Azidolysis‐thermolysis of C‐19 diastereomer tosyl esters of morphine derivatives resulted in the formation of a new substituted 4‐azatetracyclo [4.4.02,4.03,8] decane ring system. As the course of such transformations depends on the chirality of C‐19, its relative configuration was determined both in the starting materials and in the new compounds. To this end a semiquantitative 1H{1H} NOE interpretation was combined with MMX molecular mechanics. Interestingly, both in the starting materials with a rotating C‐7—C‐19 bond and in the fairly rigid aziridine ring‐containing products, the same (H‐8α{19‐Me}) NOE effects were the most informative on the stereochemistry. In addition full a priori 1H and 13C assignments of 13 compounds are presented.