SYNTHESIS, STRUCTURE AND REACTIVITIES OF PARACYCLOPHANES

Article abstract of DOI:10.1016/S0040-4020(01)87605-5

The synthesis, structure and reactivities of <6>paracyclophanes 2a-c, the smallest bridged paracyclophanes so far isolated, are described.The parent hydrocarbon 2a has been efficiently synthesized by oxidative decarboxylation of <6.2.2>propellenecarboxylic acid (7) by lead tetraacetate.The 8-carbomethoxy derivative 2b has been quantitatively synthesizedby thermal valence isomerization of the <6.2.2>propelladiene (4b) (Dewar isomer of 2b).X-Ray structure analysis of the crystalline 2c has revealed that the benzene ring of 2c is severely deformed into a boat conformation with deformation angles of α=20.5 degree and β=18.5 degree.Furthermore, the bond angles of the bridging chain (C(2), C(3), C(4) and C(5)) are considerably expanded from the normal sp³ angle.Vapor phase thermolysis of 2a gives the spiro triene 12 via homolysis at the benzyl position.Acid-catalyzed isomerization of 2a with trifluoroacetic acid takes place readily to afford the meta and ortho isomers 13a and 14a in a ratio of 1:3.UV irradiation of the ester 2b brings about valence isomerization to the Dewar isomer 4b which izomerizes slowly to the prismane derivative 15a on further irradiation.A <4+2> cycloaddition of 2a with N-phenyl-1,2,4-triazoline-3,5-dione occurs at room temperature to give mainly adduct 16.Addition of bromine to 2a takes place to furnish quantitatively the unstable 1,4-addition product 18.Oxidation of 2a with mCPBA readily takes place to give the dienone dimer 20 which affords on UV irradiation the cage diketone 21 derived from intramolecular photocycloaddition.