Step 4: Preparation of 3-isopropyl-1H-pyrazole (88b). 1-(Dimethylamino)-4-methylpent-1-en-3-one (87) (6.6 g, 1 eq.) was slowly added dropwise to a stirred mixed solution of hydrazine monochloride (3.2 g, 1 eq.), sulfuric acid (1.13 mL) and water (6 mL). The reaction mixture was stirred at 68 °C for 2 hours. Upon completion of the reaction, the reaction mixture was neutralized with 1N NaOH solution and subsequently extracted with ether. The organic layers were combined, dried with anhydrous Na2SO4, filtered and concentrated under reduced pressure to afford the target product 3-isopropyl-1H-pyrazole (88b) as a beige solid in 94% yield.1H NMR (DMSO-d6, 400 MHz) δ: 1.17 (s, 3H), 1.19 (s, 3H), 2.87-2.93 (m, 1H), 5.99 ( s, 1H), 7.40 (s, 1H), 1.39-1.43 (t, J = 7.04 Hz, 3H), 4.08-4.13 (q, J = 7.04 Hz, 2H), 5.86 (d, J = 12.40 Hz, 1H), 7.90 (d, J = 12.40 Hz, 1H).
