Synthesis of Azulene Derivatives

　3,4-Dichlorothiophene 1,1-Dioxide (1) is useful as a synthetic starting material for the preparation of azulene derivatives. More specifically, the cycloaddition reaction of 1 with furan produce adducts 2 and 3, which can then be converted into 4 by treatment with triethylamine at room temperature. This compound 4 undergoes further cycloaddtion with 5 to give intermediate 6. Subsequent elimination of sulfur dioxide and dimethylamine affords 7 which can then be elaborated into azulene derivative 9 by treatment with tetrazine derivative 8, after denitrogenation and elimination of pyridazine derivative. Due to its instability, derivative 9 is isolated and identified as its Diels-Alder adduct with N-methylmaleimide.