Synthesis of Rovafovir Etalafenamide (Part I): Active Pharmaceutical Ingredient Process Development, Scale-Up, and Impurity Control Strategy

Abstract

This manuscript describes the chemical process development and multi-kilogram synthesis of rovafovir etalafenamide (GS-9131), a phosphonamidate prodrug nucleotide reverse transcriptase inhibitor under investigation for the treatment of HIV-1 infection. Rovafovir etalafenamide is assembled in a four-step sequence beginning from the nucleoside core and an elaborated phosphonamidate alcohol. The assembly starts with a decarboxylative elimination of a β-hydroxyacid to yield the corresponding cyclic enol ether, which is subsequently coupled to a functionalized phosphonamidate alcohol in an iodoetherification reaction. Oxidative syn elimination then installs the required fluoroalkene, after which a final deprotection reaction yields the active pharmaceutical ingredient (API). Understanding the genesis, fate, and purge of the des-fluoro analog of the API, a mitochondrial toxin, proved to be a central driver in the development of the manufacturing route and impurity control strategy. Initial control strategies revolved around the use of silica gel chromatography or simulated moving bed chromatography to purge the des-fluoro impurity to an acceptable level, but ultimately a chromatography-free approach to mitigate the formation of this impurity was devised that expanded manufacturing flexibility. Design of experiments was used to improve the iodoetherification fragment coupling reaction and to reduce the level of the des-fluoro impurity formed in this step. Furthermore, several new crystalline intermediate forms were discovered and implemented as isolation points to bolster the overall impurity control strategy for standard, diastereomeric, and potentially mutagenic impurities as well as for the des-fluoro impurity. These processes were executed on multikilogram scale to produce API for clinical studies.

Synthesis of Rovafovir Etalafenamide (Part II): Dynamic Control for Successful Scale-Up of an Oxygen-Releasing Elimination Reaction Mediated by Oxone

“…The product solution was then quenched to remove residual oxidant and carried forward directly to the next step without isolation. 12 ■ THIRD-GENERATION PROCESS WITH FEEDBACK pH CONTROL Although this procedure was successfully demonstrated on over 100 kg scale, the process still presented an operational challenge due to the multiple control elements that were required during processing, including temperature, flow rates for Oxone and lithium hydroxide, reaction pH, and oxygen concentration. At laboratory scale, reactions were somewhat simplified by utilizing feedback pH control, typically accomplished by employing a Reaction Controller (J-Kem Scientific).…”

“…Then the layers were settled and separated, and the organic layer containing 3 was collected (87.0% purity by UPLC, 86% assay yield after adjusting for the purity of 5) and carried forward directly to the subsequent step. 12 A portion was purified by silica gel chromatography for spectral analysis.…”

“…The reaction performance and its context within the synthesis of 1 are covered in the previous manuscript in this series. 12 ■ BACKGROUND An initial screen of conditions for the conversion of 2 to 3 identified the use of Oxone in combination with sodium bicarbonate, acetone, and water (Scheme 2), presumably proceeding via syn elimination of an iodoso intermediate (4). 5a,c,13 Notably, the reaction was completely ineffective when alternative solvents without a ketone functionality were employed, consistent with a mechanism involving dimethyldioxirane.…”

“…Although widely utilized in bioprocessing, feedback pH control has been considered inconvenient in small-molecule synthesis, and yet real-time analysis and process automation are cornerstones of process analytical technology (PAT) and Green Chemistry, and have recently received significant attention from the pharmaceutical industry. , Herein, we describe our development activities to overcome these challenges and successfully demonstrate this process at manufacturing scale. The reaction performance and its context within the synthesis of 1 are covered in the previous manuscript in this series …”

Synthesis of Rovafovir Etalafenamide (Part III): Evolution of the Synthetic Process to the Phosphonamidate Fragment

“…Our retrosynthetic approach to 6 (Scheme ) involves disconnection to the phosphonamidate fragment 1 and the adenosine core 7 . A preceding paper details the assembly of 6 from 1 and 7 . Herein, we describe the evolution of the synthetic process to phosphonamidate 1 from its inception, requiring chromatographic separation of phosphorus diastereomers, to a more efficient, second-generation variant relying on selective crystallization.…”