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Thermal Reaction of Azulene‐1‐carbaldehydes with Dimethyl Acetylenedicarboxylate
Author(s) -
Magnussen Anette,
Uebelhart Peter,
Hansen HansJürgen
Publication year - 1993
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.19930760813
Subject(s) - chemistry , decalin , azulene , dimethyl acetylenedicarboxylate , substituent , yield (engineering) , toluene , medicinal chemistry , isomerization , anthracene , adduct , ring (chemistry) , organic chemistry , catalysis , cycloaddition , thermodynamics , physics
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(PPh 3 ) 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 1 H‐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 ).