Gravar-mail: A cycloaddition product of a chiral maleimide: 4-{(3aS*,6aS*)-4,6-dioxo-1-phenyl-5-[(1R)-1-phenyl­ethyl]-1,3a,4,5,6,6a-hexa­hydro­pyrrolo[3,4-c]pyrazol-3-yl}phenyl acetate