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\DOI{10.5802/crchim.278}
\datereceived{2023-11-24}
\daterevised{2023-12-06}
\dateaccepted{2023-12-06}
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\dateposted{2024-02-09}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{French Women in Chemistry in 2024}
%%\def\cdr@specialtitle@french{Les chimistes fran\c{c}aises en 2024}
%\makeatother

\title{Direct ring fluorination of 3-substituted 5-(1,3-dioxane) acetal
isoxazoles: application to the formal synthesis of a bioactive
fluorinated isoxazole}

\alttitle{Fluoration directe d'isoxazoles 5-(1,3-dioxane) ac\'{e}tal
3-substitu\'{e}s : application \`{a} la synth\`{e}se formelle d'un
isoxazole fluor\'{e} d'int\'{e}r\^{e}t biologique} 

\author{\firstname{Lucas} \lastname{Bacheley}}
\address{PSL University, Chimie ParisTech, CNRS UMR 8060, Institute of Chemistry for Life and Health Sciences, CSB2D Team, 75005 Paris, France}
\address{SEQENS, 2-8 rue de Rouen, ZI de Limay-Porcheville, 78440 Porcheville, France}

\author{\firstname{G\'{e}rard} \lastname{Guillamot}}
\addressSameAs{2}{SEQENS, 2-8 rue de Rouen, ZI de Limay-Porcheville, 78440 Porcheville, France}

\author{\firstname{Phannarath} \lastname{Phansavath}\IsCorresp}
\addressSameAs{1}{PSL University, Chimie ParisTech, CNRS UMR 8060, Institute of Chemistry for Life and Health Sciences, CSB2D Team, 75005 Paris, France}
\email[P. Phansavath]{phannarath.phansavath@chimieparistech.psl.eu}

\author{\firstname{Virginie} \lastname{Ratovelomanana-Vidal}\CDRorcid{0000-0003-1167-1195}\IsCorresp}
\addressSameAs{1}{PSL University, Chimie ParisTech, CNRS UMR 8060, Institute of Chemistry for Life and Health Sciences, CSB2D Team, 75005 Paris, France}
\email[V. Ratovelomanana-Vidal]{virginie.vidal@chimieparistech.psl.eu}

\thanks{This work was supported by the Minist\`{e}re de l'Enseignement
Sup\'{e}rieur et de la Recherche (MESR) and the Centre National de la
Recherche Scientifique (CNRS). LB is grateful to SEQENS for a grant
(2021--2024).}

\shortrunauthors

\keywords{\kwd{Fluorination} 
\kwd{Isoxazoles} 
\kwd{Heterocycles}}


\altkeywords{\kwd{Fluoration} 
\kwd{Isoxazoles} 
\kwd{H\'{e}t\'{e}rocycles}}


\begin{abstract}
The access to a novel variety of 4-fluorinated isoxazoles bearing an
aryl or alkyl group at the C-3 position and an acetal fragment at the
C-5 position has been achieved through a direct fluorination using
N-fluorobenzenesulfonimide (NFSI) with yields up to 75\%. This method
was transposed to gram-scale and has been applied to the formal
synthesis of a bioactive compound that increases CFTR (Cystic Fibrosis
Transmembrane Conductance Regulator) activity.
\end{abstract}

\begin{altabstract}
L'acc\`{e}s \`{a} une nouvelle vari\'{e}t\'{e} d'isoxazoles
4-fluor\'{e}s portant un groupe aryle ou alkyle en position C-3 et un
fragment ac\'{e}tal en position C-5 a \'{e}t\'{e} r\'{e}alis\'{e} par
une fluoration directe \`{a} l'aide de N-fluorobenzenesulfonimide
(NFSI), avec des rendements allant jusqu'\`{a} 75 \%. Cette m\'{e}thode
a \'{e}t\'{e} transpos\'{e}e \`{a} l'\'{e}chelle du gramme et a
\'{e}t\'{e} appliqu\'{e}e \`{a} la synth\`{e}se formelle d'un
compos\'{e} bioactif qui augmente l'activit\'{e} CFTR (Cystic Fibrosis
Transmembrane Conductance Regulator).
\end{altabstract}

\maketitle

\vspace*{6pt}

\twocolumngrid

\end{noXML}


\section{Introduction}

Heterocyclic scaffolds represent the largest family of organic
compounds and have a role in many fields of sciences such as medicinal
chemistry~\cite{1a,1b}, biochemistry~\cite{2} or agrochemistry~\cite{3a,3b}.
More specifically, the isoxazole skeleton is a very important motif
found in natural products or synthetic products such as
antibiotics~\cite{4}, analgesics~\cite{5}, antirheumatic~\cite{6},
anti-inflammatory~\cite{7} or antidepressant compounds~\cite{8}.

