Radical-mediated reactions of α-bromo aluminium thioacetals, α-bromothioesters, and xanthates for thiolactone synthesis

Article abstract of DOI:10.3390/molecules23040897

Thiolactones have attracted considerable attention in recent years as bioactive natural products, lead compounds for drug discovery, molecular probes, and reagents for polymerisation. We have investigated radical-mediated C-C bond forming reactions as a s

Full text of DOI:10.3390/molecules23040897

molecules
Article
Radical-Mediated Reactions of α-Bromo Aluminium
Thioacetals, α-Bromothioesters, and Xanthates for
Thiolactone Synthesis
Ruairí O. McCourt 1 ID , Fabrice Dénès 2 and Eoin M. Scanlan 1,
*
ID
1
School of Chemistry, Trinity Biomedical Sciences Institute (TBSI), Trinity College Dublin,
The University of Dublin, 152-160 Pearse Street, Dublin 2, Ireland; mccourro@tcd.ie
CEISAM UMR CNRS 6230, Université de Nantes, UFR des Sciences et des Techniques,
2 rue de la Houssinière, BP 92208, 44322 Nantes CEDEX 3, France; Fabrice.Denes@univ-nantes.fr
Correspondence: eoin.scanlan@tcd.ie; Tel.: +353-(1)896-2514
2
*
Academic Editors: John C. Walton and Derek J. McPhee
Received: 9 March 2018; Accepted: 10 April 2018; Published: 13 April 2018
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Thiolactones have attracted considerable attention in recent years as bioactive natural
products, lead compounds for drug discovery, molecular probes, and reagents for polymerisation.
We have investigated radical-mediated C-C bond forming reactions as a strategy for thiolactone
synthesis. Cyclisation of an α-bromo aluminium thioacetal was investigated under radical conditions.
It was found that at low temperature, a radical fragmentation and rearrangement process occurs.
A putative reaction mechanism involving a previously unreported aluminium templated thiol-ene
step for the rearrangement process is presented. Cyclisation reactions of
α-bromo thioesters and
α
-xanthate thioesters under radical mediated conditions furnished the desired thiolactones in
moderate yields.
Keywords: radical fragmentation; radical cyclisation; thiyl radical
1. Introduction
The 5-exo-trig radical cyclisation of allylic
by Ueno and Stork [ ]. Since its discovery, this reaction has seen widespread use in organic
chemistry, providing a highly efficient method for the synthesis of numerous heterocycles due to its
high regioselectivity and diastereoselectivity [ ]. The cyclic acetal formed through the Ueno-Stork
reaction can be readily oxidised to the corresponding lactones using Jones or Grieco’s conditions [ ].
-halo-allylic esters and is
α-bromoacetals was reported in 1982 independently
1–3
3,4
5
The overall process is generally higher yielding than the cyclisation of
outlined in Scheme 1.
α
Recent work in this field has aimed at improving the overall process, with respect to yield, scope
and practicality [ ]. It has been established that the -bromo acetal starting material may be generated
in situ from the corresponding esters [ ]. It has long been established that at low temperatures
(–78 ◦C), the reduction of esters with diisobutylaluminium hydride (DIBAL-H) furnishes aluminium
acetals [ ], which decompose upon heating [ ]. The reduction of allylic- -bromo esters to form
allylic- -bromo-aluminium acetals can be coupled with the Ueno–Stork cyclisation to form -lactols [ ].
This methodology has proven compatible with propargylic -bromo esters, which undergo 5-exo-dig
cyclisation to form exo-alkenes [10]. The methodology was further modified with this reaction being
6
α
7
8
9
α
α
γ
7
α
coupled with Oppenauer-type oxidation [11]. After conversion of the allylic (or propargylic)
α
-bromo
◦
aluminium acetal into the lactol-aluminium complex at low temperatures (–78 C), an excess of a
simple sacrificial aldehyde is added, which undergoes Meerwein-Ponndorf-Verley reduction and
concomitantly oxidises the aluminium acetal. This one-pot sequence allows for the synthesis of
Molecules 2018, 23, 897; doi:10.3390/molecules23040897
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Molecules 2018, 23, 897
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lactones from allylic
α
-bromo aluminium acetals, often with complete diastereoselectivity [11], as well
as polysubstituted butenolides from propargylic α-bromo aluminium acetals [12].
Scheme 1. Ueno–Stork approach to lactones and proposed radical-mediated routes to thiolactones.
We set out to investigate if the
synthesis could be applied to allylic
α
-bromo aluminium acetal methodology developed for lactone
-bromo aluminium thioacetals, in an attempt to synthesise
α
thiolactols suitable for conversion into thiolactones. Thiolactones have emerged as fascinating targets
for organic synthesis with intense interest from numerous fields of chemical research, particularly
polymer science and medicinal chemistry [13–15]. Their synthesis through radical-mediated C–C bond
forming reactions has not previously been reported.
