Catalytic reagents developing more selective

Predicting the migration ability of groups has also improved during the first 50 years and continues to be discussed today. The development of alternative reagents which are more selective and mild continues. The development and application of catalytic systems whether enzymatic or metal mediated, which will predictably provide asymmetric oxidation products, have been successfully applied but remains in its early stages. 

The development of novel materials and the search for new bioactive compounds, such as natural products and analogues, drugs, diagnostics and agrochemicals, in academic and industrial chemistry is closely connected to the efficient preparation of such compounds. Thus, the view of synthesis has altered in recent years the development of new, highly selective methods will still be an important task, but the main focus of today s chemists is on efficiency [1], Multi-step syntheses with more than 20 steps have to be avoided since they are neither economically nor ecologically justifiable. Thus, modern syntheses must deal carefully with our resources and our time, must reduce the amount of waste formed, should use catalytic transformations and finally must avoid all toxic reagents and solvents. 

Chromium-based oxidants are probably the most widely used of all oxidizing agents. Over the years they have been continually developed and modified to overcome the typical problems that occur during oxidation and to accept wider ranges of substrates with improved selectivities. They have been accepted readily by synthesis chemists since they are easy to handle and are often off the shelf reagents . However, they are not without their problems worit-up can be problematical overoxidation can occur, and, at all times, removal of the product from toxic chromium contaminants is a concern, especially with respect to large scale preparations. In an attempt to circumvent these problems the trend has been to develop the use of catalytic and/or supported reagents. Hiis review is concerned for the most part with the ai lica-tions and limitations of more recent chromium(VI) oxidants. Several other comprehensive reviews have appeared in this area and should be consulted for more detailed descriptions of older methods, chro-mium(V) oxidants, mechanism of oxidation and for typical experimental procedures. 

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