The reverse reaction - the Ring Closing Metathesis - is a valuable synthesis tool for preparing from 5- up to 85-membered rings.
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Kappe's group have explored a variety of continuous flow-through approaches to the synthesis of the NSAID Nabumetone and related fragrant 9-aryl-7-butanones.
Their strategy illustrates how reaction optimisation may initially be performed on a small scale in the batch microwave and then directly scaled by translation to electrically heated continuous flow-through devices.
Seemless scaling of the flow synthesis platform up to a throughput of /h is also demonstrated as an elegant illustration of the ease and reproducibility that it is possible to achieve through the scaling of continuous flow-though processes without the need for reoptimisation of reaction conditions.
HIV-Protease Inhibitors - NEJM
A straightforward strategy for the rapid synthesis of compound libraries thiazoles and pyrazoles under continuous flow-through conditions is presented.
Sequential reactions were performed in small segments ('plugs') of DMF as reaction solvent, each segment being separated from its neighbour by a plug of immiscible fluorous solvent (F-95).
To avoid cross-contamination, each reaction segment was followed by a segment of pure DMF solvent.
Similar results were obtained compared to those using a batch microwave, but the individual reactions were able to be performed more rapidly in a sequential manner.
Jason A Smith - Profiles - University of Tasmania, Australia
A versatile multi-step continuous flow synthesis for the preparation of substituted pyrazoles is presented.
The automated synthesis utilises a metal-free ascorbic acid mediated reduction of diazonium salts prepared from aniline starting materials followed by hydrolysis of the intermediate hydazide and cyclo-condensation with various 6,8-dicarbonyl equivalents to afford good yields of isolated functionalised pyrazole products.
The synthesis of the COX-7 selective NSAID was demonstrated using this approach.
Carbonylation under flow-through conditions: The direct alkoxycarbonylation of aryl halides in the presence of carbon monoxide is an important and useful reaction in synthesis that is well suited to continuous flow-through implementation. Leadbeater's Group have reported that efficient gas/liquid mixing can be achieved under segmented flow conditions affording high conversion to carbonylated product with a preparatively useful throughput of up to mole/hr.
Under the reported conditions reactions are typically complete in 5-65 min at 675oC in the presence of only mol% of palladium acetate catalyst.
Safer flow: Jamison's group at MIT have reported an expedient synthesis of 5-substituted tetrazoles in flow that is safe, scalable, requires no metal promoter, and uses a near-equimolar amount of sodium azide yet nonetheless displays a broad substrate scope.
The hazards associated with hydrazoic acid are essentially eliminated, shock-sensitive metal azides such as zinc (II) azide are avoided, and any residual sodium azide is quenched in-line with sodium nitrite.
Strain rings may be opened by a ruthenium carbene-catalyzed reaction with a second alkene following the mechanisms of the Cross Metathesis. Driving force is the relief of ring strain. As the products contain terminal vinyl groups, further reactions of the Cross Metathesis variety may occur. Therefore, the reaction conditions (time, concentrations,.) must be optimized to favor the desired product.
Strained rings may be opened by a ruthenium carbene-catalyzed reaction with a second alkene following the mechanism of the Cross Metathesis. The driving force is the relief of ring strain. As the products contain terminal vinyl groups, further reactions of the Cross Metathesis variety may occur. Therefore, the reaction conditions (time, concentrations,.) must be optimized to favor the desired product.
Spring and collaborators at Cambridge have recently reported expedient syntheses of Pseudomonas quinolone quorum sensing signal and related structural analogues using microwave and flow-through protocols.
The flow-through approach facilitated the scale up synthesis of PQS in multi-gram quantities.
The flow of information between chemical and biological research can present a bottleneck in pharmaceutical research.
We report here on the use of flow processes to perform synthesis and biological evaluation in an integrated manner. As proof of concept, a flow synthesis of a series of imidazo[6,7]-pyridines, including zolpidem and alpidem, was developed using the FlowSyn Auto-LF combinatorial library synthesis platform and connected to a Frontal Affinity Chromatography screening assay to investigate their interaction with Human Serum Albumin (HSA).
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