2. Results & Discussion
Computational and DOSY experiments support the formation of a dimeric aluminium acetal
species for the non-substituted and mono-substituted
α-bromo aluminium acetal intermediates
involved in the radical cyclisation [16]. Contrary to aluminium acetals derived from esters, the
related aluminium thioacetals have proven to be a relatively robust intermediates, demonstrating
stability at temperatures up to –15 ◦C [17]. A dimeric structure was also suggested for the aluminium
thioacetal species [17] and it is reasonable to assume that
stable enough to serve as precursors in a radical cyclisation. In this case, despite the formation of a
beta-sulfanyl radical intermediate susceptible to fragmentation [18 19], the precedent in the literature
α-halo aluminium thioacetals could be
,
led us to expect that this process would not compete with the desired cyclisation [20].
A suitable substrate for investigation of the feasibility of the Ueno-Stork-type cyclisation of
α
-bromo aluminium thioacetals was prepared starting from hydrocinnamaldehyde in six steps
(Scheme 2A). Horner-Wadsworth-Emmons reaction of the corresponding aldehyde gave allyl ester
1
3
in good yield. Reduction of the ester followed by bromination with PBr3 furnished alkyl bromide
. Displacement of the bromide with KSAc, followed by reduction of the thioester furnished an allyl
thiol, which underwent Steglich coupling with bromoacetic acid to give the desired α-bromo thioester
5 in 78% yield.
When the two-step reduction/cyclisation protocol developed by Dénès et al. was applied to
thioester
allyl thiol
6
5
, the thioether
was formed in approximately 60%. Repeating the reaction furnished an identical product
at low temperature
formed. 1H NMR analysis showed only the
7 was formed in 32% instead of the expected thiolactol. In addition, the
distribution. Analysis of the reaction mixture following DIBAL-H reduction of
showed that after this step there was no thioether
6
7

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formation of allyl thiol
5. The presence of thiol
5
can be explained by the collapse of the aluminium
thioacetal generated upon DIBAL-H reduction of thioester
stability of the aluminium thioacetal at low temperature.
6. This experiment tends to support the
Scheme 2.
(
A
) The synthesis of◦an allylic
α
-bromo-thioester and the attempted Ueno–Stork-type
◦
cyclisation. a: nBuLi THF, 14 h, 0 C. b: DIBAl-H, PhMe,
h. d: KSAc MeCN, rt, 18 h. e: (i) LiAlH4, Et2O, rt. 2 h. (ii) BrCH2CO2H, EDC, DMAP, THF, rt, 18 h. f:
−
78 C, 3 h, rt, 15 mins. c: PBr3, CH2Cl2, rt, 3
DIBAL-H, Et3B, nBu3SnH, air, 14 h. (B) The synthesis of an allylic α-phenylselanyl-thioester and its
attempted Ueno-Stork cyclisation. g: (i) NaBH4, MeOH, 0 ◦C (ii) BrCH2CO2H, rt, h: 5, EDC, DMAP,
THF, rt, 18 h.
In an effort to ascertain the mechanism by which alcohol
7 was formed, α-phenylselanyl carboxylic
8
was prepared from diphenyl diselenide in two steps according to a modified literature procedure [21].
The carboxylic acid was subsequently coupled with the allyl thiol
(Scheme 2B).
5 in good yield to give thioester 9
Although the phenylselanyl group reacts in a similar manner to halides under radical conditions
(displays similar radical lability), under ionic conditions the reactivities are vastly different. This
allowed us to establish by which reaction pathway the formation of
7 was occurring. Under identical
conditions, alcohol was formed in 28% yield, providing strong evidence that the transformation
6
occurs via a radical pathway. A proposed mechanism for the rearrangement process for formation of
alcohol 7 from thioester 6 is shown in Scheme 3.
In the first step, DIBAL-H reduction of the thioester to an aluminium thioacetal occurs. Following
generation of a tributyltin radical, this intermediate abstracts a bromine atom from the aluminium
thioacetal to generate a carbon-centred radical
α to the thioacetal (Scheme 2). Contrary to what was
observed during the cyclisation of trichloroacetals under atom transfer conditions [20], and despite
the fact that the reaction was carried out at low temperature, the aluminium thioacetal subsequently
collapses through a homolytic
thiyl radical, both of which may remain coordinated to the aluminium. The electron-rich alkene thus
generated reacts with the electrophilic thiyl radical to give the thiol-ene addition product [22 23].
Hydrogen atom abstraction followed by aqueous work-up furnishes alcohol . The alternative ionic
rearrangement involving the nucleophilic displacement of the bromine atom was ruled out by the
β-fragmentation pathway to generate an aluminium enolate and a
,
7

Molecules 2018, 23, 897
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attempted cyclisation of the more robust seleno-precursor
9, which led to similar results. No further
fragmentation products were observed as collapse of the aluminium intermediate complex, prior to
thiol-ene addition would only furnish the observed thiol (following hydrogen atom abstraction) and
ethanal, which is reduced by the excess of DIBAL-H to give a volatile by product. The instability of the
aluminium complex and the dissociation of the aluminium enolate are the likely factors contributing to
the low isolated yield of the radical rearrangement product 7. However, in light of the results obtained
by I. Markó et al. demonstrating the thermal stability of this type of aluminium-thioacetal species,
together with the successful cyclisation of protected thioacetals under atom-transfer conditions [20],
the fragmentation of the aluminium-thioacetal at the low temperature employed during our reaction
is more puzzling.
