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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ANaPSyS (Artificial Natural Products System Synthesis)

Teaser

Summary

\"The search for novel drugs and new medicines strongly depends on the efficiency of synthetic processes. Especially in complex molecule synthesis the challenge to access a structure synthetically can be so huge and the effort so enormous, rendering the overall process inefficient.
In the quest for new medicines natural products are very often used, since they intrinsically are biologically active and give the medicinal chemist a head start. But natural product typically display a challenging three-dimensional structure - and thus are laborious to synthesise.
This has the consequence, that pharmaceutical companies stick to simple target molecules. The problem thereby is that simple structures only allow very crude design and thus the need for more selective and specific drugs cannot be met, since these properties most often are connected with a complex three-dimensional architecture. To overcome this process and make the quest for new bioactive compounds more efficient, we chose to develop a strategic approach called \"\"artificial Natural product systems synthesis (ANaPSyS)\"\". This concept aims to develop central building blocks that are structurally embedded in different natural products - so to say comprise a core structure - or as we like to call it \"\"privileged intermediate\"\". These building blocks are on the one hand advanced intermediates but at the same time can be flexibly used for different structure types of natural products. As a consequence such an intermediate is valuable not only for one specific target structure - as is normally the case in drug research - but can be used for multiple molecules with different biological activities. This inherently enhances synthetic efficiency and renders scale up process utmost important, since the privileged intermediate can be used multiple times, thus is needed in large quantities. We were able to put this concept into practice and showcase that this strategy is feasible by the synthesis of two different natural product family congeners with different biological activity (anti-jussive and hypotensive). The design of the privileged intermediate is thereby of utmost importance, and is developed with the help of intensive database search as was performed in our case.\"

Work performed

Since the beginning of this project we have accomplished the total syntheses of 6 different sarpagine alkaloids and the stemona alkaloid parvineostemonine (Figure 1). Furthermore we have accessed 11 non-natural sarpagine derivatives which will be tested for their biological profile and submitted to SAR studies. The syntheses are outlined in schemes 1+2

As a starting point for our synthesis of sarpagine alkaloids we took advantage of Aggarwalâ€™s chiral ketene equivalentÂ 7. 7 was reacted with pyridiniumÂ 6 under basic conditions to form the corresponding ylide in situ, which then underwent a [5+2] oxidopyridinium cycloaddition (SchemeÂ 1). The regioisomersÂ 8 and regio-8 were obtained as an inseparable mixture. Upon scale-up to deca-grams the ratio of regioisomers slightly improved from 2:1 to 2.5:1 in favor of the desired cycloadduct. Reduction of the bissulfoxides was performed using trifluoroacetic acid anhydride and sodium iodide to afford dithiolanesÂ 9 and regio-9, which were separable at this stage of the synthesis. It is noteworthy, that significant improvements of yields were achieved on large scale (multi-grams) when the temperature was carefully controlled and maintained between â€“10 â€“ to â€“5 Â°C, and the equivalents were raised from 3 to 5. The changes of this protocol proved to be vital for the scale-up of the whole synthetic route. Using this sequence multigram quantities of the desired tricycle were obtained. Further 1,4â€‘reduction of enoneÂ 9 proceeded smoothly, leading to compoundÂ 5. The missing piperidine ring of the core structure was build up via a palladium catalyzed enolate coupling of vinyliodide 5 using potassium phenoxide as a weak base to access tetracycleÂ 4. Wittig reaction with triphenylmethylmethoxy-chloride and KHMDS yielded enol etherÂ 3 in multigram quantities. The dithiolane moiety was removed by treatment of first methyl Meerweinâ€™s salt and then copper sulfate solution with in situ protection of the nitrogen by protonation prior to the reaction. During this reaction partial cleavage of the enol ether was observed in the scale-up campaign. Therefore, the crude reaction mixture was subjected to hydrolysis under acidic conditions to realize full conversion to keto-aldehyde 10. Rather harsh conditions were required for this hydrolysis, nevertheless dicarbonylÂ 10 was obtained in very good yields and with the desired stereochemistry at C-16 in accordance with StÃ¶ckigtâ€™s biosynthetic studies mentioned above. With keto-aldehyde 10 in hands we met the challenge of chemoselective conversion of the five-membered cyclic ketone in presence of the aldehyde. Since most of the sarpagine alkaloids bear a -CH2OH-group at C-16, the aldehydeÂ in 10 was first chemoselectively reduced to the corresponding alcoholÂ 11 using sodium triacetoxyborohydride. In this step acetic acid turned out to be crucial for good yields and selectivities, since protonation of the tertiary amine prevented internal delivery of the reducing agent by the nitrogen. This was important in order to avoid reduction of the keto-group. To our delight, the consecutive ring enlargement did not require protection of the free alcohol in 11. The ring expansion step was triggered by nucleophilic addition of deprotonated TMS diazomethane to the ketone in 11 and further protonation by methanol. The resulting TMS enol ether was then hydrolyzed to the desired ketoneÂ 2 during aqueous acidic workup. This transformation proceeded with excellent regioselectivity and in presence of the free alcohol by the use of more than two equivalents of reagents. The ring expanded ketone 2 represents the last synthetic intermediate of the synthesis. The sequence has been optimized by us and is now robust enough that gram quantities of ketone 2 can be produced. In light of the forthcoming SAR studies this is utmost important, since we accessed 13 different congeners, natural and non-natural, from this last intermediate in a single chemical operation â€” a Fis

Final results

The synthesis of a single natural product of this complexity (such as sarpagines and parvineostemonine) by conventional strategic approaches in total synthesis typically takes 3-4 years. The fact that we were so far able to access 17 compounds in 2.5 years clearly shows the success of our main objectives proposed in this project. Up to the end of the project we will now focus on two major objectives:
1) The total synthesis of macroline alkaloids, which comprise the third group of natural products accessible by this strategy (Figure 2)
2) Design non-natural congeners of the sarpagines synthesised so far based on the preliminary SAR studies which we are currently taking out in collaboration with the HZI in Braunschweig (Prof. M. BrÃ¶nstrup)

Another issue will be the development of a methodology for cyclohepta[b]indoles, which is already envisioned as an outlook in our original proposal. This method should enable us to extend the concept of ANaPSyS to novel classes of compounds.