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\DOI{10.5802/crchim.245} 
\datereceived{2023-07-05}
\dateaccepted{2023-07-18}
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\begin{document}

\begin{noXML}

\RubricEF{Research article}{Article de recherche}

\title{Indium-catalysed ring opening of 2-hydroxybutyrolactone through
the cleavage of C(sp$^3$)--O bond}

\alttitle{Ouverture de cycle catalys\'ee par l'indium de la
2-hydroxybutyrolactone par clivage de la liaison C(sp$^3$)--O}

\author{\firstname{Sameh} \lastname{Aoun}}
\address{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[S. Aoun]{sameh.aoun@gmail.com}
\thanks{This work was supported by Adisseo (SA post-doc grant, NS internship
grant), the R\'{e}gion PACA and Centrale Marseille (JM Ph.D. grant),
the CNRS and AMU}

\author{\firstname{Joe} \lastname{Massouh}}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[J. Massouh]{joe.massouh@centrale-marseille.fr}

\author{\firstname{No\'{e}mie} \lastname{Scornet}}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[N. Scornet]{noemie.scornet@gmail.com}

\author{\firstname{Laurent} \lastname{Giordano}}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[L. Giordano]{laurent.giordano@centrale-marseille.fr}

\author{\firstname{Alphonse} \lastname{Tenaglia}}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[A. Tenaglia]{tenaglia.alfonso@orange.fr}

\author{\firstname{G\'{e}rard} \lastname{Buono}\CDRorcid{0000-0001-8505-4005}}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[G. Buono]{gerard.buono@centrale-marseille.fr}

\author{\firstname{Patrick} \lastname{Rey}}
\address{Adisseo France SAS, 
Antony Parc 2, 10 Place du G\'{e}n\'{e}ral de Gaulle, 
92160 Antony, 
France}
\email[P. Rey]{Patrick.Rey@adisseo.com}

\author{\firstname{Virginie} \lastname{Belli\`{e}re-Baca}\IsCorresp}
\addressSameAs{2}{Adisseo France SAS, 
Antony Parc 2, 10 Place du G\'{e}n\'{e}ral de Gaulle, 
92160 Antony, 
France}
\email[V. Belli\`{e}re-Baca]{virginie.belliere-baca@adisseo.com}

\author{\firstname{Damien} \lastname{H\'{e}rault}\CDRorcid{0000-0001-7753-3276}\IsCorresp}
\addressSameAs{1}{Aix Marseille Univ, 
CNRS, Centrale Marseille, iSm2, Marseille, 
France}
\email[D. H\'{e}rault]{damien.herault@centrale-marseille.fr}

\shortrunauthors

\keywords{\kwd{Methionine}
\kwd{Lactone}
\kwd{Sulfur nucleophiles}
\kwd{Indium}
\kwd{Thiol}}

\altkeywords{\kwd{M\'ethionine}
\kwd{Lactone}
\kwd{Nucl\'eophiles soufr\'es}
\kwd{Indium}
\kwd{Thiol}}

\begin{abstract}
The 
ring opening by the cleavage of the C(sp$^3$)--O bond of 
2-hydroxybutyrolactone with (ethylthio)trimethylsilane was effectively
catalysed by  indium triiodide. Screening the reaction conditions
allowed optimizing the ring opening. The effect of the
nucleophile/catalyst ratio on the reaction efficiency has also been
demonstrated. After optimization, 2-hydroxy-4-(ethylthio)butyric acid
was isolated with an excellent yield (85\%), thus making this method
useful for the preparation of methionine analogues.
\end{abstract}

\begin{altabstract}
L'ouverture de cycle par le clivage de la liaison C(sp$^3$)--O de la
2-hydroxybutyrolactone avec le (\'ethylthio)trim\'ethylsilane a \'et\'e
efficacement catalys\'ee par le triiodure d'indium. Le criblage des
conditions de r\'eaction a permis d'optimiser l'ouverture du cycle.
L'effet du rapport nucl\'eophile/catalyseur sur l'efficacit\'e de la
r\'eaction a \'egalement \'et\'e d\'emontr\'e. Apr\`es optimisation,
l'acide 2-hydroxy-4-(\'ethylthio)butyrique a \'et\'e isol\'e avec un
excellent rendement (85~\%), rendant ainsi cette m\'ethode utile pour la
pr\'eparation d'analogues de la m\'ethionine.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}
Considerable developments are underway to develop alternative
sustainable and competitive pathways for methionine and hydroxy
analogue production. Indeed, methionine is the most essential amino
acid in human and animal organisms. It plays an important role in
cellular survival and protein 
\mbox{biosynthesis}. Added to food supplements,
it improves the quantity and quality of products (meat, milk) from
herbivorous  animals~\cite{1a,1b,1c,1d,1e}. Methionine can be obtained
synthetically by cyclisation of homoserine followed by an opening of
the lactone using mercaptan  salts~\cite{2}. Methionine may be given as
a racemic compound as well as one of its analogue, the
2-hydroxy-4-(methylthio)butyric acid (HMB). Indeed the latter and
2-keto-4-(methylthio)butyric acid (KMB)
\mbox{(Figure~\ref{fig1})} are
biotransformed in the animal organisms to form methionine \textit{in 
vivo}~\cite{3a,3b,3c,3d}. The synthesis of these derivatives is
therefore an alternative to the preparation of methionine at industrial
level.

\begin{figure}
\includegraphics{fig01}
\vspace*{-5pt}
\caption{\label{fig1}Structures of methionine, HMB and KMB
derivatives.}
\end{figure}

KMB is synthetized by a 1,4 Michael addition on an
$\rmalpha$,$\rmbeta$-unsaturated  ketone~\cite{1a,1b,1c,1d,1e,4} while
the opening of 2-hydroxybutyrolactone (\textbf{1a}) using mercaptan
salts is the classic pathway to  HMB~\cite{5}, which are energy
consuming and generates a significant amount of solid waste (notably
sodium salts). To overcome this problem, the replacement of mercaptan
salts by soft nucleophiles in the presence of Lewis acids is promising.
In this context, several methods of ($\rmalpha$-substituted) lactones
ring opening have been carried out, in particular with the use of a
aluminium halides as Lewis  acids~\cite{6a,6b}, but they still have a
limitation in the application scope in terms of the nature of the 
substitution~\cite{6a,6b,6c,6d,6e}. The ring-opening reaction of
2-hydroxy-substituted lactones such as \textbf{1a} remains relatively
underdeveloped despite its great usefulness to lead to HMB with minimum
waste. To our knowledge, scarce reports of \textbf{1a} opening
reactions have appeared where the reaction occurs at the carbon atom
C(sp$^3$)--O with various strong or soft nucleophiles like 
nitrogen~\cite{6c}, selenium~\cite{7a,7b,7g},  sulphur~\cite{1a,6d,7e},
or iodide atoms~\cite{7c,7d}.

