A Thermolytic Azulene Synthesis from Dimethyl 4-Morpholino-3,3a,8,8a-tetrahydroazulene-5,6-dicarboxylates and Its 1-Substituted Derivatives
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The thermolysis of 1-substituted 4-morpholino-3,3a,8,8a-tetrahydroazulene-5,6-dicarboxylates derivatives (<b>7a</b>—<b>c</b>) under catalytic dehydrogenation conditions was studied. The reaction substrates were prepared by a several-step sequence involving the [2+2] cycloaddition of bicyclc 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-5,6-dicarboxylate (<b>7a</b>) containing a catalytic amount of Pd–C gave a 10% yield of dimethyl azulene-5,6-dicarboxylate (<b>8a</b>), accompanied by a 10% yield of dimethyl azulene-4,6-dicarboxylate (<b>9a</b>), trace amounts of dimethyl azulene-5,7-dicarboxylate (<b>10a</b>), methyl azulene-5-carboxylate (<b>12a</b>), and methyl azulene-6-carboxylate (<b>13a</b>). Under the same thermolytic conditions, a 1-phenyl-substituted compound <b>7b</b> gave corresponding similar types of phenyl-substituted azulenes (<b>8b</b>, <b>9b</b>, <b>10b</b>, <b>11b</b>, and <b>14b</b>), and a 1-cyano-substituted compound <b>7c</b> gave a mixture of corresponding similar types of cyano-substituted azulenes (<b>8c</b>, <b>9c</b>, <b>10c</b>, and <b>11c</b>). 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-(1-pyrrolidinyl)-2,7-cycloheptadiene-1,2-dicarboxylate (<b>15</b>), even in the absence of a catalyst, gave cycloheptatriene derivatives of dimethyl 1,3,5-cycloheptatriene-dicarboxylate (<b>16</b>, <b>17</b>, <b>18</b>, and <b>19</b>), bearing no morpholine molecule.
- Bulletin of the Chemical Society of Japan
Bulletin of the Chemical Society of Japan 71(2), 459-465, 1998-02-15
The Chemical Society of Japan