A low-temperature, photoinduced thiol-ene click reaction: A mild and efficient method for the synthesis of sugar-modified nucleosides

Article abstract of DOI:10.1039/c7ob02184d

Sugar-modified nucleosides are prime synthetic targets in anticancer and antiviral drug development. Radical mediated thiol-ene coupling was applied for the first time on nucleoside enofuranoside derivatives to produce a broad range of thio-substituted d-ribo, -arabino, -xylo and l-lyxo configured pyrimidine nucleosides. In contrast to the analogous reactions of simple sugar exomethylenes, surprisingly, hydrothiolation of nucleoside alkenes under the standard conditions of various initiation methods showed low to moderate yields and very low stereoselectivity. Optimizing the reaction conditions, we have found that cooling the reaction mixture has a significant beneficial effect on both the conversion and the stereoselectivity, and UV-light initiated hydrothiolation of C2′-, C3′- and C4′-exomethylene derivatives of nucleosides at -80 °C proceeded in good to high yields, and, in most cases, in excellent diastereoselectivity. Beyond the temperature, the solvent, the protecting groups on nucleosides and, in some cases, the configuration of the thiols also affected the stereochemical outcome of the additions. The anomalous l-lyxo diastereoselectivity observed upon the addition of 1-thio-β-d-gluco- and galactopyranose derivatives onto C4′,5′-unsaturated uridines is attributed to steric mismatch between the d-ribo C4′-radical intermediates and the β-configured 1-thiosugars.

Full text of DOI:10.1039/c7ob02184d

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DOI: 10.1039/C7OB02184D
Journal Name
ARTICLE
Low-temperature, photoinduced thiol-ene click reaction: a mild
and efficient method for the synthesis of sugar-modified
nucleosides
Received 00th January 20xx,
Accepted 00th January 20xx
Miklós Begea, Ilona Bereczkia, Mihály Herczega, Máté Kicsáka , Dániel Eszenyia, Pál Herczegha and
Anikó Borbása
DOI: 10.1039/x0xx00000x
www.rsc.org/
Sugar-modified nucleosides are prime synthetic targets in anticancer and antiviral drug development. Radical mediated
thiol-ene coupling was applied for the first time on nucleoside enofuranoside derivatives to produce a broad range of thio-
substituted D-ribo, -arabino, -xylo and L-lyxo configured pyrimidine nucleosides. In contrast to analogous reactions of simple
sugar exomethylenes, surprisingly, hydrothiolation of nucleoside alkenes under the standard conditions of various initiation
methods showed low to moderate yields and very low stereoselectivity. Optimizing the reaction conditions, we have found
that cooling the reaction mixture has a significant beneficial effect on both the conversion and the stereoselectivity, and UV-
light initiated hydrothiolation of C2’- C3’- and C4’-exomethylene derivatives of nucleosides at −80 °C proceeded in good to
high yields, and, in most cases, in excellent diastereoselectivity. Beyond the temperature, the solvent, the protecting groups
on nucleosides and, in some cases, the configuration of the thiols also affected the stereochemical outcome of the additions.
The anomalous L-lyxo diastereoselectivity observed upon addition of 1-thio-β-D-gluco- and galactopyranose derivatives onto
C4’,5’-unsaturated uridines is attributed to steric mismatch between the D-ribo C4’-radical intermediates and the -
configured 1-thiosugars.
the last years.6 Due to the mild conditions, atom economy and
regioselectivity, this process has been extensively utilised in
glycochemistry.7 We and others have reported that sugar-
Introduction
The application of nucleosides and nucleic acids in therapy,1 has
prompted the development of nucleoside analogues with
enhanced chemical and biological properties. The modification
of ribose oxygen in nucleosides with other elements such as
carbon, nitrogen, fluorine or sulfur is a proven strategy for
producing new drug candidates.2 For example, 5’-thio
nucleosides are studied as selective inhibitors against essential
enzymes,3 while C2’- or C3’-branched nucleosides have shown
good antitumor or antiviral activity.2 Ribose modification is also
used to control the sugar puckering and thereby increase
nucleic acid resistance.4 Versatile and stereoselective alteration
of the furanose residue in nucleosides is an important challenge
for synthetic chemists.
derived alkenes, including endo- and exoglycals, can be
employed as acceptor substrate in the photoinitiated thiol-ene
chemistry to produce various thiosugars and S-linked
glycoconjugates in excellent stereoselectivity.8-10 Although only
two examples have emerged in the literature with furanoid
alkenes, both demonstrated that hydrothiolation of 3-
exomethylene-8c and 4-exomethylene-furanosides9a showed
complete stereoselectivity.
