What retrosynthesis really is
When you look at a molecule you want to make, you cannot buy it — but you can usually buy, or already have, simpler pieces. Retrosynthesis is the discipline of mentally breaking your target apart, one bond at a time, until every piece is something you can purchase or make easily. Each imaginary bond-break is called a disconnection, and the pieces it leaves behind are called synthons. The clean trick is that you reason in the *reverse* direction of the actual chemistry: you plan backwards, then run the reactions forwards.
A good disconnection corresponds to a *real* forward reaction you trust. There is no point breaking a bond if you have no robust way to form it again. So retrosynthetic arrows (drawn as ⇒) are really shorthand for 'I know a reaction that builds this bond.' The art is choosing disconnections that lead back to cheap, stable, available building blocks.
How a medicinal chemist plans differently
A total-synthesis chemist making one complex natural product will accept a long, clever route. A medicinal chemist has a different goal: make *many* analogues of one scaffold quickly so the team can read the structure–activity relationship. That changes everything about route choice. You want a convergent plan where the part you will vary — the R-group — is added in the *last* step from a diverse menu of building blocks. Then a single late intermediate can branch into dozens of final compounds.
- Draw the target and identify the fixed core versus the variable region you want to explore.
- Place your first disconnection so the variable group comes off — ideally as an amide, amine, or coupling bond.
- Keep disconnecting the core until each fragment is a catalog building block or one short step away from one.
- Sanity-check each forward reaction for functional-group compatibility — will an earlier group survive a later step?
Target: Ar–C(=O)–NH–R (an amide) Disconnection 1 (amide bond): Ar–C(=O)–NH–R ⇒ Ar–C(=O)–OH + H2N–R forward = amide coupling (last step; R varied from a menu of amines) Disconnection 2 (biaryl C–C, on the acid piece): Ar–C(=O)–OH ⇒ Ar'–Br + (HO)2B–Ar''(CO2H) forward = Suzuki coupling Both Ar'–Br and the boronic acid are catalog building blocks.