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The hitherto unknown (±)-4-methoxydecanoic acid was synthesized in six steps and in 25% overall yield starting from commercially available 4-penten-1-ol. The title compound demonstrated seventeen fold higher antifungal activity (MIC = 1.5 mM) against Candida albicans ATCC 60193 and Cryptococcus. neoformans ATCC 66031 when compared to unsubstituted n-decanoic acid. Our results demonstrate that Mid-chain methoxylation appears to be a viable strategy for increasing the fungitoxicity of fatty acids.
The (±)-4-hydroxydecanoic acid is an elusive fatty acid to isolate from either a natural or synthetic source because of its tendency to easily cyclize to the well known γ-decalactone.1 In fact, the spectral data for 4-hydroxydecanoic acid was first reported in 1996 by G. Feron and collaborators, attesting to the difficulty of isolating the pure hydroxylated fatty acid from the γ-decalactone.1 The γ-decalactone is an important compound for the aroma and food industries since the compound imparts a peach-apricot flavor to foods2–3 and is also responsible for the fruity flower odor of gardenia perfumes.1 The γ-decalactone has also been used in <5 ppm concentrations in a selected number of cigarette brands.4
There are just a few scattered reports on the toxicity of (±)-4-hydroxydecanoic acid but it has been reported that the presence of a hydroxyl group in the acyl chain greatly decreases toxicity. However, the γ-decalactone has been reported to have higher antibacterial activity than (±)-4-hydroxydecanoic acid since it inhibits the growth of some bacteria such as Bacillus subtilis and some fungi such as Fusarium oxysporum and Trichothecium roseum at concentrations between 0.1 and 0.7 mM.1 It should also be noted here that the 3-(R)-hydroxydecanoic acid has been isolated from Lactobacillus plantarum and displayed considerable antifungal activity (MIC = 10–100 µg/mL) against different molds and yeasts.5
We have previously shown that α-methoxylation increases the antifungal activity of fatty acids,6–7 but nothing is known as to the effect of mid-chain methoxylation on the antifungal activity of these compounds. We envisioned that a compound such as the (±)-4-methoxydecanoic acid (1) could be a good starting template to study such an effect. In addition, we were expecting this methoxylated fatty acid to be more fungitoxic than the corresponding compounds, unsubstituted n-decanoic acid (capric acid) or isolated (±)-4-hydroxydecanoic acid. The choice of a 10-carbon chain length for this study is justified by the fact that capric acid kills Candida albicans at 10 mM by the postulated mechanism of the fungal plasma membrane disintegration while other saturated fatty acids are not effective.8
Furthermore, unlike (±)-4-hydroxydecanoic acid, (±)-4-methoxydecanoic acid (1) cannot be cyclized to the corresponding γ-lactone thus allowing the facile study of its fungitoxicity. This investigation was also designed to determine whether mid-chain methoxylation is a viable substitution to α-methoxylation for enhancing the antifungal activity of fatty acids such as capric acid. Previous studies by our group with the (±)-2-methoxydecanoic acid has shown that the 2-OMe-10:0 acid is only 1.5 fold more fungitoxic than capric acid (10:0) against C. albicans (ATCC 14053). However, against C. neoformans (ATCC 66031) the 2-OMe-10:0 acid was not more antifungal than capric acid.6
Our six-step synthesis started with the protection of the primary alcohol of commercially available 4-penten-1-ol (2) with dihydropyran (DHP) and catalytic amounts of p-toluenesulfonic acid (PTSA) in CHCl3 at rt for 5h to afford the 1-[(tetrahydropyran-2-yl)oxy]-2-pentene in an 88% yield (Scheme 1). The double bond was effectively epoxidized in the presence of magnesium monoperoxyphthalate (MMPP) in EtOH as solvent for 48 h, to yield the 4,5-epoxy-1-[(tetrahydropyran-2-yl)oxy]pentane (3) in 89% yield after purification of the crude product using silica gel column chromatography (60–200 mesh) and eluting with hexane/diethyl ether (8:2). MMPP turned out to be more efficient than the classical m-chloroperoxybenzoic acid (m-CPBA) in epoxidizing these alkenes, since the latter reagent only afforded moderate to low yields even after long reaction times. The THP protected epoxide 3 was then opened with 1-pentylmagnesium bromide assisted by catalytic amounts of copper (I) chloride in THF at a reaction temperature range of −78°C to −30°C, which afforded the desired 4-hydroxy-1-[(tetrahydropyran-2-yl)oxy]decane (4) in an 81% yield after silica gel (60–200 mesh) column chromatographic purification. The free hydroxyl group in 4 was then readily methylated with methyl iodide in the presence of sodium hydride in THF, which afforded the 4-methoxy-1-[(tetrahydropyran-2-yl)oxy]decane (5) in an 88% yield. Deprotection of the primary alcohol was effectively accomplished with PTSA in CHCl3 at 45°C for 2h, which afforded the (±)-4-methoxydecan-1-ol in a 73% yield (Scheme 1). Final oxidation to the acid was accomplished by reaction of the alcohol with pyridinium dichromate (PDC) in DMF for 24h, which resulted in a 63% yield of 1.9 The overall yield for this six step synthesis was 25%.
