The invention concerns a novel method for preparing substituted anthraquinone represented by the general formula (I) in which R represents a hydrogen atom or a linear or branched alkyl group of 1 to 5 carbon atoms, a chloromethyl group, a -COCl group, a COOR' group or a -CH2OR' group in which R' is a hydrogen atom or a linear or branched alkyl group of 1 to 5 carbon atoms, R1 represents a hydrogen atom, a hydroxyl group, a linear or branched alkyl group of 1 to 5 carbon atoms, or an acyloxy group of 1 to 5 carbon atoms, and R2 represents a hydrogen atom, by Diels-Alder reaction between a 1,4-naphthoquinone of general formula (II): in which R2 is as defined above, and X represents a hydrogen or halogen atom and an acyclic diene of formula (III): CH2 = CR - CH -OR2 in which R is as defined above, and R3 represents a silyl group of formula -Si(R4)3 in which R4 is a linear or branched alkyl group of 1 to 5 carbon atoms, followed by an oxidising deprotecting reaction using Jones reagent for obtaining substituted anthraquinone of general formula (I). The invention is for preparing rheins useful in pharmaceutics.