Flavan-3-ols Are Also Called Catechins

The flavan-3-ols are also called catechins and they are found primarily in the grape seeds.¹ They include (+)-catechin, (-)-epicatechin, (-)-epigallocatechin and (-)epicatechin gallate. Experiments have shown that there is an increase in catechin and epicatechin until veraison when those flavan-3-ols begin to decline. Because catechin and epicatechin have a bitter taste, the gradual ripening of seeds may be due to the decrease in the levels of catechin and epicatechin.
The structure of (+)-catechin differs from (-)-epicatechin only in their spatial arrangement of the atoms around the 2nd and 3rd carbon atoms in the pyran ring. I hope to illustrate that point with the following structural depictions.
The structures of (+)-catechin, (-)-epicatechin, and (-)-epigallocatechin shown below will consist of the following:

• first as a bond-line structural representation
• second as a 3D representation showing the relationship between the substituents on the second and third carbons
• lastly as a space filling model

The structure of (-)-epicatechin is depicted below. The structure in the middle is a 3d representation and when compared with (+)-catechin above, shows the difference in the arrangement of the atoms connected to carbons 2 and 3.

(-)-Epigallocatechin shown below differs from (-)-epicatechin in having one more hydroxyl group (circled in red) attached to the phenyl ring.

In the case of (-)-epicatechin gallate, the stereochemistry around carbons 2 and 3 of the pyran ring is the same as in (-)-epicatechin and as in (-)-epigallocatechin, however, there is a gallic acid moiety that is esterified to carbon 3, circled in red as shown on the left.²
As mentioned above, catechins are important because they are contained in the seeds of the grape and decline after veraison. If bitter characters are detected in a wine, they may have come from these catechins. In addition, catechins are the monomeric units that can polymerize and lead to tannin formation, the subject of a future blog. Stay tuned!

References:
1. Douglas Adams, VI257, Lesson 8, pg. 50.
2. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008, pg. 283.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.