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 pK_R⁺ 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.