Scheme 3. The proposed mechanism leading to rearranged product 7.
With the knowledge that the aluminium-thioacetal intermediate was unsuitable for the
radical-mediated cyclisation process, we set out to investigate alternative conditions that would
avoid this process. The cyclisation of α-bromo esters in the presence of a hydrogen atom donor such
as nBu3SnH or nBu3GeH was investigated by Beckwith in the mid-1980s and it was found that the
restricted rotation about the allylic C-O bond in the 2-oxa radical intermediate was responsible for
a relatively slow cyclisation [24,25]. As a consequence, dehalogenation prior to the expected radical
cyclisation often prevails. Optimized reaction conditions to form the lactones require slow-addition of
the nBu3SnH in order to maintain the concentration of tin hydride low enough for the intramolecular
process to compete favourably [26
6
–
28]. These optimized reaction conditions were applied to thioester
(Scheme 4). We hypothesized that the relatively low rotation barrier in thioesters [29] such
compared to the parent esters and amides [30 31], would facilitate the cyclisation process.
as
6
,
Unfortunately, the desired thiolactone 10, produced via 5-exo-trig cyclisation, was obtained in only
low yield, with the dehalogenated thioester being afforded as the major product in 66% yield.
Attempts to extend this strategy towards the synthesis of other thiolactones failed. For instance, when
4

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identical cyclisation conditions were applied to thioester derivative 11, substituted with a methyl
group to the carbonyl, dehalogenated product 12 was obtained exclusively. This is likely due to
α
the stability of the secondary radical generated, and the trans-cis rotation about the carbonyl being
more disfavoured in the more substituted 11 allowing for hydrogen atom abstraction to compete with
cyclisation. Homolytic β-fragmentation releasing a thiyl radical was not observed in either of these
attempted cyclisations, despite the high temperature of the reaction.
Scheme 4. Cyclisation of α-bromo-allylic thioesters under traditional cyclisation conditions.
Over recent decades, Zard has pioneered and driven the field of radical group transfer of
xanthates [32]. These reagents have seen extensive use in intermolecular addition processes as well
as in cyclisation reactions [33]. Zard and co-workers have applied this strategy to N,N-disubstituted
α
-(xanthyl)-acetamides to prepare lactams [34]. (The preparation of mono- and bis-γ-thiolactones by
addition of a thiolactone xanthate onto various alkenes has recently been reported) [35]. This approach
allows the cyclisation to proceed in the absence of a hydrogen atom donor and thus circumvents
the problem associated with the high rotation barrier in amides and esters (For examples of atom
transfer reaction for the formation of lactones, see ref [36]). Application of this methodology to the
α
-xanthate thioester 13, derived near quantitatively from 6, was investigated. The radical cyclisation
conducted in refluxing cyclohexane in the presence of 0.3 equivalent of dilauroyl peroxide (DLP) as an
initiator furnished the reduced lactone 10 with no xanthate being incorporated in the final product.
The reduction arises from the hydrogen atom abstraction from the solvent [37]. The use of benzoyl
peroxide for the xanthate transfer reaction furnished a complex mixture with no thiolactone product
being observed.
3. Materials and Methods
Instrumental and General Considerations
Unless otherwise stated; all commercial chemicals were obtained from Sigma-Aldrich, Fluka,
Fluorochem, Alfa-Aesar, or Fischer Scientific, without further purification. Deuterated solvents for
NMR were purchased from Apollo. Dry solvents were distilled under argon and dried over 4 Å
molecular sieves prior to use. Solvents for synthesis purposes were used at GPR grade. A Bruker
1
Advance 400 spectrometer, H (400.13 MHz) and 13C (100.6 MHz) and a Bruker Ultrashield 600,
1H (600.13 MHz) and 13C (150.6 MHz), were employed for NMR spectra. Resonances
δ, from the
centre point, are in ppm units downfield from an internal reference [38]. NMR data was processed
using TopSpin software. Infrared spectra (IR) were recorded on a Perkin Elmer spectrometer. Mass
spectrometry analysis was performed with a Q-Tof Premier Waters Maldi-quadrupole time-of-flight

Molecules 2018, 23, 897
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(Q-Tof) mass spectrometer equipped with Z-spray electrospray ionisation (ESI) and matrix assisted
laser desorption ionisation (MALDI) sources. Silica gel Florisil (200 mesh; Aldrich) was used for
column chromatography. Thin-layer chromatography (TLC) was performed using Merck 60 F254
silica gel (pre-coated, 0.2 mm thick, 20
molybdenum staining.
