Partially fluorinated heterocyclic compounds. Part 17 [1]. The preparation of fused 2H-pyran derivatives from polyfluoroaryl and -heteroaryl prop-2-enyl ethers with potassium fluoride via an electrocyclisation reaction. A novel [1,5] sigmatropic proton shift during the reaction of pentafluorophenyl prop-2-enyl sulphide with potassium fluoride

Abstract

A new synthesis of fused 2 H -pyran derivatives via an electrocyclisation reaction is described which is based on a novel route to o-quinomethide-type precursors. These transient materials are formed by the dehydrofluorination (with KF) in dipolar aprotic solvents of the Claisen rearrangement intermediates produced by the thermolyses of polyfluoroaryl and -heteroaryl prop-2-enyl ethers. 5,6,7,8- Tetrafluoro-2 H -1-benzopyran (4) is formed from the C 6F 5-ether (1) in refluxing DMF while 5,6,7,8,9,10-hexafluoro-2 H - naphtho[2,1-b]pyran (6) is obtained from the 2-naphthyl ether (5) in sulpholane at 155–162°. The 2,4,5,6-tetrafluoro-3- pyridyl ether (8) in sulpholane at 182° gave a mixture of 6,7,8-trifluoro-2 H -pyrano[3,2-b]pyridine (10) (34%) and 5,6,8-trifluoro-2 H -pyrano[2,3-c]pyridine (12) (1%), but 2,3,5,6-tetrafluoropyridyl ether underwent dealkylation to the 4-hydroxypyridine. The o-quinodimethide intermediate from pentafluorophenylprop-2-enyl sulphide (13) isomerised via a novel [1,5] sigmatropic proton shift before cyclisation to 4,5,6,7-tetrafluoro-2-methylbenzo[b]thiophen (14).