Molecular Transformation of 2‐Methylazulenes: An Efficient and Practical Synthesis of 2‐Formyl‐ and 2‐Ethynylazulenes

2‐Formylazulene derivatives have been obtained in good yields by the reactions of 2‐methylazulenes with N , N ‐dimethylformamide dimethyl acetal, followed by oxidative cleavage of the intermediately formed enamines with NaIO 4 . Vilsmeier formylation of 1‐phenyl‐3‐methylazulenes also afforded the corresponding 2‐formylazulenes in moderate yields. The treatment of a 2‐methylazulene derivative bearing a formyl group at the 1‐position with sodium methoxide led, through a self‐condensation reaction, to a trans ‐1‐(azulen‐1‐yl)‐2‐(azulen‐2‐yl)ethylene derivative, the structure of which was verified by single‐crystal X‐ray diffraction analysis. The 2‐formylazulenes obtained were transformed into 2‐ethynylazulenes in good yields by a modified Seyferth–Gilbert reaction. The reactivity of a 1‐iodoazulene bearing a 2‐formyl function in palladium‐catalyzed cross‐coupling reactions has also been examined.