Each of R1, R3 and R5 is a hydrogen atom or methyl group, R4 is a tert-butyl group, and R6 is an acid hydrolyzable group selected from the group consisting of methoxymethyl, tetrahydro-pyranyl, trialkylsilyl, isopropoxymethyl, tetrahydrofuranyl, and tert-butoxycarbonyl, groups.