In this work, we developed a ring-opening methodology of
2-hydroxybutyrolactone (\textbf{1a}) catalysed by a Lewis acid, with
soft sulphur nucleophile to obtain 2-hydroxy-4-(ethylthio)butyric acid
(\textbf{2a}), an HMB analogue (Scheme~\ref{sc1}).

\begin{scheme}
\includegraphics{sc01}
\caption{\label{sc1}Ring opening of 1a catalysed by Lewis acid.}
%\vspace*{-5pt}
\end{scheme}

Here we describe a general route to the desired product, in which a
direct coupling between \textbf{1a} and (ethylthio)trimethylsilane was
effectively catalysed by  InI$_3$ in the presence of K$_{2}$CO$_{3}$
under drastic conditions  (InI$_3$ is stored and weighted in glove-box,
Schlenck techniques are used, see Chapter 4: experimental section). Our
method was inspired by the methodology initially developed by Baba and 
co-workers~\cite{7e}  (Scheme~\ref{sc2}). They first studied the
formation of thioether starting from ester. Among several Lewis acid
catalyst, the best result was achieved with  InI$_{3}$ (77\%), which
showed a better ability to promote the substitution than InBr$_{3}$ or
InCl$_{3}$. Then, they applied their best catalytic conditions to the
ring-opening reaction of $\rmgamma$-butyrolactone (\textbf{1b}), which
gave the corresponding thioethercarboxylic acid in 73\% yield, using 
InI$_3$ as a catalyst and (phenylthiol)trimethylsilane as a
nucleophile. Better yields can be obtained with larger lactones which
highlights the relative difficulty to open butyrolactones by the
cleavage of the C(sp$^3$)--O bond.

\begin{scheme*}
\includegraphics{sc02}
\caption{\label{sc2}Formation of thioether from ester and 
$\rmgamma$-butyrolactone (\textbf{1b}) catalysed by  InI$_3$ developed
by Baba and coworkers~\cite{7e}.}
\vspace*{4.5pt}
\end{scheme*}

\begin{table*}
\caption{\label{tab1}InI$_3$-catalysed ring opening of 1a and 
1b\vspace*{4pt}\tpar{\protect\inlinefig{fx01}}\vspace*{-6pt}}
\begin{tabular}{lcccccc}
\thead
\xmorerows{1}{Entry} & \xmorerows{1}{Substrate} & 
\xmorerows{1}{Nucleophile} & \xmorerows{1}{$t$ (h)} & 
\multicolumn{3}{c}{Isolated yields (\%)}\\
\cline{5-7}
 & & & & \textbf{2} & \textbf{3} & \textbf{4} \\
\endthead
1 & \textbf{1b} & EtSH & 20 & \textbf{2b}: 15$^{\mathrm{a}}$ & 0 & 0\\ 
2 & \textbf{1b} & EtSSiMe$_{3}$ & \03 & \textbf{2b}: 99 & 0 & 0\\ 
3 & \textbf{1a} & EtSH$^{\mathrm{b}}$ & 48 & 0 & 0 & 0\\ 
4 & \textbf{1a} & EtSSiMe$_3$ & 22 & \textbf{2a}: 30 & \textbf{3a}: 7 & 0\\ 
5 & \textbf{1a} & EtSSiMe$_3${}$^{\mathrm{c}}$ & 22 & \textbf{2a}: 42 & 
0 & \textbf{4a}: 15
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$NMR yields. 
$^{\mathrm{b}}$Reaction performed with 3.8 equiv. of EtSH and 20\% of  InI$_3$.
$^{\mathrm{c}}$Reaction in the presence of 1.1 equiv. of  K$_2$CO$_3$.}
\end{table*}

\section{Results and discussion}\label{sec2}
We first began our investigations by reproducing the ring-opening
reaction of \textbf{1b}, but by replacing (phenylthiol)trimethylsilane
with ethanethiol or (ethylthio)trimethylsilane  (Table~\ref{tab1},
entries 1 and 2).\footnote{In the classical synthesis of methionine
analogues, methanethiol is used. But for practical reasons (odor and
handling), we used EtSH and more convenient TMSSEt in this study.}
Ethanethiol was stirred in toluene with \textbf{1b} and a catalytic
amount of  InI$_3$  at 90~\textdegree C to obtain \textbf{2b} in only
15\% yield after 20~h of stirring. However, the use of
(ethylthio)trimethylsilane as a nucleophile under the same conditions
led to a full conversion after only 3~h of stirring (Entry 2). When
\textbf{1a} was used, no ring opening was observed with EtSH even after
increasing the amounts of both nucleophile and catalyst, as well as the
reaction time (Entry 3). So, we leaned towards the use of EtSSiMe$_{3}$
because it gave an excellent result with \textbf{1b} (Entry 2) and its
boiling point is higher than the reaction temperature. To our delight,
the ring opening  of \textbf{1a} carried out with this soft nucleophile
led to the desired product \textbf{2a} in 30\% yield along with
\textbf{3a} in 7\% yield (ring opening and thioesterification product,
entry 4). At this point, we concluded that our system could be
different from Baba's one due to the presence of the hydroxy group at
the $\rmalpha$-position of the lactone. Indeed, the addition of a
stoichiometric amount of  K$_{2}$CO$_{3}$ to the reaction mixture
allowed to increase the yield of \textbf{2a}, although the formation of
by-product \textbf{4a}, resulting from the nucleophilic attack on the
C-1 position (ester), was also observed (Entry~5).

