Dedicated to Edgar Heilbronner on the occasion of his 80th birthday
Der Mensch kommt nicht um die Tatsache herum, dass die Sinne zwar als höchste Herren seine Erkenntnisfähigkeit bestimmen, ihm aber keine Sicherheit bieten, da sie in jeder Lage sich leicht täuschen lassen.
Michel de Montaigne, Essais (Übers. Hans Stilett)
Heating of 4,5,6,7,8-pentamethyl-2H-cyclohepta[b]furan-2-one (1a) in decalin at temperatures b 1708 leads to the development of a blue color, typical for azulenes. It belongs, indeed, to two formed azulenes, namely 4,5,6,7,8-pentamethyl-2-(2,3,4,5,6-pentamethylphenyl)azulene (4a) and 4,5,6,7,8-pentamethylazulene (5a) (cf. Scheme 2 and Table 1). As a third product, 4,5,6,7-tetramethyl-2-(2,3,4,5,6-pentamethylphenyl)-1H-indene (6a) is also found in the reaction mixture. Neither 4,6,8-trimethyl-2H-cyclohepta[b]furan-2-one (1b) nor 2Hcyclohepta[b]furan-2-one (1c) exhibit, on heating, such reactivity. However, heating of mixtures 1a/1b or 1a/1c results in the formation of crossed azulenes, namely 4,6,8-trimethyl-2-(2,3,4,5,6-pentamethylphenyl)azulene (4ba) and 2-(2,3,4,5,6-pentamethylphenyl)azulene (4ca), respectively (cf. Scheme 3). The formation of small amounts of 4,6,8-trimethylazulene (5ba) and azulene (5ca), respectively, besides 1H-indene 6a is also observed. The observed product types speak for an [8 2]-cycloaddition reaction between two molecules of 1a or between 1b and 1c, respectively, with 1a, whereby 1a plays in the latter two cases the part of the two-atom component (cf. Figs. 5 ± 7 and Schemes 4 ± 6). Strain release, due to the five adjacent Me groups in 1a, in the [8 2]cycloaddition step seems to be the driving force for these transformations (cf. Table 3), which are further promoted by the consecutive loss of two molecules of CO 2 and concomitant formation of the 10p-electron system of the azulenes. The new azulenes react with dimethyl acetylenedicarboxylate (ADM) to form the corresponding dimethyl heptalene-4,5-dicarboxylates 20, 22, and 24 (cf. Scheme 7), which give thermally or photochemically the corresponding double-bond-shifted (DBS) isomers 20', 22', and 24', respectively. The five adjacent Me groups in 20/20' and 24/24' exert a certain buttressing effect, whereby their thermal DBS process is distinctly retarded in comparison to 22/22', which carry isolated Me groups at C(6), C(8), and C(10). This view is supported by X-ray crystal-structure analyses of 22 and 24 (cf. Fig. 8 and Table 5).