Furthermore, the interest of fluorine and its methods of introduction
are still gaining increasing interest as fluorination of an organic
molecule significantly impacts its biological characteristics. In spite
of the significant fascination for fluorine, the challenge of
integrating this element into essential building blocks has postponed
the examination and comprehension of its impacts as well as its
utilization. Nonetheless, notable advancement has been achieved in the
field over the previous decade. Considering the size of the fluorine
atom and its electronegativity, the incorporation of a fluorine or a
trifluoromethyl group profoundly modifies both the pharmacodynamics and
pharmacokinetics of the compounds such as lipid solubility, metabolic
stability or acidity~\cite{9}. Moreover, it can increase the potency of
a substance's engagement with a specific target protein. 


Roughly 30\% of all agrochemicals and 20\% of all pharmaceuticals
encompass fluorine~\cite{10} within their structures.

With the aim of continuing our investigations into the synthesis of
fluorinated carbocycles and heterocycles~\cite{11a,11b,11c,11d,11e}, we envisaged to
focus on a synthetic pathway for the fluorination of an isoxazole
functionalized at C-5 by an acetal fragment, an area that has been
relatively overlooked in the literature despite numerous molecules of
interest featuring this motif (Figure~\ref{fig1})~\cite{12a,12b,12c,12d,12e,12f}. The
synthesis of 4-fluorinated 3,5-disubstituted isoxazole was reported by
either condensation of 2-fluoro-1,3-diketone~\cite{13a,13b} or
gold-catalyzed cascade cyclization-fluorination of 2-alkynone
\textit{O}-methyl oximes~\cite{14} but only one method for the direct
fluorination in this position was reported relying on the use of
Selectfluor as the fluorinating agent and delivering the desired
products in moderate chemical yields ranging from 16\% to 44\%
(Figure~\ref{fig2})~\cite{15a,15b}. Furthermore, the products generated
during the reaction cannot undergo additional post-functionalization.
In this context, the direct fluorination at the C-4 position of the
3,5-disubstituted isoxazole core, as a late-stage functionalization,
was explored. With the substitution of an acetal group at the C-5
position, the resulting fluoroisoxazoles can undergo deprotection,
opening-up numerous possibilities for additional functionalization. 


\begin{figure}
\includegraphics{fig01}
\caption{Examples of bioactive molecules containing a 4-fluorinated
isoxazole core.\label{fig1}}
\vspace*{-5pt}
\end{figure}


\begin{figure}
\includegraphics{fig02}
\vspace*{-3pt}
\caption{Synthesis of a 4-fluorinated 3,5-disubstituted isoxazoles by
direct fluorination.\label{fig2}}
\vspace*{-2pt}
\end{figure}


\section{Results and discussion} 

A range of 3,5-disubstituted isoxazoles \textbf{1a-1k} were prepared
from diversely substituted chloroximes and bromovinyl acetals,
according to a three-step procedure developed previously~\cite{16}.
Having those substrates in hand, our study started with a series of
optimization experiments on 5-(1,3-dioxan-2-yl)-3-phenylisoxazole
\textbf{1a} (Table~\ref{tab1}). The first experiment required 3.5 equiv
of base and N-fluorobenzenesulfonimide (NFSI) to afford the fluorinated
isoxazole \textbf{2a} in 47\% yield for a conversion of 63\% at
${-}$78~\textdegree C (Table~\ref{tab1}, entry 1). Other commercially
available electrophilic fluorinated agent such as Selectfluor or
\textit{N}-fluoropyridinium triflate were tested but no conversion was
observed under these conditions. Reducing the quantity of base and
fluorinating agent to 1.2 and 2.0 equiv respectively led to better
conversions (72--74\%) and higher yields in \textbf{2a} (60--62\%)
(Table~\ref{tab1}, entries 2--3). Subsequently, the impact of the
starting material concentration in THF on the reaction outcome was
examined. The concentration was gradually increased step by step from
0.1~mol/L to 1.0~mol/L (Table~\ref{tab1}, entries 4--9), leading to an
optimized yield of 75\% at 0.4~mol/L (Table~\ref{tab1}, entry 6). In
fact, for a concentration higher than 0.6~mol/L, the reaction medium
reaches such a viscosity that stirring stops after addition of the
electophile. 