×
20 cm) and visualised by UV light (254 nm), iodine, or
Ethyl (E)-5-phenylpent-2-enoate (
1
): To a solution of triethyl phosphonoacetate (2.5 mL, 12.5 mmol, 1.25
◦
equiv.) in anhydrous THF (50 mL) under an atmosphere of argon at 0 C was added 2.5 M nBuLi
(5.2 mL, 13.0 mmol, 1.3 equiv.) and the mixture was stirred for 30 min. Hydrocinnamaldehyde (1.3 mL,
10 mmol, 1.0 equiv.) was then added, and after 10 mins the reaction mixture was allowed to warm
to rt, and was stirred for a further 14 h. The reaction was diluted CH2Cl2 (250 mL) and quenched
with 2 M aqueous HCl (40 mL). After phase separation, the aqueous phase was extracted further with
CH2Cl2 (2
filtered and the solvent removed in vacuo. Purification was achieved by column chromatography on
silica gel using 3.0 7.0% EtOAc/Hexane (v/v) to afford the title compound as yellow wax (1.758 g,
8.62 mmol, 86%). H (400 MHz, CDCl3): 7.33–7.25 (m, 2H, Ar-H), 7.24–7.16 (m, 3H, Ar-H), 7.06–6.96
(m, 1H, =C ), 5.85 (d, J = 15.6 Hz, 2H, =C ), 4.19 (q, J = 7.2 Hz, 2H, -CH2CH3), 2.78 (t, J = 7.5 Hz,
×
50 mL). The combined organic layers were then washed with brine, dried over MgSO4,
→
δ
H
H
2H, -CH2Ph), 2.53 (app-q, 2H, -CH2CH=), 1.29 (t, J = 7.2 Hz, 2H, CH3). LRMS: (ESI−) m/z calcd for
C13H16O2 ([M + Cl]−): 239.1. Found: 239.1. The spectral data were in accordance with those reported
in the literature [39].
(E)-5-phenylpent-2-en-1-ol (
(40 mL) under an atmosphere of argon at
12.5 mmol 2.5 equiv.), and the reaction was stirred at this temperature for 3 h. The reaction was
then warmed to rt and H2O (2 mL), then aqueous 1 M NaOH solution (15 mL), followed again by
H2O (5 mL), were added, with 5 minutes between each addition, maintaining vigorous stirring. The
mixture was then poured into a separating funnel, and after phase separation, the aqueous phase was
2
): To a solution of
1
(◦1.02 g, 5.0 mmol, 1.0 equiv.) in anhydrous CH2Cl2
−
78 C was added 1 M DIBAL-H in CH2Cl2 (12.5 mL,
extracted with CH2Cl2 (2
over MgSO4, filtered through a plug of silica gel, and the solvent removed in vacuo to yield the title
compound (705 mg, 4.35 mmol, 87%) without the requirement for further purification. (400 MHz,
CDCl3): 7.32–7.25 (m, 2H, Ar-H), 7.22–7.15 (m, 3H, Ar-H), 5.80–5.63 (m, 2H, 2(-C
×
25 mL). The combined organic layers were then washed with brine, dried
δ
H
H
=)), 4.09 (app-t,
2H, CH2OH), 2.72 (t, J = 7.7 Hz, 2H, -CH2Ph), 2.38 (app-q, 2H, -CH2CH=). LRMS: (ESI−) m/z calcd
for C11H14ONa ([M + Na]−): 185.1. Found: 185.1. The spectral data were in accordance with those
reported in the literature [40].
(E)-(5-Bromopent-3-en-1-yl)benzene (
3
): To a solution ◦of
2 (600 mg, 3.7 mmol, 1.0 equiv.) in anhydrous
Et2O (10 mL), under an atmosphere of argon at 0 C was slowly added PBr3 (0.22 mL, 2.3 mmol,
0.6 equiv.). The mixture was stirred for 30 mins at this temperature, then poured into a separating
funnel, diluted with Et2O (25 mL), and quenched with brine. After phase separation, the aqueous
phase was extracted with Et2O (2
filtered, and the solvent removed in vacuo. Purification of the crude product was achieved by column
×
25 mL). The combined organic phases were dried over MgSO4,
chromatography on silica gel using 5% Et2O/Hexane (v/v) to give the title compound as a colourless
oil (738 mg, 3.29 mmol, 89%).
5.94–5.58 (m, 2H, 2(-C
δ (400 MHz, CDCl ): 7.34–7.24 (m, 2H, Ar-H), 7.23–7.13 (m, 3H, Ar-H),
H
3
H
=)), 3.94 (d, 2H, CH2Br), 2.71 (t, J = 7.6 Hz, 2H, -CH2Ph), 2.40 (app-q, 2H,
-CH2CH=). LRMS: (ESI+) m/z calcd for C11H13BrNa ([M + Na]+): 247.0. Found: 247.0. The spectral
data were in accordance with those reported in the literature [40].
(E)-S-(5-phenylpent-2-en-1-yl) ethanethioate (4): To a stirred solution of 3 (784 mg, 3.5 mmol, 1.0 equiv,)
in DMF (2.5 mL) at room temperature was added KSAc (1.197 g, 10.5 mmol, 3.0 equiv.), and the
suspension was stirred vigorously for 18 h. The mixture was diluted with Et2O (25 mL), poured into a
separating funnel, washed with water (20 mL), then with a 1 M HCl/brine mixture (7 × 50 mL). The
organic phase was dried over MgSO4, filtered, and the solvent evaporated in vacuo to yield the title

Molecules 2018, 23, 897
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compound as a colourless oil (678 mg, 3.08 mmol, 88%) without the requirement for further purification.
