Approach to the Blues: A Highly Flexible Route to the Azulenes

A palette of blues: Chlorobicyclo[5.3.0]decatrienones are readily prepared from cycloheptatrienes by cycloaddition of dichloroketene, ring expansion with a diazoalkane, and dehydrochlorination in dimethylformamide. These compounds are used as intermediates in the regiocontrolled synthesis of a wide variety of natural and nonnatural azulenes (see scheme).