\begin{table*}
\caption{Optimisation of the reaction
conditions$^{\mathrm{a}}$\label{tab1}
\vspace*{4pt}\tpar{\protect\inlinefig{fx01}}\vspace*{-6pt}}
\begin{tabular}{ccccccccc}
\thead
Entry & Base & Solvent & $x$ (equiv) & $c$ (mol/L) & $T$ & $t$ (h) & Conversion$^{\mathrm{b}}$ (\%) & Yield$^{\mathrm{c}}$ (\%) \\
\endthead 
\01 & \textit{n}-BuLi & THF & 3.5$^{\mathrm{d}}$ & 0.05 & ${-}$78~\textdegree C & 20  & \063 & 47 \\
\02 & \textit{n}-BuLi & THF & 2.5$^{\mathrm{e}}$ & 0.05 & ${-}$78~\textdegree C & 20  & \072 & 62 \\
\03 & \textit{n}-BuLi & THF & 2.0\me & 0.05 & ${-}$78~\textdegree C & 2  & \074 & 60 \\
\04 & \textit{n}-BuLi & THF & 2.0\me & 0.1\0 & ${-}$78~\textdegree C & 2  & \078 & 61 \\
\05 & \textit{n}-BuLi & THF & 2.0\me & 0.2\0 & ${-}$78~\textdegree C & 2  & \082 & 73 \\
\06 & \textit{n}-BuLi & THF & 2.0\me & 0.4\0 & ${-}$78~\textdegree C & 1  & \085 & 75 \\
\07 & \textit{n}-BuLi & THF & 2.0\me & 0.6\0 & ${-}$78~\textdegree C & 0.5  & \089 & 71 \\
\08 & \textit{n}-BuLi & THF & 2.0\me & 0.8\0 & ${-}$78~\textdegree C & 0.5  & \091 & 70 \\
\09 & \textit{n}-BuLi & THF & 2.0\me & 1.0\0 & ${-}$78~\textdegree C & 0.5  & \090 & 70 \\
10 & \textit{n}-BuLi & THF & 2.0\me & 0.2\0 & ${-}$40~\textdegree C & 1  & 100 & 0$^{\mathrm{f}}$ \\
11 & \textit{n}-BuLi & Et$_{2}$O & 2.0\me & 0.2\0 & ${-}$78~\textdegree C & 3  & \020 & nd \\
12 & \textit{n}-BuLi & MeTHF & 2.0\me & 0.2\0 & ${-}$78~\textdegree C & 1  & \058 & nd \\
13 & LiHMDS & THF & 2.0\me & 0.4\0 & ${-}$78~\textdegree C & 0.5  & \0\00 & nd \\ 
14 & LiHMDS & THF & 2.0\me & 0.4\0 & \mn\00~\textdegree C & 0.5  & \0\00 & nd \\ 
15 & LiTMP  & THF & 2.0\me & 0.3\0 & ${-}$78~\textdegree C & 0.5  & 100 & 0$^{\mathrm{f}}$ \\ 
16 & LDA & THF & 2.0\me & 0.1\0 & ${-}$78~\textdegree C & 1.5  & \081 & 67 \\ 
17 & NaH & THF & 2.0\me & 0.1\0 & \mn\00~\textdegree C & 2  & \0\00 & nd  
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Reaction conditions: (\textbf{1a}, 0.43~mmol,
1.0 equiv), \textit{n}BuLi (0.52~mmol, 1.2 equiv), NFSI (0.86~mmol, 2.0
equiv), THF (0.4~M, 1.1~mL), ${-}$78~\textdegree C to rt, under argon.} 
\tabnote{$^{\mathrm{b}}$After NMR \tsup{1}H analysis of the crude reaction mixture.}
\tabnote{$^{\mathrm{c}}$After isolation by column chromatography.}
\tabnote{$^{\mathrm{d}}$3.5 equiv of base were used.}
\tabnote{$^{\mathrm{e}}$1.5 equiv of base were used.}
\tabnote{$^{\mathrm{f}}$Only degradation was observed.}
\vspace*{4pt}
\end{table*}



Carrying out the reaction at ${-}$40~\textdegree C to achieve a full
conversion led to complete degradation of the\unskip\break starting material
(Table~\ref{tab1}, entry 10). Other solvents were tested such as
Et$_{2}$O or 2-MeTHF, but lower conversions were observed (20\% and
58\% respectively, Table~\ref{tab1}, entries 11 and 12). The influence
of the base was also studied. The starting material was fully recovered
when treated with LiHMDS at ${-}$78~\textdegree C and 0~\textdegree C or
with NaH at 0~\textdegree C (Table~\ref{tab1}, entries 13, 14 and 17).
Unfortunately, LiTMP led to a complete degradation of the isoxazole
(Table~\ref{tab1}, entry 15). Using LDA gave an encouraging result of
81\% conversion and 67\% yield in \textbf{2a} but was still inferior
when \textit{n}-BuLi was used (Table~\ref{tab1}, entries 16 vs 6). 

