I'm looking at a cycloaddition that produces a bicyclic intermediate, there are two different ways of doing the addition which can be described as endo/exo. However, all three bridges are the same length. How do I assign priorities and endo/exo here?

Here's a simplified example of this reaction. In my opinion on the dienophile the $\ce{NH2}$ has the higher priority, on the pyridazine I would suspect the $\ce{N-N}$ bridge to be higher in priority which means the left product would be endo, the right one exo. But I cannot find information that confirms this.

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  • $\begingroup$ To my knowledge the endo/exo rules have as yet not been incorporated into the CIP system. Your idea has merit though. [Although your DA reaction is offered as an example, it is unlikely that the enamine tautomer of the dienophile would prevail over the imine.] $\endgroup$ – user55119 Dec 4 '18 at 23:26
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    $\begingroup$ @user55119 well that would explain why I cannot find any information on this. And yes I know about the hypothetical tautomer and the product will eliminate N2 right away, but since I'm doing calculations everything is possible. ;-) That was just a very simple system with all the key features I needed. $\endgroup$ – DSVA Dec 5 '18 at 4:42
  • $\begingroup$ Endo/exo stereodescriptors cannot be used for the preferred IUPAC names anyway. $\endgroup$ – mykhal Dec 5 '18 at 16:43
  • $\begingroup$ @mykhal I need it to describe the configuration of the transition states and I don't see an easier way to do this. $\endgroup$ – DSVA Dec 5 '18 at 21:36
  • $\begingroup$ IUPAC not withstanding, a CIP-like nomenclature could be developed. Just as E/Z covers all double bonds and cis/trans does not, endo/exo could be replaced with a more inclusive CIP system. E.g., your NH2 group has precedent over C6H5 while N=N has priority over C=C. Assuming the NH2 is bold in the first structure and dotted in the second, so your first DA adduct would be Z-ish and the second structure E-ish. ;) $\endgroup$ – user55119 Dec 5 '18 at 23:21

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