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Oxochromium(VI) reagents are the most widely used oxidants in organic synthesis for the conversion of alcohols to carbonyl compounds (I) (eq 1).
Pyridinium chlorochromate (PCC), introduced by Corey and Suggs in 1976 (2), has surpassed the Jones reagent (chromic acid) (3) and the Collins reagent (4) (chromium trioxide/ pyridine complex) in frequency of usage in contemporary organic synthesis. When compared with the Jones reagent, PCC is milder and less acidic and promotes fewer side reactions that are due to overoxidation. In contrast with the Collins reagent, PCC is easier and safer to prepare, shelf-stable, and more efficient, since up to a sixfold excess of Collins reagent may be required to effect a complete reaction. Both the PCC and Collins oxidations are characterized by the formation of tarry reduced chromium by-products, which will entrain the desired products and in most cases will complicate an otherwise simple and straightforward purification procedure. The employment of adsorbents in oxochromium(VI)-mediated oxidations of alcohols has led to an improvement in the yields and versatility of these processes in which the removal of the reduced chromium tars is promoted. PCC has been used in conjunction with Celite, molecular sieves, alumina, or silica gel (5-8) to facilitate removal of polymeric reduced chromium by-products or to provide anhydrous conditions that would otherwise lead to unwanted side reactions and decreased yields. In addition, acid-sensitive protecting groups such as tetrahydropyranyl and tert-butyldimethylsilyl ethers remain intact and thus respectable yields of protected products are realized, which lends great value during the course of a complex multistep synthesis. Since our first report of the PCC/silica gel reagent system (9), with and without promotion by ultrasound, the method has been used in several synthetic efforts (10-12).
Adapting of the standard PCC protocol for an experiment in the introductory organic chemistry course or advanced organic chemistry laboratory course required streamlining the process so that the experiment may be accomplished in a 3- or 4-hour period. We found that the silica gel modification conveniently facilitated the process so that an undergraduate student could complete the entire procedure in 3 hours. In performing the exercises students work with the standard equipment found in a typical organic chemistry teaching laboratory. The model procedures included with this communication are the oxidation of cis,trans-4-tertbutylcyclohexanol (1)...