Chemoselective Epoxidation of Ene-Ynamides: Intramolecular Cyclopropanation Induced by the Intermediate α-Oxocarbene

Abstract

Chemoselective epoxidation of the triple bond of ene-ynamides generates an α-oxocarbene intermediate that effects the intramolecular cyclopropanation of the alkene.

Key words

ynamides - epoxidation - cyclopropanation - oxirene - α-oxocarbene

References and Notes

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19

For a special issue devoted to the chemistry of ynamides, see: Tetrahedron 2006, 62, issue 16.

22

Representative Procedure: 2-Hydroxy-1-{(1 S *,5 R* )-2-(4-methylbenzenesulfonyl)-2-azabicyclo[3.1.0]hex-1-yl}ethanone (10) To a solution of ynamide 9 (136 mg, 0.486 mmol) in CH₂Cl₂ (5 mL) at 0 °C were successively added VO(acac)₂ (6.4 mg, 0.024 mmol, 0.05 equiv) and TBHP (0.220 mL, 5.5M in decane, 1.21 mmol, 2.5 equiv). After 0.5 h at r.t., the reaction mixture was cooled to 0 °C and cautiously hydrolyzed with a 25% aqueous solution of Na₂S₂O₃. After extraction with CH₂Cl₂, the combined organic extracts were washed with brine, dried over MgSO₄, filtered, and concentrated under reduced pressure. The crude material was purified by flash chromatography (PE-EtOAc, 55:45) to afford 52 mg (40%) of 10 as a colorless oil. IR: 3475, 1701, 1339, 1160, 1086, 903, 808, 666 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.70 (d, J = 8.5 Hz, 2 H), 7.34 (d, J = 8.5 Hz, 2 H), 5.24 (d, AB syst, J = 19.1 Hz, 1 H), 4.56 (d, AB syst, J = 19.1 Hz, 1 H), 3.70 (ddd, J = 10.6, 9.0, 2.5 Hz, 1 H), 3.03 (br s, 1 H, OH), 2.81 (ddd, app td, J = 10.6, 9.5 Hz, 1 H), 2.45 (s, 3 H), 2.27-2.15 (m, 1 H), 1.92-1.83 (m, 3 H), 0.62 (dd, app t, J = 4.5 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 206.3 (s), 144.8 (s), 131.9 (s), 129.7 (d, 2 C), 128.9 (d, 2 C), 67.1 (t), 53.9 (s), 49.6 (t), 34.2 (d), 25.8 (t), 21.6 (q), 16.8 (t). MS (EI, 70 eV): m/z (%) = 295 (4)[M⁺], 293 (7), 264 (9), 155 (15), 141 (10), 140 (100), 139 (11), 92 (19), 91 (82), 89 (8), 82 (74), 80 (8), 65 (29), 55 (86), 54 (11), 53 (10). Anal. Calcd for C₁₄H₁₇NO₄S: C, 56.93; H, 5.80; N, 4.74. Found: C, 56.88; H, 5.93; N, 4.61.