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Synthesis, Stability, and Crystal Structure of an Azulenium Cation Containing an Adamantyl Group



Starting from the trimethylsilyl enol ether of 1-acetyl-1,3,5-cycloheptatriene, the title 1H-azulenium cation was synthesized by a five-step sequence that involved a Noyori–Mukaiyama aldol reaction, a Nazarov cyclization, a Shapiro reaction, and a hydride abstraction. The Nazarov reaction of the aldol-type adduct resulted in the formation of an unusual double-bond position isomer, which has never been obtained in similar reactions forming tetrahydroazulenones. The pKR+ value of the title cation was found to be 9.8, which is less than that expected by inductive stabilization from the number of carbons at the 1-position. The X-ray crystal structure of the title cation (ClO4 salt) reveals CH–O interactions and deformation of the azulenyl ion part.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)