Insight into Pyridinium Chlorochromate Chemistry: Catalytic Oxidation of Tetrahydrofuran Compounds and Synthesis of Umbelactone

The catalytic system PCC (cat.)/H5IO6 has been used to oxidise mono- and polytetrahydrofuran compounds. New oxidative pathways are disclosed. 2,2,5-Trisubstituted THF rings are converted into dicarbonyl compounds through oxidative cleavage of the C2–C3 bond. Cyclic enol ethers appear to be intermediate species in this process. Oxidation of 2,2,5-trisubstituted α-keto-THF compounds proceeds with the oxidative cleavage of the C2(THF)–C=O bond to give 1,4-diketones with degraded carbon backbones. Attack of the oxidant on 2,5-disubstituted THF rings leads to 1,4-diketones containing intact THF carbon frameworks. Oxidation of complex poly-THF substrates, containing up to five THF rings, allows access to new poly-THF compounds through regioselective THF oxidation along the poly-THF backbones. A mechanistic explanation of the new processes consistent with the reported reactivity of PCC and the isolation of some minor products of the process is provided. The synthesis of racemic umbelactone, an antiviral natural butenolide metabolite, has been carried out by use of the developed chemistry.