Synthesis of Isopropyldieneacetophenone from 3-Hydroxy-3-methyl-1-phenyl-1-butanone -- DMAP as Catalyst

K. Narasaka, Org. Synth., 65, 12 (1987)

1. A solution of 3-hydroxy-3-methyl-1-phenyl-1-butanone in dry dichloromethane (60 mL) is stirred under an inert a®osphere and cooled in an ice bath.  To this cooled solution is added triethylamine (28.5 g), DMAP (catalytic amount), and dichloromethane (20 mL).  A solution of trifluoroacetic acid (25.8 g) in dichloromethane (40 mL) is added dropwise over 15 min. and the reaction mixture is stirred for a further 2.5 h.  The ice bath is removed and stirring is continued for a further 21 h at room temperature (approximately 30°C).
2. The rate of stirring is increased and saturated sodium carbonate (100 mL), water (100 mL), and ether (300 mL) are added to the reaction mixture.  The organic layer is separated and the water layer is extracted with ether (1 x 100 mL).  The combined organic phases are washed with brine, dried over magnesium sulfate, and filtered.
3. The ether solution is condensed and the residue is distilled under vacuum to afford isopropylideneacetophenone (14.5-16.0 g, 88-97% yield, b.p. 73-75°C at 0.4 mm).