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

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    $\begingroup$ Do you have a reference to support your thought process? $\endgroup$ – sai-kartik Sep 5 '20 at 6:37
  • $\begingroup$ I am not good at English. I just want to know why that replacement is number three. $\endgroup$ – HONG Sep 5 '20 at 7:44
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    $\begingroup$ 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'. $\endgroup$ – 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:

IUPAC name of Doxorubicin

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.

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    $\begingroup$ I didn't downvote, but if you are OK with some critics, then there are some things I would point out: 1. Neither Wikipedia nor PubChem can be considered trustworthy sources for the nomenclature. 2. Ideally, one should be able to present information without relying on colors or substantial amounts of text on the illustrations (such post appears less professional and are a barrier for color blind and overall visually impaired users). 3. The question is borderline HW and is also based on a false premise. $\endgroup$ – andselisk Sep 5 '20 at 21:53
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    $\begingroup$ @ andselisk♦: I do believe you and your points are well taken. I have habit using color to emphasize my points (you'd realize if you look at my publications). I'd keep those in my mind in future perspectives. Only I can't stand is pointless downvoting, that's all my complain about. Thanks for the concern. $\endgroup$ – Mathew Mahindaratne Sep 6 '20 at 1:17
  • $\begingroup$ Downvoted because the explanation why A is preferred over B is not correct. It is not about (7,9)- versus (8,10)-. The parent structure of A has eight substituent prefixes (4,6,7,8,9,9,10,11) that are all considered together, not just (7,9) since there are no “major substitutions” in nomenclature. A is actually preferred simply because the locant 7 for the indicated hydrogen is lower than 10. (…) $\endgroup$ – user7951 Sep 25 '20 at 11:01
  • $\begingroup$ (…) Anyway, the name of the parent structure actually is 7,8,9,10-tetrahydrotetracene-5,12-dione and not 8,10-dihydro-7H-tetracene-5,12-dione, which makes the comparison of the indicated hydrogen in A and B irrelevant from the start. $\endgroup$ – user7951 Sep 25 '20 at 11:02

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