yloxy-3,5-diphenylphenyl, 1-t-butoxycarbonyloxynaphthyl, 4-trimethylsilyloxyphenyl or 4-trimethylsilyloxynaphthyl, or else those in which two of R1 to R3 form a ring, in particular a five- or six-membered ring, R1 and R2 forming for example tetramethylene groups and R3 being as defined above: ##STR5## or compounds in which R1 is methyl, R2 is phenyl or tolyl and R3 is substituted phenyl with acid-