On the basis of the above results, we envisioned the extension
of the photoinitiated thiol-ene reaction to nucleoside
enofuranoside derivatives.
We present here that the thiol-ene click reaction conducted at
low temperature represents a generally applicable novel
strategy for the efficient modification of nucleosides with
various thiol substituents at C2’-, C3’- and C5’-positions.
The radical-mediated addition of thiols to non-activated
alkenes,5 also known as thiol-ene coupling, had widespread
application in materials chemistry and chemical biology during
Results and discussion
We first investigated the addition of 1-propanethiol to the easily
available 4’,5’-unsaturated uridine 1, under previously established
standard conditions for the synthesis of S-glycoconjugates,8
irradiating at λmax = 365 nm at room temperature in the presence of
the cleavable photoinitiator 2,2-dimethoxy-2-phenylacetophenone
(DPAP) (Table 1, Entries 1 and 2). According to our expectation,§ thiol
2 added exclusively across the exocyclic double bond of the furanose
residue providing the 5’-S-propyl derivative 3. However, low level of
diastereoselectivity (2:1 D-ribo:L-lyxo ratio) was observed and the
yield was only 69% even with 6 equiv. of thiol due to incomplete
conversion of 1. This result was surprising because Dondoni and
a.Departement of Pharmaceutical Chemistry, University of Debrecen, H-4032
Debrecen Egyetem Tér 1. Hungary
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Experimental details,
characterization of all reported compounds, copies of NMR spectra for all
compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Marra described excellent yield and exclusive ribo-selectivity for catechol-mediated addition (Entry 10), the reaction waVsievwerAyrtisclleugOgnilisnhe
DOI: 10.1039/C7OB02184D
hydrothiolation of the methyl -D-riboside counterpart of 1 with a at −80 °C, and the overall efficacy was inferior to that of the
slight excess of thiols.9a To evaluate if the yield and the photoinitiated reaction.
stereoselectivity can be influenced by the initiation methods, the After optimizing the conditions of the thiol-ene addition, the
reaction was repeated upon various conditions including thermal substrate scope was investigated. First, compound
1
was
and
8 and dithiol
11
initiation with AIBN, photoredox activation in the presence of TiO2
or using Et3B12 as the radical initiator (Table 1, Entries 3-6).
reacted with a variety of thiols, including 1-thiosugars
amino acid derivatives and , sulfonic acid salt
4
5,
6
,
7
9
10, at −80 °C with a 1.2:1 thiol:ene ratio (Table 2, Entries 1-12).
We were pleased to find that the addition of thiols to the
Table 1. Free radical addition of propanethiol to 1 upon various initiation methodsa
exomethylene moiety of
yields (71-92%) and, except for the 1-thiosugar cases, with high
levels of -ribo diastereoselectivity. The reaction was
1 occurred with good to excellent
D
compatible with the carboxylic acid function (Entries 7, 8 and
10) and with the sensitive Fmoc group (Entry 8) and in most
cases went to completion with the slight thiol excess applied. In
this context, this method is a mild and economic alternative to
the conventional nucleophilic substitution which generally
requires strong basic conditions and a higher thiol excess.§§
Thiol
equiv.
D-ribo : Yield
L-lyxob (%)c
Entry
Initiation
Solvent
T
Time
Surprisingly, the addition of 1-thio--D-glucose
4 in toluene,
either at room temperature or at −80 °C, proceeded with a
complete lack of stereoselectivity, while running this reaction in
3×15 min
1
2
3
4
5
6
7
8
3
6
8
3
3
4
2
2
DPAP, hd
DPAP, hd
AIBN
toluene
toluene
rt
rt
2:1
2:1
60
69
54
38
59
7
3×15 min
6 h
MeOH or in a MeOH-toluene mixture at −80 °C, a modest
L-lyxo
selectivity was observed (Entries 1-4). Addition of the 2-
toluene 120 °C
1.5:1
2:1
acetamido-1-thio--D-glucopyranose derivative
5
onto
1
also
Et3B
CH2Cl2
rt
rt
rt
2 days
4 h
showed an L-lyxo selectivity which reached the 4.5:1
L-lyxo:
D-ribo
Et3B, catechol CH2Cl2
2:1
ratio at −80 °C (Entries 5 and 6). We assumed that this opposite
stereoselectivity was caused by the higher steric demand of the
TiO2, he
DPAP, hd
DPAP, hd
CH2Cl2
2 days
3×15 min
3×15 min
2:1
glycosyl thiols relative to the primary thiols
this assumption, the bulky 2-methylpropane-2-thiol 11 was
reacted with 1 §§§
Unexpectedly, a relatively high -ribo
selectivity (3:1 -ribo: -lyxo ratio) was observed at room
temperature, which, however, decreased by cooling to a 2:1
ribo: -lyxo ratio (Entries 13 and 14). Although this result
6-10. To examine
toluene −30 °C
4:1
88
89
.