(400 MHz, CDCl ): 7.31–7.25 (m, 2H, Ar-H), 7.22–7.15 (m, 3H, Ar-H), 5.75–5.65 (m, 1H, -C =),
δ
H
H
3
5.50–5.40 (m, 1H, -CH=), 3.49 (d, J = 7.1 Hz, 2H, -CH2SAc), 2.67 (t, J = 7.8 Hz, 2H, -CH2Ph), 2.37–2.29
(m, 5H, SAc, -CH2CH=). LRMS: (ESI+) m/z calcd for C13H16OSNa ([M + Na]+): 243.1. Found: 243.1.
The spectral data were in accordance with those reported in the literature [41].
(E)-5-phenylpent-2-ene-1-thiol (
(15 mL) under an atmosphere of argon was added 1 M DIBAL-H (4 mL, 3.9 mmol, 2.5 equiv.) and
the mixture was stirred for 2 h at
78 ◦C. The reaction was then warmed to rt and H2O (2 mL), then
5): To a solution of 4 (350 mg, 1.59 mmol, 1.0 equiv.) in anhydrous CH2Cl2
−
aqueous 1 M NaOH solution (15 mL), followed again by H2O (5 mL), were added, with 5 minutes
between each addition, maintaining vigorous stirring. The mixture was then poured into a separating
funnel, and after phase separation, the aqueous phase was extracted with CH2Cl2 (2
combined organic layers were then washed with brine, dried over MgSO4, filtered through a plug of
silica gel, and the solvent removed in vacuo to yield the thiol as a colourless wax (263 mg,1.479 mmol,
×
25 mL). The
93%) without the requirement for further purification.
δ (400 MHz, CDCl ): 7.35–7.28 (m, 2H, Ar-H),
H
3
7.25–7.17 (m, 3H, Ar-H), 5.64–5.59 (m, 2H, 2(=CH-)), 3.17–3.11 (m, 2H, -CH2SH), 2.72 (t, J = 7.8 Hz,
2H, -CH2Ph), 2.40–2.33 (m, 2H, -CH2CH=), 1.40 (t, J = 7.5 Hz, 1H, SH). δC (100 MHz, CDCl3): 141.7
(q, Ar-C), 131.1 (=
CH), 129.8, 128.5, 128.3 (Ar-C), 125.9 (=CH), 35.6 (-CH2Ph), 33.9 (-CH2CH=), 26.8
(-CH2SH). LRMS: (ESI−): calcd. C11H14SNa ([M + Cl]−): 213.1. Found 213.1.
(E)-S-(5-Phenylpent-2-en-1-yl)-2-bromoethanethioate ( ): To DIC (460 L, 2.96 mmol, 2.0 equiv.) in
anhydrous THF (5.5 mL), under argon was added BrAcOH (411 mg, 2.96 mmol, 2.0 equiv.). The
mixture was stirred for 15 min and then (263 mg, 1.48 mmol, 1.0 equiv.) and DMAP (90 mg,
6
µ
5
0.74 mmol, 0.5 equiv.) in anhydrous THF (5.5 mL) under an atmosphere of argon were added. The
mixture was stirred at rt for 14 h, then diluted with Et2O (150 mL), filtered through a plug of silica
and the solvent removed in vacuo. The crude product was purified by column chromatography
on silica gel using 3% Et2O/Hexane (v/v) to furnish
oil. Rf = 0.43 (5% E2tO/Hexane). (400 MHz, CDCl3): 7.23–7.07 (m, 5H, Ar-H), 5.83–5.73 (m,
1H, -C =), 5.53–5.43 (m, 1H, -C =), 4.20 (s, 2H, -CH2Br), 3.59 (d, J = 7.4 Hz, 2H, -CH2S), 2.71 (t,
J = 7.9 Hz, 2H, -CH2Ph), 2.38 (app-q, 2H, -CH2CH=). δC (100 MHz, CDCl3): 193.6 (C=O), 141.5 (q,
Ar-C), 134.6 (= H-), 128.5, 128.3, 125.9 (Ar-C), 124.3 (= H-), 48.0 (- H2Br), 35.5 (- H2Ph), 34.0 (- H2S)
31.8 (- H2CH=).
max (ATR)/cm−1: 608 (C-Br), 1084 (C-S), 1621 (C=C), 1687 (C=O). m/z HRMS (ESI+)
calcd for C13H19NOSBr = 316.0365 [M + NH4]+. Found 316.0364.
6 (344 mg, 1.15 mmol, 78%) as a colourless
δ
H
H
H
C
C
C
C
C
C
ν
(E)-2-((5-phenylpent-2-en-1-yl)thio)ethan-1-ol (
in anhydrous PhMe (10 mL) under argon at
(1.0 mL, 1.00 mmol, 1.5 equiv.). After complete disappearance of the starting material was observed by
7
): To a stirred solution of
6
(200 mg, 0.66 mmol, 1.0 equiv.)
