# The systematic (IUPAC) name for the amino acid threonine is (2S,3R)-2-amino-3-hydroxybutanoic acid. The systematic name indicates that threonine has two stereocenters, at positions 2 and 3, with S and R configurations, respectively. What is ths the Fischer projection of threonine. How many other possible stereoisomers of threonine are there?

May 7, 2015

Here's how I would draw the Fischer projection of threonine.

Step 1. Draw the structural formula of threonine.

Step 2. Draw the longest continuous chain of carbon atoms vertically, with $\text{COOH}$ at the top. Draw horizontal lines to make crosses at $\text{C-2}$ and $\text{C-3}$.

Step 3. Attach atoms to the bonds.

Arbitrarily put ${\text{NH}}_{2}$ on the right of $\text{C-2}$ and $\text{OH}$ on the right of $\text{C-3}$. Put $\text{H}$ atoms on the left of $\text{C-2}$ and $\text{C-3}$.

I have a 25 % chance of being right. If I am wrong, I will simply reverse the locations of the $\text{H}$ atoms.

Step 4. Determine the configuration at $\text{C-2}$.

The order of priority of the functional groups is ${\text{NH}}_{2} = 1$; $\text{COOH} = 2$; $\text{C-3 = 3}$; $H = 4.$

The sequence $\text{NH"_2 → "COOH" → "C-3}$ is clockwise (R).

But the $\text{H}$ atom is pointing forward, so we must reverse the assignment to (R).

We want (S), so we must must interchange $\text{H}$ and ${\text{NH}}_{2}$.

Step 4. Determine the configuration of $\text{C-3}$.

The order of priority of the functional groups is $\text{OH = 1}$; $\text{C-2 = 2}$; ${\text{CH}}_{3} = 3$; $\text{H} = 4$.

The sequence ${\text{OH → C-2 → CH}}_{3}$ is counterclockwise (S).

But the $\text{H}$ atom is pointing forward, so we must reverse the assignment to (R). That's correct!

The Fischer projection of threonine is therefore

The number of possible stereoisomers is ${2}^{n}$, where $n$ is the number of stereocentres.

Since threonine has 2 stereocentres, there are ${2}^{n} = 4$ stereoisomers.

So there are three other stereoisomers of threonine.