Triazole antifungal agents have been in widespread clinical use for the treatment of various fungal infections, not only high-risk diseases such as acute invasive aspergillosis(1) but also low-risk pathological conditions such as nail infections.(2) Although fungal infections are generally regarded as nonfatal clinical conditions, immunocompromised patients with human immunodeficiency virus (HIV) infection or undergoing cancer chemotherapy are susceptible to life-threatening fungal diseases.(3) The representative triazole antifungal agent fluconazole (Figure 1), which was developed by Pfizer, is effective orally against a range of fungal infections.(4) However, it is poorly active against Aspergillus spp., which cause life-threatening infections in immunocompromised patients, and fluconazole resistance has been reported in patients receiving long-term treatment.(5) To address these issues, an advanced triazole antifungal agent, voriconazole (Vfend), was developed.(6) This agent is active against all Candida spp., including fluconazole-resistant Candida albicans, Candida glabrata, and Candida krusei, as well as several Aspergillus spp., including the amphotericin B-resistant Aspergillus terreus.(7) Therefore, voriconazole is a primary drug in the first-line treatment of invasive aspergillosis, as either an intravenous or oral formulation.(1a, 7f, 8) The high potency of voriconazole has inspired extensive efforts devoted to the development of a variety of synthetic derivatives.(9) The replacement of one of the triazole rings in fluconazole with heteroaromatics (e.g., 5-fluoropyrimidine for voriconazole) and the installation of a methyl group next to the tetrasubstituted stereogenic center have been proved to be beneficial structural modifications,(6a) leading to the identification of advanced triazole antifungal agents. As these modifications break the molecular symmetry of achiral fluconazole, the development of an efficient enantioselective synthetic route is in high demand.
Figure 1. Structures of fluconazole and advanced triazole antifungal agents.
Considering the common substructures that are shared in these antifungal agents, the enantiomerically pure epoxide 1 bearing 2,4-difluorobenzene, 1,2,4-triazole, and a methyl group is a rational intermediate for their divergent syntheses (Scheme 1). The enantioselective construction of the consecutive tetra- and trisubstituted stereogenic centers represents a formidable task in the synthesis of epoxide 1. Because of the pivotal role of 1 in the efficient synthesis of these antifungal agents, various synthetic approaches toward epoxide 1 have been investigated.(10-12) Almost all of the synthetic routes rely on the use of d- or l-lactic acid as a chiral pool.(11a-11c, 12) In particular, Bristol-Myers Squibb has developed an excellent approach toward epoxide 1 in six steps in 25% overall yield, leading to the scalable synthesis of ravuconazole.(10) Although other approaches utilizing Sharpless asymmetric epoxidation(11e) or enzymatic resolution(11f) have been accomplished, there remains room for improvement in terms of the number of synthetic steps. The exploitation of a catalytic asymmetric C–C bond-forming reaction is a viable option for the integration of the construction of a molecular skeleton with a stereogenic center. We recently demonstrated that the catalytic asymmetric cyanosilylation of a ketone is particularly useful for the construction of the elusive tetrasubstituted stereogenic center, culminating in the enantioselective synthesis of voriconazole.(13)
Scheme 1. Enantioselective Synthesis of Epoxide 1, a Key Intermediate for Various Antifungal Agents
Herein we report a new route, the shortest reported to date, to access epoxide 1 in four steps from commercially available ketone 2 in 29% yield. The key step features the catalytic asymmetric cyanosilylation to construct the tetrasubstituted stereogenic center of 3. The utility of this synthetic approach has been demonstrated by the efficient syntheses of the significant antifungal agents ravuconazole(9b, 10) and efinaconazole (Jublia).(9d, 14) Ravuconazole, which bears a functionalized thiazole, features a broad antifungal spectrum as well as the longest half-life and has completed P2 clinical trials. Efinaconazol (Jublia), which possesses a 4-methylenepiperidine moiety and has recently received approval in Canada,(15) is the first external-use antifungal agent for the treatment of onychomycosis.(16)
