Carbon analogs of antifungal dioxane-triazole derivatives: Synthesis and in vitro activities

Article abstract of DOI:10.1016/j.bmcl.2008.10.055

A new series of triazole compounds possessing a carbon atom in place of a sulfur atom were efficiently synthesized and their in vitro antifungal activities were investigated. The carbon analogs showed excellent in vitro activity against Candida, Cryptococcus, and Aspergillus species. The MICs of compound 1c against C. albicans ATCC24433, C. neoformans TIMM1855, and A. fumigatus ATCC26430 were 0.016, 0.016, and 0.125 μg/mL, respectively (MICs of fluconazole: 0.5, >4, and >4 μg/mL; MICs of itraconazole: 0.125, 0.25, and 0.25 μg/mL).

Full text of DOI:10.1016/j.bmcl.2008.10.055

Bioorganic & Medicinal Chemistry Letters 18 (2008) 6538–6541
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Carbon analogs of antifungal dioxane-triazole derivatives: Synthesis and
in vitro activities
*
Takuya Uchida, Atsushi Somada, Yoshiko Kagoshima, Toshiyuki Konosu , Sadao Oida
Medicinal Chemistry Research Laboratories II, Daiichi Sankyo Co., Ltd, 1-16-13, Kitakasai, Edogawa-ku, Tokyo 134-8630, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2008
Revised 26 September 2008
Accepted 10 October 2008
Available online 15 October 2008
A new series of triazole compounds possessing a carbon atom in place of a sulfur atom were efficiently
synthesized and their in vitro antifungal activities were investigated. The carbon analogs showed excel-
lent in vitro activity against Candida, Cryptococcus, and Aspergillus species. The MICs of compound 1c
against C. albicans ATCC24433, C. neoformans TIMM1855, and A. fumigatus ATCC26430 were 0.016,
0.016, and 0.125
lg/mL, respectively (MICs of fluconazole: 0.5, >4, and >4
lg/mL; MICs of itraconazole:
0.125, 0.25, and 0.25
l
g/mL).
Keywords:
Ó 2008 Elsevier Ltd. All rights reserved.
Triazole
Antifungal
Grignard reaction
The growing population of immunocompromised patients due
to transplantation, AIDS and cancer chemotherapy, has resulted
in an increase in severe fungal infections.1 In many cases, it is
not the AIDS or cancer itself but the mycoses that are lethal to
these patients. Triazole compounds are an important class of anti-
fungal agents because of their generally broad antifungal spec-
trum, high potency and low toxicity.2 Triazole derivatives
displace lanosterol from lanosterol 14-demethylase, a cytochrome
P450-dependent enzyme, and block the biosynthesis of an essen-
tial component of the fungal cell membrane, ergosterol.3 Previ-
ously, we synthesized a series of dioxane-triazole compounds
possessing a sulfur atom, as depicted by general formula A (Fig. 1).4
They showed excellent in vitro antifungal activity against vari-
ous pathogenic fungi including Candida, Cryptococcus, and Aspergil-
lus species. The structure–activity relationship studies on A4,5 led
us to further investigate the role of the sulfur atom in the anti-
fungal activity. Herein, we report the synthesis and in vitro anti-
fungal activities of triazole derivatives B, in which the sulfur
atom was replaced by a carbon atom.
Figure 1. Structural formulas of dioxane-triazole derivatives.
generated in situ from acetone cyanohydrin and lithium hydride
to afford nitrile 5, in 75% yield. Nitrile 5 was converted, via amide
6 (Na2CO3, H2O2, acetone, H2O, 82%) and ester 7 (trifluoromethane-
sulfonic acid, PrOH, 74%), to diol 8 (LiBH4, EtOH, 74%). Treatment of
diol 8 with triethylamine and thionyl chloride gave cyclic sulfite
ester 9, and successive oxidation with sodium periodate in the
presence of a catalytic amount of ruthenium trichloride afforded
cyclic sulfate ester 10 in 52% yield.9,10 The cyclic sulfate was
opened by a sodium diethyl malonate attack and successive one-
pot acid treatment gave diester 11 in 71% yield. Finally, LiBH4
reduction gave triol 3 in 81% yield.
Although Route A furnished desired triol 3 in an efficient man-
ner, it uses toxic reagent ruthenium trichloride, and therefore
alternative synthetic routes were searched for. Route B (Scheme
2) involves a stereoselective Evans aldol reaction. Following the re-
port of Bartroli et al.,11 epoxide 14 was formed from commercially
available (S)-(À)-4-benzyl-2-oxazolidinone. Treatment of epoxide
14 with sodium bis(2-methoxyethoxy)aluminum hydride afforded
epoxy alcohol 15 in 85% yield. Epoxy alcohol 15 was activated with
For comparison of the antifungal activities with those of the
previous derivatives, we planned to synthesize 1a–d having vari-
ous lengths of olefinic chains and a p-(trifluoromethyl)phenyl
group (Fig. 2). Aldehydes 2a–d were prepared from 4-(trifluoro-
methyl)benzaldehyde stereoselectively as described previously.4
The precursor triol 3 was prepared in three different routes, shown
as follows.
Route A (Scheme 1) uses optically active epoxide 4.6,7 Following
Tsuruoka’s protocol,8 epoxide 4 was treated with lithium cyanide
* Corresponding author. Tel.: +81 3 3680 0151; fax: +81 3 5496 8344.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.10.055

