Why should ether O be named as OXA in a bicyclo ring when it is a substituent?
The suggested name 3-methoxy-6-methyl-7-oxabicyclo[2.2.1]hept-5-en-2-one is correct. Indeed, the ketone is the principal characteristic group in this case; however, this is not the reason why the other oxygen is named as ‘oxa’. Already the unsubstituted parent structure is named 7-oxabicyclo[2.2.1]hept-2-ene.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), such bicyclic heterocycles are named using skeletal replacement (‘a’) nomenclature (a system in which a heteroatom replaces skeletal atoms of a parent hydride).


  P-31.1.4.4 Bi- and polycyclic von Baeyer heterocycles named by skeletal replacement (‘a’) nomenclature. In heterocyclic compounds formed using skeletal replacement (‘a’) nomenclature, low locants are assigned to heteroatoms, in accord with the fixed numbering of the system, then to unsaturated sites.


Nondetachable prefixes, called replacement (‘a’) prefixes representing the element being introduced, are used to designate the replacing skeletal atoms with their standard bonding number, for example ‘aza’ (N), ‘oxa’ (O), or ‘thia’ (S).