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I can't seem to find a source for a comprehensive list of FG-priorities. I also am not aware of any system to determine the priority for more uncommon FGs. One of the two would be really helpful if one of you would know about something akin to that (I don't have access to the IUPAC blue book right now).

For more detail: I have to choose my core system; now I would naturally choose the heterocycle (in my case a oxazolidinone, with an alkyliden subst amongst others. Im not so sure about alkyliden/alkylidine subst. priorities too, that's why I mention it), but one sidechain features a guanidyl group. According to IUPAC, the system featuring the FG with the highest priority determines the core system. I am at a loss here, since I do not know the priority of guanidyl-FGs.

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    $\begingroup$ For your detailed example, a drawing of the structure could be helpful. Note that the order of seniority is only used for the principal characteristic group expressed as a suffix, so the alkylidene group cannot be relevant. $\endgroup$
    – Loong
    Commented Jan 18, 2023 at 19:03

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Blue book would be indeed the reference to go to. Chemistry.se's page resources for learning chemistry though includes a contribution about nomenclature in general, including the freely accessible online compilation by 2013 by G. P. Moss updated by 2022. (For their brevity, the brief guide and the 4-letter abstract (open access) likely miss an entry about oxazolidinone.) Chapter P-4, P-44 seniority order for parent structures may be of particular interest for you.

Last resort can be to sketch/import the structure in question for an assignment e.g., by ChemDraw, or ChemDoodle. There is a JavaScript based test site of ChemDraw retaining the structure2name function. Because the analogue test site for ChemDoodle JS is among the tutorial pages, there possibly is a limit of non-Hydrogen atoms to test the functions. (The program itself allows to adjust the nomenclature used, hence, there are occasions with different names assigned to one and the same (more complex) structure by ChemDraw and ChemDoodle.)

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  • $\begingroup$ This is a very helpful resource, thank you! I will be looking into how oxazolidinones (as cyclic amides/heterocycles) differ from guanidine in seniority. Curious (at least to me) is, that e.g. chemdraw chooses 'guanidine' as the parent structure. This could mean that oxazolones might be classified as ketones rather than amides. I am just wondering how to determine the seniority of guanidine, since it is also used as prefix. There are many ways to deduce it (urea, amidine etc). $\endgroup$
    – Katjuscha
    Commented Jan 19, 2023 at 6:45
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    $\begingroup$ @Katjuscha P44-0, Introduction, provides an overview for the rules convened. Then P-44.1 Seniority order for parent structures, P-44.2 Seniority order only for rings and ring systems, P-44.3 Seniority of acyclic chains (the principal chain), and P-44.4 Seniority criteria applicable to rings, ring systems, or acyclic chains. E.g., P-44.2.1.8 "The senior ring or ring system has the greater number of heteroatoms that occur earlier in the following sequence: F > Cl > Br > I > O > S > Se > Te > N > P > [cut for brevity] [criterion (g) in P-44.2.1]. Examples [...]: [illustrations follow]" $\endgroup$
    – Buttonwood
    Commented Jan 19, 2023 at 13:33
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    $\begingroup$ @Katjuscha To be precise, oxazolidinone is classified as pseudoketone in the seniority order. $\endgroup$
    – Loong
    Commented Jan 19, 2023 at 19:30

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