Dedicated to Dr. Jean-Louis Luche on the occasion of his retirement
Nearly a century and a half ago, Septimus Piesse applied the descriptive name azulene to azure-blue distillates from various sources, such as chamomile, yarrow, and wormwood. However, it was only in 1937 that Placidus Plattner and Alexander Pfau successfully carried out the first synthesis of azulene (1), [2] the appellation given to the parent compound of the family of bicyclo[5.3.0]decapentaenes, which are now collectively referred to as the azulenes. Since this historical event, additional syntheses of the parent azulene and preparations of various other azulenes, such as guaiazulene (2), which is the archetype of the major azulene class, have been reported. Although the approaches to these fascinating and electronically unique compounds are often ingenious, they suffer for the most part from being excessively long, low yielding, or lacking in generality. Many substitution patterns of the azulenes are still difficult to access, if they can be accessed at all. We have found that chlorotrienones 5 a and 5 b are readily prepared from cycloheptatriene and 7-methylcycloheptatriene, respectively, through a doubly regioselective cycloaddition of dichloroketene, a regioselective ring expansion with ethereal diazomethane, and dehydrochlorination in dimethylformamide (DMF; Scheme 1). [4]