For example, such a compound can be synthesized by introducing desirable substituents into a synthetic intermediate, a compound represented by formula (I) wherein R5 and R6 each are a hydroxy group, and alkylation or acylation.For example, when X1, X2, X3 and X4 each are all a sulfur atom, the synthetic intermediate represented by formula (I) wherein R5 and R6 each are a hydroxy group can be synth