Identification of the isomers was unambiguous by comparison of their 13C NMR spectra with the known spectra7,11 of the C-2 and C-6 monomethylated compounds. 20,6a-Dimethyl-trazis-decahydroqumoline: mp 38-39 °C; mp (for the picrate) 151-153 °C; 13C NMR (CDC13) 61.77 (C-8a),
Irradiation at 2537 Á of perfluorocyclooctatetraene yielded antiand syn-perfluorotricyclo[4.2.0.0y]octa-3,7-dienes la and lb, respectively, which reverted quantitatively to the tetraene at 150 °C. Bromine addition occurred exclusively in the exo, suprafacial fashion with both dienes, but only the anti isomer added 2 mol of bromine. The latter fact, together with analysis of the 19F NMR spectra of the bromine adducts, constitutes convincing evidence for the configurational assignments for la and lb. Perfluorobenzene underwent [2 + 2] photocycloaddition with chlorotrifluoroethylene, and the bicyclic adducts suffered photochemical electrocyclization to a stereoisomeric mixture of 7-chlorotricyclo[4.2.0.02,5]oct-3-enes. Analysis of the 19F NMR spectra of the major isomers permitted their assignment as exo,anti and endo.anti. Reduction of the former gave la, thus confirming the diene's configuration. An explanation based on "second-order" orbital symmetry effects is suggested for the syn/anti stereoselectivity found in the thermal and photochemical electrocyclic processes described above.
Difluordichlorethylen (II) reagiert mit Hexafluorbenzol (I) bei der Photolyse zu einem Tricyclus (III), der thermisch zum Dihydrobenzocyclobutan (IV) führt.
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