**1. Introduction**

Over the last two decades, the inhibition of protein-protein-interactions (PPI) with small molecules has emerged as a challenging but rewarding new paradigm in Chemical Biology and Drug Discovery [1–4]. The challenge is associated with the fact that—in contrast to established drug targets such as enzymes, G protein-coupled receptors (GPCRs), ion channels, etc.—protein-protein-interaction interfaces are characterized by a large, rather flat surface, in which several amino acids distributed over a wide surface area contribute synergistically to the binding of the protein partner. This requires new types of compounds being able to mimic such interaction partners. Among them foldamers [5], stapled peptides [6], and α-helix mimetics [7–13] have turned out to be of particular value. Hamilton and co-workers have demonstrated that trisubstituted linear teraryls can function as α-helix mimetics, displaying the i, i+4 and i+7 amino acid residues in angle and distance characteristic for the α-helix motif within proteins [14]. These teraryl structures have resulted in efficient inhibitors of protein-protein

interactions, with the advantages of lower molecular weight, better bioavailability, and hydrolytic stability, when compared with peptide drugs. We could show that such teraryl peptide mimetics can be assembled in a modular way using aryltriflates via Pd-catalyzed cross-coupling reactions [15–18]. In order to address an even larger part of the protein-protein interaction site, we are aiming to synthesize α-helix mimetics in the form of quateraryls featuring four amino acid residues. Ideally, these structures should be accessible from simple starting materials by an iterative cross-coupling process [19]. We envisioned that electrooxidative dehydrogenative coupling of suitably substituted phenols would produce 4,40 -biphenols as building blocks for core fragments [20]. Electroorganic synthesis activates molecules by the simple addition or removal of electrons. Consequently, this method requires no stoichiometric reagents. Currently, this methodology exhibits the lowest environmental impact and is considered as inherently safe [21–24]. Upon conversion of the biphenols into sulfonate esters, these structure motifs could be connected with pyridine boronic acids via Pd-catalyzed cross-coupling reactions. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 2 of 10 efficient inhibitors of protein-protein interactions, with the advantages of lower molecular weight, better bioavailability, and hydrolytic stability, when compared with peptide drugs. We could show that such teraryl peptide mimetics can be assembled in a modular way using aryltriflates via Pdcatalyzed cross-coupling reactions [15–18]. In order to address an even larger part of the proteinprotein interaction site, we are aiming to synthesize α-helix mimetics in the form of quateraryls featuring four amino acid residues. Ideally, these structures should be accessible from simple starting materials by an iterative cross-coupling process [19]. We envisioned that electrooxidative dehydrogenative coupling of suitably substituted phenols would produce 4,4′-biphenols as building blocks for core fragments [20]. Electroorganic synthesis activates molecules by the simple addition or removal of electrons. Consequently, this method requires no stoichiometric reagents. Currently, this methodology exhibits the lowest environmental impact and is considered as inherently safe [21–24]. Upon conversion of the biphenols into sulfonate esters, these structure motifs could be connected

In this manuscript, we report about the implementation of such a strategy, which enabled us to synthesize a quateraryl fragment, which could function as a mimic of the β-catenin/B-cell CLL/lymphoma 9 protein (Bcl9) interaction site. with pyridine boronic acids via Pd-catalyzed cross-coupling reactions. In this manuscript, we report about the implementation of such a strategy, which enabled us to synthesize a quateraryl fragment, which could function as a mimic of the β-catenin/B-cell CLL/lymphoma 9 protein (Bcl9) interaction site.

#### **2. Results and Discussion 2. Results and Discussion**

As a test case for our synthetic methodology (see Supplementary Materials), we choose the PPI between β-catenin and Bcl9, which is an important regulatory factor in the development of cancer via the Wnt signaling pathway [25]. The β-catenin/Bcl9 PPI has been well characterized, and the group of Verdine has developed stapled peptides addressing this PPI [26]. By analyzing available structural information from the β-catenin/Bcl9 interface [27], we identified Arg-359, Leu-363, Leu-366, Ile-369, and Leu-373 as relevant amino acids of an α-helix structural element. This led us to propose the following quateraryl structures as target molecules (Figure 1). As a test case for our synthetic methodology, we choose the PPI between β-catenin and Bcl9, which is an important regulatory factor in the development of cancer via the Wnt signaling pathway [25]. The β-catenin/Bcl9 PPI has been well characterized, and the group of Verdine has developed stapled peptides addressing this PPI [26]. By analyzing available structural information from the βcatenin/Bcl9 interface [27], we identified Arg-359, Leu-363, Leu-366, Ile-369, and Leu-373 as relevant amino acids of an α-helix structural element. This led us to propose the following quateraryl structures as target molecules (Figure 1).

