Periodic Reporting for period 1 - CRAFTMOL (From Simplicity to Complexity: Crafting Molecular Architectures via Cascade Polyene Cyclizations)
Berichtszeitraum: 2022-02-01 bis 2023-07-31
Polyene cyclizations belong to the most powerful and fascinating chemical transformations to rapidly assemble molecular architectures. However, the very limited substitution pattern of known substrates and nature’s inability to accommodate heteroatoms or substituents other than simple alkyl groups restrict molecular diversity, complexity and functionality. We want to address these limitations by investigating a set of novel heteroatom-substituted and tri/tetrasubstituted polyenes. The participation of these structural units in the cyclization will unlock previously inaccessible reaction pathways and enable efficient, selective and practicable routes to anticancer, anti-inflammatory and antiviral molecules.The overall objective of this project is to realize innovative polyene cyclization pathways with unparalleled diversity, thus providing a strong boost for synthetic organic chemistry. The realization of the presented tasks provides synthetic access to structurally complex and diverse molecular architectures that previously eluded synthesis or required lengthy reaction sequences.
We will first investigate substrates containing trisubstituted double bonds and allenes (WP1) and explore their behaviour in cyclizations involving a transannular/cross-termination step. Realization of this concept will provide a highly modular synthetic platform for the rapid construction of more than 15 bioactive natural products. In parallel, we will explore tetrasubstituted double bonds and allenes (WP2) to realize first-time synthesis of a structurally diverse family of natural products with unique biological activities.
We will first investigate substrates containing trisubstituted double bonds and allenes (WP1) and explore their behaviour in cyclizations involving a transannular/cross-termination step. Realization of this concept will provide a highly modular synthetic platform for the rapid construction of more than 15 bioactive natural products. In parallel, we will explore tetrasubstituted double bonds and allenes (WP2) to realize first-time synthesis of a structurally diverse family of natural products with unique biological activities.
In our early studies we were able to accomplish the first synthesis of the complex pimarane natural product pimara-15-en-3α-8α-diol in 13 steps. The key cyclization precursor was synthesized via a highly modular approach in only six steps. The developed strategy is based on a powerful polyene cyclization, which sets five stereocenters, forms four carbon-carbon bonds and generates four six-membered rings. The final step involves a unique transannular endo-termination. This transformation was unprecedented in the chemical literature and culiminated in the first high-impact publication (Angew. Chem., Int. Ed. 2020, 59, 12436–12439). We later investigated a closely related polyene cyclization cascade for the 16-nor-ent-primarane diterpenoid norflickinflimoid C (WP1, Task 1A, Transannular endo-Termination of Dual Nucleophilic Enol Ethers) by combining (a) an enantiomerically enriched epoxide as initiating group, (b) a dual nucleophilic aryl enol ether to generate a bridged pentacyclic system and (c) an allene. The endo-termination of an allylic cation was envisioned to set five new stereocenters, three of which are quaternary. Unfortunately, no tetracyclization products were observed for an electron-poor sulfinyl allene, presumably due to lowered nucleophilicity of the allene. The other three allenes, which differ mainly in their respective steric demand according to the number of methyl substituents, performed efficient tetracyclization upon treatment with tin(IV) chloride. However, exclusively the undesired exo-termination products of the allylic cations were formed and, so far, all attempts to favor an endo-termination failed. Depending on the cyclization conditions, secondary reactions like 1,3-phenolate shifts and dimerization were observed. Of note, an unprecedented reactivity for allenes in cationic polyene cyclization was discovered, namely cyclization via a highly electrophilic vinyl carbocation. Two additional publications arose from these studies (Chem. Eur. J. 2021, 27, 12410–12421; Chem. Eur. J. 2021, 27, 7017–7021). After extensive experimentation we were able to develop a synthetic platform for the construction of complex ent-pimarane natural products that were previously inaccessible via chemical synthesis. The developed synthesis benefits from robust transformations and allows for selective late-stage diversification. Highlights are a HFIP mediated bicyclization to set the trans-decalin stereochemistry and an arene hydrogenation/oxidation/alkylation sequence to install the crucial C13 quaternary stereocenter of the C-ring. The key intermediate was prepared in 10 steps and served as the late branching point to selectively modify the A- or C-ring of the tricyclic core. In total, eight natural products were accessible within 11–16 steps from commercially available starting materials. To the best of our knowledge, this works represents the first divergent total synthesis of ent-pimarane natural products.The results were published in the renowned journal Organic Letters (Org. Lett. 2022, 24, 39, 7151). In parallel a radical type cyclization of an enol ether allowed us to intersect the previous route and offer some insights into this new type of cyclization. The developed