Terpenoid-Biosynthese abseits bekannter Wege: unkonventionelle Cyclasen und ihre Bedeutung für die biomimetische Synthese

New Trends and Future Opportunities in the Enzymatic Formation of C-C, C-N, and C-O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021, 1, 1-21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021, 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C-C, C-N and C-O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.

Bifunctional Polyene Cyclizations: Synthetic Studies on Pimarane Natural Products

Polyene cyclizations generate molecular complexity from a linear polyene in a single step. While methods to initiate these cyclizations have been continuously expanded and improved over the years, the majority of polyene substrates are still limited to simple alkyl-substituted alkenes. In this study, we took advantage of the unique reactivity of higher-functionalized bifunctional alkenes. The realization of a polyene tetracyclization of a dual nucleophilic aryl enol ether involving a transannular endo-termination step enabled the total synthesis of the tricyclic diterpenoid pimara-15-en-3α-8α-diol. The highly flexible and modular route allowed for the preparation of a diverse library of cyclization precursors specifically designed for the total synthesis of the tetracyclic nor-diterpenoid norflickinflimiod C. The tetracyclization of three diversely substituted allenes enabled access to complex pentacyclic products and provided a detailed insight into the underlying reaction pathways.

Beyond the Isoprene Pattern: Bifunctional Polyene Cyclizations

Polyene cyclizations are capable of producing molecular complexity in a single step. While classical systems are limited to simple alkyl substitution patterns only, bifunctional polyenes take advantage of the unique reactivity of higher-functionalized alkenes. Here, we highlight the potential of these variants for the synthesis of structurally complex polycycles involving unprecedented termination steps. We also want to provide a stimulus for the development of novel modes of cyclization that involve bifunctional units to enable efficient synthesis of yet inaccessible natural products.

A Case for Bond-Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a "bond-network analysis" of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.

Unprecedented [5.5.5.6]Dioxafenestrane Ring Construction in Fungal Insecticidal Sesquiterpene Biosynthesis

Fenestranes, a specific class of natural products, contain four fused rings that share a central quaternary carbon atom. The fungal natural product penifulvin A (1) is a potent insecticidal sesquiterpene that features the [5.5.5.6]dioxafenestrane ring. Although the chemical synthesis of 1 has been achieved recently, the enzymes catalysing the cyclization and oxidation of FPP to 1 remain unknown. In this work, we identified a concise pathway that uses only three enzymes to produce 1. A new sesquiterpene cyclase (PeniA) generates the angular triquinane scaffold silphinene (6). A cytochrome P450 (PeniB) and a flavin-dependent monooxygenase (PeniC) catalyse a series of oxidation reactions to transform 6 into 1, including oxidation of the C15 methyl group to a carboxylate moiety, oxidative coupling of the C15 carboxylate and the C1-C2 olefin to form a γ-lactone, and Baeyer-Villiger oxidation to form a δ-lactone. Our results demonstrate the highly concise and efficient ways in which fungal biosynthetic pathways can generate complex sesquiterpene scaffolds.

Convergent Assembly of the Tetracyclic Meroterpenoid (-)-Cyclosmenospongine by a Non-Biomimetic Polyene Cyclization

The cationic cyclization of polyenes constitutes a powerful and elegant transformation, which has been utilized by nature's biosynthetic machinery for the construction of complex polycyclic terpenoids. Previous studies by chemists to mimic this cyclization in the laboratory were limited to different modes of activation using biosynthetic-like precursors, which accommodate only simple methyl-derived substituents. Here we describe the development of an unprecedented and highly efficient polyene cyclization of an aryl enol ether containing substrate. The cyclization was shown to proceed in a stepwise manner to generate three rings and three consecutive stereocenters, two of which are tetrasubstituted, in a single flask. The developed transformation is of great synthetic value and has enabled the convergent assembly of the tetracyclic meroterpenoid (-)-cyclosmenospongine.