D
toluene −80 °C
5:1
D
L
toluene-
MeOH
3×15 min
9
2
2
DPAP, hd
−80 °C
6.3:1
2.5:1
88
64
D-
L
CH2Cl2- −80 to
MeOH −20 °C
10f
Et3B, catechol
24 h
confirmed that direction of the H-abstraction by the C-4’
centered radical intermediate can be influenced not only by the
temperature but also the bulkiness of the thiol, it did not explain
aThe reactions were carried out on a 0.2–0.5 mmol scale; bratio of products
determined by 1H NMR; coverall yield of products isolated by column
chromatography, the low yield was caused by low conversion of 1; dirradiation by
UV light (max = 365 nm), the reaction was carried out in a borosilicate vessel
without any caution to exclude air or moisture; eirradiation by visible light using a
100 W domestic lightbulb; fkept in refrigerator overnight.
the lyxo-selectivity observed with the thiosugars
4 and 5. Next,
alkene 1 was reacted with the 1-thio--D-mannopyranose derivative
12 and the -thiosugars 13 and 14 (Entries 15-22). To our great
surprise, the addition of the -thiosugar 12 occurred with a
significant D-ribo selectivity at rt which was further increased to a 8:1
Unfortunately, neither the yield nor the selectivity could be
increased. The Et3B-catechol reagent system, which was developed
for the hydrothiolation of allylic double bonds,12a showed similar
efficacy as the UV-initiated reaction (Entries 5 and 1). The other
initiation methods proved to be less efficient, due to the low
conversion of 1 (Entries 4 and 6) and the thermal activation even
slightly eroded the D-ribo diastereoselectivity (Entry 3). Next, the
temperature effect on the photoinduced addition was studied. To
our great delight, both the stereoselectivity and the yield could
substantially be improved by cooling even with a much lower thiol
D-ribo:
L-lyxo ratio when the reaction was carried out at −80 °C.
However, similar reactions with the

-
congener 13 at either rt
and −80 °C proceeded with complete lack of stereoselectivity,
while the addition of the 1-thio--D-galactopyranose 14 followed
the stereoselectivity of the gluco-epimers 4 and 5. These results
clearly demonstrate that the anomeric configuration of the
thiosugars exerts profound effect on the stereochemical outcome of
the addition which can also be modified slightly by the solvent and
by the C2 configuration. To the best of our knowledge, it has not
been observed before that the configuration of the thiols can
affect the stereochemical outcome of the thiol-ene coupling.
o
excess (Entries 7-9). At −30 C, an 88% overall yield was achieved
with 2 equiv. of thiol, and the stereoselectivity was increased to a 4:1
D-ribo:L-lyxo ratio. By cooling the reaction to −80 °C the selectivity
reached the 5:1 D-ribo:L-lyxo ratio in toluene and the use of a
toluene-MeOH 1:1 solvent mixture led to an even higher D-ribo
selectivity. Although the cooling was also beneficial for the Et3B-
2 | J. Name., 2012, 00, 1-3
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The thiol-ene reactions of ribothymidine 26 (TableV2ie,wEAnrttircileeOsn2li3ne-
26) showed the same stereoselectivity trDeOndI:s10a.1s0o39b/sCe7rOvBe0d2w18i4tDh
Table 2. Photoinduced addition of various thiols to 4’-enofuranoside uridine and
ribothymidine at low temperature
the uridine analogue
slight -lyxo selectivity, while a significant
observed with the primary thiols and
1
. Addition of 1-thioglucose
-ribo preference was
. Although the yields
4 showed a
L
D
2
7
were slightly lower with ribothymidine than uridine, they still
remained in a preparatively useful range.
