Metal free C-3 chalcogenation (sulfenylation and selenylation) of 4H-pyrido[1,2-a]pyrimidin-4-ones

Article abstract of DOI:10.1039/d1ra00834j

An expeditious metal free C-3 chalcogenation of 4H-pyrido[1,2-a]pyrimidin-4-one has been devised to synthesize diversely orchestrated 3-ArS/ArSe derivatives in high yields (up to 95%). This operationally simple reaction proceeds under mild reaction condit

Full text of DOI:10.1039/d1ra00834j

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Metal free C-3 chalcogenation (sulfenylation and
selenylation) of 4H-pyrido[1,2-a]pyrimidin-4-ones†
Cite this: RSC Adv., 2021, 11, 10258
*
Prasanjit Ghosh,
Gautam Chhetri and Sajal Das
An expeditious metal free C-3 chalcogenation of 4H-pyrido[1,2-a]pyrimidin-4-one has been devised to
synthesize diversely orchestrated 3-ArS/ArSe derivatives in high yields (up to 95%). This operationally
simple reaction proceeds under mild reaction conditions, can be executed in gram scale, and also
highlights broad functional group tolerance. Preliminary experimental investigation suggests a radical
mechanistic pathway for these transformations.
Received 31st January 2021
Accepted 3rd March 2021
DOI: 10.1039/d1ra00834j
rsc.li/rsc-advances
Organosulfur species and derivatives thereof, have garnered agrochemicals, pharmaceuticals and material sciences.14 In this
a prominent position in contemporary organic synthesis.1 They family, 4H-pyrido[1,2-a]pyrimidin-4-one (Fig. 1) exhibits versa-
also have widespread applications in pharmaceuticals, bioac- tile biological activities,15 such as CXCR3 antagonism,16 HLE
tive compounds and polymer materials.2 In addition, carbon- inhibition,17 MexAB-OprM specic efflux pump inhibition,18
selenium-carbon skeletons are high value core structures for potent 5-HT6 antagonists,19 and acetylcholinesterase inhibi-
their extensive use as therapeutically active agents, such as tion.20 Meanwhile, Pd catalyzed direct arylation and alkenyla-
antioxidant, antihypertensive, antimicrobial, antibacterial, tion of 4H-pyrido[1,2-a]pyrimidin-4-one through C–H bond
antiviral, anticancer agents etc.3 Moreover, organoselenium functionalization has already been reported in the literature.21
species have been identied as non-toxic compounds.4 Some of Rather, only a single report for the insertion of –SAr group in
the biologically active C–S/C–Se linkage containing scaffolds is 4H-pyrido[1,2-a]pyrimidin-4-one molecule using sulfonyl
outlined in Fig. 1. Consequently, numerous efforts have been hydrazides as thiol surrogates is documented by Wang et al.22
devoted to develop facile and reliable methods for the instal- Nevertheless, this protocol is effective at elevated temperature.
lation of a sulfenyl/selenyl group into organic frameworks.5 Based on our research interests on the structural diversication of
Apart from the cross-coupling approach, an alternative protocol heterocyclic scaffolds, we recently reported different methodolo-
for the transition-metal-catalyzed C–S bond formation via C–H gies for the metal free direct C–H bond functionalization.23 Herein,
bond functionalization has been established, which is known as we envisaged to disclose a straightforward and efficient protocol of
a sulfenylation reaction.6 In this reaction, aryl sulfonyl hydra- sulfenylation/selenylation for 4H-pyrido[1,2-a]pyrimidin-4-one in
zides,7 arylsulfonyl chlorides,8 sulnic acids,9 and sodium sul- the presence of iodine under mild conditions. Pleasingly, several
nates10, thiols11 are mostly used as the sulfenylating agents. thiols/organodiselenides are smoothly coupled with 4H-pyrido[1,2-
Although, these methods are advantageous, certain limitations a]pyrimidin-4-one and furnished the desired anticipated products
comprising the use of metal catalyst and toxic reagents still in good to excellent yields (Scheme 1).
persist. Complete removal of trace amounts of transition-metal
We commenced our studies with the optimization of the
