The two title isomers were obtained in 1,3-dipolar cycloaddition of the same ylide 2, with dimethyl fumarate and dimethyl maleate, respectively. Their stereochemistry was established by X-ray diffraction analysis. The molecular geometries of the two isomers are compared.
Tinant, B., Declercq, J.-P., Dostal, J., & Potacek, M. (1994). X-ray Structures of Cis and Trans-diisomers of 1,2-dimethyl-3-ethyl 2,3-dihydropyrrolo[1,2-f]phenanthridine-1,2,3-tricarboxylate. Societes Chimiques Belges. Bulletin, 103(7-8), 303-307. https://doi.org/10.1002/bscb.19941030702 (Original work published 1994)