I am trying to understand the nomenclature of amoxicillin:

The IUPAC name that Wikipedia has given is (2S,5R,6R)-6-{[(2R)-2-amino-2-(4-hydroxyphenyl)-acetyl]amino}-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-24-carboxylic acid.

I think I understand most of it but I have two problems:

  1. Why is there a 24 before the carboxylic acid?
  2. There is a $\ce{C=O}$ group between two nitrogens that doesn't appear in the name. I was expecting an amide, rather than amino or oxo.

To preface this, I'm not sure if this is indeed the correct IUPAC name as per the latest 2013 recommendations. It might well be, but I don't know. However, that's not crucial to your question, which is more about understanding the name that is given on Wikipedia. Here's the structure of amoxicillin:


The amide C=O is in "acetyl", which refers to the $\ce{CH3C(=O)}$ group (see e.g. acetyl chloride, $\ce{CH3COCl}$):

acetyl substitutent

The '24' is probably a typo and should instead be '2'.

Bicyclo numbering

Following this numbering scheme you can see that the lactam C=O is in '-7-oxo'.

  • $\begingroup$ Thanks for your help. It does indeed appear to be a typo as on the pubchem website (pubchem.ncbi.nlm.nih.gov/compound/amoxicillin) it has the IUPAC name as (2S,5R,6R)-6-[[(2R)-2-amino-2-(4-hydroxyphenyl)acetyl]amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid. Apologies as I have should of course checked somewhere else first. $\endgroup$ – Davidmb Jan 16 '18 at 22:24

The answer given by orthocresol explains (and also partly corrects) the name that was suggested by Wikipedia and was mentioned in the question; however, it deliberately does not discuss the correct name according to current (2013) IUPAC recommendations.

The resulting name ‘6-{[2-amino-2-(4-hydroxyphenyl)-acetyl]amino}-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid’ (ignoring stereodescriptors for simplicity’s sake) is actually not wrong; however it is not the preferred IUPAC name.

The difference concerns the ‘[2-amino-2-(4-hydroxyphenyl)-acetyl]amino’ substituent group. For the preferred IUPAC name, the locants ‘2’ for the amino and the 4-hydroxyphenyl substituents would be omitted since all substitutable hydrogen atoms of an acetyl group ($\ce{-CO-CH3}$) have the same locant, resulting in the name ‘[amino(4-hydroxyphenyl)acetyl]amino’. However, the preferred suffix for a substituent of the type $\ce{-NH-CO-R}$ is ‘carboxamido’ (i.e. ‘acetamido’ for $\ce{-NH-CO-CH3}$) rather than ‘acylamino’ (i.e. ‘acetylamino’ for $\ce{-NH-CO-CH3}$). Thus, the preferred name for the complete substituent group is 2-amino-2-(4-hydroxyphenyl)acetamido. (Note that the ocants ‘2’ for the amino and the 4-hydroxyphenyl substituents can no longer be omitted since acetamido has substitutable hydrogen atoms with different locants.) Therefore, the preferred name for the complete compound is 6-[2-amino-2-(4-hydroxyphenyl)acetamido]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid.

(2S,5R,6R)-6-[2-amino-2-(4-hydroxyphenyl)acetamido]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

To understand this name, we first have to see that the carbocylic acid group ($\ce{-COOH}$) is the principal characteristic group in this case. Therefore, this group is chosen for citation at the end of a name by means of a suffix. Accordingly, the senior parent structure of this compound is bicyclo[3.2.0]heptane-2-carboxylic acid.

bicyclo[3.2.0]heptane-2-carboxylic acid

The name of the parent structure is modified according to the positions of the hetereoatoms to 4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid.

4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

Since the principal characteristic group is expressed by means of a suffix at the end of a name, the other substituent groups have to be expressed as prefixes. Considering only the groups that are directly attached to the parent structure leads to the name 6-acetamido-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid.

6-acetamido-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

As mentioned above, however, the acetamido group has two further substituents: (a simple substituent group ‘amino’ and a compound substituent group ‘4-hydroxyphenyl’), which changes the acetamido group to the complex substituent group ‘2-amino-2-(4-hydroxyphenyl)acetamido’.

6-[2-amino-2-(4-hydroxyphenyl)acetamido]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid


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