Our synthesis commenced with the catalytic asymmetric cyanosilylation of ketone 2, a key reaction promoted by a Gd-based asymmetric catalyst to construct the tetrasubstituted stereogenic center (Scheme 2).(17-19) A putative Gd-based polymetallic catalyst composed of Gd and the sugar-derived chiral ligand 4(20, 21) in a 2:3 ratio, as suggested by ESI-MS analysis in the presence of TMSCN,(17b) was generated by mixing Gd(HMDS)3 and 4 in a 2:3 ratio at −30 °C. The polymetallic catalyst (2 mol% based on Gd) promoted the catalytic asymmetric cyanosilylation of 2 with TMSCN at −30 °C in propionitrile to afford the desired cyanohydrin 3 with TMS protection in 92% yield with 80% ee. Because of its instability under acidic and basic conditions and silica gel column chromatography, 3 was immediately submitted to DIBAL reduction to give corresponding aldehyde 5. Our next focus was the diastereoselective installation of a methyl group and 1,2,4-triazole. Initially, we faced several undesired transformations. After the formation of secondary alcohol 6 using organometallic reagents, quenching with acidic or basic aqueous solutions led to partial migration of the TMS group to provide a complicated mixture of 6 and 7 and their diastereomers. The secondary TMS group of 7(22) was prone to deprotection under either acidic or basic conditions as well as silica gel column chromatography. Even when 7 was isolated via laborious purification and subjected to 1,2,4-triazole introduction under basic conditions at room temperature, deprotection of the TMS group occurred and the subsequent formation of epoxide 9 proceeded partially. The suppression of these unwanted transformations was intractable, and the complicated reaction mixtures made the purification in each step fruitless. Given that all of the byproducts could be converted into diol 10, we anticipated that the sequential manifestation of these undesired transformations in one-pot would allow direct access to 10. After extensive manipulations of the reaction conditions, we found that the installation of the methyl group, the deprotection of the TMS group, the formation of epoxide 9, and the installation of 1,2,4-triazole could be carried out in a one-pot operation. The initial Grignard addition to 5 gave secondary alcohol 6. Other organometallic reagents resulted in low yield or low diastereoselectivity.(23) Treatment of the reaction mixture with a 3 N aqueous NaOH solution in the same flask converted 6 into tertiary alcohol 7 via intramolecular migration of the TMS group. Successive addition of 1,2,4-triazole and TBAB as a phase-transfer catalyst initially induced the removal of TMS to give diol 8, which eventually cyclized to afford epoxide 9 under basic conditions. Ring opening of epoxide 9 proceeded slowly to furnish diol 10 in favor of the desired diastereomer in an 86:14 ratio. The diastereomers were easily separable using silica gel column chromatography to provide the requisite diol 10 as a single diastereomer in 65% yield from aldehyde 5.
Scheme 2. Short Synthesis of the Key Intermediate Epoxide 1
Diol 10 is a crystalline solid, and enantioenrichment was attempted at this stage. When a concentrated acetonitrile solution oversaturated at 60 °C was submitted to rapid nucleation with stirring at −20 °C, a nearly racemic solid (6.3% ee) appeared, and the filtrate was enriched to 97% ee.(24) The second cycle of an identical procedure (but with stirring at 0 °C) afforded the enantiopure diol 10 (>99% ee) in 74% recovery yield after two cycles.(25) With the optically pure diol 10 in hand, we examined its transformation to epoxide 1. Regioselective mesylation of diol 10 proceeded smoothly at 0 °C to provide the transient intermediate 11, which was subsequently treated with a 3 N aqueous NaOH solution and TBAB to afford the key intermediate epoxide 1 in 86% yield in one pot.
Next, we turned our attention to the synthesis of efinaconazole (Scheme 3). According to the literature procedure,(9d) epoxide 1 was subjected to a ring-opening reaction with 4-methylenepiperidine(26) at 80 °C. However, 51% of epoxide 1 remained unchanged after 24 h, and efinaconazole was isolated in 44% yield. Microwave irradiation at 120 °C solved this problem, affording efinaconazole in 90% yield.(27) The spectroscopic data of the synthesized sample were identical to those of the reported one.(9d) Moreover, we also demonstrated the synthesis of ravuconazole according to the literature procedure (Scheme 4).(9b, 10) Ring opening of epoxide 1 using Et2AlCN provided cyanide 12 in 76% yield. The nitrile functionality of 12 was transformed into a primary thioamide with diethyl dithiophosphate to give 13 in excellent yield. Treatment of 13 with 2-bromo-4′-cyanoacetophenone furnished ravuconazole in 78% yield.
Scheme 3. Synthesis of Efinaconazole (Jublia)
Scheme 4. Synthesis of Ravuconazole
In conclusion, we have developed a new route, the shortest reported to date, to access the key intermediate epoxide 1 in 29% overall yield in four steps from the commercially available ketone 2. The key step features a catalytic asymmetric cyanosilylation using Gd(HMDS)3 and a sugar-derived chiral ligand to construct the tetrasubstituted stereogenic center that is essential in advanced triazole antifungal agents. This streamlined synthetic approach led us to demonstrate enantioselective efficient syntheses of two significant antifungal agents.
