|Sometimes the solution to solutions is not a solution but a bead.
Despite the move toward using solid-phase synthesis for combinatorial chemistry (combichem), there is still much to be said for carrying out synthetic reactions in solution. Working in solution allows combinatorial researchers to use the vast body of existing synthetic chemistry methodology, which provides this technology platform with the same synthetic tools as those available to a medicinal chemist, says Carmen Baldino, director of library optimization and combinatorial operations at ArQule, Inc. (Woburn, MA; www.arqule.com) (1). The chemistries used range widely, running an alphabetical gamut from acid hydrolysis to Wittig reactions. Additionally, it is almost a truism that the ultimate compound produced in combinatorial synthesis will most likely be needed for use in solution, especially for biological applications and assays. Because solid-phase synthesis requires chemical cleavage to release the product and subsequent purification, it inevitably builds in added steps, which translate into additional cost, time, and potential loss of product.
Although reactions in solution are the historical mainstay of organic chemistry, synthesis in solution makes up only 30% of libraries produced, according to a recent review by Roland Dolle, a researcher at Pharmacopeia, Inc. (Princeton, NJ; www.pharmacopeia.com) (2). There are many reasons for this seemingly contradictory state of affairs, but perhaps the most important one for combichem is the difficulty of removing leftovers from previous reactions or unwanted products that might interfere with subsequent chemistry.
Because combichem usually requires multiple rounds of additional chemical reactions to create the final product, these unwanted co-travelers have proved, if not an insurmountable problem for solution-based approaches, certainly cumbersome. One of the greatest benefits of solid-phase synthesis has been how the technique avoids this problem. A quick rinse and a transfer of the beads on which the reaction product is held immobilized can move the desired compound easily from one chemistry to another, leaving the unwanted materials behind.
Still, there is another approach that many chemists who are loath to give up the greater flexibility of solution-phase synthesis are using. To them, the answer is not abandoning solution-based synthesis, but turning toward the productive use of scavenger or quench resins.
A benefit of such scavenging is that it permits chemical reactions to be driven further in the desired product direction by enabling the addition of a large excess of one of the reagents. Because the reagent not in excess is effectively used up to make the product, scavenging of the reagent added in excess leaves behind a solution containing the desired product, free of starting components.
Because multiple scavenger resins can be used in concert (even incompatible functional groups can be used because of their comparative isolation within the beads), a wide variety of compounds can be scrubbed from the solutions. Similarly, because a variety of resins often exist with different physical and chemical characteristics that remove the same contaminant, the choice of resin can be optimized to the particular solution-phase system.
In 1997, Daniel Flynn and his colleagues at Searle Discovery Research and Ceregen, two St. Louis-based subsidiaries of Monsanto, proposed the first use of what they referred to as complementary molecular reactivity and recognition (CMR/R) resins as a purification strategy for solution-phase combinatorial libraries (3). They proposed using the resins to remove excess reactants, reagents, and byproducts, as well as the quench phase commonly used in library purification. Their application was demonstrated for amine acylations, Moffatt oxidation, and the reaction of organometallics and carbonyl compounds. The authors pointed out that these resins were applicable to a broad spectrum of reactions and were highly amenable to automation.
Today, a variety of resins, now known as scavengersthe CMR/R nomenclature did not catch onare available to facilitate solution-phase synthesis. They vary in the physical properties of the beads (including solvent compatibility and porosity) and the chemical properties of the scavenging moieties attached to the polymers (i.e., whether they bind anions, cations, ketones, etc.). Resins used for the beads include polystyrene and polystyrenedivinylbenzene cross-links. Researchers also have investigated new polymeric supports for the scavenging side chains. For example, Alessandro Falchi and Maurizio Taddei, chemists at the University of Sassari (Italy), recently reported using polyethylene glycoldichlorotriazine as a soluble polymer-supported scavenger for alcohols, thiols, and various phosphines and phosphine oxides in combinatorial solution-phase reactions (4).
The variety of functional moieties has also expanded. The available side groups include almost the complete range of those used for the more common application of solid-phase extraction (see The Solid-Phase Attraction, Todays Chemist at Work, February 2002). As a case in point, anion exchange resinsso familiar in chromatography applicationshave been widely used to scavenge excess acidic reagents and byproducts from solution-phase synthesis of combinatorial libraries.
Other resins have been designed specifically for scavenging. Aminomethyl resins, for example, are used to scavenge carboxylates, sulfonyl halides, and isocyanates. Methylisocyanate resins scavenge amines and hydrazines, while morpholinomethyl resins are used as tertiary amines for scavenging acids. Benzenesulfonic acid is used to bind amines and other basic compounds. For further information and structures, visit the websites of Glycopep (Chicago; www.glycopep.com/Webcat/Scavenger/Scavenger.html), Rapp Polymere GmbH (Tübingen, Germany; www.rapp-polymere.com/preise/scavres.htm), and Sigma-Aldrich (St. Louis; www.sigma-aldrich.com).
The resins can be designated as either electrophilic or nucleophilic scavengers. Electrophilic scavengers include those with bound thiophenol, trisamine, or triphenylphosphine, whereas nucleophilic scavengers include those with bound benzaldehyde, isocyanate, and carbonate. In some cases, one of the reagents used in the product synthesis is bound to a resin bead and added in excess to force completion of the reaction, thereby scavenging the other component(s) of the chemical process from the mix. Typical of such reagent scavengers used for combichem are those with bound borohydride, triphenylphosphine, carbodiimide, and carbonate. (For further in formation, Argonaut Technologies (Foster City, CA; www.argotech.com).
Focusing on the use of filter disks for scavenging, Jennifer Tripp and colleagues at the University of California, Berkeley, named this technique reactive filtration (5). Similar to standard solid-phase extraction filter techniques (although in this case the unwanted materials are bound rather than the desired product), is the use of grafted macroporous polymer monolithic disks as filters to remove excess reagents or unwanted byproducts from solution-phase synthesis reactions. Because most of the pores in these monoliths are large and controllable over a wide range of sizes by variation of the polymerization conditions when the disk is formed, they are well suited to flow-through applications, following the same patterns used for solid-phase extraction. Since material can be pumped through the monoliths under pressure, the flow-through characteristics are particularly beneficial because of increased mass transport of materials to the reactive sites, and the whole process is greatly accelerated.
Tripp and her colleagues continued their research on this technique and recently reported on the development of polyethylene-encased porous poly(chloromethylstyrene-co-divinylbenzene) disks (6). The original filter material was produced through polymerization in a cylindrical glass mold and then cut into disks. Various scavenging moieties were grafted onto the disks using a selection of chemistries to produce monoliths for different purposes.
The monolithic scavenging disks were easy to reproduce and had good flow-through characteristics. The researchers presented detailed results on using them to filter amines from various solvents, including water and alcohol.
Ultimately, the use of scavenging beads or filters is not likely to lead to the abandonment of all the benefits of solid-phase applications. But for smaller libraries, and for applications in which the use of a linked substrate is impractical because of its unique chemistry, scavenging resins and disks hold high promise for reinvigorating and better automating solution-phase synthesis for combinatorial chemistry.
Mark S. Lesney is a senior associate editor of Modern Drug Discovery. Send your comments or questions regarding this article to email@example.com or the Editorial Office by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.
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