Stereoselective synthesis of two potential metabolites of cis-metconazole

Article abstract of DOI:10.1016/j.tetlet.2017.09.087

The stereoselective synthesis of compounds 5 and 6, potential hydroxylated metabolites of the agricultural fungicide cis-metconazole, is reported. In a key step of the initially surveyed synthetic route, hydrodechlorination of 12 was competitive with hydr

Full text of DOI:10.1016/j.tetlet.2017.09.087

Accepted Manuscript
Stereoselective Synthesis of Two Potential Metabolites of Cis-Metconazole
Marcin Budny, Joanna Włodarczyk, Tadeusz Muzioł, Mariusz Jan Bosiak,
Andrzej Wolan
PII:
S0040-4039(17)31255-8
https://doi.org/10.1016/j.tetlet.2017.09.087
TETL 49357
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
24 July 2017
15 September 2017
29 September 2017
Please cite this article as: Budny, M., Włodarczyk, J., Muzioł, T., Bosiak, M.J., Wolan, A., Stereoselective Synthesis
of Two Potential Metabolites of Cis-Metconazole, Tetrahedron Letters (2017), doi: https://doi.org/10.1016/j.tetlet.
2017.09.087
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Stereoselective Synthesis of Two Potential Metabolites of Cis-
Metconazole
Marcin Budny,1* Joanna Włodarczyk,1 Tadeusz Muzioł,2 Mariusz Jan Bosiak,1,2 Andrzej Wolan,1,2
1 Synthex Technologies Sp. z o.o., Gagarina 7/134B, 87-100 Toruń; 2 Nicolaus Copernicus University,
Faculty of Chemistry, Gagarina 7, 87-100 Toruń
*Corresponding author. Tel.: +48 56 646 19 63; fax: +48 56 654 24 77; e-mail: budny@synthex.com.pl
Orcid id: orcid.org/0000-0003-1300-7175
Abstract
The stereoselective synthesis of compounds 5 and 6, potential hydroxylated metabolites of the
agricultural fungicide cis-metconazole, is reported. In a key step of the initially surveyed synthetic route,
hydrodechlorination of 12 was competitive with hydrogenation of the trisubstituted olefin. Application
of a Miyaura borylation/hydrogenation/boron-to-halogen exchange reaction sequence solved the
chemoselectivity issue.
Graphical abstract
Keywords: metconazole, metabolites, 1,2,4-triazole, Miyaura borylation
Introduction

Metconazole (cis-1 and trans-1, Fig. 1A), belonging to the 1,2,4-triazole-containing fungicide family,
is extensively used in agriculture to protect cereals, vegetables, and fruits against fungal infections.1-6 In
fungi, 1 inhibits lanosterol 14α-demethylase (CYP51A1), a cytochrome P450 enzyme engaged in the
biosynthesis of ergosterol, a constituent of cell membranes which is essential for proper fungal growth.7
Numerous 1,2,4-triazoles have found applications as drugs and agrochemicals, e.g. tebuconazole (2),
fluconazole (3), and voriconazole (4) (Fig. 1B).
Figure 1. (A) Cis- and trans-metconazole; (B) Selected important triazole fungicides; (C) Potential
metabolites of cis-metconazole.
Cytochrome P450 enzymes (CYPs) also play important roles in humans, including the synthesis of
steroids and participation in the metabolism of xenobiotics. Similarly to other triazoles, 1 may inhibit
CYPs, resulting in adverse effects such as hormonal and metabolic disorders.8-11 Moreover, long-term
exposure to fungicides may be connected with the development of drug resistance for pathogenic
fungi.12-15 Concerns regarding the impact of widespread agricultural applications of 1 on human health

have arisen, and in order to address these questions, the biological activity of 1 as well as its metabolites
must be studied more closely.16
Although, the synthesis of metabolites and metabolite-like derivatives of biologically active
compounds have attracted significant attention,17-21 the preparation of metconazole metabolites have
not been reported; only the in vitro monooxygenation of 1 catalyzed by human CYP3A4, providing two
hydroxylated products with undetermined structures, has been studied.22 Among the twenty C-H bonds
in 1, one tertiary and two benzylic bonds are the most electronically prone toward monooxygenation,
although other bonds may also be reactive with specific CYPs since electronic factors are not the only
determinants of enzymatic oxidation selectivity.23
Herein, we disclose a concise synthesis of a pair of diastereomeric hydroxymetconazoles 5 and 6 (Fig.
1C), representing potential metabolites of cis-1, which is the more abundant metconazole isomer in the
manufactured product.
Results and Discussion
Our general strategy to access 5 and 6 is depicted in Scheme 1. We proposed that both compounds
could be derived from commercially available 7 and the remaining structural motifs could be introduced
using well-established chemistry. We also envisioned the relative stereochemistry between the
cyclopentane ring substituents could be controlled by a bulky protecting group on C(3)-OH.
Scheme 1. Retrosynthetic analysis of compounds 5 and 6.

