Furans pyridinium chlorochromate

Oxidation of 2,5-dialkylfurans with pyridinium chlorochromate results in high yields of a,(3-unsaturated y-dicarbonyl compounds (Scheme 25) (82S245, 80T661). Similar results are obtained by peracid oxidation of furans (see review (90MI206-01)). The acid most frequently used is MCPBA. It is assumed that the first step involves epoxidation as shown in Scheme 25 (83CL1771). 

Oxidative ring fission of furans using the commercially available reagent pyridinium chlorochromate (PCC) has been studied as well (80T661). Experimental evidence supports the preliminary formation of intermediate (87) formed by 1,4-electrophilic attack of chlorochromate anion upon the furan ring. This intermediate then breaks down by heterolytic cleavage of the Cr—O bonds to afford initially the cis enedione which isomerizes to the trans product. Treatment of (88) with sodium hydroxide in methanol effects ring closure with formation of the 4-methoxycyclopentenone (89 Scheme 22). 

Pyridinium chlorochromate in dichloromethane reacts with furans to give a range of products, but they are all formed by 1,4-electrophilic attack of chlorochromate on the furan ring the fate of the resultant intermediate (57) by heterolytic cleavage of the Cr—O bond depends on the substituents at the a-positions of the substrate. 2,5-Dialkylfurans yield a, /8-unsaturated-y-dicarbonyl compounds, 5-methyl-2-furylcarbinols yield pyran derivatives, and 5-bromo-2-furylcarbinols yield 5-hydroxyfuran-2(5//)-ones (Scheme 25) (80T661). 

The oxidation of 1,4-dihydroxybut-2-enes (pyridinium chlorochromate Mn02/H ), which are in turn available from arylsulfonylmethyloxirans, yields furans (Scheme 28) <89JOCi49i>. 

Other interesting methods can generate furans. One method treats an allylic diol such as 125 with pyridinium chlorochromate (PCC see Chapter 17, Section 17.2.3), in an oxidative-cyclization process, to give 3-ethylfuran, 126. Thiophene derivatives can also be prepared from 1,4-dicarbonyl compounds. A Paal-Knorr thiophene synthesis reacts 2,4-hexanedione (115) with phosphorus pentasulfide (P4S10) to give 2,5-dimethylthiophene, 127. 

Piancatelli G, Scettri A, D Auria M (1977) The oxidation of furan derivatives with pyridinium chlorochromate a novel synthesis of 6-hydroxy-27f-pyran-3(67f)-ones. Tetrahedron Lett 18 (25) 2199-2200. doi 10.1016/S0040-4039(01)83720-5 Pleshakov VG, Abedin MZ, Sergeeeva ND, Prostakov NS (1983) Formation of 9-methylisoindolo [l,2-c]quinoxaline by catalytic dehydrocyclization of 2-(2,4-dimethylphenyl)quinoxaline. J Org Chem, USSR (Engl Transl) 19(3) 586-587... 

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