To determine if the protecting groups on the alkene were able
to influence the stereochemical outcome of the addition, the
uridine derivatives 30 and 32 were reacted with thiol
4, initially
at room temperature. Similarly to the analogous reaction of
1
,
Entry
Thiol
Solvent
Product D-ribo Yield
lack of stereoselectivity was observed with the acetyl-protected
alkene 30, moreover, the yields were low. (Table 3, Entries 1-2).
Interestingly, the reaction of the tert-butyldimethylsilyl-
protected 32 with 4 showed a remarkable ~4:1 L-lyxo selectivity
at rt, applying either the UV-irradiation or the Et3B-catechol-
mediated conditions (Table 3, Entries 4 and 5). Repeating the
:
(%)b
L-lyxoa
1.1:1
1:1
1:3
1:2
1c
2
3
toluene, rt
toluene
toluene-MeOH
MeOH
15
15
15
15
87
89
88
81
4
photoinitiated reaction at −80 °C, an increased 6:1
dr was achieved in an excellent 98% overall yield (Entry 6). The
addition of 1-propanethiol onto 32 proceeded with a 2.5:1
lyxo preference at room temperature. However, the cooling
favoured again the formation of the -ribo isomer, as observed
in the reactions of with primary thiols and led to the complete
L-lyxo:D-ribo
toluene-MeOH,
rt
toluene-MeOH
5
6
16
16
1:1.25 77
1:4.5
10:1
80
92
L-
D
7d
toluene-MeOH
17
1
loss of stereoselectivity at −80 °C (Table 3, Entries 7 and 8).
8
9
toluene-MeOH
MeOH-DMF 5:1
18
19
10:1
14:1
89
85
Table 3. Hydrothiolation of 4’-methyleneuridine derivatives bearing different protecting
groups
10
MeOH
20
6:1
91
11c
12c
toluene
toluene:MeOH
21
21
6:1
6:1
71
70
13d
14d
toluene, rt
toluene, -40 oC
22
22
3:1
2:1
58
62
15
16
toluene, rt
toluene
23
23
3.5:1
8:1
60
89
D-ribo :
L-lyxoa
1.1:1
Yield
(%)b
56c
Entry
Alkene
30
Thiol
Initiation
T
17
18
19
toluene, rt
toluene
toluene-MeOH
24
24
24
1.2:1
1:1
1:1
56
72
66
1
2
4
5
4
4
4
4
DPAP, h
Et3B,
catechol
DPAP, h
Et3B,
catechol
DPAP, h
DPAP, h
DPAP, h
rt.
rt.
rt.
rt.
30
1.1:1
1:3.7
1:3.5
54
77
82
20
21
22
toluene, rt
toluene
toluene-MeOH
25
25
25
1:1.6
1:1.6
1:3.5
68
80
78
32
32
6
7
8
32
32
32
4
2
2
−80°C
rt.
−80°C
1:6
1:2.5
1:1
98
59
67
23
24
toluene-MeOH
27
1:2.5
80
aRatio of products determined by 1H NMR; boverall yield of products isolated by
column chromatography.
toluene-MeOH
28
5:1
64
25
toluene
toluene
29
29
5:1
5:1
59
78
To further study the alkene scope, compounds 35, 38 and 41
were subjected to thiol-ene reactions. First, 35 bearing the
exocyclic double bond at position C2’ was hydrothiolated with
26e
aRatio of products determined by 1H NMR; boverall yield of products isolated by
column chromatography; c1.5 equiv. of thiol was used; d6 equiv. of thiol was used;
e3 equiv. of thiol was used
2
and 4 (Table 4). In contrast to the C4’ exomethylene case, the
addition reactions across the C2’-positioned double bond
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-arabino selectivity using either the Surprisingly, propanethiol 2 did not react with either of the
showed the same trend of
D
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primary thiol
of diastereoselectivity was observed in favour of the
isomer at room temperature with both thiols, the yields were addition products 39 and 42 with fair yields and excellent
2 or the sugar thiol 4
. Although a fairly good level alkenes at room temperature. HoweverD, OtoI: 1o0u.1r03g9r/eCa7tOBd0e2li1g8h4Dt,
D-arabino performing the reactions at −80 °C led to the formation of the
D-xylo
only moderate (Table 4, Entries 1 and 3). We were pleased to selectivities (Entries 1 and 4). Addition of the -1-thioglucose
find that running the reactions at −80 °C significantly improved across the C3’-exomethylene moiety also proceeded with a
4
D-
the yields and also the levels of diastereoselectivity (Table 4, xylo selectivity in all cases (Entries 2, 3, 5 and 6). The low level
Entries 2 and 4).
