Decarbonylation Enables Chemodivergent Access to Guaiazulene-3-Oxalic and -Carboxylic Acid Derivatives

Guaiazulene derivatives are of interest due to the low cost of guaiazulene compared to azulene while still retaining many of the same unique electronic properties. Syntheses of guaiazulene-3-carboxylic acid and azulene-1-carboxylic acid derivatives have been described, but they typically rely on uncommon or toxic reagents such as oxalyl bromide or phosgene. We describe a new route to these compounds by functionalization with oxalyl chloride, followed by thermal or bromide-induced decarbonylation and quenching with a nucleophile. Combined experimental and computational studies suggest a concerted 1,2-halide shift mechanism for the decarbonylation.