◦
−
78 C was added 1 M DIBAL-H in PhMe dropwise
TLC (30 min), 1 M in hexanes Et3B (0.33 mL, 0.33 mmol, 0.5 equiv.), nBu3SnH (0.27 mL, 1.00 mmol,
1.5 equiv.), and air (ca 1 mL) were simultaneously added at
−
78 ◦C. The mixture was maintained at
this temperature for 14 h. The reaction mixture was then warmed to rt and quenched with saturated
NaF solution (30 mL). The mixture was stirred vigorously for 2 h, poured into a separating funnel
and diluted with CH2Cl2 (100 mL). After phase separation, without agitation, the aqueous phase was
extracted with CH2Cl2 (2
×
50 mL). The combined organic layers were then washed with brine (25 mL),
dried over MgSO4, filtered and the solvent removed in vacuo. The crude product was purified by
column chromatography on anhydrous K2CO3/silica gel (10% (v/v)) using 35% Et2O/Hexane (v/v) to
give
7
(47 mg, 0.21 mmol, 32%) as a colourless oil. Rf = 0.17 (30% Et2O/Hexane (v/v)).
δ (300 MHz,
H
CDCl3): 7.34–7.28 (m, 2H, Ar-H), 7.25–7.17 (m, 3H, Ar-H), 5.64–5.39 (m, 2H, 2 (=C
H
)), 3.67 (t, J = 6.1
Hz, 2H, -CH2OH), 3.11 (d, J = 6.9 Hz, 2H, -SCH2CH=), 2.74 (t, J = 7.2 Hz, 2H, -CH2Ph), 2.60 (t, J = 6.1
Hz, 2H, -CH2S-), 2.41 (app-q, 2H, -CH2CH=), 2.24 (bs, 1H, OH). δC (75 MHz, CDCl3): 141.5 (q, Ar-C),
133.2 (=
33.5 (-S
(ESI+) calcd for C13H18OS = 222.1078 (M)+. Found 222.1074.
C
H-), 128.5, 128.4, 126.5 (Ar-C), 125.9 (=
CH-), 60.2 (-CH2OH), 35.7 (-CH2S-), 33.9 (-CH2CH=),
CH2CH=), 33.3 (- H2Ph).
C
v
max (ATR)/cm−1: 1021 (C-S), 1048 (C-O), 3352 (O-H). m/z HRMS

Molecules 2018, 23, 897
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2-(Phenylselanyl)acetic acid (
8
): To a stirred solution of (PhSe)2 (3.48 g, 11.2 mmol, 1.0 equiv.) in MeOH
(40 mL) at 0 ◦C was added NaBH4 (2.00 g, 50.0 mmol, 2.5 equiv.) slowly over 30 mins. After the final
addition the reaction was stirred for a further 30 mins. BrAcOH (3.06 g, 11.2 mmol, 1.0 equiv.) in
MeOH (8 mL) was then added. The reaction was warmed to rt and stirred for 15 mins. The solvent
was removed in vacuo and the crude yellow residue was redissolved in Et2O (25 mL) and poured in
a separating funnel. The organic phase was extracted with water (50 mL). The aqueous phase was
then acidified with 4 M HCl and the aquesous phase was extracted with CH2Cl2 (5
combined organic phases were dried over MgSO4, filtered, and the solvent removed in vacuo to yield
as a yellow oil (1.766 g, 8.18 mmol, 73%) without further purification being required. H (400 MHz,
CDCl3): 11.3 (bs, 1H, CO2H), 7.56–7.50 (m, 2H, Ar-H), 7.26–7.20 (m, 3H, Ar-H), 3.45 (s, 2H, -CH2−).
LRMS: (ESI−) m/z calcd for C8H7O2Se ([M H]−): 215.0. Found: 215.0. The spectral data were in
×
20 mL). The
8
δ
−
accordance with those reported in the literature [21].
(E)-S-(5-Phenylpent-2-en-1-yl) 2-(phenylselanyl)ethanethioate (9): To a solution of 8 (400 mg, 2.0 mmol, 2.0
equiv.) in anhydrous THF (5.5 mL), under an atmosphere of argon was added DIC (0.6 mL, 4.0 mmol,
2.0 equiv.). The mixture was stirred for 15 min and then (178 mg, 1.0 mmol, 1.0 equiv.) and DMAP
6
(61 mg, 0.5 mmol, 0.5 equiv.) in anhydrous THF (5.5 mL) under an atmosphere of argon were added.
The mixture was stirred at rt for 14 h, then diluted with Et2O (150 mL), filtered through a plug of
silica and the solvent removed in vacuo. The crude product was purified by column chromatography
on silica gel using 5% EtOAc/Hexane (v/v) to furnish
Rf = 0.27 (5% Et2O/Hexane). (400 MHz, CDCl ): 7.60–7.55 (m, 2H, Ar-H), 7.32–7.25 (m, 5H, Ar-H),
7.22–7.13 (m, 3H, Ar-H), 5.75–5.64 (m, 1H, =C
J = 7.3 Hz, -CH2S), 2.67 (t, J = 7.8 Hz, 2H, -CH2Ph), 2.32 (app-q, 2H, -CH2CH=). δC (100 MHz, CDCl3):
195.4 (C=O), 141.6 (q, Ar-C), 133.9 (= H-), 133.6, 129.3, 128.5, 128.4 (Ar-C), 128.0 (q, Ar-C), 125.9 (Ar-C),
124.8 (= H-), 36.5 (- H2Se), 35.5 (- H2Ph), 34.1 (- H2CH=), 32.0 (- H2S).
