How to Do Retrosynthesis: Working Backward From the Target

The Vocabulary of Working Backward

Retrosynthesis has its own notation and a few terms that make the rest coherent. A disconnection is the imaginary breaking of a bond in the target to reveal simpler fragments, drawn with a special open retrosynthetic arrow that reads as 'is made from'. The idealized fragments a disconnection produces are called synthons, and they are usually charged: one piece is written as a cation and the other as an anion so that the bond you broke could re-form by a normal polar reaction.

Because synthons are idealized, you also need their real-world stand-ins, called synthetic equivalents. A simple carbanion synthon is not a bottle on the shelf, but a Grignard or organolithium reagent plays its role. Harder synthons need cleverer equivalents: an acyl anion, a carbon that is nucleophilic where a carbonyl carbon would normally be electrophilic, has no direct reagent at all, so chemists reach for cyanide or a dithiane instead. Finally, a functional group interconversion, or FGI, is a step that changes one functional group into another without breaking the carbon skeleton, such as turning an alcohol into a better handle for the next disconnection.

Where to Disconnect: Follow the Functional Groups

You do not disconnect bonds at random. The best disconnections sit next to functional groups, because those are the bonds that real reactions can form. Carbonyls are the richest hunting ground: the bond between a carbonyl carbon and the atom next to it, or the bond one carbon further along, corresponds to well-known carbon-carbon bond-forming reactions. Learning to look at a molecule and see its carbonyls as disconnection sites is most of the skill.

The reason this works is polarity. A carbonyl carbon is naturally electron-poor, an acceptor, while the carbon next to it can be made electron-rich, a donor, by removing an alpha proton. A sound disconnection pairs a donor synthon with an acceptor synthon so the forward reaction is just a nucleophile meeting an electrophile. When you break a bond and the two synthons have matching, sensible polarities, you have found a disconnection that a real reaction can execute.

• Prioritize bonds adjacent to functional groups, especially carbonyls and heteroatoms.
• A good disconnection gives one donor synthon and one acceptor synthon with sensible polarity.
• Carbon-heteroatom bonds, such as C-O in an ether or ester, are usually easy first disconnections.
• Use an FGI to install a functional group that unlocks a better disconnection.

Read Every Disconnection as a Reaction in Reverse

A disconnection is only useful if you can name the forward reaction that reassembles the pieces. This is why the forward reactions you already know are the raw material of retrosynthesis. When you disconnect the carbon-carbon bond of a secondary alcohol, you should immediately see a Grignard reagent adding to an aldehyde. Disconnect the bond beta to a carbonyl in a 1,3-relationship and you should recognize an aldol. Disconnect an ether at the carbon-oxygen bond and you should see a Williamson ether synthesis.

This mapping goes both ways, and building it is the real study task. Every named reaction you learn forward is also a disconnection you can run backward, so a strong reaction toolkit is exactly what makes retrosynthesis feel easy. Certain patterns are worth memorizing as reverse reactions: a 1,3-dioxygenated pattern such as a beta-hydroxy carbonyl points to an aldol, a 1,3-dicarbonyl such as a beta-keto ester points to a Claisen condensation, a 1,5-dicarbonyl points to a Michael addition, and a cyclohexene ring points to a Diels-Alder.

Choosing Among Several Disconnections

A target usually offers more than one place to cut, so you need guidelines for picking. Favor the disconnection that simplifies the molecule the most, ideally splitting it into two pieces of roughly similar size rather than shaving off one carbon at a time. Favor disconnections that exploit symmetry, because a symmetric target may come from two identical, cheap fragments. And favor disconnections whose precursors are commercially available or only a step or two away.

Above all, favor disconnections that correspond to reliable, high-yielding reactions with good selectivity. A theoretically valid disconnection that relies on a capricious reaction is worse than a slightly less elegant one that uses a robust reaction. Keep going recursively: apply the same reasoning to each precursor until every branch of the tree ends at a recognizable starting material. The goal is a short tree of dependable steps, not the cleverest single cut.

Turning the Plan Into a Real Synthesis

Once the retrosynthetic tree reaches available starting materials, flip it over and write the synthesis in the forward direction with actual reagents and conditions. This forward pass is where you catch problems the backward analysis hid. Check the order of steps so that each reaction leaves the groups you need for later steps intact, and confirm that each reagent is selective for the group you are targeting and not for another sensitive group in the molecule.

When two functional groups would interfere, this is where protecting groups earn their place: mask a reactive group, run the step, then unmask it. Also decide the order to install stereochemistry and any rings. A clean way to practice is to take a target, draw the full retrosynthetic tree, then write the forward synthesis and stress-test it for selectivity. In Octet you can browse reactions and compare them side by side to check that each disconnection you propose maps onto a real, dependable forward reaction.

Frequently asked questions

Keep going