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Every IUPAC Systematic name I have come across, I have understood up until now. This is Carminic Acid ($\ce{C22H20O13}$):

Carminic Acid

This is the IUPAC Systematic Name for Carminic Acid:

(1S)-1,5-Anhydro-1-(7-carboxy-1,3,4,6-tetrahydroxy-8-methyl-9,10-dioxo-9,10-dihydro-2-anthracenyl)-D-glucitol

I have 3 main questions:

  • I do not understand the numbering system inside the brackets. The numbering system implies that the numbers assigned to the carbon atoms is the following:

Carminic Acid Numbers

I would have thought the numbers assigned to the carbon atoms would be the following:

Carminic Acid Numbers Presumption

Why is the numbering system the way it is? (Why does the numbering system not go around the molecule, in a normal cycle, counting off each carbon?, Why is the number 10 carbon opposite the number 9 carbon?, Why are 4 carbons [circled in the picture below] left out in the numbering system?)

Carminic Acid Carbons Left Out

  • I also do not understand why Carbons 9 and 10 in the anthracene group are referred to as '9,10-dioxo-9,10-dihydro'. Surely, they only need to be referred to as '9,10-dioxo'? And what is a '-hydro' group?
  • Doesn't '1,5-Anhydro...-D-Glucitol' imply the following: Hexan-2,3,4,6-tetrahydroxy-1-yl, a chain, and NOT a ring structure of: Oxan-3,4,5-trihydroxy-6-hydroxymethyl-2-yl, considering D-Glucitol (shown below). Also, as well as '1,5-Anhydro...-D-Glucitol' not accounting for the ring structure of that section of the molecule, it also doesn't account for the Oxygen.

D-Glucitol D-Glucitol

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    $\begingroup$ What is your source for the supposed IUPAC systematic name? A program I use (Chemdoodle) says the IUPAC preferred name of this compound is 3,5,6,8-Tetrahydroxy-1-methyl-9,10-dioxo-7-[3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl]-2-anthracenecarboxylic acid. That is very different than what you say the systematic name is. $\endgroup$ – Curt F. Dec 6 '15 at 6:09
  • $\begingroup$ @Curt F. The source I used was the RSC's chemical database ChemSpider. This is the page for Carminic Acid: Carminic Acid in ChemSpider. The name you have quoted is one of the names listed but is not the IUPAC Systematic Name. $\endgroup$ – Saul McShane Dec 6 '15 at 11:32
  • $\begingroup$ Pubchem also lists a different name than the one you give. I guess the question is, how do you decide to name a thing as a glucitol derivative vs. a anthracene derivative if both groups are present in a molecule? $\endgroup$ – Curt F. Dec 6 '15 at 13:58
  • $\begingroup$ @Curt F. I fully understand the name you have quoted and I understand the other systematic names on ChemSpider, but my focus is on this name and the three questions listed above. To be honest, I would use the name you have quoted in a practical situation, I just would like to understand the name above from an intellectual standpoint. $\endgroup$ – Saul McShane Dec 6 '15 at 14:27
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According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the preferred name (PIN) for the parent ring component consisting of three benzene rings fused in a linear manner is the retained name anthracene.

Subsection P-25.1.1, Table 2.7, Entry (11) shows that anthracene has a special numbering:

anthracene

According to Subsection P-14.4 (a), this fixed numbering must be used in PINs and in general nomenclature.

The prefixes ‘hydro’ and ‘dehydro’ are used to indicate addition and subtraction, respectively, of hydrogen atoms (see Subsection P-31.2.1). Hence, the PIN for the ring component of the structure given in the question is 9,10-dihydroanthracene:

9,10-dihydroanthracene

The corresponding monovalent substituent groups derived from such ring systems by the removal of one hydrogen atom are named by replacing the final ‘e’ of the name of the hydrocarbon by ‘yl’ (see Section P-29). Thus, the name for the substituent group derived from 9,10-dihydroanthracene is (9,10-dihydroanthracen-2-yl):

(9,10-dihydroanthracen-2-yl)

(Note that locants are placed immediately before that part of the name to which they relate (see Subsection P-14.3.2); i.e. ‘…anthracen-2-yl’. Thus, the name ‘…-2-anthracenyl’ given in the question is not correct.)

