6-[(2S,3R)-3-(2,4-Difluorophenyl)-3-hydroxy-4-(1H-1,2,4-triazol-1-yl)butan-2-yl]-5-fluoropyrimidine-4-carbaldehyde

Abstract

Voriconazole (VN) is an antifungal drug indicated for the treatment of several fungal infections. Due to its side effects, some works involving late-stage functionalization of VN have been reported in the literature. Here, we disclose a new VN derivative, the 6-[(2S,3R)-3-(2,4-difluorophenyl)-3-hydroxy-4-(1H-1,2,4-triazol-1-yl)butan-2-yl]-5-fluoropyrimidine-4-carbaldehyde (VN-CHO). This compound results from the photoredox-catalyzed hydroxymethylation of VN, affording a hydroxymethylated derivative (VN-CH₂OH), followed by oxidation of the former CH₂OH group. VN-CHO was obtained in good yield (70% yield) and its structure was unveiled by 1D (¹H and ¹³C) and 2D (HSQC and HMBC) NMR techniques. The introduction of a formyl group in VN structure creates a very promising site for further functionalization in a molecule which originally does not have many active sites.

1. Introduction

Voriconazole (VN) is a well-known antifungal drug used to treat several fungal infections such as aspergillosis, candidiasis, coccidioidomycosis, histoplasmosis and penicilliosis, acting on fungal metabolism and fungal cell membranes. VN is the second generation of triazole antifungal drugs, derived from the former fluconazole (FN), both developed by Pfizer in the 1990s and 1980s, respectively, of the 20th century. Despite the good in vivo efficacy and excellent pharmacokinetic properties, invasive aspergillosis showed some resistance to FN [1,2,3]. The replacement of one triazole moiety in FN with a 5-fluoropyrimidine alongside the installation of a methyl group on the linker next to the stereogenic center ended up with VN (Figure 1). These structural modifications have been demonstrated to be quite favorable against all Aspergillus species, allowing to overcome the resistance associated with FN. Indeed, the VN showed to be 20-fold more potent against Candida albicans and Candida glabrata [4]. Over the years, VN was subjected to several other structural modifications, especially in the 5-fluoropyrimidine moiety, to favour its antifungal activity in terms of the enhancement of pharmacokinetic properties as well as to achieve tolerable toxicity profiles [5].

The stereochemical feature of VN is crucial for its potent antifungal activity. The installation of two contiguous chiral centers can be quite a challenge for synthetic chemists and therefore, it is very important to establish efficient enantioselective synthetic methods to prepare the VN. To the best of our knowledge, there are two main synthetic pathways for the synthesis of VN (see SI for details) [6,7].

The first synthetic route of VN involves five reaction steps (Scheme S1): (i) the Friedel-Crafts acylation of difluorobenzene with chloroacetyl chloride for the preparation of the 2-chloroacetophenone; (ii) the reaction of the 2-chloroacetophenone with 4-aminotriazole, followed by (iii) deamination to install the triazole moiety; (iv) installation of the pyrimidine moiety through the reaction of the organozinc reagent of 4-(1-bromoethyl)pyrimidine and the triazole ketone precursor (with the desired diastereoselectivity (2R,3S) over the (2R,3R)-isomer in a 12:1 ratio); and (v) catalytic hydrogenolysis in the presence of sodium acetate to give VN [6].

The second route towards VN differs from the previous one only in the final steps (Scheme S2). This means that the triazole ketone reacts with 4-chloro-6-ethyl-5-fluoropyrimidine upon deprotonation with lithium diisopropylamide (LDA) as a base in tetrahydrofuran (THF) at a low temperature, resulting in a 1:1 mixture of diastereomers (2R,3S)/(2R,3R). The final hydrogenolysis step requires an additional resolution step in order to obtain VN, limiting the synthetic utility of this route [7].

Given the synthetic difficulties of the enantioselective synthesis of VN, it is advisable for medicinal chemists the utilization of late-stage functionalization (LSF) methods, which use the C–H bonds of drug leads as points of diversification for generating new analogs. LSF approaches (Figure 2) offer interesting advantages such as the quick assessment of structure–activity relationships (SARs), optimization of on-target potency, selectivity and absorption–distribution–metabolism–excretion (ADME) properties, improvement of physical properties such as solubility and stability, the generation of oxidized metabolites, the blocking of metabolic hot spots as well as the preparation of biological probes, without resorting for arduous de novo chemical synthesis [8,9].

