Threonine is `(2S,3R) -2-`amino`-3-`hydroxybutanoic acid. Which of the following is threonine ?

Explanation

To determine which structure corresponds to threonine, we need to analyze the stereochemistry of the given options based on the provided description of threonine as (2S,3R)-2-amino-3-hydroxybutanoic acid. Here’s a step-by-step solution:

### Step 1: Understand the Structure of Threonine
Threonine is described as (2S,3R)-2-amino-3-hydroxybutanoic acid. This means:
- The carbon at position 2 (C2) has an S configuration.
- The carbon at position 3 (C3) has an R configuration.

### Step 2: Analyze Each Option
We will analyze each option provided to determine their configurations at C2 and C3.

#### Option A:
1. **Number the Carbon Atoms**:
- Assign numbers to the carbon atoms in the structure (1, 2, 3, 4).
2. **Assign Priorities**:
- For C2: Assign priorities based on the substituents attached to it.
- Determine the configuration (S or R) based on the arrangement of the groups.
3. **Configuration**:
- After analysis, C2 is found to be R.
- C3 is also found to be R.
- **Conclusion**: This option does not match threonine.

#### Option B:
1. **Number the Carbon Atoms**:
- Assign numbers to the carbon atoms (1, 2, 3, 4).
2. **Assign Priorities**:
- For C2: Assign priorities and determine the configuration.
- For C3: Repeat the process.
3. **Configuration**:
- C2 is found to be S.
- C3 is found to be R.
- **Conclusion**: This option matches (2S, 3R) configuration of threonine.

#### Option C:
1. **Number the Carbon Atoms**:
- Assign numbers to the carbon atoms (1, 2, 3, 4).
2. **Assign Priorities**:
- For C2: Assign priorities and determine the configuration.
- For C3: Repeat the process.
3. **Configuration**:
- C2 is found to be R.
- C3 is found to be S.
- **Conclusion**: This option does not match threonine.

#### Option D:
1. **Number the Carbon Atoms**:
- Assign numbers to the carbon atoms (1, 2, 3, 4).
2. **Assign Priorities**:
- For C2: Assign priorities and determine the configuration.
- For C3: Repeat the process.
3. **Configuration**:
- C2 is found to be S.
- C3 is found to be S.
- **Conclusion**: This option does not match threonine.

### Final Conclusion:
The correct structure that corresponds to threonine is **Option B**, which has the configuration (2S, 3R).

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