of diastereoselectivity observed at rt was increased to a 90-96%
range by cooling, however, the yields remained moderate
(Entries 2 vs 3 and 5 vs 6). Finally, the -thiosugar 12 was reacted
with 41 in order to study if the anomeric configuration exerts an
effect on the stereochemical outcome of the reaction (Entry 7).
In this case, the anomeric configuration did not make any
difference in the diastereoselectivity, the addition of both the
Table 4. Hydrothiolation of 2’deoxy-2’-methyleneuridine

-thiosugar 12 and the
excellent level of -xylo selectivity providing the corresponding
products 43 and 44 in the same 50:1 -xylo: -ribo ratio (Entries
6 and 7)
-thiosugar 4 onto 41 occurred at
D
D
D
.
The source of the stereoselectivity in the thiol-ene coupling of
exocyclic alkenes is the preferred H-abstraction by the carbon-
centered radical from the thiol into an axial position.6d,13 We
assume that the reactions presented herein proceed through
the equatorial radicals depicted in Scheme 1, and the
stereochemical outcome of the reactions is controlled by the
relative stability of the radical pairs. Our results suggest that the
proportion of the more stable radical intermediate and thus the
level of stereoselectivity in a given reaction can be greatly
increased by cooling the reaction to −80 oC.
D-arabino:
D-riboa
4:1
12.5:1
5:1
Yield
(%)b
39
68
68c
89
Entry
Thiol
Product
T
2
2
4
4
1
2
3
4
36
36
37
37
rt.
−80°C
rt.
−80°C
10:1
aRatio of products determined by 1H NMR; boverall yield of products isolated by
column chromatography; csimilar results were obtained with Et3B-catechol
Finally, the C3’-exomethylene derivatives 38 and 41 were
In the case of the C2’- and C3’-exomethylene derivatives, the
reactions preferably go through the more stable radicals
possessing an all-equatorial substitution pattern, leading to the
reacted with thiols 2, 4 and 12 (Table 5).
Table 5. Photoinitiated hydrothiolation of C3’-methylene derivatives of uridine (38) and
ribothymidine (41)
observed
D-arabino-selectivity from the C2’-alkene 35 and the
D-xylo-selectivity from the C3’-alkenes 38 and 41 (Scheme 1).
Although the participation of the less stable D-ribo-configured
C2’- an C3’-radicals is not negligible in the room-temperature
reactions, which explains the low/moderate levels of
diastereoselectivity at rt, it can be significantly suppressed by
cooling
diastereoselectivities.
For the 4’-exomethylene derivatives
resulting
in
the
observed
excellent
1
,
26, 30 and 32, the
stereochemical outcome of the reactions can be influenced by
many factors including the solvent, the protecting groups and
the size and configuration of the thiols. Our results demonstrate
that the low-temperature reactions of
thiols, as well as with the 1-thiomannose derivative 12, go
preferentially through the -ribo C4’-radical, existing in the C2’-
exo (3T2) conformation, leading to the good
-ribo-selectivities.
We assume that the anomalous stereochemical results
observed with the bulky thiol 11 and the -gluco-configured
1-thiosugars ( 13 and 14) can be explained by the steric
congestion of the carbon-centered radicals and, possibly, by a
steric mismatch between the thiosugars and the -ribo
configured C4’-radicals formed from Due to this
26 or 32 14
steric mismatch, the -lyxo radicals participate in the H-
1 and 26 with primary
Entry Alkene Thiol Product
T
D-xylo :D-riboa Yieldb
D
1c
2d
3d
4c,d
5d
6d
7
2
4
4
2
4
39
40
40
42
43
43
44
−80°C
rt.
−80°C
−80°C
rt.
50:1
3:1
75%
26%
49%
39%
27%
30%
60%
38
38
38
41
41
41
41
D
17:1
12.5:1
2:1
-D
4, 5,
4
12
−80°C
−80 °C
50:1
50:1
D
1
,
.
aRatio of products determined by 1H NMR; boverall yield of products isolated by
column chromatography; cno reaction observed at room temperature; dunreacted
starting compounds were recovered.
