GRL0617-VIR251 hybrid. As a first strategy, we devised a molecular design based on available structural information. As discussed above, VIR251 binds in the long narrow catalytic site making a covalent bond with the catalytic cysteine. In contrast, GRL0617 blocks the entry of this tunnel sitting at more than 7 Å from the catalytic cysteine with some overlap with the far end of VIR251. We envisioned hybrid structures featuring both GRL0617 and VIR251. Thus, from crystal structures of VIR251 and GRL0617 co-crystallized with PL pro and their respective binding modes (Figure ), we designed compound SP-5. This strategy is similar to that employed by Sanders et al. To evaluate not only the inhibitory potency of the designed hybrid, but also the impact of each group in this design, we first began with the synthesis of GRL0617 analogues. This first set of compounds would inform us of the optimal linker between the two parts of the hybrid constructs and possible changes on the GRL0617 portion. Compounds SP-1a-l were prepared from the coupling reaction of benzoic acids and (R)-1-(naphthalen-1-yl)ethan-1-amine (Scheme 1). All attempts of O-methylation of compound SP-1d were unsuccessful except the simple acetylation reaction to give SP-2 (see Supporting Information). Compound SP-1g/1k/1l were then chosen as a building block for the synthesis of hybrid PL pro inhibitors SP-5a-c, since the C-I bond is intrinsically weaker and is beneficial in cross-coupling reactions. The key intermediate SP-3a-c was synthesized through Ullmann amination, using CuI and β-alanine with Cs2CO3 as the base in DMF at 100 °C (Scheme 2). To install the glycine-like warhead, N-Boc-2aminoacetaldehyde was reacted with triethylphosphonoacetate using the Horner-Woodward-Emmons reaction to form the Boc-protected product. This was followed by Boc deprotection using TFA to provide the unprotected amine salt (SP-4) in quantitative yield. SP-3a-c and SP-4 were coupled to give the GRL0617-VIR251 hybrid analogues (SP-5a-c).