First, precursor 7 was alkylated with MeI to afford 8 in 36% yield.24 Then, a single carbonyl group in 8
was selectively reduced using a substoichiometric amount of NaBH4, affording alcohol 9 in 94% yield.25
Cross-aldol condensation of alcohol 9 and 4-chlorobenzaldehyde followed by silylation of the hydroxyl
group, furnished 11 in good overall yield. The Corey-Chaykovsky epoxidation of 11, performed under
Danishefsky conditions,26 gave the corresponding epoxide which was directly subjected to the next step
due to its instability. Treatment of the epoxide with 1,2,4-triazole/K2CO3 in DMF at 85 °C afforded 12 in
50% yield as a single diastereomer; the relative stereochemistry was confirmed by X-ray crystallography.
The stereochemical outcome of the epoxidation reaction can be explained by shielding the top face of
the cyclopentane ring by the TBS-protecting group, which forces attack of the sulfonium ylide on the C=O
bond from the opposite side (see ESI, Fig. S1 for details).
Scheme 2. Synthesis and attempted hydrogenation of key intermediate 12. Reagents and conditions:
i. a) NaOH (1 eq), H2O, rt, 15 min; b) MeI (1.5 eq), DMF, rt, overnight; c) HCl(aq), 1 h, reflux, 36% (3 steps)
ii. NaBH4 (0.27 eq), MeOH/H2O (4:1), 0 °C -> rt, 30 min, 94%; iii. 4-chlorobenzaldehyde (1 eq), NaOH (1
eq), MeOH/H2O (4:1), rt, 1 h, 89%; iv. TBSCl (1.5 eq), imidazole (3.0 eq), DMF, rt, overnight, 91%; v.
trimethylsulfonium iodide (1.2 eq), KHMDS (1.1 eq), THF, 0 °C, then 11, 0 °C, 1.5 h; vi. 1,2,4-triazole (1.5
eq), K2CO3, (1.5 eq), DMF, 60 °C, overnight, 50% (2 steps); vii. 10% Pd/C (16% w/w), HCO2NH4 (10 eq),
EtOH, reflux, 1 h, 75%, dr = 88:12.

Hydrogenation of the trisubstituted alkene bond in 12 was challenging. Under typical conditions
(Pd/C, 1 atm H2, EtOH or EtOAc, rt; Pd/C, 1 atm H2, EtOAc, 65 °C; Pd/C, 100 atm H2, EtOH) compound 12
reacted sluggishly with low conversions, leading to dechlorinated 13 as the main product. Attempts to
modify the catalyst activity by using Ph2S27,28 as a catalyst poison failed. With low Ph2S loadings, no
changes in the chemoselectivity were observed; however, higher loadings stopped the reaction
completely. Remarkably, in a weakly basic environment with ammonium formate as the hydrogen
source, 12 was reduced smoothly, providing 13 in 75% yield as a mixture of diastereomers. Cationic
reduction of 12 was also attempted, however, this approach led to partial deprotection of the TBS-
protecting group (Et3SiH/TFA) or to complete decomposition of the starting material (Et3SiH, TfOH).
To avoid chloroarene hydrodechlorination, we decided to exchange chlorine with the
pinacolboronate group, which is considered to be stable in Pd-catalyzed hydrogenations.29,30 We
envisioned chlorine could be easily reintroduced after olefin reduction via a copper-mediated process.31
Thus, Suzuki-Miyaura borylation of 12 furnished pinacolboronate 14 on a multigram scale in 79% yield.32
Hydrogenation of 14 proceeded smoothly (Pd/C, EtOAc, 65 °C) without significant formation of the
protodeboration product, affording 15a/b in 93% yield with good diastereoselectivity (dr = 83:17) (see
ESI, Fig. S2 for conformational analysis). Treatment of 15a/b with excess of CuCl2 in iPrOH/H2O led to
hydroxymetconazoles 5 and 16 in 79% yield (61% combined yield after separation by fractional
crystallization). The relative stereochemistry of diastereomer 5 was confirmed by X-ray crystallography
(Scheme 3).

Scheme 3. Synthesis of hydroxymetconazole 5. Reagents and conditions: i. pin2B2 (1.05 eq), Pd2(dba)3
(2.5 mol%), XPhos (10 mol%), KOAc (3 eq), 1,4-dioxane, 110 °C, overnight, 79%; ii. 10% Pd/C (30% w/w),
H2 (balloon), EtOAc, 65 °C, 6 h, 93%, dr = 83:17; iii. CuCl2×H2O (4.0 eq), iPrOH/H2O (3:1), reflux, 48 h, 79%;
55% for 5 and 6% for 16 after fractional crystallization.
Finally, we applied a two-step sequence to convert 5 into epimeric 6. In the first step, Swern
oxidation of 5 furnished ketone 17 in 84% yield, which was stereoselectively reduced with NaBH4 to
provide 6 in 74% yield (Scheme 4).

Scheme 4. Synthesis of hydroxymetconazole 6. Reagents and conditions: i. (COCl)2 (5 eq), DMSO (5 eq),
DCM, -78 °C, 30 min, then 5, -78 °C, 90 min, then Et3N (50 eq), rt, 30 min, 84%; ii. NaBH4 (4 eq),
MeOH/H2O (15:1), 30 min, rt, 74%.
Conclusions
In summary, we have developed a stereoselective, multigram synthesis of hydroxymetconazole 5
and proposed a two-step sequence for the conversion of 5 to 6. These compounds may find applications
as chromatographic standards in the detection of metconazole metabolites. During the synthesis, we
tackled the undesired chloroarene hydrodechlorination which was competitive with alkene bond
hydrogenation. We proposed a three-step route involving Miyaura borylation/hydrogenation/boron-to-
chlorine exchange reactions as a solution for this issue. Further applications of this method for the
synthesis of other metconazole metabolites are ongoing in our laboratory.
Supplementary data
1
13
Experimental procedures, copies of H, C, 2D NMR spectra and detailed crystallographic data
for compounds 6 and 13. CIF files containing crystallographic data for compounds 6 and 13.
Author information
Corresponding author
*E-mail: budny@synthex.com.pl
ORCID
Marcin Budny 0000-0003-1300-7175
Notes
Authors declare no competing financial interest.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-
for-profit sectors.

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Highlights:
•
•
Synthesis of two possible metabolites of cis-metconazole
Chloroarene was temporary converted into the corresponding pinacolboronate to prevent
hydrodechlorination
•
The developed method was applied to the multigram synthesis of 5