The reactions were performed in a round-bottom flask with a Teflon-coated magnetic stirring bar and a three-way glass stopcock under an Ar atmosphere, unless otherwise noted. Air- and moisture-sensitive liquids were transferred via a gastight syringe and a stainless steel needle. All workup and purification procedures were carried out using reagent-grade solvents under ambient atmosphere. Flash chromatography was performed using silica gel 60 (230–400 mesh). Chemical shifts (δ) for protons are reported in units of parts per million downfield from tetramethylsilane and are referenced to residual protons in the NMR solvent (CDCl3, 7.24 ppm). For 13C NMR, chemical shifts are reported on the scale relative to the NMR solvent (CDCl3, 77.0 ppm) as an internal reference. For 19F NMR, chemical shifts are reported on the scale relative to trifluoroacetic acid (76.5 ppm) as an external reference. NMR data are reported as follows: chemical shifts (multiplicity, coupling constant in Hz, integration). Multiplicities are denoted as follows: s, singlet; d, doublet; dd, doublet of doublets; t, triplet; q, quartet; sep, septet; m, multiplet; br, broad signal. Optical rotation was measured using a 2 mL cell with a 1.0 dm path length. Compounds 1, 3, 5, 8, 9, 10, 11, 12, and 13 are known compounds (CAS registry numbers 127000-90-2, 861718-83-4, 861718-85-6, 832151-94-7, 126918-35-2, 133775-25-4, 133775-26-5, 170862-36-9, and 170863-34-0, respectively).
To a solution of 5 (1.24 g, 4.23 mmol) in THF (6.70 mL) was added 0.92 M MeMgBr solution in THF (6.44 mL, 5.92 mmol) at −78 °C, and the reaction mixture was stirred at the same temperature for 35 min. The reaction mixture was quenched with saturated aqueous NH4Cl, and the resulting mixture was warmed to room temperature and stirred for 20 min. The aqueous layer was extracted twice with EtOAc. The combined organic layers were washed with H2O, dried over MgSO4, filtered, and concentrated under reduced pressure. The resulting residue was purified using silica gel column chromatography (n-hexane/EtOAc = 85:15) to give 740 mg of (2R,3S)-6 (57% yield) as a colorless oil and 117 mg of (2S,3S)-6 (9% yield) as a colorless oil. 1H NMR for (2R,3S)-6 (400 MHz, CDCl3) δ 7.53–7.47 (m, 1H), 6.90–6.85 (m, 1H), 6.77–6.71 (m, 1H), 4.21 (d, J = 12.1 Hz, 1H), 4.20–4.15 (m, 1H), 4.12 (dd, J = 12.1, 1.1 Hz, 1H), 0.90 (d, J = 6.0 Hz, 3H), 0.29 (s, 9H); 13C NMR for (2R,3S)-6 (100 MHz, CDCl3) δ 162.4 (dd, J = 249, 13 Hz), 158.2 (dd, J = 246, 12 Hz), 131.0 (dd, J = 9.1, 6.2 Hz), 124.8 (dd, J = 13, 3.8 Hz), 111.0 (dd, J = 21, 3.4 Hz), 103.9 (dd, J = 29, 26 Hz), 83.2 (d, J = 6.7 Hz), 71.4 (d, J = 3.8 Hz), 50.1 (d, J = 6.7 Hz), 18.6, 2.41; 19F NMR for (2R,3S)-6 (376 MHz, CDCl3) δ −108.7, −111.6; 1H NMR for (2S,3S)-6 (400 MHz, CDCl3) δ 7.47–7.41 (m, 1H), 6.90–6.85 (m, 1H), 6.81–6.75 (m, 1H), 4.27 (d, J = 12.1 Hz, 1H), 3.97 (q, J = 6.3 Hz, 1H), 3.88 (dd, J = 12.1, 1.4 Hz, 1H), 1.10 (d, J = 6.3 Hz, 3H), 0.24 (s, 9H); 13C NMR for (2S,3S)-6 (100 MHz, CDCl3) δ 162.5 (dd, J = 249, 13 Hz), 159.1 (dd, J = 248, 12 Hz), 131.3 (dd, J = 9.6, 5.8 Hz), 124.0 (dd, J = 13, 4.8 Hz), 110.8 (dd, J = 22, 4.3 Hz), 104.3 (dd, J = 29, 25 Hz), 83.2 (d, J = 4.8 Hz), 72.8 (d, J = 1.9 Hz), 48.6 (d, J = 7.7 Hz), 17.6, 2.33; 19F NMR for (2S,3S)-6 (376 MHz, CDCl3) δ −107.1, −111.2; IR for (2R,3S)-6 (CHCl3, cm–1) ν 3588, 3467, 2956, 1615, 1498, 1419, 1253; HRMS for (2R,3S)-6 (ESI-TOF) calcd for C13H19O2ClF2SiNa [M + Na]+m/z 331.0703, found 331.0702.
