4-Acetoxy-5-^tbutyldimethylsilyloxy-cyclopent-2-en-1-one (245 mg, 0.91 mmol)
Sodium borohydride (powdered, 50 mg, 1.42 mmol)
Cerium trichloride heptahydrate (dried in vacuum, 450 mg, 1.22 mmol)
Methanol (distilled from CaH₂, 4mL)

This procedure is used for the chemoselective reduction of enones to produce allylic alcohols, in this case (and many others) with a good degree of stereospecificity. The enone and cerium trichloride are stirred in methanol at 0C and the sodium borohydride is added portionwise over about 15 minutes; some effervescence should be seen. Stirring is continued at this temperature until completion (TLC), usually about 1.5-2 hours, then quenched with ammonium chloride solution, extracted 4 times with ethyl acetate, washed with brine, dreid over magnesium sulphate, filtered, concentrated and purified by column chromatography eluting with 20% EtOAc/PE

The yield is 80% of the desired trans, trans isomer in this case, with 8% of the undesired cis, trans isomer also being isolated. No reduction of the alkene is observed. The reaction is slightly moisture sensitive and thus fresh, powdered sodium borohydride should be used, and the cerium trichloride should be dried for a few hours under vacuum first. The reaction has also been carried out in the presence of a free hydroxy group in a slightly reduced yield (60-70%).

¹HNMR (300MHz, CDCl₃) 5.83 (1H, d), 5.72 (1H, d), 5.23 (1H, s), 4.39 (1H, s), 4.04 (1H, t), 1.98 (3H, s), 0.81 (9H, s), 0.03 (3H, s), 0.00 (3H, s).

Luche JL, Rodriguez-Hahn L, Crabbe P. J. Chem. Soc. Chem. Comm. 1978 p.601