The substitutive prefixes for the remaining groups are
‘methyl’ $(\ce{-CH3})$,
‘hydroxy’ $(\ce{-OH})$,
‘oxo’ $(\ce{=O})$, and
‘carboxy’ $(\ce{-COOH})$.

The name for the complete substituent group is (7-carboxy-1,3,4,6-tetrahydroxy-8-methyl-9,10-dioxo-9,10-dihydroanthracen-2-yl):

(7-carboxy-1,3,4,6-tetrahydroxy-8-methyl-9,10-dioxo-9,10-dihydroanthracen-2-yl)

The bases for carbohydrate names are the structures of the parent monosaccharides in their acyclic form (see Subsection P-102.2). Alditols are named by changing the ending ‘ose’ in the name of the corresponding aldose into ‘itol’ (see Subsection P-102.5.6.5):

D-glucitol

The prefix ‘anhydro’ expresses the corresponding intramolecular ether, which is formally obtained by elimination of water from two hydroxy groups of a single molecule of a monosaccharide or monosaccharide derivative (hence the structure is commonly called an intramolecular anhydride). The prefix ‘anhydro’ is preceded by a pair of locants identifying the two hydroxy groups (see Subsection P-102.5.6.7.1).

1,5-anhydro-D-glucitol

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    $\begingroup$ Great answer (as usual) Loong. This explains the carbon numbering mystery that the OP was wondering about. However I'm unclear on how to decide whether the IUPAC systematic name would regard the molecule as a glucitol-substituted anthracene or an anthracene-substituted glucitol. $\endgroup$ – Curt F. Dec 6 '15 at 15:10
  • $\begingroup$ @CurtF. In “normal” organic nomenclature, the carboxylic acid group has the highest seniority. Thus, the preferred name should be “3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-7-(3,4,5,6-tetrahydroxyoxan-2-yl)-9,10-dihydroanthracene-2-carboxylic acid”. However, the section on carbohydrate nomenclature (P-102) is based on parent monosaccharides having retained names, which are modified to indicate the present characteristic groups. According to the introduction given in P-100, preferred IUPAC names (PINs) are not identified for the compounds in this chapter. $\endgroup$ – Loong♦ Dec 6 '15 at 16:31
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    $\begingroup$ Thanks again Loong. If I understand you correctly you are saying that there is no clear choice for the PIN for this compound? The "systematic" name would seem to be the one that ends dihydroanthracene-2-carboxylic acid, because, the P100 chapter you allude to says When the full structure is known, a ‘systematic name’ may be generated in accordance with Rules described in Chapters 1 through 8 of these recommendations. $\endgroup$ – Curt F. Dec 6 '15 at 17:59
  • $\begingroup$ And it goes on to say that naming things as natural product derivatives, e.g. as a D-glucitol derivative, is a convenience and a "semisystematic" name. I think the idea is that semisystematic names are supposed to be much shorter than the full official systematic name. But in this case, the difference is only 30 letters saved in a full systematic name of 100 characters, so the "semisystematic" name is probably not worth it. $\endgroup$ – Curt F. Dec 6 '15 at 18:05
  • $\begingroup$ Thank you very much. Very decisive, and very informative answer. I now fully understand the IUPAC Systematic name for Carminic acid. You're answer is very much appreciated. I haven't heard of the Blue Book before and will use it in future. $\endgroup$ – Saul McShane Dec 6 '15 at 20:30
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Just note that 1,5-anhydro-ᴅ-glucitol would be the correct name for the carbohydrate part itself (without the anomeric C-1 OH group). However, using this name type for the whole compound is rather wrong.

This is an example of C-glycoside, or more correctly, C-glycosyl compound. The nomenclature is described in P-102.6.1.4.

Related carbohydrate (as glycosyl group is formed by removal of the anomeric hydroxy group from the cyclic monosaccharide, as stated in P-102.6.1.1.1) is β-ᴅ-glucopyranose.

The correct “natural product” IUPAC name would be
7-(β-ᴅ-glucopyranosyl)-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid.

Fig.

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