Within the LSF toolbox, several C–H functionalization reactions have been reported such as borylation, halogenation, oxidation and carbon–carbon bond formation reactions. One of the most important is perhaps the photoredox catalysis which enabled the development of completely new reaction mechanisms, facilitating the construction of challenging carbon–carbon and carbon–heteroatom bonds [10]. The installation of the hydroxymethyl group (-CH₂OH) is often desirable as it can modulate physical properties and solubility, and through hydrogen-bonding interactions, the binding mode of the pharmacophore. However, the hydroxymethyl is not the primer functional group for the generation of other target compounds. Luckily, this primary alcohol could be oxidized to its corresponding aldehyde, creating an appealing point of lead diversification through aldol reaction, Wittig reaction, imine formation, reductive amination and so on.

Herein, we installed a formyl group at C-4 of the pyrimidine unit of VN through oxidation of its hydromethylated precursor (VN-CH₂OH), which was obtained by photoredox-catalyzed hydroxymethylation of VN (Scheme 1).

2. Results and Discussion

Firstly, VN was converted into VN-CH₂OH through a photoredox-catalyzed hydroxymethylation procedure. This methodology consisted in the generation of hydroxymethyl radicals from methanol and their addition to VN using a photoredox approach [11]. To do so, we used an irradiation source based on blue LEDs in the presence of an Ir(III)-photocatalyst–[Ir(dF-CF₃-ppy)₂(dtbpy)]PF₆–and benzoyl peroxide (BPO) as oxidant (Scheme 2). The desired hydroxymethylated VN (VN-CH₂OH) was obtained together with a byproduct resulting from the dihydroxymethylation of VN at C-2 and C-4 of the pyrimidine ring [VN-(CH₂OH)₂].

Next, VN-CH₂OH was oxidized to VN-CHO in the presence of activated manganese dioxide as an oxidizing agent (Scheme 2). Using an excess of MnO₂, in ethyl acetate, at reflux for 4 h, VN-CHO was obtained in 70% yield. It is noteworthy that this methodology is regioselective as the hydroxymethylation step occurs predominantly at C-4 of the pyrimidine ring of VN and consequently, only one regioisomer was obtained. On the other hand, if a standard Vilsmeier–Haack formylation procedure was performed directly in the VN, the desired VN-CHO might not be obtained since the preferred sites for formylation would be the electron-rich 1,2,4-triazole and difluorophenyl moieties (Figure 3).

VN-CHO was fully characterized by 1D (¹H and ¹³C) (Figures S1 and S2) and 2D (HSQC and HMBC) (Figures S3 and S4) NMR techniques. In its ¹H NMR spectrum (Figure S1), the most characteristic signals are:

(i)

The doublet (J_H-F 0.6 Hz) at δ_H 10.19 ppm, which corresponds to the resonance of the proton of the 4-CHO group.

(ii)

The doublet (J_H-F 1.6 Hz) at δ_H 9.08 ppm, which corresponds to the resonance of H-2 of the pyrimidine ring.

(iii)

Two singlets at δ_H 7.54 and 7.96 ppm, which correspond to the resonance of H-3′′′ and H-5′′′ of the 1,2,4-triazole moiety.

(iv)

The broad singlet at δ_H 6.19 ppm, which corresponds to the resonance of the proton of the 3′-OH group.

(v)

The doublet (J_1′-2′ 7.1 Hz) at δ_H 1.15 ppm, which corresponds to the resonance of the protons of the 2′-CH₃ group.

In addition, through the observed correlations in the HMBC spectrum of VN-CHO (Figure S4), we were able to unequivocally assign all nonprotonated carbons (Figure 4):

(i)

C-4 (δ_c 144.2 ppm) as a doublet (²J_4-F 7.0 Hz) due to its connectivities with 4-CHO and H-2.

(ii)

C-5 (δ_c 153.8 ppm) as a doublet (¹J_5-F 282.3 Hz) due to its connectivities with 4-CHO and H-2′.

(iii)

C-6 (δ_c 163.6 ppm) as a doublet (²J_6-F 13.3 Hz) due to its connectivities with H-2, H-2′ and 2′-CH₃.

(iv)

C-3′ (δ_c 77.6 ppm) due to its connectivities with 2′-CH₃, H-2′, H-4′ and H-6′′.

(v)

C-1′′ (δ_c 123.4 ppm) as a doublet of doublets (⁴J_1′′-F 4.0 and ²J_1′′-F 12.3 Hz) due to its connectivities with 3′-OH, H-4′, H-3′′ and H-5′′.