**Figure 1.** Proposed design for quateraryl mimetics of Bcl9 based on available crystal structure **Figure 1.** Proposed design for quateraryl mimetics of Bcl9 based on available crystal structure information [27]. The protein structure has been generated using Pymol [28].

information [27]. The protein structure has been generated using Pymol [28].

*2.1. Electrooxidative Cross-Coupling of Phenols* 

#### *2.1. Electrooxidative Cross-Coupling of Phenols Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 10

According to our retrosynthetic reasoning, the quateraryls will be assembled from 4,40 -biphenols. In order to have maximum flexibility in the selection of side-chain substituents, the core fragments should be synthesized from differently substituted phenols. In earlier work we could show that symmetric or non-symmetric 4,40 -biphenols can be prepared from suitable ortho-blocked phenols by direct anodic dehydrogenative coupling, using boron-doped diamond (BDD) electrodes and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as suitable solvent in yields up to 77% [20]. We reasoned that we could improve the selectivity in the electrochemical cross-coupling if we would offer one reaction partner as a free phenol and the second one as a protected phenol [29–31]. For synthetic efficiency, we considered a silyl-protecting group as a good choice. In the event, we tried *tert*-butyldimethylsilyl (TBDMS)-protected phenols **2** and **6** and *tert*-butyldiphenylsilyl (TBDPS)-protected phenol **4** in the coupling with 2,6-dimethoxyphenol (**1**). The yields for the cross-coupling reactions were with 23–27% rather low (Scheme 1, left column). As a comparison, the yields for the unprotected building blocks are displayed (Scheme 1, right column), which, except for **13**, provided the 4,40 -biphenols in yields >60%. According to our retrosynthetic reasoning, the quateraryls will be assembled from 4,4′ biphenols. In order to have maximum flexibility in the selection of side-chain substituents, the core fragments should be synthesized from differently substituted phenols. In earlier work we could show that symmetric or non-symmetric 4,4′-biphenols can be prepared from suitable ortho-blocked phenols by direct anodic dehydrogenative coupling, using boron-doped diamond (BDD) electrodes and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) as suitable solvent in yields up to 77% [20]. We reasoned that we could improve the selectivity in the electrochemical cross-coupling if we would offer one reaction partner as a free phenol and the second one as a protected phenol [29–31]. For synthetic efficiency, we considered a silyl-protecting group as a good choice. In the event, we tried *tert*butyldimethylsilyl (TBDMS)-protected phenols **2** and **6** and *tert*-butyldiphenylsilyl (TBDPS) protected phenol **4** in the coupling with 2,6-dimethoxyphenol (**1**). The yields for the cross-coupling reactions were with 23-27% rather low (Scheme 1, left column). As a comparison, the yields for the unprotected building blocks are displayed (Scheme 1, right column), which, except for **13**, provided the 4,4′-biphenols in yields >60%.

**Scheme 1.** Electrooxidative cross-coupling of phenols and phenolethers forming 4,4′-biphenol **Scheme 1.** Electrooxidative cross-coupling of phenols and phenolethers forming 4,40 -biphenol building blocks.

In addition to its poor coupling yields, the silyl-monoprotected building blocks could also not be successfully used in the subsequent nonaflation/Pd-cross coupling steps. Therefore, we preferred to use the unprotected 4,4′-biphenols as core fragments for the assembly of our target quateraryls, which would make the synthetic route even more efficient and shorter. In addition to its poor coupling yields, the silyl-monoprotected building blocks could also not be successfully used in the subsequent nonaflation/Pd-cross coupling steps. Therefore, we preferred to use the unprotected 4,40 -biphenols as core fragments for the assembly of our target quateraryls, which would make the synthetic route even more efficient and shorter.

