Approximately, 1.1–7 equiv., typically1.5 equiv. of solid PCC are added over a ca. 0.01–0.25 M solution of thestarting alcohol in dry DCM. The resulting mixture is stirred at roomtemperature. Very often, ca. 0.2–1.2 g of activated MS per mmol of alcohol areadded in order to accelerate the reaction. In order to moderate the acidity ofPCC, it is very common to add ca. 0.3–1 equivalents of NaOAc. A solid support,such as silica gel, Celite, Florisil or magnesium sulfate, is added, very oftenin a proportion of ca. 0.3–2 g of solid support per mmol of alcohol, in orderto facilitate the work-up. Occasionally alumina, working both as a solidsupport-used to facilitate the work-up and as an accelerant, mixed with PCC isadded, in a proportion of ca. 0.4–1.5 g of aluminaper mmol of alcohol. Normally, PCC is deposited over the alumina. Occasionally,ca. 10–20 equivalents of acetic acid are added in order to accelerate thereaction.
Sometimes, the reaction flask issonicated with ultrasound in order to fragment the surface of the PCC particlesand, therefore, accelerate the reaction.
Although in PCC oxidations, it is verycommon to add simultaneously to the reaction an accelerant, a buffer and awork-up-facilitator; it is not common to employ simultaneously two materialsbelonging to the same kind, with the exception of the combination of the twoaccelerants molecular sieve and acetic acid, which are very often usedtogether.
When a TLC analysis shows that most ofthe starting alcohol is consumed, the solids suspended in the reaction andthe chromium species are removed by filtration through a pad of Florisil1,silica gel, alumina or Celite, and the pad is washed with an organic solvent,such as ether, DCM, or EtOAc. Sometimes, the solids can be removed bydecantation. Other times, it is advisable to add some diethyl ether to thereaction mixture before the filtration, in order to promote the separation ofreduced chromium species in a granular form. Occasionally, the reaction mixtureis concentrated before the addition of diethyl ether. Finally, the collectedorganic phases are concentrated at the rotary evaporator, giving a crudealdehyde or ketone that may need some further purification.
A 500-mL, round-bottomed flask equipped with a 4.5-cm, egg-shaped Teflon-coated magnetic stir bar ischarged with 130 mL of CH2Cl2, the alcohol prepared in Step A (10.4 g, 40.0 mmol), and 15 g of freshly powdered 3 Å molecularsieves. Pyridinium chlorochromate (21.5 g, 100mmol) is added portionwise over 10 min and the resulting mixture isstirred at room temperature for 15 hr. Ether (200 mL) is added slowly with vigorous stirring and thesolution is filtered under vacuum through a pad of 35 g of Celite. The solids remaining in the reactionflask are transferred to the Celite pad by scraping with a spatula and washing with three 50-mLportions of ether. The resulting cloudybrown filtrate is concentrated by rotary evaporation at room temperature togive a brown solid. To this solid is added 25 mL of 1:1 ether:hexaneand the solids are scraped with a spatula. The mixture is then poured onto 60 g of Whatman 60 Å (230-400 mesh) silicagel packed in a 4-cm diameter chromatography column and the liquid is adsorbedonto the silica gel by gravity. The material remaining in the flask is furtherwashed with 1:1 ether:hexane and transferred ontothe silica gel; this process is repeated until all the material has been loadedonto the silica gel. The ketone is eluted using 500 mL of1:1 ether:hexane and the eluent is concentrated by rotaryevaporation to afford the crude ketone as a white solid. This material isdissolved in 40-45 mL of boiling hexane. Upon cooling the solution to room temperature,the ketone begins to crystallize. The flask is then cooled to −25 °C for 2 hr. The resulting solids arecollected by filtration, washed with three 25-mL portionsof cold (−25 °C) hexane, and dried to afford 8.84-9.08 g, (86-88%)of the ketone as a white solid.
Notes：PCC is prepared by addition of pyridine to a solution of chromium trioxide (CrO3) in aqueous HCland crystallization.
Reference:OrganicSyntheses, Vol. 80, p.1
【Synlett 2004, 738–740】