NOVEL DIELS-ALDER CYCLOADDITION INVOLVING 2ALPHA,BETA-UNSATURATED ESTERS IN THE FORMATION OF ISOCOUMARINS FROM THE REACTION OF ETHYL PROPYNOATE WITH 3,4-BIS(TRIFLUOROMETHYL)FURAN

Reaction of ethyl propynoate with 3,4-bis(trifluoromethyl)furan (4) at 150°C affords a mixture of ethyl 6,7-bis(trifluoromethy)isocoumarin-3- carboxylate (7) and the corresponding 4-carboxylate (8) in the ratio 52:9, produced by Diels-Alder cycloaddition of the alkyne to the α,β-unsaturated ester function of the initially-formed 1:1 adduct, the oxanorbornadiene (9), followed by opening of the oxygen bridge and elimination of ethanol. The corresponding reaction involving dimethyl acetylenedicarboxylate gives the expected oxanorbornadiene (5). Thermal reactions of furan and its derivatives with acetylenic esters have been widely investigated since that between furan and dimethyl acetylenedicarboxylate (DMAD) was first reported in 1931 [1]. Products from the DMAD reactions are temperature and reactant ratio dependent, e.g. Diels-Alder 1:1, 2:1, and 3:1 furan : DMAD adducts have been isolated and characterised [1,2]. Reactions involving ethyl propynoate (EP) are less facile than those with DMAD and the only product isolated from the reaction with furan at 130°C was the 1:2 adduct (1) [3]; 2,5-dimethylfuran afforded the analogous compound (2) as the major product [4]. Compounds (1) and (2) were considered to arise by dimerisation of the initially-formed 1:1 adduct, the oxanorbornadiene (3), by a π2s + π2s + π2s mechanism followed by retrocleavage of furan or 2,5-dimethylfuran [4] (Scheme 1). In the present work the reactions of 3,4-bis(trifluoromethyl)furan (4) [5,6] with DMAD and EP have been carried out as part of a general investigation of the chemistry of this furan derivative. © 1990.