PARTIALLY FLUORINATED HETEROCYCLIC COMPOUNDS. PART 17 . THE PREPARATION OF FUSED 2H-PYRAN DERIVATIVES FROM POLYFLUOROARYL AND -HETEROARYL PROP-2-ENYL ETHERS WITH POTASSIUM FLUORIDE VIA AN ELECTROCYCLISATION REACTION. A NOVEL SIGMATROPIC PROTON SHIFT DURING THE REACTION OF...

Article abstract of DOI:10.1016/S0022-1139(00)81169-0

A new synthesis of fused 2H-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-2H-1-benzopyran (4) is formed from the C6F5-ether (1) in refluxing DMF while 5,6,7,8,9,10-hexafluoro-2H-naphtho<2,1-b>pyran (6) is obtained from the 2-naphthyl ether (5) in sulpholane at 155-162 deg.The 2,4,5,6-tetrafluoro-3-pyridyl ether (8) in sulpholane at 182 deg gave a mixture of 6,7,8-trifluoro-2H-pyrano<3,2-b>pyridine (10) (34percent) and 5,6,8-trifluoro-2H-pyrano<2,3-c>pyridine (12) (1percent), but 2,3,5,6-tetrafluoropyridyl ether underwent dealkylation to the 4-hydroxypyridine.The o-quinodimethide intermediate from pentafluorophenylprop-2-enyl sulphide (13) isimerised via a novel <1,5> sigmatropic proton shift before cyclisation to 4,5,6,7-tetrafluoro-2-methylbenzothiophen (14).