IUPAC Name to Structure Matching for Ethynyl Hydroxy Acid

To determine the correct chemical structure corresponding to the IUPAC name 3-ethynyl-2-hydroxy-4-methylhex-3-en-5-ynoic acid, we break down the name into its structural components:

1. Identify the Principal Functional Group and Main Chain Numbering

• Principal Functional Group: Carboxylic acid ($-\text{COOH}$ or $-\text{CO}_2\text{H}$), which takes highest priority. Thus, the carboxylic carbon is designated as $\text{C-1}$.
• Main Chain Length: "hex" indicates a 6-carbon principal chain containing $\text{C-1}$ to $\text{C-6}$.
• Position of Unsaturation:
  • "3-en" indicates a double bond between $\text{C-3}$ and $\text{C-4}$ ($\text{C-3}=\text{C-4}$).
  • "5-yn" indicates a triple bond between $\text{C-5}$ and $\text{C-6}$ ($\text{C-5}\equiv\text{C-6}$).

2. Identify the Substituents and their Positions

• $\text{C-2}$: Has a hydroxyl group ($-\text{OH}$), giving the fragment $-\text{CH(OH)}\text{CO}_2\text{H}$.
• $\text{C-3}$: Has an ethynyl substituent ($-\text{C}\equiv\text{CH}$).
• $\text{C-4}$: Has a methyl substituent ($-\text{CH}_3$).

3. Construct the Complete Structure

Combining these parts gives the structural formula: $\text{HO}_2\text{C}-\overset{2}{\text{C}}\text{H(OH)}-\overset{3}{\text{C}}(\text{C}\equiv\text{CH})=\overset{4}{\text{C}}(\text{CH}_3)-\overset{5}{\text{C}}\equiv\overset{6}{\text{C}}\text{H}$

• At $\text{C-3}$: The alkene carbon is attached to the $-\text{CH(OH)}\text{CO}_2\text{H}$ group and an ethynyl group ($-\text{C}\equiv\text{CH}$).
• At $\text{C-4}$: The alkene carbon is attached to a methyl group ($-\text{CH}_3$) and the alkynyl chain ($-\text{C}\equiv\text{CH}$ which constitutes $\text{C-5}$ and $\text{C-6}$).

4. Evaluating the Options

• Option (A): The substituent at $\text{C-4}$ is an ethyl group ($-\text{CH}_2\text{CH}_3$), not a methyl group.
• Option (B): The ethyl group is at the upper carbon, giving incorrect numbering and connectivity.
• Option (C): The methyl group is attached to $\text{C-3}$, and $\text{C-4}$ has two ethynyl groups attached, which violates the main chain structure.
• Option (D):
  • The upper alkene carbon ($\text{C-3}$) is attached to $-\text{CH(OH)}\text{CO}_2\text{H}$ and an ethynyl group ($-\text{C}\equiv\text{CH}$).
  • The lower alkene carbon ($\text{C-4}$) is attached to a methyl group ($-\text{CH}_3$) and an ethynyl group ($-\text{C}\equiv\text{CH}$) forming $\text{C-5}$ and $\text{C-6}$.

Thus, the structure in Option (D) perfectly matches the IUPAC name.

Correct Answer: (D)