L
abstraction step in an increased extent compared to the
4 | J. Name., 2012, 00, 1-3
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reactions of primary thiols, thus leading to the increased ratio predict, probably due to the comparable stability of the
View Article Online
DOI: 10.1039/C7OB02184D
of the
L-lyxo isomer in the products.
corresponding
Nevertheless, good levels of
in the low-temperature reactions of the isopropylidene-
protected uridine with primary thiols. Interestingly, the
configured thiosugars did not follow this trend, instead, they
tend to react with an -lyxo selectivity at low temperature.
D
-ribo and
L-lyxo radical intermediates.
D
-ribo selectivity could be reached
1
-
L
Beside enhancing the stereoselectivity, the low temperature
also enhances the yield of the thiol-ene coupling, assumedly, by
exerting a beneficial effect on the overall kinetics of the
reaction.
The investigation of the scope and potential of the low-
temperature thiol-ene coupling on purine nucleosides and on
sugar-alkenes with an endocyclic double bond is underway.
Experimental
General informations
2,3,4,6-Tetra-O-acetyl-1-thio-β-
acetamido-3,4,6-tri-O-acetyl-2-deoxy-1-thio-β-
),17 N-(9-fluorenylmethoxycarbonyl)-
-cystein
mercaptoethyl)thioacetate (10),19 2,3,4,6-tetra-O-acetyl-1-thio-
D
-glucopyranose
-glucopyranose
),18 S-(2-
(
4
),16
2-
Scheme 1. Equatorial C2’, C3’ and C4’ radical intermediates formed in the first, reversible
step of the thiol-ene reactions. All-equatorial radicals are highlighted in red.
D
(5
L
(7
The most unusual finding of our study is the beneficial effect of
cooling on the degree of conversion of the thiol-ene click
reaction. Recent kinetic analysis of thiol-ene coupling has
revealed the importance of the stability of the carbon-centered
radical intermediate, which directly influences not only the
activation barrier of the hydrogen abstraction step but also the
reversibility of the propagation (thiyl addition) step.15 In our
case, the reaction can be accomplished through several radicals
of different stability, and the reaction path involving the most
stable radical is preferred at low temperature. We assume that
the equilibrium of the rapidly reversible propagation step lies
toward the product (carbon-centered radical) in a greater
extent for a more stable radical than for a less stable one. The
other beneficial effect of cooling is that it significantly
suppresses the disulfide formation from the thiyl radical, which
is one of the undesired termination steps of the thiol-ene
coupling. Thereby, the thiol excess applied can substantially be
reduced at low temperature.

-
D
-mannopyranose (12),20 2,3,4,6-tetra-O-acetyl-1-thio-β-
D
-
-
mannopyranose (13),20 and 2,3,4,6-tetra-O-acetyl-1-thio-β-
D
galactopyranose (14)21. were prepared according to literature
procedures. 2,2-Dimethoxy-2-phenylacetophenone (DPAP) and
thiols
Chemical Co. and used without further purification. Synthesis of
nucleoside enofuranosides 26 30 32 35 38 and 41 is
2, 6-9 and 11 were purchased from Sigma Aldrich
1
,
,
,
,
,
described in ESI. Optical rotations were measured at room
temperature with a Perkin-Elmer 241 automatic polarimeter.
TLC was performed on Kieselgel 60 F254 (Merck) with detection
by UV-light (254 nm) and immersing into sulfuric acidic
ammonium-molibdenate solution or 5% ethanolic sulfuric acid
followed by heating. Flash column chromatography was
performed on Silica gel 60 (Merck 0.040-0.063 mm). Organic
solutions were dried over anhydrous Na2SO4 or MgSO4, and
1
concentrated in vacuum. The H NMR (360 and 400 MHz) and
13C NMR (90 and 100 MHz) spectra were recorded with Bruker
DRX-360 and Bruker DRX-400 spectrometers at 25 oC. Chemical
1
shifts are referenced to Me4Si (0.00 ppm for H) and to the
residual solvent signals (CDCl3: 77.2, DMSO-d6: 39.5, CD3OD:
49.0 for 13C). Two-dimensional COSY and 1H−13C HSQC
experiments were used to assist NMR assignments and 2D
ROESY spectra were used for configurational assignments.
