A team from Merck & Co has developed a five-step method for the synthesis of uprifosbuvir with a 50-fold increase in yield compared to the previous manufacturing process. The scalable strategy minimizes unwanted by-products and could be adapted to synthesize a range of nucleosides to meet global demand for antiviral therapies.

Upifosbuvir is used to treat chronic hepatitis C infections. It is a nucleoside-based molecule that inhibits a specific RNA polymerase, thereby limiting viral replication. The original route to uprifosbuvir requires 12 steps, some of which suffer from poor regio- and diastereoselectivity, resulting in a 1% yield of the final product. By shortening the synthesis route to five steps and carefully optimizing the reaction conditions to maximize selectivity, the Merck team, led by Artis Klapars, achieved an overall yield of 50% in the production of over 100 kg of the compound.

A picture that shows the new synthesis route

The team chose a readily available uridine as the starting material for the nucleoside component. Sophisticated functionalization in the 2 ′ position was then required. They then explored a commercially viable protecting group strategy using acyl chlorides, and careful selection of the solvent and Lewis acidic additives gave the desired protected isomer prior to oxidation to the required ketone. This method of complexation-driven selective acyl migration and oxidation enabled a selectivity of 50: 1 compared to the uridine starting material.

Methyl and chlorine substituents were then introduced in the 2 ‘position via an olefination and hydrochlorination sequence. The process of installing the methyl substituent was then adjusted to improve cost, safety and environmental impact by using an organomanganese reagent. To selectively convert the tertiary alcohol, the team came across FeCl3· 6H2 O and tetramethyldisiloxane as inexpensive reagents to drive the equilibrium towards the tertiary alkyl chloride with high diastereoselectivity, revealing the full nucleoside structure of uprifosbuvir.

Finally, a phosphoramidate side chain was attached using a specially designed chiral nucleophilic imidazole carbamate catalyst that delivered uprifosbuvir in an impressive overall yield of 50%.

“The most difficult step in terms of optimization was the complexation-driven selective acyl migration and oxidation due to the number of transformations that are connected there,” emphasizes Klapars. “The reaction selectivity is certainly an important factor. Ensuring the robustness and scalability of reactions is one of the guiding principles of our work. We go to great lengths to understand how the responses work and to define areas of operation that will deliver a quality product on any scale required. ‘

Katherine Seley-Radtke, an expert in nucleoside drug design at the University of Maryland, Baltimore County, USA, is impressed with the improved protocol. “They managed to take something quite complex and make it very simple, very straightforward and very economical with readily available materials. They went from extremely poor yields to a 50X increase in yield. I think the main aspect is the stereo specificity they were able to achieve. I think it could have a huge impact. ‘

Erik De Clercq from the Rega Institute for Medical Research in Belgium, co-inventor of some of the first anti-HIV drugs, also says the method could have broad applicability. “The approach described could prove useful in derivatizing antiviral agents in general, and certainly C-nucleoside analogues such as remdisivir and others such as pyrazofurin.”

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