Thermal Reactions of Guaiazulene and Its 3‐Methyl Derivative with Dimethyl Acetylenedicarboxylate

Abstract

AbstractThe 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.