The reaction is preferably carried out in an organic solvent, e.g. acetic acid, methanol, ethanol, 2-propanol, 1-propanol, 1-butanol, 2-butanol or tert-butanol, under pressures of 1 bar (105 Pa) to 100 bar (107 Pa) and at temperatures of about 20??? C. to about 100??? C. The tertiary alcohols of formula III are preferably employed here in the form of one of their salts.