l group of 1 to 10 carbon atoms or an aryl group of 6 to 10 carbon atoms, or a pair of R2 and R3 and a pair of R4 and R5 may bond together to form a benzene, naphthalene or pyridine ring with the carbon atoms to which they are bonded; R6 is a hydrogen atom, a straight, branched or cyclic alkyl group of 1 to 10 carbon atoms or an aryl group of 6 to 10 carbon atoms; R7 and R8 are each a hydrogen ato