EtSH has a boiling point about  35~\textdegree C, which is below the
reaction temperature. Its volatility can lead to a loss of reagent
while heating at  90~\textdegree C. To this end, reactions with EtSH
were performed  in an autoclave system (closed Schlenk tube of 5~mL)
containing only 2/5 of dead volume instead of a conventional reflux
system. No real improvement was observed with this nucleophile. But
when reactions were carried out with EtSSiMe$_{3}$ (boiling point about
134~\textdegree C, above the reaction temperature) using the same
conditions, the yields have been improved  (Table~\ref{tab2}).

\begin{table*}
\caption{\label{tab2}InI$_3$-catalysed ring opening of 1a in an
autoclave 
system\vspace*{4pt}\tpar{\protect\inlinefig{fx02}}\vspace*{-6pt}}
\begin{tabular}{lcccccc} 
\thead
\xmorerows{1}{Entry} & \xmorerows{1}{Nucleophile} 
& \xmorerows{1}{InI$_3$ (\%)} & \xmorerows{1}{K$_2$CO$_3$ (equiv.)} & 
\multicolumn{3}{c}{Isolated yields (\%)}\\
\cline{5-7}
 & & & & \textbf{2a} & \textbf{3a} & \textbf{4a}\\
\endthead 
\01 & EtSSiMe$_3$ & - & - & 0 & 0 & 0 \\ 
\02 & EtSSiMe$_3$ & - & 1.1 & 0 & 0 & 0 \\ 
\03 & EtSH & - & 1.1 & Traces & 0 & 0 \\ 
\04 & EtSSiMe$_3$ & 10 & - & 35 & 4 & Traces \\ 
\05 & EtSSiMe$_3$ & 10 & 1.1$^{\mathrm{a}}$ & 66 & 0 & 9 \\ 
\06 & EtSH & 10 & - & 0 & 0 & 0\\ 
\07 & EtSH & 10 & 1.1$^{\mathrm{a}}$ & 18 & 0 & 0\\ 
\08 & EtSH & 10 & 1.1$^{\mathrm{b}}$ & 26 & 0 & 0\\ 
\09 & EtSSiMe$_3$ & 10 & 1.1$^{\mathrm{b}}$ & 85 & 0 & 11\\ 
10 & EtSSiMe$_3$ & 10 & 1.1$^{\mathrm{c}}$ & 16 & 0 & 9
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Without preliminary drying of  \textbf{1a} and 
K$_2$CO$_3$. 
$^{\mathrm{b}}$With preliminary drying of \textbf{1a} and  K$_{2}$CO$_{3}$.
$^{\mathrm{c}}$With addition of 0.25~mL of water in the reaction mixture.}
\end{table*}

First, a control experiment in the absence of the catalyst did not lead
to the formation of products \textbf{2}--\textbf{4a} (Entries 1--3). On
the other hand, with 10\% of InI$_{3}$ and in the absence of
K$_{2}$CO$_{3}$, there was no significant efficiency compared to a
conventional reflux system (Entry 4, compared to entry 4 in 
Table~\ref{tab1}). When the base was present (Entry 5), the yield of
\textbf{2a} was increased significantly (66\%). As expected, EtSH
instead of EtSSiMe$_{3}$ was less efficient in producing \textbf{2a}
(18\% yield) in the presence of K$_{2}$CO$_{3}$ (Entry 7) while it did
not generate the desired product without a base (Entry~6). In addition,
the presence of a residual amount of water in \textbf{1a} or
K$_{2}$CO$_{3}$ is suspected to decrease the reactivity of the
nucleophile reagent, and consequently, of leading to lower yields.
Drying \textbf{1a} and K$_{2}$CO$_{3}$ before their use confirmed this
hypothesis (Entries 8 and 9). Finally with optimised conditions,
EtSSiMe$_{3}$ gave the desired product. Indeed, \textbf{2a} was
successfully isolated with 85\% yield along with by-product \textbf{4a}
in 11\% yield, which corresponds to an overall conversion of 96\%. The
introduction of additional water into the reaction mixture logically
led to a dramatic decrease in yield (Entry 10).

\break
With optimal reaction conditions in hand, based on the Baba's
study~\cite{7e}, further screening of catalysts in autoclave systems
was realized  (Table~\ref{tab3}).\footnote{In comparison to the study
of Yamakasi and  Shibuya~\cite{7c,7d}, the Br\"onsted acid HI (1
equiv.) was used in EtSH as solvent (C $=$ 0.5~M), at 35~\textdegree C
without base. Mixture of iodide substituted compounds were detected by
mass spectroscopy, but no desired SEt substituted products were
observed.} As~expected, InI$_{3}$ provided the best result for the
ring opening  of \textbf{1a} (Entry 1). But In(OTf)$_{3}$ (less hard LA
than InI$_{3}$) also showed good activity (Entry 2). However, other
Lewis acids based on metal triflate have been tested. The soft and
larger Bi(OTf)$_{3}$ was found to be unsatisfactory (Entry 3), as well
as the unreactive oxophile  Al(OTf)$_{3}$ (Entry 4). Also, AlBr$_{3}$
allowed the formation of only a small quantity of the thioester
\textbf{4a} (10\%). Therefore, the reactivities are clearly dependent
on the nature of the Lewis acid as observed by Baba.

\begin{table*}
\caption{\label{tab3}Catalysts screening for the ring opening of 
\textbf{1a}\vspace*{4pt}\tpar{\protect\inlinefig{fx03}}\vspace*{-6pt}}
\tabcolsep 10pt
\begin{tabular}{lcccc} 
\thead
\xmorerows{1}{Entry} & \xmorerows{1}{Catalyst} & 
\multicolumn{3}{c}{Isolated yields (\%)}\\
\cline{3-5}
 & & \textbf{2a} & \textbf{3a} & \textbf{4a}\\
\endthead
1 & InI$_{3}$ & 85 & 0 & 11\\ 
2 & In(OTf)$_{3}$ & 58 & 8 & \00\\ 
3 & Bi(OTf)$_{3}$ & \04 & 7 & \00\\ 
4 & Al(OTf)$_{3}$ & \00 & 0 & \00\\ 
5 & AlBr$_{3}$ & \00 & 0 & 10 
\botline
\end{tabular}
\vspace*{10pt}
\end{table*}

