Formation of Unusual Products from the Acid-Catalyzed Reaction of Azulenes with Dimethyl Acetylenedicarboxylate

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).