One of R4 and R5 may be a hydrogen atom and another may be an unsubstituted alkyl group, preferably, one of R4 and R5 is a hydrogen atom and another is a t-butyl group; or one of R4 and R5 may be a hydrogen atom and another may be a phenylalkyl group, preferably, one of R4 and R5 is a hydrogen atom and another is a benzyl group.