Flexibilene, a compound isolated from marine coral, is the...

Name: Problem 32, Chapter 27: Organic Chemistry
Uploaded: 2020-08-04T19:32:28-08:00
Duration: 3 min 10 s
Channel: Kendrick Buford
Description: Flexibilene, a compound isolated from marine coral, is the first known terpenoid to contain a 15 -membered ring. What is the structure of the acyclic biosynthetic precursor of flexibilene? Show the mechanistic pathway for the biosynthesis.

Flexibilene, a compound isolated from marine coral, is the first known terpenoid to contain a 15 -membered ring. What is the structure of the acyclic biosynthetic precursor of flexibilene? Show the mechanistic pathway for the biosynthesis.

Key Concepts

Macrocyclization

Macrocyclization in terpenoid biosynthesis refers to the formation of large ring systems from linear precursors. In these reactions the enzyme’s active site holds the flexible, acyclic molecule in a conformation that favors the formation of a macrocycle via intramolecular bond formation. This controlled cyclization is essential for constructing unusual ring sizes such as the 15-membered ring found in flexibilene, highlighting the diversity and complexity of terpene cyclases.

Carbocation Cyclization Mechanism

The carbocation cyclization mechanism is a key step in transforming acyclic isoprenoid precursors into cyclic terpenoid structures. This process involves ionization of the pyrophosphate group to form an allylic carbocation, which then undergoes a series of intramolecular electrophilic additions. These reactions, guided by the enzyme’s active site, facilitate the closure of rings through attack on multiple double bonds, allowing formation of complex and often large ring systems.

Terpenoid Biosynthesis

Terpenoid biosynthesis refers to the formation of a wide variety of natural products built from isoprene units. These molecules are assembled by sequential coupling of five?carbon building blocks and often undergo cyclization reactions to form complex cyclic structures. The process starts from simple acyclic precursors and is catalyzed by enzymes that promote formation of reactive intermediates such as carbocations, which then rearrange and cyclize into diverse frameworks.

Acyclic Isoprenoid Precursors

Acyclic isoprenoid precursors, such as farnesyl pyrophosphate (FPP) in the case of sesquiterpenes, are linear molecules constructed from multiple isoprene units. These precursors typically feature alternating double bonds and diphosphate leaving groups, and they serve as the starting point for the biosynthesis of cyclic terpenoids by providing the carbon skeleton that can be reorganized through enzyme?mediated reactions.

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