Synthesis and Antifungal Evaluation of Novel N-Alkyl Tetra- and Perhydroquinoline Derivatives

A series of novel N-alkyl tetra- and perhydroquinoline derivatives and their hydrochlorides were prepared from tetrahydro- or trans-perhydroquinoline by direct alkylation with alkyl halides and subsequent precipitation with HCl gas. The antimicrobial activity of the resulting amines was evaluated in an agar diffusion assay. The minimal inhibitory concentrations (MIC) of the active compounds were determined by the microdilution method. In contrast to the tetrahydroquinolines, the perhydro analogues showed significant antifungal activity. In an assay for the detection of target enzymes in ergosterol biosynthesis, N-undecylperhydroquinoline was identified as an inhibitor of Δ8,7-isomerase.


Introduction
In the last few decades, a dramatic increase in fungal infections was observed in the Northern Hemisphere. Especially cancer patients, organ-engrafted patients, and immunecompromised patients (e.g. AIDS patients) are predisposed to systemic fungal infections by Candida or Aspergillus species with a high lethality. Only a few drugs from three classes can be used in the treatment of these life-threatening systemic infections: azoles (e.g. fluconazole, posaconazole, or voriconazole), polyene macrolides (e.g. amphotericin B), and echinocandins (e.g. caspofungin, anidulafungin, or micafungin) [1,2] (Fig. 1 Fig. 1.

Drugs used for the treatment of systemic fungal infections
Ergosterol biosynthesis is an extremely important target in the development of new antimycotic drugs [3]. Ergosterol and the enzymes in the post-squalene part of ergosterol biosynthesis are specific for fungi, so specific inhibitors should be selective for the fungal pathway. By now, only the four enzymes, squalene epoxidase (by allylamines), C14 demethylase (by azoles), and Δ8,7-isomerase and Δ14-reductase (both by morpholines), of this complex pathway are targeted by antimycotics used in human medicine.
The mimicry of carbocationic high energy intermediates of this biosynthesis pathway by protonated amines is an often-used approach towards inhibitors of Δ8,7-isomerase and Δ14-reductase. The most important drug in human medicine using this concept is the morpholine amorolfine (A), but this drug can only be used in topical formulations [4] (Fig. 3). Several morpholine antifungals like fenpropimorph or tridemorph, showing the same mechanism of action, are used in agrochemistry.
The same mechanism of action could be shown for N-alkylpiperidines like fenpropidin [5][6][7]. Rahier and coworkers found the same mechanism of action for complex N-alkylperhydroisoquinolines (B). In our group, N-n-undecyl-trans-decahydroisoquinoline (C) was identified as a potent antifungal inhibiting the enzyme Δ8,7-isomerase [8] (Fig. 3). Related imidazol-5-yl carbinols showed comparable antifungal activities, but surprisingly did not interfere with ergosterol biosynthesis [9]. The surprisingly high antifungal activities of simple N-alkyldecahydroisoquinolines and imidazol-5-yl carbinols, the observed outstanding role of the length of the alkyl side chain in both series, and the surprising differences in molecular mechanisms of actions prompted us to perform analogous investigations on a third heterocyclic scaffold, the quinoline ring system.