We studied in more detail the loading of InI$_{3}$ on the outcome of
the reaction. As shown in  Table~\ref{tab4}, the ring opening of
\textbf{1a} was examined with different catalytic amounts of this Lewis
acid.

\begin{table*}
\caption{\label{tab4}Ring opening of 1a with various loadings of 
InI$_3$\vspace*{4pt}\tpar{\protect\inlinefig{fx04}}\vspace*{-6pt}} 
\begin{tabular}{lccccc}
\thead
\xmorerows{1}{Entry} & \xmorerows{1}{Nucleophile} & \xmorerows{1}{InI$_3$ (\%)} &
\multicolumn{3}{c}{Isolated yields (\%)}\\
\cline{4-6}
 & & & \textbf{2a} & \textbf{3a} & \textbf{4a}\\
\endthead
\01 & EtSH & 10 & 26 & 0 & \00\\ 
\02 & EtSH & 20 & 23 & 0 & \00\\ 
\03 & EtSSiMe$_3$ & \05 & 59 & 6 & \00\\ 
\04 & EtSSiMe$_3$ & 10 & 85 & 0 & 11 \\ 
\05 & EtSSiMe$_3$ (1.1 equiv) & 10 & 24$^{\mathrm{a}}$ & 15$^{\mathrm{a}}$ & \00\\ 
\06 & EtSSiMe$_3$ (0.3 equiv) & 10 & Traces$^{\mathrm{a}}$ & Traces$^{\mathrm{a}}$ & \00\\ 
\07 & EtSSiMe$_3$ & 20 & 91 & Traces & \03\\ 
\08 & EtSSiMe$_3$ & 33 & 61 & 6 & \00\\ 
\09 & EtSSiMe$_3${}$^{\mathrm{b}}$ & 33 & 47  & 2 & \00\\ 
10 & EtSSiMe$_3$ (3 equiv) & 33 & 90 & 0 & \00
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$NMR observations. 
$^{\mathrm{b}}$The putative catalytic intermediate  
InI$_{(3-n)}$(SEt)$_n$ was
heated to 90~\textdegree C (2~h) then cooled to  r.t. before the
addition of \textbf{1a} and  K$_2$CO$_{3}$.  Formation of
InI$_{(3-n)}$(SEt)$_n$: $\mathrm{InI}_3 + 
n\;\mathrm{EtSSiMe}_3\rightarrow \mathrm{InI}_{(3-n)}(\mathrm{SEt})_n +
n\; \mathrm{ISiMe}_3$.}
\end{table*}

\begin{table*}
\caption{\label{tab5}Reactivity study of different 
$\rmalpha$-substituted-$\rmgamma$-butyrolactones in InI$_3$-catalysed
ring-opening 
reactions\vspace*{4pt}\tpar{\protect\inlinefig{fx05}}\vspace*{-6pt}}
\begin{tabular}{lcccccc}
\thead
\xmorerows{1}{Entry} & \xmorerows{1}{Substrate} & 
\xmorerows{1}{Nucleophile} & \xmorerows{1}{Base (equiv.)} & 
\multicolumn{3}{c}{Isolated yields (\%)}\\
\cline{5-7}
 & & & & \textbf{2} & \textbf{3} & \textbf{4}\\
\endthead
\01 & \textbf{1a} & EtSH & K$_{2}$CO$_{3}$ (1.1) & 
\textbf{2a}: 26 & 0 & 0\\ 
\02 & \textbf{1c} & EtSH & K$_{2}$CO$_{3}$ (1.1) & 
\textbf{2a}: 21 & \textbf{3a}: 4 & \textbf{4a}: Traces\\ 
\03 & \textbf{1d} & EtSH & K$_{2}$CO$_{3}$ (1.1) &
\textbf{2d}: 39 & 0 & 0\\ 
\04 & \textbf{1a} & EtSSiMe$_3$ & - & \textbf{2a}: 35 & \textbf{3a}: 4 & 
\textbf{4a}: Traces \\ 
\05$^{\mathrm{a}}$ & \textbf{1a} & EtSSiMe$_3$ & - & \textbf{2a}: 34 & \textbf{3a}: 4 & 
\textbf{4a}: Traces \\ 
\06 & \textbf{1c} & EtSSiMe$_3$ & - & \textbf{2a}: 55 & \textbf{3a}: 5 & 
\textbf{4a}: 17\\ 
\07 & \textbf{1d} & EtSSiMe$_3$ & - & \textbf{2d}: 2 & \textbf{3d}: 17 & 
\textbf{4d}: 39 \\ 
\08 & \textbf{1a} & EtSSiMe$_3$ & K$_{2}$CO$_{3}$ (1.1) & \textbf{2a}: 85 & 
0 & \textbf{4a}: 11\\ 
\09 & \textbf{1c} & EtSSiMe$_3$ & K$_{2}$CO$_{3}$ (1.1) & \textbf{2a}: 47 & 
0 & \textbf{4a}: Traces\\ 
10 & \textbf{1d} & EtSSiMe$_3$ & K$_{2}$CO$_{3}$ (1.1) &
\textbf{2d}: 11$^{\mathrm{b}}$ & 0 & \textbf{4d}: 53$^{\mathrm{b}}$\\ 
11 & \textbf{1a} & EtSSiMe$_3$ & DBU (1.1) & \textbf{2a}: 40 & 
0 & \textbf{4a}: Traces
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Reaction performed with the addition of 
molecular sieves. 
$^{\mathrm{b}}$NMR yields.}
\vspace*{3pt}
\end{table*}

