Estradiol is also metabolized via hydroxylation into catechol estrogens. Tyrosinases and catechol oxidases are collectively termed polyphenol oxidases. Aside from sulfate conjugation, EE is mainly metabolized by hydroxylation into catechol estrogens. Iron-containing dioxygenase enzymes catalyze the cleavage of catechol. The activity of tyrosinase is similar to catechol oxidase, a related class of copper oxidase. Hydroquinone, benzenetriol and catechol are converted to polyphenols. The free catechol hydroxyl groups keep it susceptible to enzymatic metabolism. Arthropod cuticle consists of chitin linked by a catechol moiety to protein. The meta-hydroxyl group of catechol is methylated by catechol-O-methyltransferase. Many of these active ingredients work through inhibiting catechol oxidase. Key products include vanillin and ethylvanillin, but also hydroquinone or catechol. The enzyme protocatechuate decarboxylase uses 3,4-dihydroxybenzoate to produce catechol and CO. Synthesis from catechol requires an additional step, Williamson ether synthesis using dichloromethane. The catechol group is a dihydroxy benzene ring. Guaiacol can be synthesized by the dimethylation of catechol followed by selective mono-demethylation.