Having these optimized conditions in hand, we further explored the
scope and limitations of the reaction with 3,5-disubstituted isoxazoles
previously synthesized. This process allowed us to obtain the
corresponding fluorinated isoxazoles bearing various electron-donating
or -withdrawing substituents in the \textit{para}-position of the aryl
ring such as \textit{t}-butyl- \textbf{2b}, chlorine \textbf{2c},
fluorine \textbf{2d} or trifluoromethyl \textbf{2e} groups in yields
between 59--70\%. Compounds \textbf{2g} and \textbf{2h} having a
3-chloro-4-fluorophenyl or a 2,6-dichlorophenyl substituent were
obtained in 44\% and 21\% yield, respectively (Figure~\ref{fig3}).
Having a methyl in \textit{meta}-position (\textbf{1f)} led to 75\%
isolated yield of the corresponding fluorinated product \textbf{2f}. To
extend the feasibility of our process, a bulky group such as a
naphthyl or a 2,2-difluorobenzodioxole was also investigated and
afforded the products \textbf{2i} and \textbf{2j} in 20\% and 60\% yields,
respectively. The reaction was also suitable for isoxazoles bearing a
non-aromatic substituent at the C-3 position and compound \textbf{2k},
bearing a \textit{n}-butyl chain, was synthesized in 25\% yield.
\looseness=-1


\begin{figure}
\includegraphics{fig03}
\caption{Scope of the fluorination.\label{fig3}}
\end{figure}

Furthermore, our goal was to obtain a range of fluorinated isoxazoles
that could be subsequently modified after deprotection of the acetal
fragment at the C-5 position. In this context, the acetal was
deprotected in acidic media (HCl 1N, 5 equiv), in a mixture of
water/acetone (1/1) for 4 days at 90~\textdegree C affording the
corresponding aldehyde \textbf{3a} in 65\% yield (Figure~\ref{fig4}).

\begin{figure}
\vspace*{3pt}
\includegraphics{fig04}
\vspace*{3pt}
\caption{Formal synthesis of a bioactive compound that increases CFTR
activity.\label{fig4}}
\vspace*{1pc}
\end{figure}

Isoxazole \textbf{3a} is an intermediate in the preparation of a
bioactive compound that increases Cystic Fibrosis Transmembrane
Conductance Regulator (CFTR) activity~\cite{11c}. Cystic fibrosis is a
result of genetic mutations in the CFTR gene, responsible for encoding
a chloride channel that spans multiple cell membranes in epithelial
tissues. The loss of a functional CFTR channel leads to a reduced lung
function and increases mucus viscosity resulting in chronic infection
and inflammation~\cite{17}. Besides respiratory impairment, CFTR also
affects the regular operation of additional organs such as the
pancreas, intestine, or gall bladder. 


The effectiveness of this fluorination process was supported by a
scale-up experiment conducted under the defined standard conditions
using 5-(1,3-dioxan-2-yl)-3-phenylisoxazole \textbf{1a}
(Figure~\ref{fig5}) to obtain the desired fluorinated product
\textbf{2a} in 61\%\unskip\break yield.


\begin{figure}
\includegraphics{fig05}
\vspace*{3pt}
\caption{Scale-up experiment of the fluorination of \textbf{1a}.\label{fig5}}
\vspace*{6pt}
\end{figure}


\section{Conclusion}

In conclusion, we succeeded in synthesizing a range of new
4-fluorinated 3,5-disubstituted isoxazoles bearing various substituents
on the aromatic ring at the C-3 position in 20--75\% yields. This process
was also tolerant to bulkier substituents such as a  naphthyl  or a
difluorodioxobenzene and even a non-aromatic substituent such as a
\textit{n}-butyl fragment. The objective of this project was also to
obtain a substrate that could be subsequently functionalized. This was
achieved through the deprotection of the acetal, resulting in a much
more advantageous aldehyde for subsequent post-functionalization. The
usefulness of this method was demonstrated through the formal synthesis
of a bioactive compound that increases CFTR activity.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgement}

We thank Dr. C. Fosse (Chimie ParisTech - PSL) for HRMS analysis.

\back{}

\section*{Supplementary data}
Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-278-suppl.pdf}}

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