The InI$_{3}$ quantity has no influence on the yield when EtSH was used
(Table~\ref{tab4}, entries 1 and 2). Nevertheless, with the EtSSiMe$_3$
and 5\% of InI$_{3}$ (Entry 3), the desired product \textbf{2a} was
isolated with 59\% yield, while the yield reached 91\% with 20\% of
Lewis acid (Entry 7). Surprisingly, a remarkable decrease in yield was
observed with 33\% of the catalyst loading (Entry 8), which could
indicate either an insufficient activation of plausible intermediates
InI$_{(3-n)}$(SEt)$_{n}$ formed \textit{in situ}, or an insufficient
amount of nucleophile. The second hypothesis was confirmed by a rise in
the yield with 3 equivalents of EtSSiMe$_3$\unskip\break
(Entries 9 and 10). This
suggests that likely at least (3 $*$ n
(catalyst) $+$ 1) equivalents of the
nucleophile are required to achieve the maximum conversion. This
hypothesis was also confirmed by the experiment with 0.3 equiv. of
EtSSiMe$_3$ and 10\% of InI$_{3}$ (Entry~6), where only traces (instead
of expected 20--30\%) of expected products were observed. This lack of
conversion suggests a plausible interaction between the catalyst and
the nucleophile. The ratio nucleophile/catalyst appears to be important
and the putative InI$_{(3-n)}$(SEt)$_{n}$ species could be formed
before acting as a catalyst. It should be noted that Baba and
co-workers used an InI$_{3}$/ester/PhSSiMe$_{3}$\ $=$ 1/10/20 ratio in
their experiments, so more than (3*n (catalyst) +1) equivalents of
nucleophile.

Finally, the outcome of the ring opening of several
$\rmalpha$-substituted-$\rmgamma$-butyrolactones was examined under
various reaction conditions  (Table~\ref{tab5}).

\textbf{1c}~\cite{8} and \textbf{1d}~\cite{13a,13b} were prepared from
\textbf{1a} according to the protocols of the literature and isolated
with 96\% and 86\% yields respectively. Of the two compounds, the ether
\textbf{1d} was found to be the most reactive with EtSH in the presence
of K$_{2}$CO$_{3}$  (Table~\ref{tab5}, entries 1--3). On the other
hand, when the nucleophile is EtSSiMe$_3$ and in the absence of a base,
\textbf{1c} was found to be the most reactive substrate with a yield of
55\% of \textbf{2a} (Entries 4--7). Moreover, the addition of
K$_{2}$CO$_{3}$ promoted the reactivity of \textbf{1a} to \textbf{2a},
while \textbf{1d} mainly provided the thioester 
\textbf{4d}\unskip\break (Entries
8--10). The presence of the -OTMS group on the substrate \textbf{1c}
(Entries 2, 6 and 9) did not seem to have a beneficial influence in the
selective formation of the compound \textbf{2a} in these conditions. It
is important to note that this type of K$_{2}$CO$_{3}$ base is not
crippling for industrial purposes due to the possibility of 
regeneration~\cite{9}. Based on the above results, we decided to keep a
base, but we replaced K$_{2}$CO$_{3}$ with the more soluble DBU (Entry
11), which resulted in a disappointing loss of yield.

\section{Conclusions}\label{sec3}
In conclusion, we have developed an efficient and practical synthesis
of one methionine analogue, the HMB, by the InI$_{3}$-catalysed
ring-opening reaction between 2-hydroxybutyrolactone (\textbf{1a}) and
a thiosilane in the presence of a base (K$_{2}$CO$_{3}$) which can be
regenerated. This could be advantageous for further industrial
developments. The reaction occurred selectively at the carbon atom
C(sp$^3$)--O. The method allows easy access to the desired product
\textbf{2a} in a high isolated yield (85\%) with 10\%
InI$_{3}$ catalyst, when the reaction is
carried out in an
autoclave and in the absence of water. To our knowledge, the current
one-pot reaction system is the only one, among the reported procedures,
that produces HMB analogues in excellent yields from \textbf{1a} using
catalytic electrophilic activation.

\section{Experimental section}\label{sec4}

\subsection{General methods}\label{sec4.1}
All reagents were obtained from commercial sources and used as received
unless otherwise noted. K$_{2}$CO$_{3}$ was dried at  85~\textdegree C
under vacuum in a B\"{u}chi glass oven B-585. THF and toluene were
purified and dried over Braun solvent purification system (MB-SPS-800).
InI$_{3}$ and the other Lewis acids were stored and weighted in
glove-box in oven-dried glassware. All reactions were performed in
oven-dried glassware under an atmosphere of argon using Schlenk
techniques unless otherwise noted. Analytical Thin Layer Chromatography
(TLC) was carried out on Merck silica gel60 F$_{254}$. Products were
stained with potassium permanganate dyeing reagent solution followed by
gentle heating. Flash chromatography was performed on
Combiflash\tsup{\text{\textregistered}} Companion or with Merck silica
gel 60 (230--400 mesh).  $\mbox{}^{1}$H and $\mbox{}^{13}$C NMR spectra
were recorded in CDCl$_{3}$, at ambient temperature on Bruker Avance
III 300 or 400 spectrometers operating at 300 and 400~MHz respectively
for $\mbox{}^{1}$H.  $\mbox{}^{13}$C nuclei were observed with
$\mbox{}^{1}$H decoupling. Solvent residual signals were used as
internal  standard~\cite{11}. Chemical shifts  $(\rmdelta)$ and
coupling constants  $(J)$ are given in ppm and Hz respectively. The
peaks patterns are indicated as the following format multiplicity 
(s:
singlet; d: doublet; t: triplet; q: quartet; qt: quintuplet; m:
multiplet; dd: doublet of doublet). The prefix br. indicates a broadened
signal. IR spectra were obtained using a Bruker Alpha Platinum ATR.
HRMS were recorded on SYNAPT G2 HDMS (Waters) or on QStar Elite
(Applied Biosystems SGIEX) equipped with an Atmospheric Pressure
Ionization (API) source or on MaXiS 4G (Bruker) equipped with an
Electron Spray Ionization (ESI) source. Mass spectra were obtained
using a Time Of Flight (TOF) analyser. Elementary analyses were
recorded on a Thermo Finnigan EA 1112 apparatus.