(vi)

C-2′′ and C-4′′ (δ_c 156.7–164.6 ppm) due to its connectivities with H-6′′. However, their signals are hidden by the background noise in the ¹³C NMR spectrum, which prevented their accurate assignment.

3. Materials and Methods

3.1. General Remarks

Melting points were measured with a Büchi Melting Point B-540 apparatus and are uncorrected. NMR spectra were recorded with a Bruker Avance 300 spectrometer (300 MHz for ¹H and 75 MHz for ¹³C), in CDCl₃ as solvent. Chemical shifts are reported in ppm and coupling constants (J) in Hz; the internal standard was tetramethylsilane (TMS). Unequivocal ¹³C assignments were made with the aid of 2D gHSQC and gHMBC (delays for one-bond and long-range J C/H couplings were optimized for 145 and 7 Hz, respectively) experiments. Positive ESI mass spectra were acquired with a QTOF 2 spectrometer. Preparative thin layer chromatography (TLC) was performed with Macherey–Nagel silica gel G/UV254. All chemicals and solvents were obtained from commercial sources and used as received or dried by standard procedures. VN was obtained by liquid–liquid extraction of the lyophilized powder of Vfend^® for intravenous infusion provided by the Hospital de Santa Maria, Lisbon, Portugal. VN-CH₂OH was synthesized according to a procedure described in the literature [11].

3.2. General Procedure for the Synthesis of 6-[(2S,3R)-3-(2,4-Difluorophenyl)-3-hydroxy-4-(1H-1,2,4-triazol-1-yl)butan-2-yl]-5-fluoropyrimidine-4-carbaldehyde (VN-CHO)

MnO₂ (349 mg; 4.02 mmol) was added to a solution of VN-CH₂OH (127 mg; 0.335 mmol) in EtOAc (6 mL). The reaction mixture was stirred at reflux for 4 h. After this time, the reaction mixture was filtered through a celite pad and then, the reaction crude was purified by preparative TLC using a mixture of 5% MeOH/CH₂Cl₂ as eluent. VN-CHO was obtained as a white solid (88.5 mg, 70% yield). Mp 91–94 °C. ¹H NMR (300 MHz, CDCl₃): δ 1.15 (d, J 7.1 Hz, 3H, 2′-CH₃), 4.22–4.29 (m, 1H, H-2′), 4.40 and 4.73 (2 d, J 14.2 Hz, 2H, H-4′), 6.19 (br s, 1H, 3′-OH), 6.80–6.91 (m, 2H, H-3′′,5′′), 7.54 (s, 1H, H-3′′′), 7.64 (dt, J 6.6 and 9.2 Hz, 1H, H-6′′), 7.96 (s, 1H, H-5′′′), 9.08 (d, J 1.6 Hz, H-2), 10.19 (d, J 0.6 Hz, 1H, 4-CHO) ppm. ¹³C NMR (75 MHz, CDCl₃): δ 16.1 (2′-CH₃), 37.6 (d, J 5.2 Hz, C-2′), 57.1 (d, J 5.4 Hz, C-4′), 77.6 (C-3′), 104.3 (dd, J 25.8 and 27.5 Hz, C-3′′), 111.8 (dd, J 3.4 and 20.6 Hz, C-5′′), 123.4 (dd, J 4.0 and 12.3 Hz, C-1′′), 130.6 (dd, J 5.6 and 9.5 Hz, C-6′′), 144.1 (C-5′′′), 144.2 (d, J 7.0 Hz, C-4), 151.2 (C-3′′′), 153.5 (d, J 9.9 Hz, C-2), 153.8 (d, J 282.3 Hz, C-5), 163.6 (d, J 13.3 Hz, C-6), 188.4 (d, J 2.2 Hz, 4-CHO) ppm. HRMS-ESI⁺ m/z calcd for [C₁₇H₁₄F₃N₅O₂ + CH₃OH + H]⁺: 410.1440, found: 410.1435.

4. Conclusions

In conclusion, we synthesized a new VN derivative employing a LSF approach. VN-CHO was obtained in good yield through a regioselective methodology, which involved a photoredox-catalyzed hydroxymethylation reaction followed by an oxidation step with manganese dioxide. The installation of the formyl group occurred at C-4 of the pyrimidine ring of VN, creating a new active site for further functionalization through many well-known transformations involving aldehydes (for instance, Witting reaction, aldol condensation, reductive amination, among others).