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Oxidation of 4,6,8-trimethylazulene (1) with hydrogen peroxide in pyridine at 25°C for 18 h gave 4,6,8-trimethyl-1,5- and -1,7-azulenequinones, their 2-(4,6,8-trimethyl-1-azulenyl) derivatives, and 2,3-dihydro-4,6- and -4,7-dimethyl-2-(4,6,8-trimethyl-1-azulenyl)-1H-inden-1-ones in much higher yields than those by autoxidation of 1 in DMF at 120°C. The same oxidation of guaiazulene afforded sixteen separable products, among which there were three new dimeric condensates.

Various 3,3′-methylenebis azulenes were prepared in high yields upon treatment of 1-substituted azulenes with a solution of 0.1–0.2 wt% paraformaldehyde in glacial acetic acid at 25°C for 1 h. Thus, 1-methyl-, 1-isopropyl-, 1,5-diisopropylazulene, guaiazulene, and methyl 2-hydroxy-1-azulenecarboxylate readily provided the corresponding 3,3′-methylenebisazulenes, respectively. Acetic acid which had been refluxed with 5 wt% KMnO4 for 2 h and then distilled according to a purification method was found to contain 0.1–0.2 wt% formaldehyde and exert the same 3,3′-methylenebisazulene formation. 3,3′-Dideuteriomethyl-enebis(guaiazulene) was obtained by employing the similar distillate from acetic acid-d4.