Thanks for your question:
First some background: Indoles as you all know react first at C3 if substitued C2 and only in the benzene ring in special circunstances. The N-H is heavily prone to electrophylic substitution.
I could not share which compound or family of compounds, but it was interesting as the compound was already substitued at C3 and C5. Leaving only C2 unsubstitued. Reaction of indoles and C3, C2 substitued indoles seems to be very complex as highlighted by Reactions of indoles with nitrogen dioxide and nitrous acid in an aprotic solvent - Organic & Biomolecular Chemistry (RSC Publishing). However, 3 alkyl substitued indoles could form N-Nitroso compounds based on this study but stability is an issue and perhaps why the N-nitrosated indole is so reactive.
Additionally, in our studies the N-nitroso seemed to form very easily in with certain solvents at certain pH with the API in solution. While more stable in others w/o a significant increase from base. But, you leave the solution in the bench exposed to air and you start seeing quite a bit of by-products, including the N-nitroso regardless of the buffer used after some time. Our marker (standard) by the way needs refrigerated conditions at least or lower °T and by the way aid us to differentiate between the C-nitroso and N-nitroso. Again instability.
Fortunatelly, this reactivity ends in a no issue for N-nitrosated indoles in a case by case approach. But, of course may open a chapter for C-nitrosation. This is highlighted in efpia-nitrosamines-risk-management-workflows-jun-24-udpate.pdf as a risk coming from NAP test where it says: “Where MS confirms the potential presence of a nitrosamine and further action is likely to be required, it is recommended to manufacture or isolate an authentic sample for structural confirmation to rule out potential false positives (e.g. C-nitroso, O-nitroso, oxime, etc.). If significant alternative products (i.e. not the expected nitrosamine) are formed from these screening conditions, then identification of the structures may be considered to understand any potential mutagenicity risk (these impurities can be considered as “reasonably expected” from an ICH M7 perspective).”