T. Uchida et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6538–6541
6539
searched for further synthetic routes. In Route C (Scheme 3), the
same intermediate 16 used in route B could be synthesized starting
from methyl (S)-(+)-3-hydroxy-2-methylpropionate (18). Treat-
ment of 18 with a mixture of sulfuric acid and acetic acid afforded
carboxylic acid 19. The carboxylic acid 19 was transformed into
acyl chloride 20 by treatment with oxalyl chloride and subjected
to a Friedel–Crafts reaction to afford propiophenone 21 in 55%
yield (3 steps). The acetyl protecting group was removed using sul-
furic acid to give keto alcohol 22 in 90% yield. A Grignard reaction
of 22 furnished a diastereomeric mixture of 1,2-oxasilinanes 23
and 24, which were treated with H2O2 under a basic condition12
to afford triol 25 in 78% yield (2 steps), after chromatographic sep-
aration. The diastereomeric ratio observed in triol 25 was >20:1. In
contrast, a similar Grignard reaction of the acetic acid ester 21 or
the tetrahydropyranyl ether analog gave almost no diastereoselec-
tivity (ratio both ca. 3:2; determined after converting to triol 25).
The high diastereoselectivity observed in the Grignard reaction of
alcohol 22 was presumably brought about by either of the follow-
ing mechanisms (Fig. 3). Nucleophilic attack occurred selectively
from the less hindered face of the carbonyl group of the magne-
sium-chelated intermediate X, or, after exchange of the alkoxy
group on the silicon atom in the Grignard reagent, the attack pro-
ceeded through the Felkin-Anh transition state Y with chair-like
conformation.13 Triol 25 was treated with trifluoromethanesulfo-
nic anhydride to afford a ditriflate intermediate, which was treated
with sodium bis(trimethylsilyl)amide and then with sodium
diethyl malonate to give diester 16 in 43% yield.
Figure 2. Structural formulas of carbon analogs 1a–d and their precursors 2a–d, 3.
trifluoromethanesulfonic anhydride and allowed to react with so-
dium diethyl malonate to give epoxy diester 16 in 83% yield.
Reduction of the diester moiety of 16 with LiBH4 gave epoxy diol
17 in 86% yield. Finally, the epoxy ring was opened by an attack
of sodium salt of triazole and triol 3 was obtained in 73% yield.
Since the above described Evans aldol reaction requires a low
temperature and the diastereoselectivity was unsatisfactory, we
The triol 3 thus obtained was coupled with aldehydes 2a–d to
give 1a–d. The reaction was driven in the presence of p-toluenesul-
fonic acid and molecular sieves 4A in dichloromethane. The trans
Scheme 1. Synthesis of triol 3 (Route A). Reagents and conditions: (a) Ref. 8; (b) Na2CO3, H2O2, acetone, H2O, rt, 82%; (c) CF3SO3H, PrOH, reflux, 74%; (d) LiBH4, EtOH, rt, 74%;
(e) NEt3, SOCl2, 0 °C, CH2Cl2; (f) NaIO4, RuCl3, CH3CN, H2O, rt, 52% (2 steps); (g) 1—NaH, CH2(CO2Et)2, DMF, 50 °C, 2—HCl, 1,4-dioxane, EtOH, rt, 71%; (h) LiBH4, EtOH, rt, 81%.
Scheme 2. Synthesis of triol 3 (Route B). Reagents and conditions: (a) Ref. 11; (b) Na(MeOCH2CH2O)2AlH2, THF, À50 °C, 85%; (c) 1—i-Pr2NEt, (CF3SO2)2O, toluene, rt, 2—NaH,
CH2(CO2Et)2, DMF, rt, 83%; (d) LiBH4, EtOH, rt, 86%; (e) NaH, triazole, DMF, 80 °C, 73%.

