Decarbonylation Reaction of 1-Azulenecarbaldehydes under Mild Conditions. A Strategy for the Protection of 1- and/or 3-Position of Azulene Rings

Abstract The reaction of 1-azulenecarbaldehydes 9a and b with pyrrole (8) in acetic acid resulted in decarbonylation to afford azulenes even at room temperature in 49 and 78% yields, respectively. 1,3-Azulenedicarbaldehydes also reacted with 8 to give azulenes in 36 and 52% yields, respectively. This decarbonylation reaction was adopted to the selective synthesis of 3,3′-unsubstituted di(1-azulenyl)methane derivatives 7a—d. Acid-catalyzed condensation of 9a and b with paraformaldehyde or benzaldehyde afforded 3,3′-methylenedi(1-azulenecarbaldehyde)s, following this decarbonylation reaction gave the desired 7a—d as a sole product in 33—59% yields starting from 9a and b. Such decarbonylation is because of the ability of protonation of azulene ring in acidic condition and because of electron-donating properties of pyrrole ring. This reaction would serve as a new strategy for the protection of 1- and/or 3-positions of azulene ring.