Investigating a semi-continuous synthetic pathway for the preparation of the antiretroviral drugs tenofovir disoproxil and tenofovir alafenamide

Abstract

This dissertation details the investigation of the synthetic pathway of the antiretroviral drug, tenofovir, and its prodrug forms tenofovir disoproxil and tenofovir alafenamide, in attempt to develop a more efficient route by exploring both batch and flow-chemistry avenues for each synthetic step. The initial step of the synthesis was an N-9 alkylation of adenine which, due to its solubility challenges, limited our reaction solvent options to only polar aprotic solvents (dimethylformamide, dimethyl sulfoxide and N-methyl-2-pyrrolidone) under basic conditions. The results showed that the presence of water in the solvent system produced both N-9 hydroxypropyl adenine and minor hydroxypropyl adenine regioisomers (N-7 and N-3). Hydroxypropyl adenine was isolated in one of two ways: trituration or newly implemented “catch-and-release” resin work-up. Trituration with ethanol or a mixture of ipropanol/methanol (1:1) produced the purest product while the ion-exchange work-up with Amberlyst 15 ® resin gave the highest yields and required less solvent. The translation of step 1 to a flow reactor required an addition of a glass mixing chip and a steel coil as accessories before we could obtain any conversion of adenine to hydroxypropyl adenine. The conversion to hydroxypropyl adenine was 50% faster than that obtained in batch, however, we were unable to pursue an in-line resin work-up because the glass Omnifit™ column accessory was incompatible with our reaction solvent, dimethyl sulfoxide. The second step of the synthetic pathway was split into two sub-steps: a) the addition of a phosphonate ester to produce phosphonatepropyl adenine and b) the hydrolysis of the ester, producing active antiretroviral drug tenofovir. We managed to successfully synthesize and isolate phosphonatepropyl adenine by replicating the method employed by Riley et al. This method was particularly problematic in terms of its work-up because of the sticky salt cake that made it difficult to extract our product. We, therefore, attempted to solve the problem in one of two ways: i) an alternative synthetic route for the synthesis of phosphonatepropyl adenine which involved an Arbuzov reaction. We were unsuccessful in this approach and so moved on in attempt to ii) develop a polymer-supported reagent (Polymer-supported diethyl (tosyloxy) methyl phosphonate) that we believed would allow us to isolate phosphonatepropyl adenine from the sticky cake while trapping the troublesome magnesium salts on the spent solid support. The results on the formation of polymer-supported diethyl (tosyloxy) methylphosphonate were inconclusive, leaving step 2a as a batch reaction. Step 2b involved hydrolysis of the phosphate ester using classical reagents, trimethylsilyl bromide and trimethylsilyl chloride under high pressure in order to produce the phosphonic acid. The reaction worked best with trimethylsilyl bromide but the cost effective alternative trimethylsilyl chloride produced satisfactory results, especially in the presence of NaBr. This step, although unoptimized, was successfully translated to flow with the addition of a glass Omnifit™ column and coil reactor accessories. The final steps of the synthetic pathway diverged into the synthesis of prodrugs tenofovir disoproxil and tenofovir alafenamide. Alkylative esterification of tenofovir (using chloromethyl isopropylcarbonate) successfully afforded tenofovir disoproxil using the batch method reported by Ripin et al.

The synthesis of tenofovir alafenamide was more challenging, requiring two separate steps as reported by Chapman et al.: the first being esterification of tenofovir using phenol to afford phenol-tenofovir, and the second was the addition of isopropyl L-alanine to form tenofovir alafenamide. We successfully synthesized phenol-tenofovir in batch but unfortunately were unsuccessful in the synthesis of tenofovir alafenamide.

In entirety we managed to establish a semi-continuous synthetic pathway for the synthesis of antiretroviral active drug, tenofovir and its prodrug tenofovir disoproxil as well as precursor to tenofovir alafenamide, phenol- tenofovir. This success provides a useful stepping stone towards future research for a proficient and elegant pathway for the synthesis of antiretroviral prodrugs tenofovir disoproxil and tenofovir alafenamide which might include coupling of an already-existing flow synthesis of essential reagent, (R)-propylene carbonate.