6540
T. Uchida et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6538–6541
Scheme 3. Synthesis of triol 3 (Route C). Reagents and conditions: (a) H2SO4, AcOH, reflux; (b) (COCl)2, DMF, CH2Cl2, 0 °C; (c) 1,3-difluorobenzene, AlCl3, rt, 55% (3 steps); (d)
H2SO4, MeOH, H2O, rt, 90%; (e) i-PrOSiMe2CH2MgCl, THF, rt, (23:24 = >20:1); (f) NaHCO3, H2O2, THF, MeOH, 60 °C, 78% (2 steps); (g) 1—i-Pr2NEt, (CF3SO2)2O, toluene, À20 °C,
2—NaHMDS, THF, rt, 3—NaH, CH2(CO2Et)2, DMF, rt, 43%; (h) 2 steps as in Scheme 2, 63%.
Minimum inhibitory concentrations (MICs) of the test com-
pounds were determined against Candida, Cryptococcus, and Asper-
gillus species by the broth microdilution method in accordance
with the guidelines in the National Committee for Clinical Labora-
tory Standards (NCCLS) documents.15 The activities of 1a–d were
compared with those of our former sulfur derivatives 26a–d,4
fluconazole and itraconazole (Table 1).
When the compounds with the same lengths of olefinic chains
were compared, most of the MICs of the carbon analogs 1a–d were
Figure 3. Possible mechanisms of the Grignard reaction of 22.
about two to eight times better than those of the corresponding
sulfur compounds 26a–d. Among compounds 1a–d, compound
1c, which has a side chain with two olefinic double bonds, showed
the best MICs. The difference was most clear in the activity against
dioxane isomers 1a–d were predominantly produced and were
easily separated from the cis isomers by silica gel column
chromatography.14
Table 1
Minimum inhibitory concentrations (MICs) of compounds 1a–d, 26a–d, fluconazole (FLCZ), and itraconazole (ITCZ)
Straina
MIC (
l
g/mL)b
Compoundc
n
1a
0
1b
1
1c
2
1d
3
26a
0
26b
1
26c
2
26d
3
FLCZ
ITCZ
Z
CH2
CH2
CH2
CH2
S
S
S
S
C. albicans ATCC24433
C. albicans SANK51486
C. albicans TIMM3164
C. albicans ATCC64550
C. parapsilosis ATCC90018
C. glabrata ATCC90030
C. krusei ATCC6258
C. tropicalis ATCC750
C. neoformans TIMM1855
A. fumigatus ATCC26430
A. fumigatus SANK10569
A. flavus SANK18497
0.016
60.008
0.016
0.5
0.016
1
0.125
0.125
0.031
0.25
0.25
1
0.031
60.008
0.016
1
0.031
1
0.063
0.125
0.016
0.25
0.25
0.5
0.016
60.008
0.016
0.5
0.031
1
0.063
0.125
0.016
0.125
0.125
0.5
0.125
0.031
0.125
0.5
0.125
2
0.25
0.25
0.125
0.5
0.016
60.008
0.063
1
0.031
1
0.25
0.5
0.031
1
0.063
0.016
0.125
2
0.031
1
0.25
0.031
0.016
0.25
0.25
1
0.125
0.016
0.125
1
0.063
1
0.25
0.125
0.016
0.25
0.25
0.5
0.25
0.031
1
>4
0.25
>4
1
2
0.5
2
0.5
0.25
>4
>4
0.5
>4
>4
2
>4
>4
>4
>4
0.125
0.031
0.25
1
0.125
1
0.5
0.5
0.25
0.25
0.25
0.5
0.5
1
1
2
2
4
a
C. albicans, Candida albicans; C. parapsilosis, Candida parapsilosis; C. glabrata, Candida glabrata; C. krusei, Candida krusei; C. tropicalis, Candida tropicalis; C. neoformans,
Cryptococcus neoformans; A. fumigatus, Aspergillus fumigatus; A. flavus, Aspergillus flavus.
b
MICs were determined at 35 °C (30 °C for Aspergillus spp.) in RPMI1640 medium (yeast nitrogen base for C. neoformans) at pH 7.0. MICs were defined as the minimum
concentrations of the test compounds that inhibit the growth of the fungi by 80%.
c
For compounds 1a–d, 26c, and 26d, their oxalic acid salts were subjected to the test.

