - carbenes - ruthenium

References

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16

Müller, P.; Chappellet, S. Helv. Chim. Acta, to be published.

18

Sample Run. Cycloaddition of 1 to 2 in PhCH ₃ . Ethyl 4,5-dihydro-3a H -furo[2,3- b ]furan-2-carboxylate ( 3): (S,S)-2,6-Bis(4-isopropyl-2-oxazolin-2-yl)pyridine (86 mg, 0.294 mmol) was added to a solution of [RuCl₂(p-cymene)]₂ (58 mg, 0.095 mmol) in CH₂Cl₂. The dark red mixture was stirred at r.t. for 1 h under argon atmosphere. The solvent was removed under reduced pressure. Dihydrofuran (1.4 mL) and a solution of ethyl diazopyruvate (100 mg, 0.950 mmol) in PhCH₃ (3.0 mL) was added. The resulting suspension was stirred until complete consumption of starting material (thin layer chromatography). The mixture was concentrated, and the residue was purified by column chromatography on silica gel using EtOAc-pentane (60:40) as eluent afforded a pale yellow oil in 68% yield (104 mg, 0.620 mmol); [α]_D ²⁰ 17 (c 1.12, CDCl₃) for 74% ee. IR: 2983, 2281, 1737, 1633 cm^-1. ¹H NMR (300 MHz, CDCl₃): d = 6.25 (d, J = 8.0 Hz, 1 H), 5.84 (d, J = 3.5 Hz 1 H), 4.32 (q, J = 9.3 Hz, 2 H), 4.11 (m, 1 H), 3.78 (m, 2 H), 2.13 (m, 1 H), 1.96 (m, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 159.7 (s), 148.8 (s), 111.7 (d), 110.5 (d), 67.1 (t), 61.4 (t), 47.6 (d), 31.2 (t), 14.2 (q). HRMS: m/z calcd for C₉H₁₂O₄ ⁺: 184.0738; found: 184.0718. Enantiomer separation: Supelco β-dex column (isothermal 140 °C) τ₁: 43.9 min (87%), τ₂: 45.7 min (13%).