The Diels-Alder Reaction: A Natural Example

Based upon our previous knowledge of the Diels-Alder class of reactions, we can make some general statements regarding this style of synthesis:

• A conjugated diene in an s-cis conformation reacts with
• A dienophile to form
• A 6-membered ring with one double bond

These basic facts and more are summarized in the following video:

My Little s-Cis Diene (Diels- Alder Reaction Animated) 

We know that this reaction can occur in the lab, but can it happen naturally?

The answer is yes. A photosynthetic fungus, Macrophoma commelinae, synthesizes the compound macrophomate synthase in complex with pyruvate through a three step proccess, the second of the reactions in this process is a diels alder reaction. The entire reaction process is illustrated below:

It can be observed in the figure the starting molecule (first molecule in the second row of the figure), which reacts to form a six- membered ring.
The diene of this reaction is known as 2-pyrone (IUPAC: pyran-2-one) and is illustrated below:

The dienophile of this reaction is ethyl propiolate (IUPAC:  Ethyl acetylenecarboxylate) and is illustrated below:

The reaction conditions of this process also include that the reaction occurs entirely within the fungus itself, and must be catalyzed by the enzyme MPS. 

Sources (In order of appearance):

Video: My Little s-Cis Diene (Diels-Alder Reaction Animated). YouTube. http://www.youtube.com/watch?v=8SfGsNdVV0Q (accessed Feb 03, 2014)

Reaction Article: Ose, Toyoyuki; Watanabe, Kenji; Mie, Takashi; Honma, Mamoru; Watanabe, Hiromi; Yao, Min; Oikawa, Hideaki; and Tanaka, Isao. Inisght into a natural Diels-Alder reaction from the structure of macrophomate synthase. Nautre. 2003, 422, 185-189.

 

2-pyrone: 2H-Pyran-2-one. Pub Chem Compound. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=68154#itabs-2d (accessed Feb 03, 2014)

Ethyl propriolate: ethyl propiolate. Pub Chem Compound. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=12182#itabs-2d (accessed Feb 03, 2014)