(ATR)/cm−1: 1679
(C=O). m/z HRMS (ESI+) calcd for C19H21OSSe = 377.0473 [(M + H)]+. Found 377.0469.
9 (267 mg, 0.71 mmol, 71%) as a yellow oil.
δ
H
3
H
-), 5.47–5.38 (m, 1H, =CH-), 3.76 (s, 2H, -CH2Se), 3.50 (d,
C
C
C
C
C
C
ν
max
4-(3-Phenylpropyl)dihydrothiophen-2(3H)-one (10): To a stirred solution of (150 mg, 0.50 mmol, 1.0 equiv.)
6
in degassed anhydrous C6H6 (50 mL) under an atmosphere of argon was added AIBN (8.2 mg,
10 mol%). The mixture was heated at reflux for 5 mins and then a solution of nBu3SnH (0.20 mL,
0.75 mmol, 1.5 equiv.) in degassed anhydrous C6H6 (1.8 mL) and a solution of AIBN (20.2 mg, 25 mol%)
in degassed anhydrous C6H6 (2.0 mL) were added separately and simultaneously over 16 h (using a
syringe pump) to the refluxing solution. The solvent was removed in vacuo and the crude product
was purified by column chromatography on anhydrous K2CO3/silica gel (10% (v/v)) using 10%
Et2O/Hexane (v/v)) to give the title compound as a colourless oil (34.5 mg, 0.16 mmol, 31%). Rf = 0.31
(10% Et2O/Hexane (v/v)).
δ (300 MHz, CDCl ): 7.68–7.24 (m, 5H, Ar-H), 3.32–3.29 (m, 1H), 2.99–2.96
H
3
(m, 1H), 2.59–2.53 (m, 2H), 2.52–2.48 (m, 1H), 2.46–2.42 (m, 1H), 2.40–2.36 (m, 1H), 1.63–1.58 (m, 2H),
1.53–1.49 (m, 2H). δC (75 MHz, CDCl3): 208.1 (C=O), 141.7 (q, Ar-C), 128.6, 128.3, 126.0 (Ar-C), 47.4
(CH2), 39.4 (CH), 38.1, 35.9, 33.7, 29.9 (C
H2). vmax (ATR)/cm−1: 1163 (C-S), 1702 (C=O). m/z HRMS
(ESI+) calcd for C13H16OS = 220.0922 ([M])+. Found 220.0920.
(E)-S-(5-Phenylpent-2-en-1-yl) 2-bromopropanethioate (11): To EDC (310 µL, 2.0 mmol, 2.0 equiv.) in
anhydrous THF (5.5 mL), under argon was added MeBrAcOH (306 mg, 2.0 mmol, 2.0 equiv.). The
mixture was stirred for 15 min and then (178 mg, 1.0 mmol, 1.0 equiv.) and DMAP (61 mg, 0.50 mmol,
6
0.5 equiv.) in anhydrous THF (5.5 mL) under an atmosphere of argon were added. The mixture was
stirred at rt for 14 h, then diluted with Et2O (150 mL), filtered through a plug of silica and the solvent
removed in vacuo. The crude product was purified by column chromatography on silica gel using
4% EtOAc/Hexane (v/v) to furnish 11 (253 mg, 0.81 mmol, 81%) as a colourless oil. Rf = 0.43 (10%
EtOAc/Hexane).
1H, =C -), 5.52–5.41 (m, 1H, =C
(t, J = 7.7 Hz, -C 2Ph), 2.35 (app-q, 2H, -C
196.0 (C=O), 141.6 (q, Ar-C), 134.5 (= H-), 128.5, 128.4, 125.9 (Ar-C), 124.3 (=
δ
H (400 MHz, CDCl3): 7.33–7.25 (m, 2H, Ar-H), 7.23–7.15 (m, 3H, Ar-H), 5.80–5.70 (m,
-), 4.51 (q, J = 6.9 Hz, 1H, -C Br), 3.55 (d, J = 7.1 Hz, 2H, -C 2S), 2.69
2CH=), 1.85 (d, J = 6.9 Hz, 3H, -C 3). δC (100 MHz, CDCl3):
H-), 48.1 (- HBr), 35.5
H
H
H
H
H
H
H
C
C
C

Molecules 2018, 23, 897
9 of 11
(-
C
H2Ph), 34.1 (-
C
H2CH=) 32.1 (-
C
H2S), 22.1 (-
C
H3).
ν
max (ATR)/cm−1: 641 (C-Br), 1096 (C-S), 1690
(C=O). m/z HRMS (APCI+) calcd for C14H18OSBr = 313.0257 [(M + H)]+. Found 313.0256.
