Reaction Steps
General Ketalization. Ketone 9E (0.257 g, 0.535 mmol, 1.0 equiv) was dissolved in benzene (27 mL). After the ketone was completely dissolved, p-toluenesulfonic acid (0.0102 g, 0.0535 mmol, 0.1 equiv) and ethylene glycol (0.298 mL, 5.35 mmol, 10 equiv) were added and the solution was heated to reflux for 20 h using a modified Dean-Stark apparatus. The mixture was then allowed to cool to room temperature, and was then concentrated using a rotary evaporator. The residue was then diluted with CH2Cl2 (30 mL) and washed with saturated sodium bicarbonate solution (3×30 mL). The organic layer was washed with a saturated brine (30 mL), dried with MgSO4, and vacuum filtered through a pad of Celite. The filtrate was concentrated with a rotary evaporator to give 10E as a pure white crystalline product (0.246 g, 88% yield). 10Z was prepared by an analogous procedure in 83% yield. (6R,E)-Ethyl-2-methyl-6-((10S,13R,14R,17R)-4,4,10,13,14-pentamethyl-1,2,4,5,6,10,12,13,14,15,16,17-dodecahydrospiro[cyclopenta[a]phenanthrene-3,2′-[1,3]dioxolan]-17-yl)hept-2-enoate (10E): mp 75-79° C. IR (KBr) νmax 3047, 2963, 2924, 2898, 2888, 1705, 1652, 1197, 1141, 1101, 1098, 1050, 1028, 964, 952 cm−1. 1H NMR (CDCl3) δ 6.75 (dt, J=1.3, 7.5 Hz, 1H), 5.45 (m, 1H), 5.31 (d, J=6.3 Hz, 1H), 4.18 (q, J=7.1 Hz, 2H), 3.96 (m, 3H), 3.88 (m, 1H), 2.27-2.17 (m, 2H), 2.11-1.84 (m, 8H), 1.81 (d, J=1.1 Hz, 3H), 1.75-1.31 (m, 7H), 1.29 (t, J=7.1 Hz, 3H), 1.22-1.1 (m, 2H), 1.04 (s, 3H), 1.02 (s, 3H), 0.92 (d, J=6.5 Hz, 3H), 0.88 (s, 3H), 0.86 (s, 3H), 0.60 (s, 3H). 13C{1H} (CDCl3) δ 168.3, 145.9, 142.8, 142.5, 127.4, 120.2, 116.0, 113.1, 64.9, 64.7, 60.4, 50.8, 50.3, 47.4, 43.8, 41.8, 37.8, 37.0, 36.2, 34.9, 34.0, 31.5, 27.9, 27.2, 25.7, 25.6, 23.1, 22.9, 22.7, 20.4, 18.3, 15.6, 14.3, 12.3. HRMS calcd for C34H5304 (M+H)+525.3938. found 525.3922.
