In the case where only one of R6 to R8 is a hydrogen atom or none of these is a hydrogen atom (e.g., in the case where R6 is a hydrogen atom and R7 and R8 each is a cycloalkyl group or where R6 to R8 each is a cycloalkyl group), then the cycloalkyl groups represented by the remaining two-or all of R6 to R8 each is preferably a cycloalkyl group having 3 to 16, more preferably 3 to 14 carbon atoms.