(E)-S-(5-Phenylpent-2-en-1-yl) propanethioate (12): To a stirred solution of 11 (156 mg, 0.50 mmol, 1.0
equiv.) in degassed anhydrous C6H6 (50 mL) under an atmosphere of argon was added AIBN (8.2 mg,
10 mol%). The mixture was heated at reflux for 5 min and then a solution of nBu3SnH (0.20 mL,
0.75 mmol, 1.5 equiv.) in degassed anhydrous C6H6 (1.8 mL) and a solution of AIBN (20.2 mg,
25 mol %) in degassed anhydrous C6H6 (2.0 mL) were added separately and simultaneously over 16 h
(using a syringe pump) to the refluxing solution. The solvent was removed in vacuo and the crude
product was purified by column chromatography on anhydrous K2CO3/silica gel (10% (v/v)) using
10% Et2O/Hexane (v/v)) to give the title compound as a colourless oil (106 mg, 0.455 mmol, 91%).
Rf = 0.42 (10% Et2O/Hexane (v/v)).
3H, Ar-H), 5.76–5.67 (m, 1H, =CH−), 5.52–5.42 (m, 1H, =C
(t, J = 7.7 Hz, 2H, -CH2Ph), 2.58 (q, J = 7.5 Hz, 2H, -CH2C=O), 2.34 (app-q, 2H, -
J = 7.5 Hz, 3H, - H3). δC (100 MHz, CDCl3): 199.8 (C=O), 141.7 (q, Ar-C), 133.5 (=
125.9 (Ar-C), 125.4 (= H-), 37.3 (- H2C=O), 35.6 (- H2Ph), 34.1 (- H2CH=), 31.0 (-
max(ATR)/cm−1: 1692 (C=O). m/z HRMS (APCI+) calcd for C14H19OS = 235.1152 ([M + H])+. Found
235.1151.
δ (400 MHz, CDCl ): 7.33–7.25 (m, 2H, Ar-H), 7.23-7.25 (m,
H
3
H
-), 3.51 (d, J = 7.1 Hz, 2H, -CH2S), 2.69
H2CH=), 1.98 (t,
H-), 128.5, 128.3,
H2S), 9.7 (- H3).
C
C
C
C
C
C
C
C
C
v
(E)-S-(5-phenylpent-2-en-1-yl) 2-((ethoxycarbonothioyl)thio)ethanethioate (13): To a stirred solution of
6
(298 mg, 1.0 mmol, 1.0 equiv.) in EtOH (10 mL) at 0 ◦C was added KSCSOEt (160 mg, 1.0 mmol, 1.0
equiv), and the reaction was stirred at this temperature for 1.5 h. The mixture was then filtered, and
the solvent removed in vacuo to yield 13 as a colourless wax (320 mg, 0.94 mmol, 94%) without the
requirement for further purification.
Ar-H), 5.79–5.70 (m, 1H, =C -), 5.51–5.42 (m, 1H, =C
-CH2C=O), 3.56 (d, J = 7.1 Hz, 2H, -CH2S), 2.69 (t, J = 7.5 Hz, 2H, -CH2Ph), 2.35 (app-q, 2H,
1.45 (t, J = 7.1 Hz, 3H, CH3). δC (100 MHz, CDCl3): 211.8 (C=S), 193.7 (C=O), 141.6 (q, Ar-C), 134.3
(= H-), 128.4, 128.3, 125.9 (Ar-C), 124.5 (= H-), 70.9 (O H2-), 45.6 (- H2C=O), 35.5 (- H2Ph), 34.0
(-
H3). vmax (ATR)/cm−1: 1050 (C=S), 1683 (C=O). m/z LRMS ESI−:
δ (400 MHz, CDCl ): 7.34–7.26 (m, 2H, Ar-H), 7.24–7.16 (m, 3H,
H
3
H
H
-), 4.68 (q, J = 7.1 Hz, 2H, -C
H2O), 4.13 (s, 2H,
CH2CH=),
C
C
C
C
C
C
H2CH=), 32.0 (-CH2S),13.7 (-C
calcd. C16H19O2S3 ([M − H]−): 339.1. Found 339.1.
4. Conclusions
In conclusion, Ueno-Stork cyclisation of
radical fragmentation that occurs upon formation of the aluminium thioacetal. Evidence supporting
this theory can be derived from the fragmentation/rearrangement of selanyl compound . However, a
method for the radical cyclisation of -bromo-allylic thioesters has been reported, though this produces
thiolactones in poor yield. Additionally, substitution does not appear to be tolerated and the reaction
α-halo thioacetals cannot be achieved, likely due to the
9
α
α
does not appear to be general. Work is currently ongoing for the optimisation of this cyclisation
process and the xanthate group cyclisation strategy. Furthermore, alternative cyclisation routes, such
as atom-transfer-cyclisation methods are also being pursued.
Supplementary Materials: The following are available online, 1H- and 13C-NMR Spectra for Compounds
9, 11, 12 and 13.
5, 6, 7,
Acknowledgments: This work was supported by Science Foundation Ireland (SFI) under grant number
15/CDA/3310 (E.M.S.) and by philanthropic donation to Trinity College Dublin by Beate Schuler (R.O.M.)
and ANR grant (ANR Blanc 13 BS07-0010-01 “UZFul-Chem”), (F.D.). French Embassy in Dublin, travel bursary
(E.M.S. and R.O.M.).
Author Contributions: R.O.M. synthesized and characterized the compounds. F.D. and E.M.S. generated
and managed the project and wrote the manuscript. All authors discussed the results and commented on
the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2018, 23, 897
10 of 11
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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