Electrocyclic Reactions in Biological Systems
Electrocyclic reactions occur frequently in nature. One of the most common such electrocyclizations is the biosynthesis of vitamin D3.
The first step involves a photochemically induced conrotatory ring opening of 7-dehydrocholesterol to form pre vitamin D3. A -hydride shift then forms vitamin D3.
Another example is in the proposed biosynthesis of aranotin, a naturally occurring oxepine, and its related compounds.
Enzymatic epoxidation of phenylalanine-derived diketopiperazine forms the arene oxide, which undergoes a 6π disrotatory ring opening electrocyclization reaction to produce the uncyclized oxepine. After a second epoxidation of the ring, the nearby nucleophilic nitrogen attacks the electrophilic carbon, forming a five membered ring. The resulting ring system is a common ring system found in aranotin and its related compounds.
The benzonorcaradiene diterpenoid (A) was rearranged into the benzocycloheptatriene diterpenoid isosalvipuberlin (B) by boiling a methylene chloride solution. This transformation can be envisaged as a disrotatory electrocyclic reaction, followed by two suprafacial 1,5-simatropic hydrogen shifts, as shown bellow.
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