A versatile synthesis of multiply substituted cyclic dipeptides has been designed, based on the stepwise construction of the piperazine-2,5-dione ring using mol­ecular fragments from four different precursor mol­ecules. Starting from substituted 2-allyl­anilines, reaction with methyl 2-bromo-2-phenyl­acetate yields the corresponding methyl 2-(2-allyl­anilino)-2-phenyl­acetates, which react with haloacetyl chlorides to give methyl 2-[N-(2-allyl­phen­yl)-2-haloacetamido]-2-phenyl­acetates, which then undergo ring closure with benzyl­amine to yield the corresponding cyclic dipeptides of type 4-(2-allyl­phen­yl)-1-benzyl-3-phenyl­piperazine-2,5-dione. (3RS)-4-(2-Allyl-3,5-di­methyl­phen­yl)-1-benzyl-3-phenyl­piperazine-2,5-dione, C28H28N2O2, (IIId), crystallizes with Z? = 2 in the space group P21/c; the allyl groups in the two independent mol­ecules adopt different conformations and, in one of them, the allyl group is disordered over two sets of atomic sites having occupancies of 0.534?(4) and 0.466?(4). In both mol­ecules, the piperazine-2,5-dione ring adopts a boat conformation, with the 3-phenyl ring in a quasi-axial site. The mol­ecules of (IIId) are linked into a three-dimensional framework structure by a combination of three C?H...O hydrogen bonds and three C?H...?(arene) hydrogen bonds. Comparisons are made with some related structures.