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.

2 Answers 2


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, 2018 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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