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The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° yields, beside the expected heptalene- and azulene-1,2-dicarboxylates, tetracyclic compounds of type ‘anti’-V and tricyclic compounds of type E (cf. Schemes 2–4 and 8–11). The compounds of type ‘anti’-V represent Diels-Alder adducts of the primary tricyclic intermediates A with ADM. In some cases, the tricyclic compounds of type E also underwent a consecutive Diels-Alder reaction with ADM to yield the tetracyclic compounds of type ‘anti’- or ‘syn’-VI (cf. Schemes 2 and 8–11). The tricyclic compounds of type E, namely 4 and 8, reversibly rearrange via [1,5]-C shifts to isomeric tricyclic structures (cf. 18 and 19, respectively, in Scheme 6) already at temperatures > 50°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20via a di-π-methane reaction. The observed heptalene- and azulene-1,2-dicarboxylates as well as the tetracyclic compounds of type ‘anti’'-V are formed from the primary tricyclic intermediates Avia rearrangement (heptalenedicarboxylates), retro-Diels-Alder reaction ( azulenedicarboxylates), and Diels-Alder reaction with ADM. The different reaction channels of A are dependent on the substituents. However, the main reaction channel of A is its retro-Diels-Alder reaction to the starting materials (azulene and ADM). The highly reversible Diels-Alder reaction of ADM to the five-membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)-controlled, in contrast to the at 200° irreversible ADM addition to the seven-membered ring of the azulenes to yield the Diels-Alder products of type E. This competing reaction must occur on grounds of orbital-symmetry conservation under SHOMO(azulene)/LUMO(ADM) control (cf. Schemes 20–22). Several X-ray diffraction analyses of the products were performed (cf. Chapt. 4.1).

The thermal reaction of 7-isopropyl-1,3,4-trimethylazulene (3-methylguaiazulene; 2) with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° leads to the formation of the corresponding heptalene- (5a/5b and 6a/6b; cf. Scheme 3) and azulene-1,2-dicarboxylates (7 and 8, respectively). Together with small amounts of a corresponding tetracyclic compound (‘anti’-13) these compounds are obtained via rearrangement ( 5a/5b and 6a/6b), retro-Diels-Alder reaction ( 7 and 8), and Diels-Alder reaction with ADM ( ‘anti’-13) from the two primary tricyclic intermediates (14 and 15; cf. Scheme 5) which are formed by site-selective addition of ADM to the five-membered ring of 2. In a competing Diels-Alder reaction, ADM is also added to the seven-membered ring of 2, leading to the formation of the tricyclic compounds 9 and 10 and of the Diels-Alder adducts ‘anti’-11 and ‘anti’-12, respectively of 9 and of a third tricyclic intermediate 16 which is at 200° in thermal equilibrium with 9 and 10 (cf. Scheme 6). The heptalenedicarboxylates 5a and 5b as well as 6a and 6b are interconverting slowly already at ambient temperature (Scheme 4). The thermal reaction of guaiazulene (1) with excess ADM in decalin at 190° leads alongside with the known heptalene- (3a) and azulene-1,2-dicarboxylates (4; cf. Schemes 2 and 7) to the formation of six tetracyclic compounds ‘anti’-17 to ‘anti’-21 as well as ‘syn’-19 and small amounts of a 4:1 mixture of the tricyclic tetracarboxylates 22 and 23. The structure of the tetracyclic compounds can be traced back by a retro-Diels-Alder reaction to the corresponding structures of tricyclic compounds (24--29; cf. Scheme 8) which are thermally interconverting by [1,5]-C shifts at 190°. The tricyclic tetracarboxylates 22 and 23, which are slowly equilibrating already at ambient temperature, are formed by thermal addition of ADM to the seven-membered ring of dimethyl 5-isopropyl-3,8-dimethylazulene-1,2-dicarboxylate (7; cf. Scheme 10). Azulene 7 which is electronically deactivated by the two MeOCO groups at C(1) and C(2) shows no more thermal reactivity in the presence of ADM at the five-membered ring (cf. Scheme 11). The tricyclic tetracarboxylates 22 and 23 react with excess ADM at 200° in a slow Diels-Alder reaction to form the tetracyclic hexacarboxylates 32, ‘anti’-33, and ‘anti’-34 (cf. Schemes 10–12 as well as Scheme 13). A structural correlation of the tri- and tetracyclic compounds is only feasible if thermal equilibration via [1,5]-C shifts between all six possible tricyclic tetracarboxylates (22, 23, and 35–38; cf. Scheme 13) is assumed. The tetracyclic hexacarboxylates 32, ‘anti’-33, and ‘anti’-34 seem to arise from the most strained tricyclic intermediates (36–38) by the Diels-Alder reaction with ADM.

Azulene-1-carbaldehydes which have Me substituents at C(3) and C(8) and no substituent at C(6) react with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° to yield exclusively the Diels-Alder adduct at the seven-membered ring (cf. Scheme 3). The corresponding 1-carboxylates behave similarly (Scheme 4). Azulene-1-carbaldehydes which possess no Me substituent at C(8) (e.g.11, 12 in Scheme 2) gave no defined products when heated with ADM in decalin. On the other hand, Me substitutents at C(2) may also assist the thermal addition of ADM at the seven-membered ring of azulene-1-carbaldehydes (Scheme 6). However, in these cases the primary tricyclic adducts react with a second molecule of ADM to yield corresponding tetracyclic compounds. The new tricyclic aldehydes 16 and 17 which were obtained in up to 50% yield (Scheme 3) could quantitatively be decarbonylated with [RhCl(PPh3)3] in toluene at 140° to yield a thermally equilibrated mixture of four tricycles (Scheme 8). It was found that the thermal isomerization of these tricycles occur at temperatures as low as 0° and that at temperatures > 40° the thermal equilibrium between the four tricycles is rapidly established via [1,5]-C shifts. The establishment of the equilibrium makes the existence of two further tricycles necessary (cf. Scheme 8). However, in the temperature range of up to 85° these two further tricycles could not be detected by 1H-NMR. When heated in the presence of excess ADM in decalin at 180°, the ‘missing’ tricyclic forms could be evidenced by their tetracyclic trapping products ‘anti’-45 and ‘anti’-48, respectively (Scheme 9).

