Four kinds of biazulenes, 1,1′- (1a), 2,2′- (2a), 1,2′- (3a), and 2,6′-biazulenes (4a), and their derivatives were synthesized by utilizing Ullmann-type coupling of haloazulene derivatives. Ullmann reaction of ethyl 3-iodo-(5a) and 2-iodoazulene-1-carboxylates (6b) gave diethyl 1,1′-biazulene-3,3′-dicarboxylate (1b) and diethyl 2,2′-biazulene-1,1′-dicarboxylate (2b), respectively, in excellent yields. The reaction of 2-iodoazulene gave 2a directly. Diethyl 2-chloroazulene-1,3-dicarboxy late (7a) and its 5-alkyl derivatives also reacted to give tetraethyl 2,2′-biazulene-1,1′,3,3′-tetracarboxylate (2d) and its 5,5′-dialkyl derivatives, respectively. A mixed Ullmann reaction of 5a and 6b afforded a mixture of 1b, 2b, and diethyl 1,2′-biazulene-1′,3-dicarboxylate (3b). Similarly, a mixed Ullmann reaction of 7a and diethyl 6-iodoazulene-1,3-dicarboxylate gave a mixture of 2d and tetraethyl 2,6′-biazulene-1,1′,3,3′-tetracarboxylate (4b). The parent hydrocarbons of biazulenes, 1a, 2a, 3a, and 4a, could be derived from the corresponding ethoxycarbonyl derivatives, 1b, 2b, 2d, 3b, and 4b, respectively, by decarboxylation. The planarity of biazulenes and their ester derivatives is also discussed on the basis of the spectral data.
