Corey’s PCC

The Collins/Sarett oxidation (chromium trioxide-pyridine complex), and Corey s PCC (pyridinium chlorochromate) and PDC (pyridinium dichromate) oxidations follow a similar pathway as the Jones oxidation. All these oxidants have a chromium (VI), normally yellow, which is reduced to Cr(IV), often green. 

The mechanism of the Sarett oxidation, Collins oxidation, with Corey s PCC and with PDC, follows a similar mechanism as shown in Scheme 7.1. The alcohol reacts with CrOs to give a chromate ester. Either a base (Py) removes a proton from the chromate ester to give an oxidized product (aldehyde or ketone) and HCrOs" or a proton is transferred by the intramolecular mechanism to give an aldehyde or ketone and H2Cr03. 

The Collins oxidation, Jones oxidation, and Corey s PCC (pyridinium chlorochromate) and PDC (pyridinium dichromate) oxidations follow a similar pathway. 

These authors also described a three-step synthesis of 13Z-retinoic acid [56], The obtained hydroxydihydropyrane (66%) was oxidized either by Jones s reagent (CrC>3, water, H2SO4, 90%) or Corey s reagent (pyridinium chlorochromate (PCC), 65%). Finally, the dihydropyranone was transformed into retinoic acid (as a mixture of 9E, 13Z, and 9Z,13Z), by /BuOK, according to a known procedure [57], Fig. (26). 

In the final stages of the total synthesis of (+)-cephalotaxine by M.E. Kuehne et al., a tetracyclic c/s-vicinal diol was oxidized to the a-diketone. Using PCC, pyridine/SOs or the Swem protocol did not yield the desired product. However, by applying the Corey-Kim protocol, NCS-DMS in dichloromethane at -42 °C, afforded the diketone in 89% yield. 

Pyridinium Chlorochromate. The need for improved oxidation of primary alcohols and greater ease for isolation of products prompted further research into the nature of Cr(VI) reagents. Corey found that addition of pyridine to a solution of chromium trioxide in aqueous HCl allowed crystallization of a solid reagent characterized as 31, pyridinium chlorochromate (PCC). This reagent was superior for the conversion of primary alcohols to aldehydes in dichloromethane but less efficient than the Collins oxidation when applied to allylic alcohols. Oxidation of 1-heptanol with PCC in dichloromethane gave 78% of heptanal, for example. As stated by Corey, PCC is an effective oxidant in dichloromethane although aqueous chlorochromate species are not very effective oxidants. Oxidation of secondary alcohols to ketones is straightforward, as in Banwell s synthesis of y-lycorane, in which 32 was oxidized by PCC to the ketone (33). ... 

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