MALDI-TOF MS analyses of the compounds were carried out in
Conclusion
In conclusion, we have demonstrated that the low-temperature
photoinitiated thiol-ene reaction provides a facile approach to
various sugar-modified nucleosides including 5’-thiosubstituted
the positive reflectron mode using
a BIFLEX III mass
D-ribo or
L-lyxo derivatives, as well as valuable C2’- and C3’-
spectrometer (Bruker, Germany) equipped with delayed-ion
extraction. 2,5-Dihydroxybenzoic acid (DHB) was used as matrix
and F3CCOONa as cationising agent in DMF. ESI-TOF MS spectra
branched compounds with
The low or moderate stereoselectivity observed at rt upon
hydrothiolation of the C2’- and C3’-exomethylene nucleosides
D
-arabino- or -xylo configuration.
D
were recorded by
a microTOF-Q type QqTOFMS mass
35
corresponding C2’-branched
-xylosyl derivatives could be produced with good to excellent
selectivity.
,
36 and 41 could be greatly increased by cooling, and the
spectrometer (Bruker) in the positive ion mode using MeOH as
the solvent. Elemental analysis (C, H, N and S) was performed
on an Elementar Vario MicroCube instrument.
D-arabinosyl and the C3’-branched
D
Our study revealed that the stereoselectivity of the thiol-ene
coupling of the 4’,5’-unsaturated nucleosides is not easy to
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The photoinitiated reactions were carried out in a borosilicate
vessel by irradiation with a Hg-lamp giving maximum emission
at 365 nm, without any caution to exclude air or moisture.
Conflicts of interest
View Article Online
There are no conflicts of interest to declare.DOI: 10.1039/C7OB02184D
Representative example for the photoinduced addition of thiols to
alkenes at −80 °C in the presence of DPAP:
Acknowledgements
The authors gratefully acknowledge financial support for this
research from the National Research, Development and
Innovation Office of Hungary (OTKA K 109208 and TÉT_15_IN-
1-2016-0071). The research was also supported by the EU and
co-financed by the European Regional Development Fund under
the project GINOP-2.3.2-15-2016-00008.
2’,3’-O-isopropylidene-5’-S-n-propyl-5’-thiouridine (3a) and 1-
(2’,3’-O-isopropylidene-5’-S-n-propyl-5’-thio--L-lyxofuranosyl)-
uracil (3b)
To a solution of alkene
1 (90 mg, 0.3 mmol) and thiol 2 (0.6
mmol, 2 equiv., 60 µL) in toluene (1 mL) 2,2-dimethoxy-2-
phenylacetophenone (7.7 mg, 0.03 mmol) was added. The
reaction mixture was cooled to −80 °C and irradiated with UV
light for 15 min. After 15 min DPAP (7.7 mg, 0.03 mmol)
dissolved in toluene (0.3 mL) was added, and the mixture was
cooled to –80 °C and irradiated for another 15 min. The addition
of DPAP and irradiation at this temperature was repeated once
more. Then the solution was concentrated and the crude
product was purified by flash column chromatography (gradient
elution 8:2→7:3 n-hexane–acetone) to give a 5:1 mixture of 3a
and 3b (103 mg, 89%). A second flash column chromatography
(95:5 CH2Cl2–acetone) of the diastereomeric mixture gave pure
3a (Rf = 0.31, 7:3 n-hexane–acetone) as a colourless syrup and
pure 3b (Rf = 0.30, 7:3 n-hexane–acetone) as a colourless syrup.
Notes and references
§ As terminal double bonds react much faster than internal ones
and conjugated double bonds are nonreactive under thio-click
conditions (see ref 6a and 6c) chemoselective addition to the
exocyclic double bond was expected.
§§ We have studied the synthesis of 17 and 18 by nucleophilic
substitution starting from the corresponding 5’-deoxy-5’-iodo
uridine derivative. Compound 17 could be prepared in 76% yield
with 4 equiv. of
6 in the presence of Cs2CO3. However, analogous
reactions of using various bases gave 18 in a yield of up to 20%.
7
§§§ Due to the low reactivity of the t-butylthiyl radical formed
(See ref 8c), a higher excess of 2-methylpropane-2-thiol was
required for the efficient thiol-ene reaction.
