Process for the preparation of hydroquinones

A process for the preparation of a hydroquinone compound of formula (I)

• wherein
• R2, R3, R5, R6 is independently from one another H, A, Cyc, Hal, CN, —(CYY)n—OA, —(CYY)n—NYY, —O(CYY)n—OA, —O(CYY)n—NYY, —NH(CYY)n—OA or —NH(CYY)n—NYY;
• Y is H, A or Hal;
• A is unbranched or branched alkyl having 1-10 C atoms, in which 1-7 H atoms can be replaced by Hal, or in which one or two adjacent CH2 groups can be replaced independently of one another by a —CH═CH— or —C≡C— group;
• Cyc is cycloalkyl having 3-7 C atoms, in which 1-4 H atoms can be replaced independently from one another by A, Hal or OY;
• Hal is F, Cl, Br or I;
• and
• n is 0, 1, 2, 3, 4, 5 or 6;
• comprising the steps of:
• (a) heating to reflux temperature a phenol compound of formula (II)
• wherein R2, R3, R5 and R6 have the meaning as defined above,
• with 0.3 to 0.4 equivalents of hexamethylenetetramine in an organic acid,
• followed by adding a hydrolysis medium and heating the resulting mixture to yield a 4-hydroxy-benzaldehyde compound of formula (III)
• wherein R2, R3, R5 and R6 have the meaning as defined above,
• and
• (b) reacting the compound of formula (III) with an oxidizing agent in a solvent under acidic conditions to yield the compound of formula (I).