Chromium trioxide-3,5-dimethylpyrazole preparation

Some fine examples of the synthetic use of this reagent are available in the literature for example in a total synthesis of vemolepin, intermediate (70), containing a fairly sensitive lactol ether unit, was selectively prepared by the use of chromium trioxide-dimethylpyrazole with formation of only 5% of the allylically rearranged product (equation 36). 

A 3,5-dimethylpyrazole-chromium trioxide complex, prepared at -20 °C by quickly adding 3,5-dimethylpyrazole to chromium trioxide in CH2CI2 has been shown to be an efficient reagent for allylic oxidation. A -Steroids (e.g. choles-teryl benzoate) are oxidized very rapidly in about 75% yield to the corresponding A -ketone, there being a rate increase of 100-fold compared with using a pyridine-chromium trioxide complex. This increased activity is thought to be partly due to the increased solubility of the complex and more importantly to the possibility of intramolecular acceleration due to the pyrazole nucleus. 

The chief difficulty with this reagent is that the complex is highly hygroscopic. However, it can be prepared in situ, thus avoiding this major drawback. Pyridinium dichromate and chromium trioxide 3,5-dimethylpyrazole are also effective as selective oxidizing agents for these reactions. 

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