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Formation of Unusual Products from the Acid‐Catalyzed Reaction of Azulenes with Dimethyl Acetylenedicarboxylate
Author(s) -
Brügger Paul,
Uebelhart Peter,
Kunz Roland W.,
Sigrist Rolf,
Hansen HansJürgen
Publication year - 1998
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/(sici)1522-2675(19981216)81:12<2201::aid-hlca2201>3.0.co;2-7
Subject(s) - azulene , chemistry , dimethyl acetylenedicarboxylate , trifluoroacetic acid , tetralin , catalysis , toluene , medicinal chemistry , derivative (finance) , molecule , organic chemistry , cycloaddition , financial economics , economics
The reaction of guaiazulene ( 4 ) and dimethyl acetylenedicarboxylate (ADM) in tetralin or toluene, catalyzed by 5 mol‐% of trifluoroacetic acid (TFA) at ambient temperature, leads to the formation of the corresponding heptalene‐4,5‐dicarboxylate 6 and a guaiazulenyl‐substituted 2,2a,4a,8b‐tetrahydrocyclopent[ cd ]azulene derivative 7 beside the expected guaiazulenyl‐substituted ethenedicarboxylates ( E )‐ 5 and ( Z )‐ 5 as main products ( Scheme 2 ). The structure of 7 was unequivocally established by an X‐ray crystal‐structure analysis ( Fig. 1 ). Precursor of 7 must be the 2a,4a‐dihydrocyclopent[ cd ]azulene‐3,4‐dicarboxylate 9 which reacts, under TFA catalysis, with a second molecule of 4 ( Scheme 3 ). No formation of products of type 7 has been observed in the TFA‐catalyzed reaction of 4,6,8‐trimethyl‐ and 1,4,6,8‐tetramethylazulene ( 13 and 16 , respectively) and ADM ( Scheme 4 ). On the other hand, the TFA‐catalyzed reaction of azulene ( 18 ) itself and ADM at ambient temperature gives rise to a whole variety of new products ( Scheme 5 ), the major part of which is derived from dimethyl 2a,4a‐dihydrocyclopent[ cd ]azulene‐3,4‐dicarboxylate ( 25 ) as the main intermediate ( Scheme 6 ). Nevertheless, for the formation of the 2a,4a,6,8b‐tetrahydrocyclobut[ a ]azulene derivatives ( E )‐ 24a and ( E )‐ 24b , a corresponding 2a,8b‐dihydro precursor 29 has to be postulated as crucial intermediate ( Scheme 8 ).