The most significant absorption in the NMR spectrum of 1 was observed for the carbons and hydrogens bearing the methoxy functionality. For example, the methoxy protons resonated at δ 3.32 ppm and the methoxy carbon was observed at δ 56.5 ppm, while the methine hydrogen (CHOCH3) resonated at δ 3.20 ppm and the methine carbon (CHOCH3) at δ 79.9 ppm. These 1H NMR and 13C NMR displacements seem to be characteristic for saturated mid-chain methoxylated fatty acids and useful as a future reference for other similar analogs. It is also interesting to mention that in the 70 eV electron impact (EI) mass spectrum of 1 the typical McLafferty rearrangement of fatty acids at m/z = 60 was greatly reduced (1% relative abundance) by the presence of the methoxy functionality at C-4. In the mass spectrum of 1 the α fragmentation at both sides of the methoxylated carbon predominated, but the fragments containing the carboxyl end (at m/z =117 corresponding to C5H903+ and at m/z = 85 corresponding to C4H502+) were the most abundant.
The antifungal activity of 1 was determined against a fluconazole-resistant strain of Candida albicans (ATCC 60193) and against Cryptococcus neoformans (ATCC 66031) following our previously published protocol (Table 1).6–7 n-Decanoic acid was also tested as a control. As can be seen from the data in Table 1 the (±)-4-methoxydecanoic acid (1) was approximately seventeen fold more antifungal against both fungal strains (MIC = 1,457 µM) when compared to n-decanoic acid (MIC = 25,478 µM). Therefore, the antifungal results clearly show that C-4 methoxylation increased the antifungal activity of the parent n-decanoic acid. In fact, for n-decanoic acid C-4 methoxy substitution seems to be more effective than C-2 methoxy substitution in increasing the antifungal activity of n-decanoic acid.6
As to the reasons for the better antifungal activity of 1 over that of n-decanoic acid we can only speculate at this stage and more mechanistic studies are required. The addition of the C-4 methoxy functionality possibly makes the fatty acid more soluble than unsubstituted n-decanoic acid thus facilitating its interaction with the target sites. In addition, based on the previously published antifungal mechanism of decanoic acid8 we can also speculate that acid 1 seems to be able to more efficiently disrupt the fungal membranes due to the mid-chain methoxy substitution. Moreover, the title compound 1 may also inhibit fatty acid biosynthesis within the fungi interacting with some key enzymes. In summary, our results clearly demonstrate that mid-chain methoxylated fatty acids are valuable compounds that can be optimized for developing more potent antifungal agents and thus merit further scrutiny in the search for better antifungal analogs.
The project described was supported by Award Number SC1GM084708 from the National Institutes of General Medical Sciences. We also acknowledge the financial supports from National Science Foundation, Grant Number CHE 0748555, and American Cancer Society grant number RSG-07-290-01-CDD. We thank Dr. Fred Strobel (Emory University) for the high resolution mass spectral data.
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