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Thermal Reactions of Guaiazulene and Its 3‐Methyl Derivative with Dimethyl Acetylenedicarboxylate
Author(s) -
Uebelhart Peter,
Hansen HansJürgen
Publication year - 1992
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19920750804
Subject(s) - chemistry , decalin , dimethyl acetylenedicarboxylate , azulene , tricyclic , aryne , medicinal chemistry , ring (chemistry) , derivative (finance) , electrocyclic reaction , adduct , bicyclic molecule , stereochemistry , organic chemistry , cycloaddition , catalysis , financial economics , economics
Abstract 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.