Azulene (1) and its alkyl derivatives 2–15 were oxidized in a UV-irradiated stationary system by mercury(II) trifluoroacetate in dichloromethane. These derivatives are: 4,6,8-trimethyl-(2), 1,4,6,8-tetramethyl-(3), 6-tert-butyl-1,4,8-trimethyl-(4), 2,4,6,8-tetramethyl-(5), 1,3-dimethyl-(6), 1,3-ditert-butyl-(7), 1,3,5-tri-tert-butyl-(8), 1,3,6-trimethyl-(9), 6-tert-butyl-1,3-dimethyl-(10), 1,3,5,7-tetramethyl-(11), 1,3,4,6,8-pentamethyl-(12), 1,3,4,8-tetramethyl-6-propyl-(13), 6-tert-butyl - 1,3,4,8-tetramethyl-(14) and 1,2,3,4,6,8-hexamethyl-azulene (15). Only radical cations substituted in both reactive positions C-1 and -3 (6˙+-15˙+) were sufficiently persistent to be characterized by their hyperfine data with the use of EPR spectroscopy. Those bearing tert-butyl substituents at C-1 and -3 (7˙+ and 8˙+) or having three alkyl groups at the seven-membered ring in addition to the 1,3-dimethyl substituents (12˙+-15˙+) were also amenable to ENDOR and TRIPLE resonance studies. In contrast, the radical cations with none (1˙+, 2˙+ and 5˙+) or only one methyl group in the positions C-1 and -3 (3˙+ and 4˙+) rapidly reacted to yield follow-up products. For 1˙+-4˙+these products were identified by EPR and EN DOR spectroscopy as the radical cations of correspondingly substituted 1, 1′-biazulenyls (1a–4a). A mechanism is proposed for the formation of the primary (6˙+-15˙+) and the secondary radical cations (1a˙+-4a˙+). The failure to detect the radical cation of biazulenyl 5a˙+, starting from 5, must be due to instability of 5a˙+ in which two pairs of methyl substituents sterically interact. The high reactivity of the azulene radical cations in the positions C-1 and -3 is consistent with the unusually high π-spin populations ρ1.3 at these centres, as manifested by the largest by far coupling constants |aH1,3| of the α and methyl β-protons.

The dehydrogenation reaction of the heptalene-4,5-dimethanols 4a and 4d, which do not undergo the double-bond-shift (DBS) process at ambient temperature, with basic MnO2 in CH2Cl2 at room temperature, leads to the formation of the corresponding heptaleno[1,2-c]furans 6a and 6d, respectively, as well as to the corresponding heptaleno[1,2-c]furan-3-ones 7a and 7d, respectively (cf. Scheme 2 and 8). The formation of both product types necessarily involves a DBS process (cf. Scheme 7). The dehydrogenation reaction of the DBS isomer of 4a, i.e., 5a, with MnO2 in CH2Cl2 at room temperature results, in addition to 6a and 7a, in the formation of the heptaleno[1,2-c]-furan-1-one 8a and, in small amounts, of the heptalene-4,5-dicarbaldehyde 9a (cf. Scheme 3). The benzo[a]heptalene-6,7-dimethanol 4c with a fixed position of the CC bonds of the heptalene skeleton, on dehydrogenation with MnO2 in CH2Cl2, gives only the corresponding furanone 11b (Scheme 4). By [2H2]-labelling of the methanol function at C(7), it could be shown that the furanone formation takes place at the stage of the corresponding lactol [3-2H2]-15b (cf. Scheme 6). Heptalene-1,2-dimethanols 4c and 4e, which are, at room temperature, in thermal equilibrium with their corresponding DBS forms 5c and 5e, respectively, are dehydrogenated by MnO2 in CH2Cl2 to give the corresponding heptaleno[1,2-c]furans 6c and 6e as well as the heptaleno[1,2-c]furan-3-ones 7c and 7e and, again, in small amounts, the heptaleno[1,2-c]furan-1-ones 8c and 8e, respectively (cf. Scheme 8). Therefore, it seems that the heptalene-1,2-dimethanols are responsible for the formation of the furan-1-ones (cf. Scheme 7). The methylenation of the furan-3-ones 7a and 7e with Tebbe's reagent leads to the formation of the 3-methyl-substituted heptaleno[1,2-c]furans 23a and 23e, respectively (cf. Scheme 9). The heptaleno[1,2-c]furans 6a, 6d, and 23a can be resolved into their antipodes on a Chiralcel OD column. The (P)-configuration is assigned to the heptaleno[1,2-c]furans showing a negative Cotton effect at ca. 320 nm in the CD spectrum in hexane (cf. Figs. 3–5 as well as Table 7). The (P)-configuration of (–)-6a is correlated with the established (P)-configuration of the dimethanol (–)-5avia dehydrogenation with MnO2. The degree of twisting of the heptalene skeleton of 6 and 23 is determined by the Me-substitution pattern (cf. Table 9). The larger the heptalene gauche torsion angles are, the more hypsochromically shifted is the heptalene absorption band above 300 nm (cf. Table 7 and 8, as well as Figs. 6–9).