Isomorphous diethyl 1-(4-chloro-benz-yl)-4-(4-chloro-phen-yl)-2,2-dioxo-3,4,6,7,8,8a-hexa-hydro-1

Accurate studies on the effect of substituents on the crystal packing are essential for understanding the inter-molecular inter-actions and thus paving the way to crystal structure prediction. The crystal structures of diethyl 1-(4-chloro-benz-yl)-4-(4-chloro-phen-yl)-2,2-dioxo-3,4,6,7,8,8a-hexa-hydro-1H-pyrrolo-[2,1-c][1,4]thiazine-1,3-di-carboxyl-ate, C26H29Cl2NO6S, (I), and its isomorphous pair diethyl 1-(4-methyl-benz-yl)-4-(4-methyl-phen-yl)-2,2-dioxo-3,4,6,7,8,8a-hexa-hydro-1H-pyrrolo-[2,1-c][1,4]thia-zine-1,3-di-carboxyl-ate, C28H35NO6S, (II), are described. The mol-ecular aggregation patterns appear to be strikingly similar despite changes in the substituents, with a Cl atom in (I) being replaced by a methyl group in (II). Inspite of the chemical modifications, the structures of (I) and (I) are isomorphous, isostructural and found to obey the chlorine-methyl exchange rule. Both the structures feature C-H⋯O hydrogen bonding. However, a distinguishing feature between (I) and (II) is observed in the conformation of the pyrrole rings where the twist occurs on different C-N bonds. Hirshfeld analysis of both structures is presented and discussed.