Question

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?

Answer 1

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 `"COOH"` at the top. Draw horizontal lines to make crosses at `"C-2"` and `"C-3"`.

Step 3. Attach atoms to the bonds.

Arbitrarily put `"NH"_2` on the right of `"C-2"` and `"OH"` on the right of `"C-3"`. Put `"H"` atoms on the left of `"C-2"` and `"C-3"`.

I have a 25 % chance of being right. If I am wrong, I will simply reverse the locations of the `"H"` atoms.

Step 4. Determine the configuration at `"C-2"`.

The order of priority of the functional groups is `"NH"_2 = 1`; `"COOH" = 2`; `"C-3 = 3"`; `H = 4.`

The sequence `"NH"_2 → "COOH" → "C-3"` is clockwise (R).

But the `"H"` atom is pointing forward, so we must reverse the assignment to (R).

We want (S), so we must must interchange `"H"` and `"NH"_2`.

Step 4. Determine the configuration of `"C-3"`.

The order of priority of the functional groups is `"OH = 1"`; `"C-2 = 2"`; `"CH"_3 = 3`; `"H" = 4`.

The sequence `"OH → C-2 → CH"_3` is counterclockwise (S).

But the `"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.