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General procedure for the synthesis of spirodihydrofurans: A solution of CAN (2.2 mmol) in methanol (10 mL) was added to a methanolic solution
(10 mL) containing exocyclic olefin (1 mmol) and 1,3-dicarbon-yl compound (1.2 mmol)
at 0 °C with stirring under an argon atmosphere. The reaction proceeds rapidly and
the reddish brown color of CAN disappears by the time the addition was over. After
decolorization, the reaction mixture was diluted with water (50 mL) and extracted
using dichloromethane
(3 × 30 mL). The combined organic extracts were washed with water and brine, dried
over anhydrous magnesium sulfate and concentrated. The residue was purified through
silica gel column chromatography (10% ethyl acetate-hexane) to furnish spirodihydrofurans
(8a-p).
<A NAME="RD02302ST-16">16</A>
All new compounds gave satisfactory spectroscopic data consistent with their structures.
Selected spectral data, 8a: Yellow solid, mp 69-70 °C; IR (CHCl3): = 2909, 1667, 1598, 1452, 1390, 1245, 1154, 1102, 1064, 983, 937, 918, 733 cm-1; 1H NMR (200 MHz, CDCl3): δ = 1.57-1.62 (m, 2 H), 1.74-1.83 (m, 10 H), 2.15-2.23 (m, 8 H), 2.78 (s, 2 H);
13C NMR (50.3 MHz, CDCl3): δ = 16.1 (CH3), 26.1 (CH), 29.8 (CH3), 33.6 (CH2), 35.1 (CH2), 37.7 (CH), 38.0 (CH2), 41.0 (CH2), 92.9 (quat-C, C-5), 112.5 (quat-C, C-3), 169.2 (quat-C, C-2), 195.1 (quat-C, C=O); MS: m/z = 246 (M+); Anal. Calcd for C16H22O2: C, 78.01; H, 9.00. Found: C, 78.05; H, 8.98. 8m: Yellow solid; mp 83-84 °C; IR (CHCl3): = 2928, 1679, 1603, 1467, 1378, 1258, 1144, 1073, 937, 631 cm-1; 1H NMR (200 MHz, CDCl3): δ = 0.89 (s, 3 H, CH3), 0.97 (s, 3 H, CH3), 1.01 (s, 3 H, CH3), 1.09-1.63 (m, 9 H), 1.65-1.80 (m, 3 H), 2.18-2.23 (m, 8 H), 2.88 (d, 1 H, J = 6.0 Hz); 13C NMR (50.3 MHz, CDCl3, mixture of diastereomer, * denotes minor isomer): δ = 15.4* (CH3), 15.6 (CH3), 21.3* (CH2), 22.1 (CH2), 24.0* (CH2), 24.6 (CH2), 24.8 (CH2), 24.9* (CH2), 25.5 (CH3), 28.5* (CH2), 29.6 (CH3), 29.7 (CH3), 30.8* (CH3), 31.4* (CH3), 32.9* (CH2), 33.4 (quat-C), 33.5 (quat-C), 34.1 (CH3), 35.5* (CH2), 37.6* (CH2), 38.5 (CH2), 38.8 (CH2), 39.3* (CH2), 41.0* (CH2), 45.7 (CH), 48.0 (quat-C), 49.5 (CH), 51.4* (CH), 63.5 (CH), 113.1 (quat-C), 166.5 (quat-C), 194.7 (quat-C, C=O). MS: m/z = 303 (M + 1, 43.5), 302 (M+, 76.5), 287 (22.6), 260(100), 241 (35.7), 217 (28.7), 190 (40.9), 189(72), 164(44),
150 (61.7), 119 (69.6), 105(79), 91 (74.8), 67 (74.8); Anal. Calcd for C20H30O2: C, 79.42; H, 10.00. Found: C, 79.48; H, 9.97.