To a solution of (2R,3S)-6 (21.4 mg, 0.0693 mmol) in THF (115 μL) was added 3 N NaOH (46.0 μL, 0.139 mmol) at room temperature, and the reaction mixture was stirred at the same temperature for 10 min. The reaction mixture was quenched with saturated aqueous NH4Cl, and the aqueous layer was extracted twice with EtOAc. The combined organic layers were dried over MgSO4, filtered, and concentrated under reduced pressure. The resulting residue was purified using preparative TLC (n-hexane/EtOAc = 7:1) to give 12.4 mg of 7 (58% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.72–7.65 (m, 1H), 6.92–6.87 (m, 1H), 6.78–6.72 (m, 1H), 4.31 (q, J = 6.2 Hz, 1H), 4.04 (d, J = 11.4 Hz, 1H), 3.84 (d, J = 11.4 Hz, 1H), 3.12 (s, 1H), 0.90 (d, J = 6.2 Hz, 3H), 0.15 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 162.5 (dd, J = 249, 13 Hz), 158.6 (dd, J = 247, 13 Hz), 130.7 (dd, J = 9.6, 6.7 Hz), 123.4 (dd, J = 13, 3.8 Hz), 111.3 (dd, J = 21, 3.4 Hz), 103.8 (dd, J = 27, 25 Hz), 77.9 (d, J = 5.8 Hz), 70.7 (d, J = 4.8 Hz), 51.5 (d, J = 5.8 Hz), 18.5, 0.21; 19F NMR (376 MHz, CDCl3) δ −109.7, −111.2; IR (CHCl3, cm–1) ν 3545, 2959, 1619, 1503, 1422, 1254; HRMS (ESI-TOF) calcd for C13H19O2ClF2SiNa [M + Na]+m/z 331.0703, found 331.0703.
To a solution of (2R,3S)-6 (31.7 mg, 0.103 mmol) in THF (343 μL) was added 1.0 M TBAF solution in THF (113 μL, 0.113 mmol) at 0 °C, and the reaction mixture was stirred at the same temperature for 15 min. The reaction mixture was quenched with saturated aqueous NH4Cl, and the aqueous layer was extracted twice with EtOAc. The combined organic layers were dried over MgSO4, filtered, and concentrated under reduced pressure. The resulting residue was purified using preparative TLC (CHCl3/MeOH = 10:1) to give 18.3 mg of 8 (75% yield) as a colorless crystal. Mp 86–87 °C; 1H NMR (400 MHz, CDCl3) δ 7.63–7.57 (m, 1H), 6.93–6.88 (m, 1H), 6.81–6.75 (m, 1H), 4.27–4.14 (m, 3H), 3.09 (brs, 1H), 2.17 (brs, 1H), 0.96 (d, J = 6.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 162.7 (dd, J = 250, 12 Hz), 158.6 (dd, J = 247, 12 Hz), 130.1 (dd, J = 9.6, 6.7 Hz), 123.8 (dd, J = 13, 3.8 Hz), 111.4 (dd, J = 21, 3.8 Hz), 104.2 (dd, J = 28, 25 Hz), 77.8 (d, J = 4.8 Hz), 70.0 (d, J = 4.8 Hz), 51.7 (d, J = 5.8 Hz), 18.6; 19F NMR (376 MHz, CDCl3) δ −109.2, −110.7; IR (CHCl3, cm–1) ν 3433, 3266, 2979, 1617, 1500, 1272; HRMS (ESI-TOF) calcd for C10H11O2ClF2Na [M + Na]+m/z 259.0308, found 259.0310.
