Speeding up drug discovery

In the last two decades the use of microwave irradiation to perform economically organic synthesis has attracted considerable interest. Since in 1986 Gedye and co-workers reported remarkable rate accelerations for microwave-mediated syntheses[1], a steadily growing number of researchers started to examine this new discipline. The possibility to drastically shorten reaction times by employing sealed vessels in microwave cavities established this modern technique as a sophisticated procedure in chemical research and drug development. The initial use of simple domestic ovens for these applications had substantial drawbacks in performance and reliability, mainly low reproducibility of experiments due to inhomogeneous field density and lack of appropriate parameter sensing as well as stirring opportunities. Accordingly, the development of so-called “dedicated” microwave instruments for synthesis applications, serving the fundamental requirements for reasonable research, was essential. Especially the combination of rapid heating with simplified set-up for autoclave-like systems turned out to be one of the most important features of modern microwave equipment. Since the late 1990s various sophisticated instruments from different vendors enable users to elaborate new methods and protocols for chemical reactions leading to cleaner, more efficient, and highly reproducible reactions, even in scale-up approach.

Extending the Limits

Instruments utilizing either multi-mode or single-mode technology fulfill the demands of preparative chemists for precise reaction control by accurate temperature measurement, pressure sensing and software-aided experiment monitoring[2]. Although the popular monomode instruments are frequently used in the initial development/optimization stage in the drug development process, these microwave reactors show several limits when scale up is required. The production of more than 10 g product still involves time-consuming techniques. Employing monomode reactors, such gram quantities can either be generated sequentially or utilizing a flow-through accessory. Dependent on the chemistry and optimized reaction time both methodologies serve a turnover of approximately 3-5 g per hour. Comparing identical experimental set-ups the more powerful multi-mode instruments allow the preparation of several dozens of grams of product within one run, utilizing large vessels in parallel rotors[3]. Therefore, reactors like the Anton Paar Synthos 3000 represent the intermediate stage from method development to process scale. On the other hand, the different parallel rotors with up to 48 positions enable preparation of various derivatives of a promising scaffold within one experiment, resulting in the utmost diversity with minimum time required.

In this context, microwave chemistry has successfully altered the bottleneck in drug development. Whereas several years ago the most crucial point was the preparation of potential targets, it is nowadays the screening and analysis of the rapidly generated compounds to find promising leads. Unattended, automated processing of short-time protocols drastically reduces the overall experimental time for reaction scouting as well as lead optimization. For drug discovery and pharmaceutical research, speed equals efficient use of resources, faster investigation of structure-activity relationships, and can finally determine positioning in the marketplace. Moreover, since direct scalability of microwave protocols has been verified, also the multi-gram production of valuable targets can be performed without the need for tedious transformation of the method from microwave irradiation to classic thermal heating. Consequently, to move microwave technology closer towards plant processing, the next step for the technicians will be the investigation of appropriate instrumentation for kg production, which will be very likely a flow through system employing large reaction containers.

Synthos 3000 – New Dimensions in Microwave Synthesis

With the Synthos 3000 Anton Paar not only offers a microwave instrument for scale-up towards multi-gram synthesis but for the first time an instrument which enables technicians to run reactions at maximum temperature AND maximum pressure at the same time.

The high performance 8-position rotor serves 80 mL quartz vessels especially designed for extreme reaction conditions of 300 °C and 80 bar. This allows the performing of organic reactions even under microwave-mediated near critical water conditions, which is a field of growing interest in organic synthesis. In a scientific cooperation these special features, parallel multi-gram synthesis as well as high-pressure applications, have been investigated at the Karl-Franzens University Graz, Austria, where Prof. C.O. Kappe supervises one of Europe’s leading groups in microwave synthesis (see: www.maos.net). Besides numerous different reaction types to verify the direct scalability in varying ranges Kappe and co-workers recently presented several microwave-mediated reactions in near critical water like a classical Diels-Alder condensation.[4]

Simplified handling of the accessories makes it easy to prepare the Synthos equipment for high-pressure applications. The unique hydraulic system, integrated in the top plate of the 8-position rotor, allows simultaneous pressure sensing of all reaction vessels. Reaction control for those near critical water experiments is easily achieved by following the vapor pressure curve of water. To prove the reaction conditions, the inside temperature is precisely sensed by an immersing gas balloon thermometer in one reference vessel.

Another important issue is the use of low-absorbing solvents in microwave-mediated processes. Solvents like dioxane or toluene are frequently used in organic syntheses but show difficulties when applied in microwave protocols. Due to their weak coupling efficiency hardly any heat can be introduced into the system by microwave irradiation. Thus, such low-absorbing mixtures have to be doped with good coupling agents like polar reagents, ionic compounds or passive heating elements. Whereas any additional chemicals (especially ionic liquids) may lead to more difficult work-up procedures, the latter interact nicely with microwaves and introduce heat effectively by thermal convection but do not interfere chemically with the samples. Anton Paar provides passive heating elements made of silicon carbide, available in various lengths to aid efficient heating of different volumes and providing all the above-mentioned features.

With these tools microwave-mediated reactions employing unpolar solvents can be performed at much higher temperatures than usual, leading to improved and more economical protocols. Besides the drastically reduced reaction times, microwave protocols often consist of rather simple mixtures with less excess of reagents and minimized quantities of catalysts or additives used – definitely an important issue when scaling up reactions.

Furthermore, several other accessories for special applications like solid-phase synthesis, photochemistry and the use of gaseous reagents are available, making everyday laboratory work more convenient.

Economic and Safety Aspects

For appropriate and immediate customer support Anton Paar provides a global network with several subsidiaries and local distribution partners in more than 80 countries. Regularly held trainings and service seminars guarantee well educated technicians to ensure customer satisfaction. In case of new method development, application support can be requested from experienced microwave synthesis chemists. This supportive network and the proven reliability of the instruments ensure remarkably short or almost no downtimes in processing, even when the equipment is used extensively.

User safety in case of unexpected spontaneous reactions is one of the major concerns when performing chemistry under such elevated conditions, therefore the Synthos 3000 provides a comprehensive safety package with several active and passive safety measures. Each vessel is equipped with a durable metal rupture disk to release the overpressure in case of a thermal runaway. Software-controlled pressure rate enhancement and temperature limitations protect the equipment efficiently from damage. Finally, the cage-like design of the rotors with an additional protective lid precludes debris from being released outside the cavity in case of a – very unlikely – vessel rupture.

Summary

Microwave synthesis far beyond the usual operation limits enables the rapid development of completely new reaction pathways using the Synthos 3000 and its sophisticated accessories. Rotor systems especially designed for extreme reaction conditions guarantee the highest level of safety in case of unforeseen side-reactions. Furthermore, multi-gram synthesis of valuable pharmaceutical scaffolds can easily be accomplished with time-saving procedures in the various rotors of the Synthos 3000 providing a total reaction volume of 1000 mL as well as a combinatorial approach in up to 48 vessels. Thus, this modular system is highly applicable for the scale up approach in the microwave-assisted drug development process.

References

[1] R. N. Gedye et al., Tetrahedron Lett.1986, 27, 279-282; R. J. Giguere et al., Tetrahedron Lett.1986, 27, 4945-4947.

[2] for a comprehensive equipment review see C. O. Kappe, A. Stadler: Chapter 3 (p.29-56) in Microwaves in Organic and Medicinal Chemistry, Wiley-VCh 2005

[3] A. Stadler et al., Org. Process Res. Dev. 2003, 707-716

[4] J. M. Kremsner, C. O. Kappe, Eur. J. Org. Chem. 2005, 3672-3679.