Reaction of Hydroxymethyl‐ and Alkyl‐Substituted Azulenes with Manganese Dioxide

It is shown that the 2‐(hydroxymethyl)‐1‐methylazulenes 6 are being oxidized by activated MnO 2 in CH 2 Cl 2 at room temperature to the corresponding azulene‐1,2‐dicarbaldehydes 7 ( Scheme 2 ). Extension of the MnO 2 oxidation reaction to 1‐methyl‐ and/or 3‐methyl‐substituted azulenes led to the formation of the corresponding azulene‐1‐carbaldehydes in excellent yields ( Scheme 3 ). The reaction of unsymmetrically substituted 1,3‐dimethyl‐azulenes ( cf. 15 in Scheme 4 ) with MnO 2 shows only little chemoselectivity. However, the observed ratio of the formed constitutionally isometric azulene‐1‐carbaldehydes is in agreement with the size of the orbital coefficients in the HOMO of the azulenes. The reaction of guaiazulene ( 18 ) with MnO 2 in dioxane/H 2 O at room temperature gave mainly the expected carbaldehyde 19 . However, it was accompanied by the azulene‐diones 20 and 21 ( Scheme 5 ). The precursor of the demethylated compound 20 is the carbaldehyde 19 . Similarly, the MnO 2 reaction of 7‐isopropyl‐4‐methyalazulene ( 22 ) as well as of 4,6,8‐trimethylazulene ( 24 ) led to the formation of a mixture of the corresponding azulene‐1,5‐diones and azulene‐1,7‐diones 20 / 23 and 25 / 26 , respectively, in decent yields ( Schemes 6 and 7 ). No MnO 2 reaction was observed with 5,7‐dimethylazulene.