To a solution of (2R,3S)-6 (33.6 mg, 0.109 mmol) in THF (363 μL) was added 1.0 M TBAF solution in THF (272 μL, 0.272 mmol) at room temperature, and the reaction mixture was stirred at the same temperature for 23 h. The reaction mixture was quenched with saturated aqueous NH4Cl, and the aqueous layer was extracted twice with EtOAc. The combined organic layers were dried over MgSO4, filtered, and concentrated under reduced pressure. The resulting residue was purified using preparative TLC (CHCl3/MeOH = 15:1) to give 7.4 mg of 9 (34% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.42–7.36 (m, 1H), 6.89–6.84 (m, 1H), 6.81–6.76 (m, 1H), 4.07 (qd, J = 6.6, 1.6 Hz, 1H), 3.28 (d, J = 5.3 Hz, 1H), 2.78 (dd, J = 5.3, 0.5 Hz, 1H), 1.14 (dd, J = 6.6, 1.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 162.8 (dd, J = 249, 13 Hz), 160.4 (dd, J = 249, 12 Hz), 130.6 (dd, J = 10, 6.2 Hz), 120.6 (dd, J = 15, 3.8 Hz), 111.4 (dd, J = 21, 3.8 Hz), 103.7 (dd, J = 25, 25 Hz), 68.4 (d, J = 1.9 Hz), 60.6, 51.9, 19.1; 19F NMR (376 MHz, CDCl
One of the triazole antifungal agents that inhibits cytochrome P-450-dependent enzymes resulting in impairment of ergosterol synthesis. It has been used against histoplasmosis, blastomycosis, cryptococcal meningitis & aspergillosis. [PubChem]
- Itrizole (tn)
- Sporanox (tn)
CCC(C)N1N=CN(C1=O)C1=CC=C(C=C1)N1CCN(CC1)C1=CC=C(OC[[email protected]]2CO[[email protected]@](CN3C=NC=N3)(O2)C2=C(Cl)C=C(Cl)C=C2)C=C1
For the treatment of the following fungal infections in immunocompromised and non-immunocompromised patients: pulmonary and extrapulmonary blastomycosis, histoplasmosis, aspergillosis, and onychomycosis.
Itraconazole is an imidazole/triazole type antifungal agent. Itraconazole is a highly selective inhibitor of fungal cytochrome P-450 sterol C-14 α-demethylation via the inhibition of the enzyme cytochrome P450 14α-demethylase. This enzyme converts lanosterol to ergosterol, and is required in fungal cell wall synthesis. The subsequent loss of normal sterols correlates with the accumulation of 14 α-methyl sterols in fungi and may be partly responsible for the fungistatic activity of fluconazole. Mammalian cell demethylation is much less sensitive to fluconazole inhibition. Itraconazole exhibits in vitro activity against Cryptococcus neoformans and Candida spp. Fungistatic activity has also been demonstrated in normal and immunocompromised animal models for systemic and intracranial fungal infections due to Cryptococcus neoformans and for systemic infections due to Candida albicans.
Itraconazole interacts with 14-α demethylase, a cytochrome P-450 enzyme necessary to convert lanosterol to ergosterol. As ergosterol is an essential component of the fungal cell membrane, inhibition of its synthesis results in increased cellular permeability causing leakage of cellular contents. Itraconazole may also inhibit endogenous respiration, interact with membrane phospholipids, inhibit the transformation of yeasts to mycelial forms, inhibit purine uptake, and impair triglyceride and/or phospholipid biosynthesis.
The absolute oral bioavailability of itraconazole is 55%, and is maximal when taken with a full meal.
Itraconazole is extensively metabolized by the liver into a large number of metabolites, including hydroxyitraconazole, the major metabolite. The main metabolic pathways are oxidative scission of the dioxolane ring, aliphatic oxidation at the 1-methylpropyl substituent, N-dealkylation of this 1-methylpropyl substituent, oxidative degradation of the piperazine ring and triazolone scission.
Itraconazole is metabolized predominately by the cytochrome P450 3A4 isoenzyme system (CYP3A4) in the liver, resulting in the formation of several metabolites, including hydroxyitraconazole, the major metabolite. Fecal excretion of the parent drug varies between 3-18% of the dose. Renal excretion of the parent drug is less than 0.03% of the dose. About 40% of the dose is excreted as inactive metabolites in the urine. No single excreted metabolite represents more than 5% of a dose.
- 381 +/- 95 mL/minute [IV administration]
No significant lethality was observed when itraconazole was administered orally to mice and rats at dosage levels of 320 mg/kg or to dogs at 200 mg/kg.
- Fungi, yeast and protozoans