Reaction Steps
Part A. A mixture of 2-ethyl-1-bromobutane (10.0 mL, 71.4 mmol), potassium cyanide (14.0 g, 215 mmol) and aliquat 336 (10 drops) in 50 mL water was heated to reflux overnight with vigorous stirring. The mixture was cooled, and extracted with dichloromethane (2×50 mL). The extracts were combined, dried over magnesium sulfate, filtered and evaporated. The residual liquid was distilled bulb-to-bulb to afford pure product, 3-ethylpentanenitrile (5.50 g, 49.5 mmol, 69%). b.p. 40-45 ° C. (5 mm Hg). Spectral data: 1H NMR (300 MHz, CDCl3): δ2.33 (2H, d, J=5.8 Hz), 1.62-1.36 (5H, m), 0.92 (6H, t, J=7.3 Hz). MS (H2O-GC/MS): m/e 112 (100). Part B. A solution of diisopropylamine (7.50 mL, 57.2 mmol) in THF (100 mL) was cooled to −78° C., and treated with n-butyllithium (34.0 mL of a 1.6 M solution in hexane). The solution was warmed briefly to 0° C., and then recooled to −78° C. The nitrile compound from Part A was then added by syringe, and the solution was allowed to stir for 1 hour. Then, ethyl propionate (6.50 mL, 56.7 mmol) was added by syringe, and the resulting mixture was allowed to stir and warm to ambient temperature for 12 hours. It was poured into 200 mL of satd. aq. NH4Cl solution, and this was extracted with ethyl acetate (2×200 mL). The extracts were combined, dried over magnesium sulfate, filtered and evaporated. The residual oil was separated by column chromatography (silica gel, 10:90 ethyl acetate-hexane) to afford the product, 4-cyano-5-ethyl-3-heptanone, as an oil 4.06 g, 24.3 mmol, 49%). TLC RF 0.47 (20:80 ethyl acetate-hexane). Spectral data: 1H NMR (300 MHz, CDCl3): δ 3.49 (1H, d, J=4.4 Hz), 2.74 (2H, q, J=7.3 Hz), 2.08-1.98 (1H, m), 1.70-1.58 (1H, m), 1.50-1.20 (3H, m), 1.12 (3H, t, J=7.3 Hz), 0.95 (3H, t, J=7.3 Hz), 0.91 (3H, t, J=7.3 Hz). MS (H2O-GC/MS): m/e 167 (100). Part C. A solution of the ketonitrile from Part B (4.06 g, 24.3 mmol), hydrazine hydrate (2.70 mL, 55.7 mmol) and acetic acid (5.00 mL, 83.7 mmol) in benzene (50 mL) was heated to reflux under a Dean-Stark trap with azeotropic distillation of water. After being heated for 12 hours, the mixture was cooled and poured into 100 mL 1 N aq. NaHCO3 solution. This was extracted with ethyl acetate (2×100 mL), and the extracts were washed in sequence with brine, combined, dried over sodium sulfate, filtered and evaporated to afford sufficiently-pure product, 3-amino-5-ethyl-4-(3-pentyl)pyrazole, as a viscous oil (2.48 g, 13.7 mmol, 56%). Spectral data: 1H NMR (300 MHz, CDCl3): δ 3.48 (2H, br), 2.54 (2H, q, J=7.3 Hz), 2.25-2.14 (1H, m), 1.71-1.49 (4H, m), 1.20 (3H, t, J=7.3 Hz), 0.83 (6H, t, J=7.3 Hz), 1H missing. MS (NH3-CI): m/e 183 (12), 182 (100). Part D. A solution of 3-amino-5-ethyl-4-(3-pentyl)pyrazole (0.750 g, 4.14 mmol) was suspended in 4 mL water and made acidic with conc. aq. HCl (2 mL). This was cooled in an ice bath, and a conc. aq. solution of sodium nitrite (0.286 g, 4.14 mmol) was added dropwise. After stirring for 30 min., the mixture was diluted with ice-cold dichloromethane (40 mL) and made alkaline with a saturated solution of sodium carbonate in water. The organic layer was separated, dried over sodium sulfate and filtered. The filtrate was then delivered dropwise to a stirring solution of 2,4-dichlorobenzoyl)methylenetriphenylphosphorane (Bauer, et al., J. Het. Chem. 1998, 35, 81-87) (1.86 g, 4.14 mmol) in dichloromethane at 10° C. After stirring for 10 hrs. with warming to ambient temperature, the reaction mixture was evaporated, and the residual material was separated by column chromatography (silica gel, 15:85 ethyl acetate-hexane) to afford the title product (0.124 g, 8.2%) as orange crystals (m.p. 116.7-117.8° C.). TLC Rf 0.51 (20:80 ethyl acetate-hexane).