Results and Discussion
In continuation of our above-mentioned work [8,9], we evaluated the antifungal potency of simple N-alkyl tetrahydro-and perhydroquinoline derivatives in the present work. As we found in previous work that N-n-alkyl substitutents with nine to twelve carbons led to the highest antifungal potency, we focused on side chains with a length of C 9 to C 12 .
In a first series, 1,2,3,4-tetrahydroquinoline (1a), first deprotonated with NaH, was alkylated with unbranched C 9 to C 12 alkyl halides to give the tertiary amines 2a-d. The amines were dissolved in dry diethyl ether and precipitated with HCl gas to give the more stable hydrochlorides 3a-d. In an alternative approach, 2d was prepared from 1a and dodecanoyl chloride to give the amide 6, which was reduced with LiAlH 4 to give 2d (Scheme 1).
In a second series, (±)-trans-perhydroquinoline (1b) was alkylated with C 9 to C 12 alkyl halides in the same way as described above to give the tertiary amines 4a-d, which were precipitated with HCl gas to give the hydrochlorides 5a-d (Scheme 1). The trans stereochemistry was selected since it resembles the stereochemistry typically found in the connections of the rings in ergosterol intermediates. The trans configuration of 1b was confirmed by comparison of 13 C-NMR data with literature data [10].
The resulting compounds and known compounds, 1-undecylpiperidine (7) [14] and 1-decylpiperazine (8a) [15], as well as 1-methyl-4-undecylpiperazine (8b) [16] (prepared in the same way), were tested in an agar diffusion assay [11]   The minimal inhibitory concentrations (MIC) of the most active compounds from the agar diffusion assay were determined in a microdilution assay on Candida glabrata, Yarrowia lipolytica, and Saccharomyces cerevisiae [11]. For comparison, the N-undecylperhydroisoquinoline (C) [8] was also tested here. For solubility reasons, the hydrochlorides were used in this assay (  [17] fluconazole 32 (MIC 90 Candida spp.) [17] nystatine B 4 (MIC 90 Candida spp.) [17] The cytotoxicity of the compounds 3a-d, 5a-d, and 8b was determined in an MTT test [13] against a human leukemia cell line (HL 60). All compounds tested showed a moderate cytotoxicity against this cell line with IC 50 values between 6 to 46 µM ( The compounds 5c and 7 were also subjected to our whole-cell assay for identification of the target enzyme in ergosterol biosynthesis [12]. In this assay, the strains Candida glabrata and Saccharomyces cerevisiae were incubated with the test compounds, and after cell lysis, the changes in the sterol pattern were analyzed by GLC-MS. The accumulation of the Δ8(9)-sterol lichesterol (ergosta-5,8,22-trien-3β-ol) clearly indicates an inhibition of the enzyme Δ8,7-isomerase. Both compounds 5c and 7 showed an accumulation of lichesterol, so one mechanism of action is an inhibition of Δ8,7-isomerase.

Conclusion
The N-alkyl tetrahydroquinoline compounds 2a-d showed no antibiotic or antimycotic activity against the tested microorganisms. Their corresponding hydrochlorides 3a-d showed weak cytotoxicity. In contrast, the (±)-trans-N-alkylperhydroquinolines showed high antimycotic activity comparable to the commonly used drug clotrimazole. The maximum of activity was found with the C 10 alkyl chain in the agar diffusion assay and with the C 12 alkyl chain in the MIC determination, shorter alkyl chains led to a decrease in activity, as already found for other N-alkyl heterocycles [8,9]. Compared to the recently described [8] N-alkyl perhydroisoquinolines (e.g. C), the new perhydroquinoline compounds showed similar antifungal activity, but higher cytotoxicity against a human cell line. Both perhydroquinolines and perhydroisoquinolines target the same enzyme in ergosterol biosynthesis (Δ8,7-isomerase), but the latter chemotype seems to have benefits in selectivity.

General Procedure I (N-alkylation)
The quinoline derivative (about 4 mmol) was dissolved in 50 mL dry THF and 3 equiv. of NaH were added. The suspension was refluxed for 1 h. Then 1.4 to 2 equiv. of the alkyl halide in 5 mL dry THF was added and the mixture was refluxed for 6 h. The mixture was quenched with 50 mL 10% aqueous NaOH and was extracted with ethyl acetate (3 × 50 mL). The combined organic layers were dried over Na 2 SO 4 , the solvent was evaporated, and the residue was purified by flash column chromatography.

General Procedure II (Preparation of Hydrochlorides)
1.0 mmol of the N-alkyl quinoline derivative was dissolved in 30 mL of dry diethyl ether and the solution was flushed with HCl gas for five minutes. The solvent was evaporated and the residue was disperged in 30 mL of dry diethyl ether, the suspension was placed in a fridge for 3 h and the precipitate was separated to give the analytically pure hydrochlorides.