61846-10-4Relevant articles and documents
Synthese, Thermolyse und Photolyse substituierter 3,4-Epoxy-cycloalkene
Eberbach, Wolfgang,Carre, Jean Claude
, p. 1027 - 1047 (2007/10/02)
The substituted epoxy-cycloalkenes 5 - 7 are transformed thermally at 200 - 250 deg C into the benzene derivatives 10, 15, and 18 after initial C/O-bond cleavage of the oxirane ring.At temperatures higher than 300 deg C (short time pyrolysis) 6 and 7 give, in addition to the aromatic compounds 15/18, the furans 16 and 19, respectively.On photochemical excitation 5 - 7 react with C/C-ring opening to the annelated 2,3-dihydrofurans 11, 17, and 20, respectively.The epoxycycloheptene 8 undergoes analogous ring expansion leading to 21 on electronic as well as thermal activation (ca. 80percent yield).Whereas 21 does not form any defined product up to 380 deg C, and the monosubstituted bicyclus 11 suffers fragmentation affording methyl 3-furancarboxylate (12) and ethylene, both dihydrofurans 17 and 20 are quantitatively converted back into the epoxy-cycloalkenes 6 and 7, respectively, at temperatures below 200 deg C; the cycloreversion reaction affording 16/19 competes only above 300 deg C.The cyclic dipoles of type IV, assumed to occur as intermediates during the 2,3-dihydrofuran-vinyloxirane isomerisations 17 . 6/20 . 7, can be trapped with dimethyl acetylenedicarboxylate or N-phenylmaleimide at significantly lower temperature (150/120 deg C) to give the 1:1-cycloadducts 38 - 40.However, because no such products could be obtained in case of the photoreactions of 5 - 8, a concerted -C-migration has to be considered as a mechanistic alternative of the light-induced formation of 11, 17, 20, and 21.