\subsection{Starting materials}\label{sec4.2}
The 2-hydroxybutyrolactone \textbf{1a} is commercially available and
was given by our industrial partner Adisseo. It is dry over 3~\AA{}\ MS
for 72~h. 

\subsubsection{3-(Trimethylsilyl)oxydihydrofuran-2(3H)-one 
(\textbf{1c})}
This compound was prepared according to a procedure reported 
elsewhere~\cite{8}. To a stirred solution of \textbf{1a} (0.074~mL,
1~mmol, 1.0 equiv.), in CH$_{3}$NO$_{2}$ (1~mL) under argon was added
freshly distilled HMDS (0.21~mL, 1~mmol,  1.0 equiv.). The reaction
mixture was stirred for 30 min at room temperature. After
concentration, the crude product was purified by column chromatography
using a gradient eluent (from 100\% petroleum ether to 90:10 $=$
petroleum ether:ethylacetate). \textbf{1c} was recovered as colorless
oil (167~mg, 96\%). These data are consistent with those described in
the  literature~\cite{12}.

$\mbox{}^{1}$H NMR (400~MHz, CDCl$_{3}$) $\rmdelta$ 4.43--4.36  (m, 2H,
CH$_2$OC(O)),  4.23--4.16 (m,1H, CHOSi),  2.51--2.43 (m, 1H, CH$_2$CHOSi),
2.28--2.18  (m, 1H, CH$_2$CHOSi),  0.20 (s, 9H, Si(CH$_{3}$)$_{3}$).

\subsubsection{3-Methoxydihydrofuran-2(3H)-one (\textbf{1d})}
NaH (60\% in oil, 302~mg, 7.54~mmol, 1.5 equiv.) was introduced in a
double-neck round bottom flask. The oil is removed by several washings
with pentane. Then, dry THF (10~mL) under argon atmosphere is added. At
0~\textdegree C, \textbf{1a} (0.37~mL, 5.02~mmol, 1.0 equiv.) is added
dropwise to the stirred solution. After 1~h, while warming to room
temperature, freshly distilled MeI (0.47~mL, 7.54~mmol, 1.5 equiv.) is
added. Subsequently, the reaction mixture was stirred for 22~h and
then, 20~mL of saturated NH$_{4}$Cl aqueous solution is
added. The layers were separated, and the aqueous layer was extracted
with $3\times25$~mL of Et$_{2}$O. The combined organic layer was
dried over Na$_{2}$SO$_{4}$, filtered and
concentrated. The crude product was purified by column chromatography
using a gradient eluent (from 100\% petroleum ether to 60:40 $=$ petroleum
ether:ethylacetate). \textbf{1d} (501~mg, 86\%) was obtained as yellow
oil.

$R_{f} = 0.55$ (AcOEt/petroleum ether 8:2).  $\mbox{}^{1}$H NMR
(400~MHz,  CDCl$_{3}$) $\rmdelta$ 4.39--4.34  (m, 1H, CH$_{2}$OC(O)),
4.23--4.17  (m,1H, CH$_{2}$OC(O)), 4.00  (t, $J= 7.8$~Hz, 1H,
CHOCH$_{3}$), 3.51  (s, 3H, OCH$_3$), 2.53--2.46  (m, 1H,
CH$_{2}$CHOCH$_{3}$), 2.23--2.14  (m, 1H, 
CH$_{2}$CHOCH$_{3}$).\unskip\break 
$\mbox{}^{13}$C NMR (100~MHz,  CDCl$_{3}$): $\rmdelta$ 174.8, 75.0,
65.4, 58.2, 29.4. These data are consistent with those described in the
literature~\cite{13a,13b}.

\subsection{Experimental procedures and characterizations
data}\label{sec4.3}

\subsubsection{4-(Ethylthio)butyric acid (\textbf{2b})}
To a stirred suspension of InI$_{3}$ (100~mg, 0.2 mmol, 0.1 equiv.) in
anhydrous toluene (2~mL) was added EtSSiMe$_3$ [0.680~mL (90\% purity),
3.8~mmol, 1.9 equiv.]. The reaction mixture was stirred for a few
minutes under N$_{2}$ atmosphere at room temperature before the
addition of $\rmgamma$-butyrolactone \textbf{1b} (0.15~mL, 2 mmol, 1.0
equiv.). The mixture was then heated to  90~\textdegree C and stirred
for 3~h. After cooling to room temperature, the resulting mixture was
poured into 20~mL of HCl$_{\mathrm{aq}}$ (1M) and the layers were
separated. The aqueous layer was extracted with  $3\times20$~mL of
ethylacetate, and the combined organic layers were washed with  brine,
dried over
Na$_{2}$SO$_{4}$ and filtered. After concentration \textit{in vacuo},
carboxylic acid \textbf{2b} was obtained as a yellow liquid (295~mg, 
99\%) without further purification.  $R_f = 0.63$ (AcOEt/petroleum ether:
8:2). $\mbox{}^{1}$H NMR (400~MHz, CDCl$_{3}$) $\rmdelta$ 10.66
(CO$_{2}$H), 2.59--2.47 (m, 6H, CH$_2$SCH$_{2}$ and CH$_{2}$CO$_{2}$H),
1.91 (qt,  $J= 7.19$, 7.25~Hz, 2H, CH$_{2}$CH$_{2}$CH$_{2}$),  1.24 (t,
$J= 7.4$~Hz, 3H, CH$_{3}$). $\mbox{}^{13}$C NMR (100~MHz, CDCl$_{3}$): 
$\rmdelta$ 179.7, 32.9, 30.8, 25.8, 24.4, 14.8. MS  (ESI$^{+}$):  $m/z
= 155$  [M$+$Li]$^{+}$; 171  [M$+$Na]$^{+}$. These data are
consistent with those described in the  literature~\cite{6b}.

