Alcohols, unsaturated with pyridinium chlorochromate

A novel one-pot tandem oxidation-cyclization-oxidation process was successfully applied in the transformation of unsaturated alcohols 250 [Eq. (5.315)].860 The intermediate aldehyde formed by oxidation with pyridinium chlorochromate (PCC) undergoes a carbonyl-ene cyclization followed by an additional oxidation to form 3-substituted piperidinones. 

Having obtained the desired cis stereochemistry, the alcohol was oxidized to the aldehyde with pyridinium chlorochromate and subjected to Wolff-Kishner reduction to yield the CDE moiety in overall yield of 30% from the unsaturated tricylic ester. Lithium-ammonia reduction of the aromatic nucleus gave the 3,7 enone 206. 

Oxidation of the tertiary allylic alcohol with pyridinium chlorochromate (PCC) gave the unsaturated ketone. 

The allyl alcohol in the seven-membered ring is oxidized to the corresponding a,P-unsaturated ketone as expected, whereas the allyl alcohol in the five-membered ring is oxidized to afford the transposed a,P-unsaturated ketone after [3,3]-sigmatropic rearrangement. This behavior is only observed with cyclic, tertiary allyl alcohols upon treatment with one equivalent of pyridinium chlorochromate. The desired oxidation product 34 is obtained in 90 % over two steps. 

Oxidation of primary alcohols carrying unsaturation at the 5 or 6 position by means of the pyridinium chlorochromate provides the cyclized products. The same products are obtained by oxidation of analogously substituted aldehydes, with this reagent. ... 

A useful application of chromium-based oxidants, especially pyridinium chlorochromate, is in the conversion of allylic tertiary alcohols to their transposed a,(3-unsaturated ketones. For example, treatment of the allylic alcohol 24 with PCC gave the a,p-unsaturated ketone 25 (6.23). The reaction is thought to proceed by rearrangement of the chromate ester of the allylic alcohol to give a new allyl chromate ester that is oxidized to the ketone. 

This section deals with oxidation reactions as an initiative step of the domino process in combination with many other organic transformations. In 2004, Snaith and coworkers [3] demonstrated a simple and efficient method to synthesize 3-substituted 4-piperidinones 4 using a domino oxidation/carbonyl-ene/oxidation reaction (Scheme 9.1). This domino reaction comprises oxidation of unsaturated alcohol 1 using PCC (pyridinium chlorochromate) to give the corresponding aldehyde 2, followed by a carbonyl-ene type reaction to yield the secondary cyclic alcohol 3, which was oxidized under the reaction conditions to give 3-substituted 4-piperidinones 4 in good yields. 

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