| http://www.w3.org/ns/prov#value | - One preferred method is by reacting a phenol derivative of formula (6) with a vinyl ether of the following formula (9): ##STR12## wherein R12 and R13 are as defined previously, or R12 and R13, taken together, may form a ring, and R20 is a divalent group obtained by eliminating one hydrogen atom from R11, in the presence of an acid catalyst.
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