T. Uchida et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6538–6541
6541
6. Konosu, T.; Tajima, Y.; Takeda, N.; Miyaoka, T.; Kasahara, M.; Yasuda, H.; Oida,
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12. Tamao, K.; Ishida, N. Tetrahedron Lett. 1984, 25, 4245.
13. Of the two proposed mechanisms, the one depicted by Y seems more plausible,
since, although ester and ether derivatives would also be able to assume a
chelated transition state like X, they demonstrated almost no
diastereoselectivity. Furthermore, treatment of similar ketone (RS)-
PhC(@O)CHMeCH2OH with MeMgBr, wherein the mechanism depicted by Y
cannot occur, is known to give lower (70% de) diastereoselectivity. See Bartoli,
G.; Bellucci, M. C.; Bosco, M.; Marcantoni, E.; Sambri, L. Chem. Eur. J. 1998, 2,
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14. The stereochemistry of the 1,3-dioxane ring was elucidated by the coupling
constants in the 1H NMR spectra. The trans isomers showed characteristic
signals of the axial methylene protons on the C4 and C6 positions in the 1,3-
dioxane ring with large coupling constants (triplet, J = ca. 11 Hz). In contrast,
the corresponding signals of cis isomers appeared as multiplets.
15. National Committee for Clinical Laboratory Standards. 1997. M27-A; 1995.
M27-T; 1998. M38-P. National Committee for Clinical Laboratory Standards,
Wayne, PA. See Supplementary data.
A. fumigatus. This tendency was similar to that of the sulfur com-
pounds 26a–d. The activity of the strongest compound, 1c, was
slightly better than of its sulfur analog 26c with an olefinic side
chain of the same length and surpassed that of fluconazole and
itraconazole.
In conclusion, we have presented novel carbon analogs of tria-
zole compounds, which could be synthesized stereoselectively.
Further investigation of these compounds as antifungal agents
for systemic use is currently underway.16
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2008.10.055.
References and notes
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16. In
a preliminary in vivo study, all the infected mice (ddY strain, n = 10,
systemically infected using intravenous invection of C. albicans 427,
5.5 Â 106 CFU/mouse) survived after 21 days after po or ip administration of
1c (given 1, 4, 24 h after infection, 20 mg/kg/administration), whereas similar
treatment with fluconazole (po) resulted in
a survival rate of 60%. In a
preliminary toxicity study, all the mice (ddY strain, n = 5) survived after oral
dosing of 500 mg/kg of 1c once daily for 4 days.