The synthesis of 6,7-benzo-4,9-oxido[11]annulenone and aromatic 6,7-benzo-4,9-oxido[11]annulenyl cations☆

Abstract The titled annulenone 6 , the first benz-annelated 11-membered fully conjugated ketone, has been prepared by the aldol type condensation of 3-benzoxepin-2,4-dialdehyde 3 and dimethyl acetone-dicarboxylate. Some evidence for the nonaromatic character of 6 could be obtained from the NMR and IR spectra as well as protonation behaviour of 6 . The NMR spectrum of 6 in conc H 2 SO 4 confirmed the presence of an aromatic 1-hydroxy-6,7-benzo-4,9-oxido[11]annulenyl cation 7 . 6,7-Benzo-4,9-oxido-1-hydroxy-1-homo[10]annulene 11 was prepared by the NaBH 4 reduction of 6 . The treatment of 11 with CF 3 COOH regenerated another completely delocalized [11]annulenyl cation 12 . The pk R + of 12 was obtained as −4·1, spectrophotometrically. The electronic spectra of these novel cations ( 7 and 12 ) were found to be similar to those of 4,5-(2′,3′-naphtho)tropylium cations ( 13a and 13b ), respectively.