At the end of 2021, Pfizer's new crown oral drug Paxlovid was approved by the US FDA for marketing, which aroused widespread concern. One of the main ingredients of Paxlovid is the 3CL protease inhibitor code-named PF-07321332. PF-07321332 plays an important role in the life cycle of a variety of coronaviruses, and its potential advantage is that it can work on all current variants of the new coronavirus. Therefore, this small molecule drug has attracted strong interest from many pharmaceutical manufacturers.
Paxlovid is an oligopeptide-like drug whose synthetic steps involve multiple reactions to prepare amide bonds (Figure 1). When preparing the N-1 key intermediate 9 in the penultimate step, the production process uses EDCl/HOPO as a condensation reagent combination to condense the two fragments 3 and 8 into an amide bond . Here EDCl plays the role of activating carboxylic acid and condensation dehydration, while HOPO plays the role of increasing the yield and reducing racemic by-products (Figure 2). With the launch of Paxlovid, HOPO, a racemization inhibitor that is usually not very eye-catching, has also been "out of the circle" and has gained a lot of attention. This article briefly introduces this niche product.
Figure 1 PF-07321332 synthetic route
Figure 2 EDCl and HOPO
It is well known that carbodiimide condensing agents are the earliest developed and most commonly used condensation reagents. Since Sheehan et al. developed the first carbodiimide-type condensing agent - N,N'-dicyclohexylcarbodiimide (DCC) in 1955 , chemists have designed and developed a series of new Carbodiimide condensing agents, such as N,N'-diisopropylcarbodiimide (DIC), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride ( EDCI) etc. However, when carbodiimide-type condensing agents activate α-amino acids, their high activity often leads to a certain degree of racemization at the chiral center of the α-position of amino acids. In order to suppress the occurrence of racemization side reactions, chemists have developed a series of racemization inhibitors and used them in combination with carbodiimide-type condensing agents. Racemization inhibitors can not only inhibit racemization to a certain extent, but also reduce the occurrence of other side reactions, so that the efficiency of peptide bond formation can be greatly improved. At present, the common racemization inhibitors include: HOSu, HOBT, HOAT and more than ten kinds (Figure 3). The most commonly used of these are HOBT and HOAT.
In most literature reports, carbodiimide condensation reagents are often used in combination with HOBT or HOAT. For example, scholars such as Carpino studied the performance comparison of DIC with or without the addition of racemization inhibitors in a literature . The author studied the yield and impurities of a polypeptide——ACP(65-74) by solid-phase synthesis (Table 1). Studies have shown that without HOAT or HOBT, the condensation effect of DIC is very poor, and the product yield is only 14%. When HOAT or HOBT is added, the yield increases significantly. In another document, the author used four condensation reagents to synthesize three polypeptides: Z-Gyl-Leu-Val-OBzl (10), Z-Gyl-Val-Val-OBzl (11), Z-Gyl-Phe- Val-OMe (12). It was found that after DCC was added with HOBT, the racemic impurities were significantly reduced (Table 2) .
Table 1 The effect of different condensation reagents on the preparation of polypeptide ACP(65-74)
Table 2 Comparison of the amount of racemic impurities generated during the preparation of peptides by different condensation reagents
For the synthesis of PF-07321332, the author did not find relevant studies on the difference in the effect of HOPO and HOBT or HOAT. However, we checked some other literatures and found that HOPO may have advantages over HOBT or HOAT in some peptide bond synthesis. For example, Yasuda et al. used the combination of EDCl/HOPO when synthesizing the CGRP receptor antagonist ubrogepant. The authors noted that 1% of racemic by-products were produced with HOBt, whereas there were no racemic by-products with HOPO (Fig. 4) . In addition, when David et al. synthesized glecaprevir, they also used the combination of EDCl/HOPO . The authors found that no racemic isomers were observed on proline under these conditions (Fig. 5). In addition, Young et al. also used the combination of EDCl/HOPO to synthesize peptide bonds when producing an API product (Figure 6) . It can be seen that in some cases HOPO has more advantages than the commonly used HOBT or HOAT, which may be the reason why chemists use HOPO in the synthesis of PF-07321332.
HOPO is a light yellow crystalline powder with a melting point of 149-152°C. From the point of view of safety, HOPO is a safer compound than HOBT or HOAT (HOBT and its derivatives are identified as the first class of explosives, and safety accidents in the production process occur from time to time!). Haofan Biotechnology has been committed to the research and development and production of condensation reagents and protective reagents for nearly 20 years, and has accumulated rich experience and formed its own characteristics. Currently, we are able to produce almost all racemization inhibitors including HOPO. The HOPO we produce is at the leading level in the industry in terms of various quality indicators. Welcome friends in need to inquire!
 Owen, DR; Allerton, CMN; Aanderson, AS; et al. Science, 2021: 374, 1586.
 Sheehan, JC; Hess, GPJ Am. Chem. Soc. 1955, 77, 1067.
 Carpino; LA; El-Faham, A.; Minor, CA; et al. J. Chem. Soc., Chem. Commun., 1994, 201.
 Alericio, F.; et al. SYNTHESIS OF PEPTIDES AND PEPTIDOMIMETICS. Methods of Organic Chemistry.
 Yasuda, N.; Cleator, E.; Kosjek, B.; et al. Org. Process Res. Dev. 2017, 21, 1851.
 Hill, DR; Abrahamson, MJ; Lukin, KA; et al. Org. Process Res. Dev. 2020, 24, 1393.
 Young, IS; Qiu, Y.; Smith, MJ et al. Org. Process Res. Dev. 2016, 20, 2108.