synthesis allowed us to prepare ample amounts for biological screening, which is currently performed in collaboration with the Koeberle group (MPI Innsbruck). Initial efforts to realize a 6pi electrocyclization cascade to access (neo)triperifordin (Task 1B), were met failure and we therefore started our investigations on a powerful cationic polyene cyclization. In this context we will expand our studies to internal allenes and study their behaviour in innovative cyclization reactions. The terpene alkaloid greenwaylactam A (Task 2, alkaloids) possess an unusual 8-membered subunit that is derived from an oxidative ring-opening of the indole scaffold. We found that the C3 position of the indole (C3 position of the "enamine" motifd) does undergo a low yielding (<10%) radical cyclization to the pentacyclic scaffold. However, careful screening of reactions conditions enabled an HFIP-assisted cationic cyclization in 57% yield. Blocking the C3 position of the indole allowed for a switch in the regioselectivity and provided access to polyavolensinol via an unprecedented N-termination. This regiodivergent synthetic platform enables for the first-time selective diversification via C or N-termination should provide access to nine natural products (ACIE 2023, e202307719). Xenibellol (WP2, task 3) and waixenicin A are unique Xenia deiterpenoids that share a common biosynthesis. Waixenicin A is a selective and highly potent TRPM7 ion channel inhibitor (16 nM) and features the type of enol ether which are currently studied for their behaviour in polyene cyclizations. We were now able to realize the first chemical synthesis of Waixenicin A. A sequential elongation strategy enabled selective sidechain installation and gave access to waixenicin A as well as 9-deacetoxy-14,15-deepoxyxeniculin, which underwent one-step rearrangement to xeniafaraunol A. In addition, we introduced a valuable alternative to the classical Stork–Jung vinyl silane. In hand with this synthetic strategy, the stage is now set to access closely related natural products as well as derivatives with deep seated structural modifications. Initial efforts to access dysiherbol (task 4a) via cyclization of an 1,5-enyne led to exclusive 5-endo-dig cyclization followed by 1,2-migration. To circumvent this issue, we extended the linker length by one methylene unit to enable a 6-exo-dig cyclization via the 1,6-enyne and also looked into an acetal functional group as an alternative initiating group. For both systems either no reaction or decomposition was observed for these systems. In addition, the required Suzuki coupling only delivered a 1.1 mixture of regioisomeric tetrasubstituted alkenes. Current work is dedicated to a novel class of cyclization precursors, that are internal allenes. For dactyloquinone (task 4b), the initial cyclization precursor carrying the central tetrasubsituted enol ether was not accessible from a readily available trichloroenol ether as only two Cl-substituents could be replaced. We therefore want to follow now a similar allene strategy as for dysiherbol.
We have already discovered innovative reaction pathways, that were previously unknown. This has allowed for efficient synthetic access to structurally diverse molecular architectures that were previously inaccessible and enabled deep-seated structural modifications of natural molecules that are currently inaccessible via fermentation or semi-synthesis. We expect to find interesting biological activity for the generated compound library and are planning to intensify our screening activity in collaboration with Prof. Zierler and Prof. Koeberle.
We have obtained a better understanding of polyene cyclizations and their mode of cyclization and want to transfer this knowledge to novel substrates such as internal allenes. This might also unlocks innovative strategies to bioactive terpenoids such as mitrephorone C, which is currently inaccessible via chemical methods. The conducted work also served as an inspiration to develop powerful methods and synthetic strategies beyond the polyene cyclizations studied so far. For instance, a cationic cyclization as key-step in the presence of a delicate enol ethers enabled the first chemical synthesis of Waixenicin A almost 40 years after its first appearance in the literature. We want to generate a library of bioactive pimarane natural products that will be used in an anti-inflammatory screening campaign at the MPI Innsbruck (Prof. Koeberel). Natural and non-natural xenicin natural products will be studied for their TRPM inhibitory activity (Prof. Zierler, JKU Linz).
We have obtained a better understanding of polyene cyclizations and their mode of cyclization and want to transfer this knowledge to novel substrates such as internal allenes. This might also unlocks innovative strategies to bioactive terpenoids such as mitrephorone C, which is currently inaccessible via chemical methods. The conducted work also served as an inspiration to develop powerful methods and synthetic strategies beyond the polyene cyclizations studied so far. For instance, a cationic cyclization as key-step in the presence of a delicate enol ethers enabled the first chemical synthesis of Waixenicin A almost 40 years after its first appearance in the literature. We want to generate a library of bioactive pimarane natural products that will be used in an anti-inflammatory screening campaign at the MPI Innsbruck (Prof. Koeberel). Natural and non-natural xenicin natural products will be studied for their TRPM inhibitory activity (Prof. Zierler, JKU Linz).