Thermal Reaction of Azulene and Some of Its Symmetrically Substituted Methyl Derivatives with Dimethyl Acetylenedicarboxylate

Article abstract of DOI:10.1002/hlca.19930760111

It is shown that azulene (1) and dimethyl acetylenedicarboxylate (ADM) in a fourfold molar excess react at 200 deg C in decalin to yield, beside the known heptalene- (5) and azulene-1,2-dicarboxylates (6), in an amount of 1.6percent tetramethyl (1RS,2RS,5SR,8RS)-tetracyclo<6.2.2.2^2,50^1,5>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 deg C led to the formation of ca. 1percent 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 deg C yielded 20percent 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 deg C gave a yield of 37percent of 'anti'-15 beside small amounts of the corresponding heptalene- 13 and azulene-1,2-dicarboxylates 14 (cf.Scheme 2).