Compound 3a (D
-ribo product): 1H NMR (400 MHz, CDCl3) δ
(ppm) 9.89 (s, 1H, NH), 7.37 (d, J = 8.1 Hz, 1H, H-6 uracil), 5.75
(d, J = 8.1 Hz, 1H, H-5 uracil), 5.69 (d, J1’,2’ = 2.2 Hz, 1H, H-1’),
1
2
L. P. Jordheim, D. Durantel, F. Zoulim and C. Dumontet, Nat.
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=
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6.6 Hz, J3’,4’ = 4.2 Hz, 1H, H-3’), 4.27 (td, J4’,5’ = 6.1 Hz, J3’,4’ = 4.3
Hz, 1H, H-4’), 2.92-2.80 (m, 2H, H-5’a,b), 2.55 (t, J = 7.5 Hz, 2H,
CH3CH2CH2), 1.63 (dt, J = 14.7 Hz, J = 7.4 Hz, 2H, CH3CH2CH2),
1.57 (s, 3H, i-propylidene CH3), 1.36 (s, 3H, i-propylidene CH3),
0.98 (t, J = 7.3 Hz, 3H, CH3CH2CH2); 13C NMR (100 MHz, CDCl3) δ
(ppm) 163.8, 150.1 (2C, 2 x CO uracil), 142.5 (1C, C-6 uracil),
114.7 (1C, i-propylidene Cq), 102.7 (1C, C-5 uracil), 94.2 (1C, C-
1’), 86.7 (1C, C-4’), 84.5 (1C, C-2’), 83.2 (1C, C-3’), 35.1 (1C,
CH3CH2CH2), 34.5 (1C, C-5’), 27.2, 25.4 (2C, 2 x i-propylidene
CH3), 23.0 (1C, CH3CH2CH2), 13.5 (1C, CH3CH2CH2); MALDI-TOF
MS: m/z calcd for C15H22N2NaO5S [M+Na]+ 365.114, found
365.119.
3
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-lyxo product): 1H NMR (400 MHz, CDCl3) δ
(ppm) 9.31 (s, 1H, NH), 7.22 (d, J = 8.0 Hz, 1H, H-6 uracil), 5.73
(dd, J = 8.1 Hz, J = 1.8 Hz, 1H, H-5 uracil), 5.36 (s, 1H, H-1’), 5.24
(d, J2’,3’ = 6.0 Hz, 1H, H-2’), 4.99 (dd, J2’,3’ = 5.9 Hz, J3’,4’ = 3.9 Hz,
1H, H-3’), 4.59 (td, J4’,5’ = 6.8 Hz, J3’,4’ = 3.9 Hz, 1H, H-4’), 2.88-
2.76 (m, 2H, H-5’a,b), 2.57 (td, J = 7.2 Hz, J = 0.8 Hz, 2H,
CH3CH2CH2), 1.65-1.57 (m, 2H, CH3CH2CH2), 1.52 (s, 3H, i-
propylidene CH3), 1.36 (s, 3H, i-propylidene CH3), 0.99 (t, J = 7.3
Hz, 3H, CH3CH2CH2); 13C NMR (100 MHz, CDCl3) δ (ppm) 163.7,
150.8 (2C, 2 x CO uracil), 143.7 (1C, C-6 uracil), 113.3 (1C, i-
propylidene Cq), 102.5 (1C, C-5 uracil), 97.4 (1C, C-1’), 85.8 (1C,
C-4’), 85.5 (1C, C-2’), 81.6 (1C, C-3’), 35.1 (1C, CH3CH2CH2), 31.1
(1C, C-5’), 26.4, 24.9 (2C, 2 x i-propylidene CH3), 23.1 (1C,
CH3CH2CH2), 13.6 (1C, CH3CH2CH2); ESI-TOF MS: m/z calcd for
C15H22N2NaO5S [M+Na]+ 365.114, found 365.122.
7
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C7OB02184D
Low-temperature, photoinduced thiol-ene click reaction: a mild and
efficient method for the synthesis of sugar-modified nucleosides
Miklós Bege, Mihály Herczeg, Ilona Bereczki, Máté Kicsák, Dániel Eszenyi, Pál Herczegh,
and Anikó Borbás*
Department of Pharmaceutical Chemistry, University of Debrecen, Egyetem tér 1, H-4032
Debrecen
While studying the radical mediated hydrothiolation of nucleoside enofuranosides unusual
temperature effect was observed by exploitation of which various thio-substituted
arabino, -xylo and -lyxo configured nucleoside analogues were produced.
D-ribo, -
L