PROCESS DEVELOPMENT Fig 1. Therapeutically important nucleoside analogues. NH2 RMgX N steps steps
core nucleoside synthesis. Some nucleosides tend to be highly polar, water soluble molecules, and so present the synthetic chemist with purification problems that we had to solve. For example, we learnt how to efficiently separate anomers and developed preparative HPLC methods for polar nucleosides. This technique provided us with an efficient process to hit the 300-compound target set by Pfizer. Finally, once the compounds were made and isolated it was not uncommon to have to prove absolute stereochemistry by NMR studies or X-ray crystallography. More recently, as we have taken on nucleotide work, we have developed expertise and invested in ion-pair chromatography, ion-exchange chromatography and counter-current chromatography for the purification of highly polar substrates.”
Fig 2. Peakdale's synthesis of enantiomerically pure 4-substituted nucleosides. single diastereoisomer
Fig 3. Examples of carbocyclic nucleosides and 2-cyano-7-deaza/aza-8-azapurine derived nucleosides synthesised by Peakdale.
Some examples of the novel chemistry requested by Pfizer and devised, developed and delivered by Peakdale are highlighted below. Analogues of 4’-substituted riboses have been shown to possess anti-HCV and anti-HIV properties. Existing methods for the introduction of groups into this position were lengthy and lacked stereoselectivity, so Peakdale developed high-yielding (42% over 8 steps) and selective routes to the substituted riboses such as 3 (Fig 2). These were successfully reacted with a number of bases using the silyl Hilbert-Johnson coupling to give the novel nucleosides (Fig 2).2 Also illustrated are Peakdale’s novel syntheses of several carbon-substituted nucleoside analogues, such as compounds 7 and 8,3 as well as the synthesis of 2-cyano-7- deaza/aza-8-azapurine derived nucleosides 9 (Fig 3).4
Synthesis of phosphorylated nucleosides
Fig 4. Peakdale synthesis of carbocyclic phosphonic acid nucleosides.
to help set up the project, this allowed us to put in place much of the initial literature searching, synthetic plans, reagent purchases, and a few early chemistry trials. The full team was then set up to go in around January 2003. The goal of the project was to deliver 300 nucleosides each year and Pfizer would provide a set of targets of increasing synthetic difficulty.
“Most of the nucleosides were novel, although some of the base and sugar units were in the literature. Other targets were well beyond existing literature and required novel routes to be developed. This allowed for a significant sized library while at the same time enriching it with novel attractive nucleosides. The project was run from two Pfizer sites, La
Jolla, California, USA, and from Sandwich, UK, which presented Peakdale with the opportunity to hone its communication skills with project leaders in different time zones. “Initially the whole team was focused on synthesising the sugar and base units, but as the nucleoside coupling steps ramped up the team was split. Four chemists focused on the individual sugar and base chemistry, while two largely concentrated on utilising classic nucleoside coupling techniques to synthesise the nucleosides. This allowed us to develop in-house expertise in large-scale sugar synthesis (multi-hundred gram scale) including overcoming the challenges of stereospecific synthesis of carbosugars, separation of isomers and so on, and also on
The difficulty in synthesising nucleosides, because they are highly polar, water-soluble and densely packed with functional groups, is further compounded when they are phosphorylated to give nucleoside monophosphates. However, these compounds are desirable medicinal chemistry targets. When nucleoside analogues are administered as drugs, intracellular processing of the nucleoside to its triphosphate generates the active form of the drug. Often the difficult step in this process is the first phosphorylation, the formation of the monophosphate. Therefore by-passing this step, by administering the nucleoside as a pro-drug of the pre-formed monophospate, is desirable.
This strategy has recently been used to great effect by Professor Chris McGuigan of
July/August 2012 sp2 Inter-Active 21
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