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A
key component of satisfying the high throughput capability
and demands of drug discovery has been the implementation
of combinatorial chemistry techniques to synthesize, purify
and confirm the identity of a large number of compounds displaying
wide chemical diversity within a class. In place of traditional
serial compound synthesis, libraries of compounds are created
in 24-, 48- or 96-well plates by interconnecting a set or
sets of small reactive molecules, called building blocks,
in many different permutations.
Automation aids the chemist in the high throughput synthesis
of these compound libraries, as well in the subsequent purification
steps required to isolate synthesized compound from reaction
starting materials, reagents and by-products. The popular
strategic options for the synthesis of combinatorial libraries
include solid-phase, solution-phase and liquid-phase synthesis.
Solid-phase
parallel synthesis uses resins to which the starting material
is attached in order to produce a large number of compounds
via split and mix methods. The solid support matrix used consists
of a base polymer, a linker to join the base polymer to the
reactive center, and a functionalized reactive site. The immobilized
reactant is then subjected to a series of chemical reactions
to prepare the desired end product. The use of excess reagents
drives reactions to completion. However, the need for deconvolution
approaches to determine the active components within a pool
has limited the utility of solid-phase synthesis. Since the
synthesized compounds are attached to the solid support, this
approach does offer simplified reagent removal via filtration,
and impurities are washed away easily during purification.
The compound of interest is released from the polymer support
in a final chemical release step.
Solution-phase
parallel synthesis techniques are more flexible than solid-phase
techniques and are often used to create focused chemical libraries.
Using this approach, the reactions occur in solution (and
are thus easily monitored by TLC or NMR) and the synthesized
compound is isolated in one liquid phase; all non-product
species are fractionated into an immiscible liquid phase.
However, in order to automate this technique, a purification
step following the reaction is required. Common approaches
to purify reaction mixtures are liquid-liquid extraction,
liquid chromatography, solid-phase extraction and the use
of solid-phase scavengers to remove excess reagents and/or
reaction impurities from crude solutions. These solid-phase
scavengers (functionally modified polymers of polystyrene
or bonded silica) are chosen for their inertness to the reaction
products but affinity for reagents and unwanted by-products.
Scavengers are becoming more popular since they can easily
be adapted to automated purification techniques via filtration.
The
procedure for use of scavengers is as follows. Scavenger beads
are placed into the wells of a flow-through 96-well filtration
plate. A flow-through reaction block (consisting of individual
wells of a flow-through 96-well plate in which the top and/or
bottom of the wells can be blocked or opened to allow flow
and reagent addition) is placed on top of the filtration plate,
so that when vacuum is applied the reaction mixture flows
out of the reaction block and through the scavenger bed. A
collection plate centered below the filtration plate collects
the solution.
Liquid-phase
parallel synthesis combines the strategic features of
both solid-phase synthesis and solution-phase synthesis. This
method uses a supporting polymer (e.g., polyethylene glycol)
that is soluble in the reaction media. This polymer can be
precipitated selectively for the purposes of isolation and
purification. Excess reagents and byproducts are removed by
simple filtration.
Learn
about automation choices for combinatorial chemistry.
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