ving 1 to 6 carbon atoms, a tert-butoxycarbonyloxy group, a tetrahydropyranyloxy group, an acetyloxy group or a group of ???OCH2COOC(CH3)3, R6 is a hydrogen atom or a cyano group, R7 is a group of ???COOR8 (in which R8 is a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a bridged alicyclic hydrocarbon group having 7 to 12 carbon atoms), a cyano group or a group show