What is Coumarin? - Structure, Synthesis & Derivatives

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The focus of this lesson will be on a privileged organic compound known as coumarin. Our main points of discussion will be on coumarin's basic structure, a couple of the more common methods by which it's made synthetically, and some important derivatives that come from the parent molecule itself.

Block that Sun

Let's say you're headed out to the beach on a warm sunny day to soak up some rays and splash in the ocean. Your mother calls, and of course, makes sure you don't forget something to keep from getting sunburned. What does she want you to bring and wear? Why, sunscreen, of course! Have you ever wondered what's actually in sunscreen that helps block the sun's powerful and dangerous ultra-violet rays? Although there are a lot of chemical compounds that are used in sunscreens to help protect us, some contain a molecule that belongs to a class of organic compounds that's the focal point of our lesson today--coumarin.

Coumarin is an organic compound that has two six-membered rings fused together, with one of the rings being a benzene ring and the other containing an alkene functionality and an ester functional group. Coumarins play an important role in both natural systems like plants and also in medicinal applications as drug molecules. The primary topics in our lesson will be the general structure of coumarin, how it's made synthetically, and some important derivatives of coumarin in terms of application.

Structure of Coumarin

Let's get started by introducing coumarin in terms of its general structure so we'll know how to recognize it. Coumarin actually belongs to a broader class of compounds called the benzopyrones. A benzopyrone is simple to identify, it's just two six-membered rings fused together with one of the rings being a benzene ring, and the other ring containing an alkene and ester functional group inside the ring. Notice these three distinct structural features in the coumarin molecule.

General structure of coumarin

Synthesis of Coumarin

Now that we know how to identify a coumarin based on its structure, let's talk about how it's made synthetically. (Note: coumarin and derivatives of coumarin can be isolated from plant sources in nature but here we are going to focus on man-made methods of its preparation.)

Pechmann Condensation

One of the most popular methods of making coumarins is through what's called a Pechmann condensation reaction. This reaction is of great utility since it utilizes cheap and commercially available chemical building blocks. The Pechmann condensation involves taking phenol and reacting it with ethyl acetoacetate in the presence of a catalyst (usually aluminum chloride is used). The reaction is called a 'condensation' because literally our two reactant molecules condense with one another to form our single product molecule. Once the condensation happens, we get our coumarin product!

Pechmann condensation of phenol and ethyl acetoacetate to make 4-methylcoumarin

Kostanecki Acylation

Another popular method of making coumarins is by a reaction called the Kostanecki acylation, which involves the use of an Ortho-hydroxyaryl ketone (an aromatic ketone with an -OH group adjacent to it) with an anhydride of some sort. Much like the Pechmann condensation, this transformation is a great way to make coumarins because it has pretty mild reaction conditions and uses cheap and abundant starting materials.

Note: in this reaction any anhydride can be used, which is why we use the 'R' group designation for the anhydride reactant.

Kostanecki acylation of an ortho-hydroxyaryl ketone with an anhydride to make a 4-methylcoumarin product

Important Coumarin Derivatives

Let's now take our attention to a couple of the more important derivatives of coumarin. These molecules contain our coumarin core structure, but also have some other groups attached to it. This is why we call them derivatives because they are derived from coumarin itself. By far, one of the most publicized coumarin derivatives is called warfarin. Warfarin is a molecule that's used as a blood thinning agent to help prevent blood clots and prevent stroke. Notice the coumarin skeleton highlighted in red in the structure of warfarin.

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