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A typical Procedure for the Formation of Cyclobutanes 13:
A solution of the freshly distilled dioxinone 5 (7.10 g, 50.0 mmol)and N-trifluoroacetyl-3-pyrroline 12 (16.50 g, 100.0 mmol) in EtOAc (400 mL), under continuous degassing with nitrogen,
was irradiated with a 125 W medium pressure mercury lamp. After 72 h TLC analysis
indicated full consumption of the dioxinone. The reaction mixture was concentrated
under reduced pressure and purified by column chromatography (50% diethyl ether/petroleum
ether eluent), to give two cyclobutane products.
13-endo: Colourless crystals (7.27 g, 47%); TLC, (diethyl ether) Rf = 0.20; mp 75-77 °C; IR (thin film): 2926 (s, CH3/CH2), 1730 (s, C3=O), 1687 (s, N-C=O), 1157 (s, C-O) cm-1; 1H NMR (300 MHz; CDCl3): δ = 4.46 and 4.43 (1 H, d, J = 12.7, 9-H), 4.32 and 4.24 (1 H, 2 × d, J = 12.3, 11-H), 3.52-3.20 (3 H, m, 9-H, 11-H and 1-H) 3.08 and 3.04 (1 H, 2 × dd,
J = 9.8 and 1.9, 2-H), 2.91 and 2.84 (1 H, 2 × dd app 2 × t, J = 8.3, 8-H), 1.63 and 1.62 (3 H, 2 × s, 7-Me), 1.56 and 1.55 (6 H, 2 × s, 5-Me);13C NMR (75 MHz; CDCl3): δ = 165.9 and 165.6 (C-3), 155.4 (q, J = 37.4, COCF3), 115.9 (q, J = 300.6, COCF3), 103.8 and 103.4 (C-5), 69.5 and 69.4 (C-7), 47.7 (C-8), 46.2 and 45.8 (C-9 or C-11),
44.6 (C-8), 44.5 (C-9 or C-11), 43.5 (C-9 or C-11), 39.0 and 38.8 (C-2), 34.6 and
32.1 (C-1), 27.0, 26.9, 26.8 and 26.4 (5-Me and 7-Me);MS (EI): m/z (%) = 308(39) [M + H], 292(8) [M-Me], 250(77) [M-CO(CH3)2], 222(25) [M-CO2(CH3)3], 165(83) [M-dioxinone], 143(87) [dioxinone], 85(95), 43(100) [CH(CH3)2]; HRMS (EI) found: 308.1127, [M + H], C13H17NO4F3 requires 308.1110.
13-exo: Colourless crystals (1.80 g, 12%); TLC, (Et2O) Rf = 0.47; mp 99-103 °C; IR (thin film): 2924 (s, CH3/CH2), 2854 (s, CH), 1735 (s, C3=O), 1689 (s, N-C=O), 1463 (s, CH3/CH2), 1385 and 1354 (s, C(CH3)3)cm-1; 1H NMR (300 MHz; CDCl3): δ = 4.25 and 4.05 (1 H, 2 × d, J = 14.0, 9-Hu), 4.08-3.98 (1 H, m, 11-H), 3.72-3.47 (2 H, m, 9-Hd and 11-H), 3.29-3.00 (2 H, m, 1-H and 8-H), 2.71 and 2.62 (1 H, 2 × d, J = 3.4, 2-H), 1.67-1.61 (6 H, m, 5-Me), 1.45 and 1.39 (3 H, 2 × s, 7-Me); 13C NMR (75 MHz; CDCl3): δ = 170.0 (C-3), 155.4 (q, J = 36.7, COCF3), 116.0 (q, J = 287.3, COCF3), 106.73 and 106.67 (C-5), 76.0 (C-7), 53.1 and 52.0 (C-11), 47.6 and 47.3 (C-9),
46.9 and 46.8 (C-8), 45.1 and 44.6 (C-2), 40.3 and 44.6 (C-1), 27.0, 29.6, 29.1 and
28.9 (5-Me), 22.0 and 21.7 (7-Me); MS (EI): m/z (%) = 308(12) [M + H], 292(5) [M-Me], 250(52) [M-CO(CH3)2], 165(80) [M-dioxinone], 143(87) [dioxinone], 69(62) [CF3], 59(49) [(CH3)2CHO] 43(100) [CH(CH3)2]; HRMS (EI) found: 308.1128, [M + H], C13H17NO4F3 requires 308.1110.
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