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Insight into the Symbiotic Chemical Communication of Algae and Bacteria: Thallusin and Dedicated Analogues

Periodic Reporting for period 1 - THALLMORPHAL (Insight into the Symbiotic Chemical Communication of Algae and Bacteria: Thallusin and Dedicated Analogues)

Reporting period: 2018-09-01 to 2020-08-31

Thallusin is a terpenoid hybrid that triggers the growth of leaf-like structure in alga Monostroma oxyspermum at a concentration as low as 10-15 g/L. Thallusin is produced by the bacteria that live in a symbiotic relationship with algae in their natural habitat. However, its general impact on the underlying communities and its mode of actions remain to be clarified. Recently, it has been demonstrated that thallusin induces the rhizoid and cell wall formation in Ulva mutabilis in the presence of Roseovarius sp. bacteria. Compared to Monostroma, thallusin has a different function in Ulva. It induced cell differentiation and formation of the holdfast organ < 10-10 g/L concentrations. The genus Ulva has a profound economic and scientific value. It grows mainly in intertidal zones. Eutrophication of coastal waters results in the rapid growth of such species, significantly increasing their biomass and thus forming, e.g. green tides, which leads to the impairment of the coastal marine ecosystem.
Access to thallusin is highly essential for biological experiments as its retrieval from natural sources is difficult. For biological work, the only isolated material in µ quantities has been used so far, so data beyond mass spectrometry was challenging to obtain. We, therefore, aimed at developing a synthetic route to thallusin, which enables scalable access to thallusin and its derivatives that can be used as a tool for studying the morphogenesis in Ulva. A 6-endo-cyclization was envisioned for the one-pot assembly of the pyridine substituted dihydropyran ring-system found in thallusin. Synthetic thallusin, its analogs, and labeled tool compounds were envisioned to significantly contribute to a better understanding of thallusin activity and relevance for algae growth. At last, data collected from all the above-described experiments will be crucial for identifying the mode-of-action and yet-unknown biosynthesis of naturally occurring thallusin. In the long term, these data will collectively stimulate our understanding of the growth of macroalgae, that have the highest relevance for carbon fixation in marine ecosystems, as well as for using algae in the production of chemicals and for biotechnology.
Our efforts began by targeting the construction of thallusin, relying on the connection of selector (terpenoid part) and chelator (pyridine carboxylic acid) through implementing an unprecedented 6-endo-dig arylative cyclization. The effect of different transition metal catalysts, ligands, bases, solvents, and substrate designs were studied on specifically designed model systems; however, only 5-exo-dig arylative cyclization was observed. On the other hand, thallusin was thought to be accessible from transition metal-catalyzed diastereoselective 6-endo-trig cyclization of stereochemically defined allenol. Toward this end, we have developed a novel method for the Au(I) catalyzed 6-endo trig cyclization of β-hydroxy allenols and sequential iododeauration of a σ-gold intermediate to generate 3-iodo-3,4-dihydropyrans.
Two different approaches were developed for the synthesis of the selector (terpene framework) of thallusin in enantiopure form 1) diastereoisomer separation by using chiral auxiliary 2) asymmetric polyene cyclization. In the 1st approach, an enantiomerically pure selector (terpenoid part) was obtained from dihydro--inone by using a chiral auxiliary in good yield. In the 2nd approach, an enantioselective polyene cyclization was established by using a substrate for a chiral iridium-based catalyst, inducing an enantioselective cyclization cascade. Finally, an excellent ee of >98 % was achieved and enabled a highly efficient synthesis of the terpene framework of (‒)-thallusin. Both approaches could lead to a common intermediate (-hydroxy aldehyde) for the synthesis of (‒)-thallusin. For the 6-endo-trig-cyclization, a method was developed for an efficient installation of an allenol on the -hydroxy aldehyde scaffold. The developed Au(I) catalyzed 6-endo trig cyclization of the β-hydroxy allenol furnished the tricyclic vinyl-iodo-alcohol in excellent yield. A Negishi coupling of zincated pyridine dicarboxylates was then developed for the completion of the total synthesis. In summary, we have developed an efficient and modular synthetic route for the enantiopure synthesis for (‒)- thallusin and its derivative. (+)-Thallusin and its derivatives were also synthesized by following the same sequence. Synthesized thallusin and its derivatives were tested for the morphogenesis activity of Ulva mutabilis in collaboration, generating key SAR data for exploring thallusin, and allowing to determine the potency of (-)-thallusin in Ulva mutabilis (EC50 = 5 pM).

The results of the project were presented in a different conference by Dr. Seema Rani. [a) GDCh Wissenschaftsforum Chemie held at RWTH Aachen, Germany, during September 2019; title of the poster: Synthetic studies towards Thallusin. b) Early Career Investigators (ECIs) meeting held at Brussels, Belgium, during February 2019. Title of the presentation: Total synthesis and SAR studies of thallusin derivatives in green macroalgae]. Dr. Rani has also presented these results in The Long Night of Science Festival. The results will be published in three publications, one on the first stereoselective total synthesis of (-)-thallusin and initial profiling of analogs (in preparation for Angew. Chem.), a second one on methodological details of the 6-endo cyclization (in preparation for Eur. J. Org. Chem.), and a third one on the polyene cyclization methodology (in preparation for Org. Lett.). Further contributions to forthcoming publications of the host lab are anticipated, especially to biological testing initiatives that are currently intensified. These results will also be published on the IOMC website of the FSU and on the host group’s website (www.arndtgroup.uni-jena.de).
The knowledge obtained during the synthesis and biological testing of thallusin will provide a better understanding for the designing of tool compounds. Labeled tool compounds will allow tracing the metabolism and degradation of thallusin. The synthetic thallusin analogs will significantly facilitate ongoing mode-of-action studies for thallusin by correlating genomic, proteomic and metabolic responses with SAR data. The host gropups will apply the tool compounds in MS, for studying ulva growth and development, and for microscopy experiments. Furthermore, data collected from all the above-described experiments will inform mode-of-action studies and future studies on the biosynthesis of thallusin. Finally, beyond the Chemical Biology investigations, the results of the project hold promise for allowing the synthesis of thallusin in sufficient quantity, especially in racemic form, for dedicated stimulation of axenic algae cultures on large scale in future biotechnology applications, where coculturing with growth-stimulationg bacteria is impossible or prohibited (e.g. GMP production).