SOME REACTIONS OF 3,4-BIS(TRIFLUOROMETHYL)FURAN AND ITS PRECURSOR, 2,3-BIS(TRIFLUOROMETHYL)-7-OXABICYCLO[2,2,1]HEPTA-2,5-DIENE - NOVEL ISOCOUMARIN FORMATION FROM THERMAL-REACTION OF THE FURAN WITH ETHYL PROPYNOATE

Thermolysis at 450-degrees-C of 2,3-bis(trifluoromethyl)-7-oxabicyclo[2,2,1]hepta-2,5-diene (2) gives 3,4-bis(trifluoromethyl)furan (1). Treatment of oxanorbomadiene (2) with triflic acid (in CH2Cl2 under reflux) and then with water affords a mixture of 2,3-bis(trifluoromethyl)phenol (4) (6%), tris-(6-trifluoromethylsalicylide) (5) (45%) and 2-trifluoromethyl-6-hydrobenzoic acid (6) (16%); analogous reaction with conc. sulphuric acid gives only the phenol 4 (44%). Diels-Alder reaction either of furan 1 with dimethyl acetylenedicarboxylate (DMAD) or of dimethyl 3,4-furandicarboxylate with hexafluorobut-2-yne gives the oxanorbornadiene (8) (ca. 80%). In contrast, reaction of furan 1 with ethyl propynoate (EP) (1:1 molar ratio) at 150-degrees-C affords a mixture of ethyl 6,7-bis(trifluoromethyl)isocoumarin-3-carboxylate (12) (52%) and its 4-carboxylate isomer 13 (9%), presumed to arise via a novel Diels-Alder cycloaddition of the dienophile EP to the C=C-C=O system of the 1:1 adduct ethyl 2,3-bis(trifluoromethyl)-7-oxabicyclo[2,2,1]hepta-2,5-diene-5-carboxylate (14). Diene 14 is conveniently prepared (93%) by the reaction of a large excess of hexafluorobut-2-yne with ethyl 3-furoate at room temperature, and when heated with EP (1:1 molar ratio) at 150-degrees-C the isocoumarin 12 (84%) is obtained; at 60-degrees-C, however, 14 decomposes to give furan 1 and EP.