# Anthracycline nomenclature - doxorubicin [closed]

Why $$\ce{-OCH3}$$ is Number 3? I think that $$\ce{-OCH3}$$ Appropriates Number one.

• Do you have a reference to support your thought process? – sai-kartik Sep 5 '20 at 6:37
• I am not good at English. I just want to know why that replacement is number three. – HONG Sep 5 '20 at 7:44
• The red numbers shown in the picture are not related to nomenclature. Clearly, the locant for the methoxy group is '1', and there is no locant '13'. – user7951 Sep 5 '20 at 7:51

According to both PubChem and Wikipedia, the IUPAC name of Doxorubicin is (7S,9S)-7-[(2R,4S,5S,6S)-4-Amino-5-hydroxy-6-methyloxan-2-yl]oxy-6,9,11-trihydroxy-9-(2-hydroxyacetyl)-4-methoxy-8,10-dihydro-7H-tetracene-5,12-dione, which probably is the correct naming. To answer your question about why $$\ce{-OCH3}$$ is number 4 (actually it is number 4, not 3 as you indicated), I put both numbering and and corresponding names in following image:
I think you have enough knowledge to understand why numbering in $$\bf{A}$$ is preferred. For instance, the principle functional group is 5,12-dione, which does not change in both numbering. Yet, for the numbering of two major substitutions in tetracene nucleus are changed: it is (7,9)- versus (8,10)-. Further, complete locant set is (4,5,6,7,9,9,11,12) versus (1,5,6,8,8,10,11,12). In addition, it is (7H) versus (10H). I hope you got the point.