Reaction Steps
Prepared according to general procedure A from 6-methylhept-4-en-3-one (89 μL, 0.60 mmol), cyclopentadiene (99 μL, 1.2 mmol), 70% aqueous perchloric acid (10.3 μL, 0.12 mmol) and (2S, 5S)-5-benzyl-3-methyl-2-(5-methyl-furan-2-yl)-imidazolidin-4-one (32 mg, 0.12 mmol) in water (120 μL) for 2.5 days at 0° C. Then, an additional 1.2 mmol of cyclopentadiene was added and the mixture was allowed to stir for an additional 3.5 days at 0° C. Purification by silica gel chromatography (18:1 pentane:ether) provided the title compound as a colorless oil in 78% yield (90 mg, 0.47 mmol); 6:1 endo:exo, endo 90% ee. For the purpose of characterization, the more quickly eluting exo diastereomer was removed by silica gel chromatography as above. IR (film) 3057, 2961, 2869, 1709, 1460, 1367, 1334, 1136, 1028, 904.2, 716.6 cm−1; 1H NMR (300 MHz, CDCl3) δ 6.26 (dd, J=5.7, 3.3 Hz, 1H, CH═CH), 5.79 (dd, J=5.7, 2.7 Hz, 1H, CH═CH), 3.14 (m, 1H, CHCH═CH), 2.77 (m, 1H, CHCH═CH), 2.63 (dd, J=3.6, 3.6 Hz, 1H, CHCO), 2.49 (m, 2H, CH2CO), 1.40 (m, 3H, CH(CH3)2 and CHCH2CH), 1.05 (d, J=7.5 Hz, 3H, one of CH(CH3)2), 0.99 (dd, J=4.8, 4.8 Hz, 3H, CH2CH3), 0.84 (d, J=7.5 Hz, 3H, one of CH(CH3)2), 13C NMR (75 MHz, CDCl3) δ 211.6, 139.6, 131.9, 57.8, 49.1, 47.4, 47.2, 45.5, 35.2, 33.1, 22.7, 22.2, 8.5; HRMS (CI) exact mass calc'd for (C13H20O) requires m/z 192.1514, found m/z 192.1513. [α]D=+20.1 (c=1.0, CHCl3). Product ratios were determined by GLC analysis (162° C., 23 psi); (1R, 2R, 3S, 4R) endo isomer tr=4.4 min, and (1S, 2S, 3R, 4S) endo isomer tr=4.2 min, exo isomers tr=3.9, 4.1 min.
