Synthesis of Aryldifluoromethyl Aryl Ethers via Nickel-Catalyzed Suzuki Cross-Coupling Reactions

An efficient synthesis of aryldifluoromethyl aryl ethers was established via nickel-catalyzed aryloxydifluoromethylation. Using this protocol, difluoromethylated PD-1/PD-L1 immune checkpoint inhibitors were comveniently achieved with enhanced antitumor efficacy over the non-fluoro congeners.

Fluorine-containing drug molecules approximately account for over 30% of the total marketed drugs and crossing diverse disease indications.[1] It is noteworthy that gem-difluoromethylated groups have gained tremendous importance in the field of drug discovery due to the capability of difluoromethyl motif both in creating new patentable intellectual property and in improving druglike properties.[2] Among these, the aryldifluoromethyl aryl ether module (ArCF2OAr') has attracted extensive applications recently due to its optimal metabolic stability against benzylic oxidation by CYP450 enzyme in liver and similar or slightly improved biological activity, compared to the non-fluorinated precursors ArCH2OAr' (Figure 1a). Previously, our group has reported a series of PD-1/PD-L1 (Programmed death-1/programmed death ligand-1) inhibitors featuring a difluoromethyleneoxy linkage (Figure 1b).[3] However, key intermediates containing aryldifluoromethyl aryl ethers were difficult to prepare using traditional oxidative desulfurization-difluorination method (less than 10% yield). Therefore, development of a mild,  substrate compatible, and site-selective method for convenient construction of ArCF2OAr' is highly desirable.

Figure 1. (a) Examples of biologically active aryl aryloxydifluoromethyl ethers I-V. (b) Synthesis for aryl aryloxydifluoromethyl ether VI.

So far, available synthetic strategies of aryldifluoromethyl aryl ethers are limited. The early strategy was reported by Zupan and coworkers in 1990,[4] which demonstrated a XeF2-introduced fluorination-rearrangement reaction of diarylketones under strong acidic condition. However, this strategy was restricted by the use of tedious XeF2 as fluorination agent and suffered from narrow substrate compatibility. Oxidative desulfurization-difluorination of thiol-esters or dithianylium salts is a classic synthetic strategy,[5,6] but suffers from several disadvantages, such as using smelly reagents and unstable fluorination reagents. Direct nucleophilic substitution of aromatic gem-difluoromethyl halides or its equivalents by phenols provides an alternative approach. As an improvement, a radical nucleophilic substitution of phenanthridine difluoromethyl sulfones by phenolates was reported recently to access ArCF2OAr' through a single-electron transfer (SET) process. Later, the Young group[7] developed a frustrated Lewis-pair-meditated C-F activation of trifluoromethylarenes with 2,4,6-triphenylpyridine (TPPy) leading to active ArCF2-TPPy salts, which then underwent substitution with suitable lithium aryloxides. More recently, Qing and co-workers[8] developed a direct C-H aryloxydifluoromethylation of heteroarenes through Ag-catalyzed decarboxylation of aryloxydifluoroacetic acids to afford ArCF2OAr' species, although this approach is limited to heteroaryl compounds. Despite these available approaches to access ArCF2OAr', they are generally limited to harsh reaction conditions, complex fluorine agents, and narrow substrate scopes.

Herein, we reported a Ni-catalyzed Suzuki cross-coupling reaction by using aryloxydifluoromethyl bromides (ArOCF2Br) as a unique halide species to undergo coupling with arylboronic acids, leading to highly efficient synthesis of various aryl aryloxydifluoromethyl ethers (Figure 2). The reaction showed wide substrate scope in both substrates containing various functional groups, and allowed late-stage difluoromethylation of many pharmaceuticals and natural products (Figure 2).

Fig. 2 Ni-catalyzed aryloxydifluoromethylation reactions with diversified arylboronic acids. (a) Reaction conditions: unless otherwise noted, a solution of 1 (0.2 mmol), 2a (0.4 mmol), Ni-2 (10 mol%), DABCO (10 mol%), and K2CO3 (0.5 mmol) in dry acetone (3.0 mL) was performed at 80 oC under argon for 10 h. The yields are isolated yields by column chromatography on silica gel. (b) Dry acetone (2.5 mL) and DMF (0.5 mL) were used for solvent. (c) 100 mg 4A MS was added. (d) Dry acetone (1.5 mL) and DMF (1.5 mL) were used for solvent. 

With the encouraging results, we succefully prepared a new difluoromethylated PD-1/PD-L1 checkpoint inhibitor 10 via 7 steps with good efficiency by using the coupling reaction as the key step. The diaryldifluoromethyl ether key intermediate D (Figure 1b) was obtained in 53% yield under standard conditions, which was much better than traditional method in our previous report (less than 10% yield). As shown in Figure 3a, compared to the non-fluoro precedent 11, difluorinated 10 showed three-fold higher potency against PD-1/PD-L1 interaction (IC50 for 10/11: 12.93 vs 39.06 nM). In vivo, inhibitor 10 displayed a significant reduction in tumor burden than control group with no significant loss of body weight or other common toxic effects in MC38 subcutaneous transplanted tumor model (Figure 3b,c,d). Furthermore, to explore the effect of 10 on tumor immunity, we measured the percentage of T lymphocytes in tumors and spleens of mice treared with 10. As shown in Figure 3e and 3f, injection of 10 significantly increased the population of CD8+ T cells in both tumors and spleens, which indicated that 10 activated antitumor immune response in MC38 xenograft model.

Fig. 3 Treatment with compound 10 inhibited tumor growth in vivo and remodeled facilitated the infiltration of CD8+ T cells. (a) PD-1/PD-L1 immune checkpoint inhibitor activity of difluorinated compound 10 and non-fluoro compound 11. (b) The growth of transplanted MC38 tumors after local injection of 20mg/kg compound 10 or vehicle daily. Data are presented as mean ± SEM (n=8, *** P < 0.001, two-way ANOVA). (c) Image of excised tumors from. (d) weight of excised tumors from. (e) Representative plots (left) and frequency (right) of CD4+ T cells and CD8+ T cells in tumors. (f) Representative plots (left) and frequency (right) of CD4+ T cells and CD8+ T cells in spleens, data are presented as mean ± SD (n=4, ns P > 0.05, * P <0.05, ** P <0.01, t-test).

For more details, especially on product derivatization and further discussions of the mechanistic studies, please see our article: https://www.nature.com/articles/s42004-022-00694-4

References

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