Hydrochloric Acid → 4-Hydroxy-4-methyl-5-(prop-2-en-1-yl)cyclopent-2-en-1-one

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In a reaction vessel, water (1 liter) and 5-methyl-2-furylallylcarbinol (25 g) were charged, and the pH value was adjusted to 5.5 with an aqueous 1 N NaOH solution and an aqueous 1 N HCl solution. The temperature was elevated up to 100° C. to reflux, and the mixture was stirred under reflux for 12 hours while maintaining a pH value of 5.0 to 5.5 by the addition of an aqueous 1 N NaOH solution and an aqueous 1 N HCl solution. After cooling to 40° C., the reaction mixture was neutralized with an aqueous 1 N NaOH solution, and sodium chloride (300 g) was added thereto. The mixture was extracted with toluene (100 ml) five times. From the extract, toluene was removed by distillation at 60° C. under reduced pressure to give an oily substance (23 g), which was subjected to distillation under reduced pressure to obtain 2-allyl-3-hydroxy-3-methyl-4-cyclopentenone (21.3 g). Yield, 85.2%. B.P., 77° C./0.1 mmHg. NMR spectrum (CCl4, internal standard TMS, δ ppm, 60 MHz): 7.32 (d, 1H, 4-H); 5.92 (d, 1H, 5-H); 5.81 (complex m, 1H, --CH2 --CH=CHaHb); 5.12 (m, 1H, --CH2 --CH=CHaHb); 4.93 (m, 1H, --CH2 --CH=CHaHb); 4.12 (broad s, 1H, 3-OH); 2.37 (m, 3H, 2-H, s, --CH2 --CH=CHaHb); 1.28 (s, 3H, 3--CH3).

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