\subsubsection{General procedure for the ring opening of compounds 1}
In an autoclave system (5~mL closed Schlenk tube) with a minimised dead
volume, the nucleophile was added to a stirred suspension of Lewis acid
in anhydrous toluene (2.4~mL). The reaction mixture was stirred for a
few minutes under N$_{2}$ atmosphere at room temperature before the
addition of dried \textbf{1} (2.44~mmol, 1.0 equiv.) followed by the
eventual addition of base. The mixture was then heated to 
90~\textdegree C and stirred for 22~h. After cooling to room
temperature, the resulting mixture was filtered, and then was poured
into 20~mL of HCl$_{\mathrm{aq}}$ (1M) and the layers were separated. After
extraction of the aqueous phase with  $3\times20$~mL of ethylacetate,
the combined organic layer was washed with 
brine, and dried over
Na$_{2}$SO$_{4}$, filtered and 
concentrated.
The crude product was\unskip\break 
purified by column 
chromatography using a gradient eluent (from 100\%
petroleum ether to 100\% ethyl\unskip\break 
acetate).

\subsubsection{4-(Ethylthio)-2-hydroxybutyric acid (\textbf{2a})}
This compound was prepared following the general procedure starting
from EtSSiMe$_3$ (0.83~mL, 4.64~mmol, 1.9 equiv. (90\% purity)),
InI$_{3}$ (242~mg, 0.48~mmol, 0.2 equiv.), \textbf{1a} (0.18~mL, 
2.44~mmol, 1.0 equiv.) and anhydrous K$_{2}$CO$_{3}$ (371~mg,
2.68~mmol, 1.1 equiv.). The crude product was purified by column
chromatography using a gradient eluent (from 100\% petroleum ether to
100\% ethyl acetate).

\textbf{2a} (365~mg, 91\%) was obtained as yellowish oil. $R_{f} = 0.52$
(AcOEt/ petroleum ether: 8:2).  $\mbox{}^{1}$H NMR (400~MHz,
CDCl$_{3}$) $\rmdelta$ 5.52 (br s, 2H, OH and CO$_{2}$H), 4.43 (dd, 
$J= 8.0$, 3.8~Hz,1H, CHOH), 2.70 (t,  $J= 6.7$~Hz, 2H,
SCH$_{2}$CH$_{2}$), 2.56 (q, $J= 7.4$~Hz, 2H, CH$_{3}$CH$_{2}$S),
2.18--2.09 (m, 1H, CH$_{2}$CHOH), 2.01--1.93 (m, 1H, CH$_{2}$CHOH), 1.26
(t,  $J= 7.4$~Hz, 3H, CH$_{3}$). $\mbox{}^{13}$C NMR (100~MHz,
CDCl$_{3}$): $\rmdelta$ 179.1, 69.4, 33.6, 27.3, 25.9, 14.8. IR (film):
3400, 2966, 2926, 1718, 1261, 1213, 1166,  1089~cm$^{-1}$. HRMS (ESI): 
$m/z$  [M$+$H]$^{+}$ Calcd for C$_{6}$H$_{12}$O$_{3}$S 165.0580;
Found 165.0579.

\subsubsection{$S$-Ethyl 4-(ethylthio)-2-hydroxybutanethioate 
(\textbf{3a})}
This compound is a by-product formed during the synthesis of
\textbf{2a} and was isolated during the chromatography. \textbf{3a}
(traces,  ${<}$1\%) was obtained as colorless oil. 

$R_{f} = 0.37$ (AcOEt/ petroleum ether 1:9).  $\mbox{}^{1}$H NMR
(300~MHz, CDCl$_{3}$) $\rmdelta$ 4.41 (dd,  $J= 8.3$, 3.6~Hz, 1H, CHOH),
2.92 (q,  $J= 7.4$~Hz, 2H, CH$_{3}$CH$_{2}$S(O)), 2.84 (br s, 1H, OH),
2.70--2.65 (m, 2H, SCH$_{2}$CH$_{2}$), 2.56 (q, $J= 7.4$~Hz, 2H,
CH$_{3}$CH$_{2}$SCH$_{2}$), 2.17--2.06  (m, 1H, CH$_{2}$CHOH), 1.98--1.87
(m, 1H, CH$_{2}$CHOH), 1.29--1.23 (m, 6H, 2CH$_{3}$). $\mbox{}^{13}$C
NMR (100~MHz, CDCl$_{3}$): $\rmdelta$ 203.8, 76.9, 34.5, 27.2, 25.9,
23.1, 14.8, 14.7.  IR (film): 3420, 2966, 2929, 1678, 1263, 
1088~cm$^{-1}$. HRMS (ESI):  $m/z$ [M$+$Na]$^{+}$ Calcd for
C$_{8}$H$_{16}$O$_{2}$S$_{2}$  231.0484;  Found 231.0484.

\subsubsection{$S$-Ethyl 2,4-dihydroxybutanethioate  (\textbf{4a})}
This compound is a by-product formed during the synthesis of
\textbf{2a} and was isolated during the chromatography. \textbf{4a}
(10~mg, 3\%) was obtained as colorless oil. 

$R_{f} = 0.55$ (AcOEt/ petroleum ether 8:2). $\mbox{}^{1}$H NMR
(500~MHz, CDCl$_{3}$) $\rmdelta$ 4.41
(dd,  $J= 9.3$, 3.0~Hz,1H, CHOH),
3.83 (t, $J= 4.1$~Hz, 2H, CH$_{2}$OH), 2.84 (q,  $J= 7.5$~Hz, 2H,
CH$_{3}$CH$_{2}$S(O)), 2.10--2.04 (m, 1H, CH$_2$CHOH), 1.85--1.79 (m, 1H,
CH$_{2}$CHOH), 1.22 (t,  $J= 7.3$~Hz, 3H, CH$_{3}$). $\mbox{}^{13}$C
NMR (125~MHz, CDCl$_{3}$): $\rmdelta$ 206.3, 75.9, 59.4, 35.8, 22.8,
14.6. IR (film): 3328, 2973, 1417, 1086, 1044,  879~cm$^{-1}$. HRMS
(ESI$^{+}$): $m/z$  [M$+$Na]$^{+}$ Calcd for
C$_{6}$H$_{12}$O$_{3}$SNa$^{+}$ 187.0399; found 187.0398.

