THERMAL-REACTION OF AZULENE AND SOME OF ITS SYMMETRICALLY SUBSTITUTED METHYL-DERIVATIVES WITH DIMETHYL ACETYLENEDICARBOXYLATE

It is shown that azulene (1) and dimethyl acetylenedicarboxylate (ADM) in a fourfold molar excess react at 200-degrees in decalin to yield, beside the known heptalene- (5) and azulene- 1,2-dicarboxylates (6), in an amount of 1.6% tetramethyl (1RS,2RS,5SR,8RS)-tetracyclo[6.2.2.2(2,5)0(1,5)]tetradeca-3,6,9,11,13-pentaene-3,4,9, 10-tetracarboxylate ('anti'-7) as a result of a SHOMO(azulene)/LUMO(ADM)-controlled addition of ADM to the seven-membered ring of 1 followed by a Diels-Alder reaction of the so formed tricyclic intermediate 16 (cf. Scheme 3) with a second molecule of ADM. The structure of 'anti'-7 was confirmed by an X-ray diffraction analysis. Similarly, the thermal reaction of 5,7-dimethylazulene (3) with excess ADM in decalin at 120-degrees led to the formation of ca. 1% of 'anti'-12, the 7,12-dimethyl derivative of 'anti'-7, beside of the corresponding heptalene- 10 and azulene-1,2-dicarboxylates 11 (cf. Scheme 2). The introduction of Me groups at C(1) and C(3) of azulene (1) and its 5,7-dimethyl derivative 3 strongly enhance the thermal formation of the corresponding tetracyclic compound. Thus, 1,3-dimethylazulene (2) in the presence of a sevenfold molar excess of ADM at 200-degrees yielded 20% of 'anti'-9 beside an equal amount of dimethyl 3-methylazulene- 1,2-dicarboxylate (8; cf. Scheme 1), and 1,3,5,7-tetramethylazulene (4) with a fourfold molar excess of ADM at 200-degrees gave a yield of 37% of 'anti'-15 beside small amounts of the corresponding heptalene- 13 and azulene-1,2-dicarboxylates 14 (cf. Scheme 2).