A Thermolytic Azulene Synthesis from Dimethyl 4-Morpholino-3,3a,8,8a-tetrahydroazulene-5,6-dicarboxylates and Its 1-Substituted Derivatives

Article abstract of DOI:10.1246/bcsj.71.459

The thermolysis of 1-substituted 4-morpholino-3,3a,8,8a-tetrahydroazulene-5,6-dicarboxylates derivatives (7a-c) under catalytic dehydrogenation conditions was studied. The reaction substrates were prepared by a several-step sequence involving the [2+2] cycloaddition of bicycle morpholino enamines with dimethyl acetylenedicarboxylate, and a subsequent electrocyclic opening of the cyclobutene ring as a key skeletal construction, starting from bicyclo[3.3.0]octane-2,6-dione mono(ethylene acetal). Refluxing of a diphenyl ether solution of dimethyl 4-morpholino-3,3a,8,8a-tetrahydroazulene-5,6dicarboxylate (7a) containing a catalytic amount of Pd-C gave a 10% yield of dimethyl azulene-5,6-dicarboxylate (8a), accompanied by a 10% yield of dimethyl azulene-4,6-dicarboxylate (9a), trace amounts of dimethyl azulene-5,7-dicarboxylate (10a), methyl azulene-5-carboxylate (12a), and methyl azulene-6-carboxylate (13a). Under the same thermolytic conditions, a 1-phenyl-substituted compound 7b gave corresponding similar types of phenyl-substituted azulenes (8b, 9b, 10b, 11b, and 14b), and a 1-cyano-substituted compound 7c gave a mixture of corresponding similar types of cyano-substituted azulenes (8c, 9c, 10c, and 11e). This sequence provides a new method for synthesizing substituted azulenes suffering from migration and removal of one ester group. Also the thermolysis of a similar system of dimethyl 3-(l-pyrrolidinyl)-2,7-cycloheptadiene-l,2-dicarboxylate (15), even in the absence of a catalyst, gave cycloheptatriene derivatives of dimethyl 1,3,5-cycloheptatriene-dicarboxylate (16, 17, 18, and 19), bearing no morpholine molecule.