\subsubsection{4-(ethylthio)-2-methoxybutanoic acid  (\textbf{2d})}
This compound was prepared following the general procedure starting
from \textbf{1d} (290~mg, 2.5~mmol, 1.0 equiv.), EtSH (0.355~mL, 
4.75~mmol, 1.9 equiv.) and 0.1 equiv. of InI$_{3}$ (124~mg, 0.25 mmol).
The crude product was purified by column chromatography using~DCM, then
DCM with 5\% MeOH as the eluent. \textbf{2d} (172~mg, 39\%) was
obtained as yellowish oil.

$R_{f} = 0.56$ (DCM/MeOH 9:1). $\mbox{}^{1}$H NMR (400~MHz, 
CDCl$_{3}$) $\rmdelta$ 3.98 (dd,  $J= 7.0$, 5.0~Hz, 1H,  CHOCH$_{3}$),
3.45 (s, 3H,  OCH$_{3}$), 2.68--2.60 (m, 2H, SCH$_{2}$CH$_{2}$), 2.53
(q, $J= 7.5$~Hz, 2H,  CH$_{3}$CH$_{2}$S), 2.08--2.00  (m, 2H,
CH$_{2}$CHOCH$_{3}$), 1.24 (t,  $J= 7.4$~Hz, 3H, CH$_{3}$CH$_{2}$S).
$\mbox{}^{13}$C NMR (100~MHz,  CDCl$_{3}$): $\rmdelta$ 177.7, 78.7, 
58.6, 32.4, 27.1, 25.8, 14.8. IR (film): 2928, 1718, 1450, 1193, 
1117~cm$^{-1}$. HRMS (ESI$^{+}$): $m/z$ [M$+$H]$^{+}$ Calcd for
C$_{7}$H$_{14}$O$_{3}$S 179.0736; Found 179.0737. 

\subsubsection{$S$-Ethyl 4-(ethylthio)-2-methoxybutanethioate 
(\textbf{3d})}
This compound is a by-product formed during the synthesis of
\textbf{2d}. This compound was prepared following the general procedure
starting from \textbf{1d} (290~mg, 2.5~mmol, 1.0 equiv.), 0.1 equiv. of
InI$_{3}$ (124~mg, 0.25~mmol) and no base was added. The crude product
was purified by column chromatography using a gradient eluent (from
100\% petroleum ether to 100\% 	ethylacetate). \textbf{3d} (93~mg, 17\%)
was obtained as\unskip\break 
yellow oil.

$R_{f}= 0.75$ (AcOEt/ petroleum ether 1:9).  $\mbox{}^{1}$H NMR
(400~MHz, CDCl$_{3}$) $\rmdelta$ 3.89 (dd, $J= 7.8$, 4.8~Hz, 1H,
CHOCH$_{3}$), 3.46 (s, 3H, OCH$_{3}$), 2.86 (q,  $J= 7.4$~Hz, 2H,
CH$_{3}$CH$_{2}$S(O)), 2.69--2.58 (m, 2H, SCH$_{2}$CH$_{2}$), 2.52 (q,
$J= 7.3$~Hz, 2H, CH$_{3}$CH$_{2}$SCH$_{2}$), 2.01--1.89 (m, 2H, 
CH$_{2}$CHOCH$_{3}$), 1.27--1.22 (m, 6H,  2CH$_{3}$CH$_{2}$S).
$\mbox{}^{13}$C NMR (100~MHz,  CDCl$_{3}$): $\rmdelta$ 203.5, 85.9,
59.4, 33.7, 27.1, 25.7, 22.4, 14.8, 14.7. IR (film): 2929, 1678, 1452,
1264, 1116,  1100~cm$^{-1}$. HRMS (ESI$^{+}$):  $m/z$ [M$+$Na]$^{+}$
Calcd for C$_{9}$H$_{18}$O$_{2}$S$_{2}$ 245.0640; Found 245.0643.

\subsubsection{$S$-ethyl~4-hydroxy-2-methoxybutanethioate\\(\textbf{4d})}
This compound is another by-product formed during the synthesis of
\textbf{2d} and was isolated during the chromatography. \textbf{4d}
(174~mg, 39\%) was obtained as brown oil.

$R_{f} = 0.65$ (AcOEt/petroleum ether 8:2). $\mbox{}^{1}$H NMR
(400~MHz, CDCl$_{3}$) $\rmdelta$ 3.93--3.85 (m, 1H, CHOCH$_{3}$),
3.73--3.65 (m, 2H, CH$_{2}$OH), 3.42 (s, 3H, OCH$_{3}$), 2.81 (q,  $J =
7.4$~Hz, 2H, CH$_{2}$CH$_{3}$), 1.91--1.76 (m, 2H,  CH$_{2}$CH$_{2}$OH),
1.20 (t, $J = 7.4$~Hz, 3H, CH$_{2}$CH$_{3}$).  $\mbox{}^{13}$C NMR
(101~MHz,  CDCl$_{3}$): $\rmdelta$ 174.7, 75.0, 65.3, 58.1, 29.4, 19.5,
19.0. IR (film): 3408, 2931, 2880, 1417, 1376, 1177, 1105~cm$^{-1}$.
HRMS (ESI$^{+}$): $m/z$ [M$+$Na]$^{+}$ Calcd for
C$_{7}$H$_{14}$O$_{3}$SNa$^{+}$ 201.0556; found 201.0562.

\vspace*{-5pt}
\section*{Conflicts of interest}
The authors declare no conflict of interest.

\vspace*{-4pt}
\section*{Acknowledgments}
We thank Dr. Christophe Chendo and Dr. Val\'{e}rie Monnier for mass
spectrometry analyses (Spectropole, F\'{e}d\'{e}ration des Sciences
Chimiques de Marseille), Arnaud Treuvey (Centrale Marseille) for IR
spectroscopy and Yoann Cotelle and Maxime Huchede for reading the
manuscript.

\back{}

\vspace*{-4pt}
\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-245-suppl.pdf}}

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