### *2.2. Synthesis of Pyridine Boronic Acids 2.2. Synthesis of Pyridine Boronic Acids*

building blocks.

For the final assembly of our target structures we would need pyridine boronic acids featuring the side chain of leucine (**16**), isoleucine (**19**), and arginine (Scheme 2). The leucine pyridine boronic acid was produced in an efficient two-step synthesis starting from 3,5-dichloropyridine (**14**). Fecatalyzed Kochi-Fürstner cross-coupling [32] of **14** with isobutyl-Grignard delivered **15** in a 53% For the final assembly of our target structures we would need pyridine boronic acids featuring the side chain of leucine (**16**), isoleucine (**19**), and arginine (Scheme 2). The leucine pyridine boronic acid was produced in an efficient two-step synthesis starting from 3,5-dichloropyridine (**14**). Fe-catalyzed Kochi-Fürstner cross-coupling [32] of **14** with isobutyl-Grignard delivered **15** in a 53% yield, which could be converted to leucine pyridine boronic acid **16** via Miyaura-borylation with Pd/XPhos in an excellent yield of 93%. For the synthesis of the isoleucine pyridine boronic acid ester **19** a Negishi-coupling strategy was chosen. Starting from 3,5-dibromopyridine (**17**) Negishi-coupling with the in-situ prepared 2-butyl zinc reagent furnished pyridine **18** in a 34% yield. Activation of **18** with a Knochel-Grignard [33] and an electrophilic quench with (pin)BO*i*Pr resulted in an isoleucine boronic acid ester **19** in a 50% yield. In previous work, we have realized that an arginine building block would be very difficult to handle, not only in the synthesis of building blocks, but also in the assembly of the oligoarenes. Therefore, we preferred to incorporate this building block in a latent alkylnitrile form **23**, which, after oligoarene assembly, can be efficiently converted to the arginine side chain by nitrile reduction, and converting the resulting primary amine with (Boc)2N-guanylation reagent **24**. The Heck reaction of 3,5-dibromopyridine (**17**) with acrylonitrile furnished **20** in a 58% yield. Chemoselective alkene reduction with diimide in situ generated from tosylhydrazide produced **21** in an 86% yield. Building on earlier experience, we chose to convert the bromopyridine **21** to the iodopyridine **22** using the Buchwald–Finkelstein reaction [34] in order to facilitate the planned metalation, with the Knochel–Grignard forming a pyridinyl-Grignard intermediate. Indeed, this transformation and subsequent electrophilic quench with (pin)BO*i*Pr allowed the isolation of latent Arg-building block (Arg\*) **23** in a good yield of 62%. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 4 of 10 Pd/XPhos in an excellent yield of 93%. For the synthesis of the isoleucine pyridine boronic acid ester **19** a Negishi-coupling strategy was chosen. Starting from 3,5-dibromopyridine (**17**) Negishi-coupling with the in-situ prepared 2-butyl zinc reagent furnished pyridine **18** in a 34% yield. Activation of **18** with a Knochel-Grignard [33] and an electrophilic quench with (pin)BO*<sup>i</sup>* Pr resulted in an isoleucine boronic acid ester **19** in a 50% yield. In previous work, we have realized that an arginine building block would be very difficult to handle, not only in the synthesis of building blocks, but also in the assembly of the oligoarenes. Therefore, we preferred to incorporate this building block in a latent alkylnitrile form **23**, which, after oligoarene assembly, can be efficiently converted to the arginine side chain by nitrile reduction, and converting the resulting primary amine with (Boc)2N-guanylation reagent **24**. The Heck reaction of 3,5-dibromopyridine (**17**) with acrylonitrile furnished **20** in a 58% yield. Chemoselective alkene reduction with diimide in situ generated from tosylhydrazide produced **21** in an 86% yield. Building on earlier experience, we chose to convert the bromopyridine **21** to the iodopyridine **22** using the Buchwald–Finkelstein reaction [34] in order to facilitate the planned metalation, with the Knochel–Grignard forming a pyridinyl-Grignard intermediate. Indeed, this transformation and subsequent electrophilic quench with (pin)BO*<sup>i</sup>* Pr allowed the isolation of latent Arg-building block (Arg\*) **23** in a good yield of 62%.

**Scheme 2.** Synthesis of the pyridine boronic acid ester building blocks.
