The synthesis of [15]annulenone 4,7:10,13-dioxides and aromatic 4,7:10,13-dioxido[15]annulenium cations : Novel 14]gp ‘odd’ annulene systems

Abstract 2,15 - Dimethoxycarbonyl - [15]annulenone 4,7:10,13 - dioxide ( 14 ) has been prepared by the condensation of cis -α,β -bis(5 - formyl - 2 - furyl)ethy]ene ( 13 ) with dimethyl acetonedicarboxylate. Treatment of 14 with conc H 2 SO 4 led to 4,7:10,13-dioxido[15]annulenone 2,15-dicarboxylic acid anhydride ( 17 ), which was subsequently converted to the corresponding dicarboxylic acid ( 18 ) by dilute KOH. Decarboxylation of 18 gave rise to two isomeric [15]annulenone 4,7:10,13 - dioxides, i.e., the tri- cis isomer ( 7 ) and the mono - trans - di - cis isomer ( 8 ). Regarding to the ring current effects, the proton chemical shifts of these [15]annulenones were compared with those of a reference model, 4,7:10,13 - dioxido - cyclopentadecaheptaene (2.4.6.8.10.12.14) ( 3 ). Both of the parent [15]annulenones ( 7 and 8 ) have been interpreted as nondiatropic, while the anhydride ( 17 ) has been shown to be diatropic, sustaining an induced diamagnetic ring current. The enforced planarity and symmetrical geometry of the anhydride have been discussed. As expected, when 7 , 8 , 14 , 17 and 18 were dissolved in CF 3 COOH or conc H 2 SO 4 , completely delocalized [15]annulenium cations were produced, all of which proved to be diatropic. Three possible geometrical isomers of these 14π cations were established experimentally.