Favipiravir was first synthesized from an inexpensive and commercially available starting material, 2-aminopyrazine. The preferred route embedded within Scheme 4 consisted of seven steps, and was highlighted by the novel and efficient synthesis of 3,6-dichloropyrazine-2-carbonitrile 8. This intermediate was prepared in four successive steps which were regioselective chlorination of the pyrazine ring, bromination, Pd-catalyzed cyanation, and Sandmeyer diazotization/chlorination. This protocol eliminated the hazardous POCl 3 of previous synthetic methods and offered a better yield (48%) which was 1.3-fold higher than a recently published procedure. From intermediate 8, the subsequent nucleophilic fluorination, nitrile hydration and hydroxyl substitution efficiently afforded the target product. Another synthetic approach with the same starting material was also investigated to bypass the allergy-causing dichloro intermediate 8. However, the key step of monofluorination at the pyrazine C6 position of intermediate 19 or 22 was not achieved.

A novel synthetic route for making (−)-CBD and its derivatives bearing various C4′-side chains is developed by a late-stage diversification method. Starting from commercially available phloroglucinol, the key intermediate (−)-CBD-2OPiv-OTf is efficiently and regioselectively prepared and further undergoes Negishi cross-coupling to furnish (−)-CBD. This approach allowed an efficient synthesis of (−)-CBD in a five-step total 52% yield on a 10 g scale. Furthermore, diversification on the C4′-side chain with this method can be realized in a wide range.

Currently, remdesivir is the first and only FDA-approved antiviral drug for COVID-19 treatment. Adequate supplies of remdesivir are highly warranted to cope with this global public health crisis. Herein, we report a Weinreb amide approach for preparing the key intermediate of remdesivir in the glycosylation step where overaddition side reactions are eliminated. Starting from 2,3,5-tri-O-benzyl-d-ribonolactone, the preferred route consisting of three sequential steps (Weinreb amidation, O-TMS protection, and Grignard addition) enables a high-yield (65%) synthesis of this intermediate at a kilogram scale. In particular, the undesirable PhMgCl used in previous methods was successfully replaced by MeMgBr. This approach proved to be suitable for the scalable production of the key remdesivir intermediate.

A one-pot process for preparing molnupiravir from cytidine was developed. The advantages of this synthesis were as follows: (1) The presence of N,N-dimethylformamide dimethyl acetal (DMF-DMA) facilitated the selective protection of 2′,3′-dihydroxyls and amino of cytidine, which eliminated the negative impact of these groups on the following isobutyrylation at 5′-hydroxyl. (2) Degradations of the product in the deprotection stage were avoided since a mild condition was used. (3) The achievement of deprotection and hydroxyamination in one single step improved the synthetic efficiency. (4) Molnupiravir with high purity (purity up to 99.7% analyzed by high-performance liquid chromatography (HPLC)) was obtained in a yield of 63% through crystallization.

A novel and efficient route of synthesis for making flibanserin via 2-ethoxy-1H-benzo[d]imidazole (12) was described with excellent yield. This protocol provided a more facile approach to flibanserin.