Retrosynthesis: Planning Synthesis Backwards

The many chemical reactions that a Chemist has in their arsenal helps them to manipulate molecules and interchange functional groups. These chemical reactions can then be used to form synthetic pathways, useful to produce what is known as “target molecules”. This idea of producing synthetic pathways has become very important to many chemists as more complex molecules are being research. Since most commercially available and inexpensive molecules contain six or fewer carbon atoms, the ability to form larger more complex molecules require synthetic pathways.

Since most target molecules are far more complex than six or fewer carbons, synthetic pathways are the best way for researchers to obtain their target molecule in enough quantity to analyze.

Retrosynthesis

Some people may think that the easiest way to produce a synthetic pathway would be to go in the forward direction, adding on to finally get the target molecule. The truth is that this is not the easiest  way of composing synthetic pathways. The best way to produce a synthetic pathways is by starting with the target molecule and taking things off to produce a viable starting material. This backwards method is known as retrosynthesis. Retrosynthesis helps to reduce dead-end pathways. Imagine trying to put together a puzzle without instructions. Now imagine putting together the same puzzle if you have already dissembled it. The later allows you to eliminate a large amount of the dead-end routes that can be taken in the forward direction.

3 Retrosynthesis Guidelines

1. Try to minimize the number of transformations used in the synthetic pathway. In some cases choosing a lower yielding step may be more pertinent if it will lessen the amount of steps in the pathway. This is mainly due to the fact that even fewer low-yield steps can produce more product than more high-yield steps is some cases.
2. Do not use Reagents that have functional groups that will interfere with the desired reaction. This would lead to unintended reactions different from the desired.
3. Take into account any mechanistic and Structural constraints affecting the reactions. In many cases certain reactions have more likely outcomes do to different types of hinderances.