residues from the anticipated bioactive products is quite chal- sulfenylation reaction where 2-phenyl-4H-pyrido[1,2-a]pyrimidin-4-
lenging task, and this contamination of a transition metal also one (1a) and thiophenol were used as a model coupling partner
inhibits the sustainable development.12 Despite these accom-
plishments, development of an efficient and practical method
for transition-metal-free C–S/Se bond forming reactions using
thiols/diselenide as the sulfenylation/selenation reagent is an
attractive and synthetically desirable.
On the other hand, N-fused bicyclic heterocycles13 has
received enormous interest from synthetic chemists as well as
medicinal researchers due to their profound impact in
Department of Chemistry, University of North Bengal, Darjeeling-734013, India.
E-mail: sajal.das@hotmail.com; Fax: +91-0353-2699-001; Tel: +91-0353-2776-381
† Electronic supplementary information (ESI) available. See DOI:
Fig. 1 Representative examples of some biologically active 4H-pyrido
[1,2-a]pyrimidin-4-one and diarylsulfide/diselenide scaffold.
10.1039/d1ra00834j
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1; entry 4). Subsequently, several others inexpensive and readily
available iodine/iodide additives were screened under aerobic
condition (Table 1; entries 5–9). Delightedly, one equiv. of iodine
provided the desired thiolated product 2a in excellent (91%) yield
at 70 ꢀC (entry 5), aer 12 h, while other forms of iodine additives
were unable to promote this transformation effectively (Table 1;
entries 6–9). It is noteworthy that except TBHP all others oxidant
appears to be redundant in this reaction (Table 1; entries 10–13).
Lowering the reaction temperature (50 ꢀC and 30 ꢀC) had a detri-
mental result on the reaction outcome (Table 1; entries 14–15).
Notably, in absence of potassium persulfate, no desired product
was detected in TLC, indicating that the oxidant has a decisive role
for this sulfenylation reaction (Table 1; entry 16). Interestingly,
consistent with our previous observation (Table 1; entry 5), using of
50 mol% of iodine also afforded the 92% yield of thiolated deriv-
ative 2a (Table 1; entry 17). Other solvents were inefficient to
provide decent yields (Table 1; entries 18–22). So it is evident from
the optimization table that a combination of 50 mol% of iodine
and K2S2O8 (2 equiv.) as an oxidant in MeCN at 70 ꢀC was found to
be optimal for the sulfenylation reaction of 2-phenyl-4H-pyrido[1,2-
a]pyrimidin-4-one, which resulted in the formation of the corre-
sponding 3-ArS derivative 2a in excellent yield (92%) aer 12 h
(Table 1, entry 17).
Scheme 1 Previous approaches and the present route of C–H bond
functionalization of 4H-pyrido[1,2-a]pyrimidin-4-ones.
(Table 1). Initial attempts to couple them, using NaI and TBHP (as
oxidant) in DMSO as a solvent at 100 ꢀC remained unfruitful (Table
1, entry 1). No improvement was observed even aer switching the
solvent from DMSO to acetonitrile (Table 1; entry 2). However, yield
of the expected product 2a was raised to 50% upon using TBAI/
K2S2O8 in water (Table 1; entry 3). The reaction efficiency was
further enhanced using one equiv. of TBAI in MeCN at 70 ꢀC (Table
Table 1 Screening of the reaction conditions: effect of reaction parametersa
Entry
Reagent (equiv.)
Oxidant (equiv.)
Temperature (ꢀC)
Solvent (ml)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
NaI (3)
NaI (3)
TBAI (2)
TBAI (1)
I2 (1)
KI (1)
NaI (1)
NH4I (1)
NIS (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
TBHP (3)
TBHP (3)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
TBHP (2)
DTBP (2)
TBPB (2)
H2O2 (2)
K2S2O8 (2)
K2S2O8 (2)
—
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
100
100
70
70
70
70
70
70
70
70
70
70
70
50
30
70
70
70
70
70
70
70
DMSO
CH3CN
H2O
24
24
24
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
NR
NR
50
67
91
NR
NR
60
63
85
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Toluene
Dioxane
DCE
9
10
11
12
13
14
15
16
17
18
19
20
21
22
NR
NR
71
53
NR
NR
92
35
41
66
81
NR
I2 (50 mol%)
I2 (50 mol%)
I2 (50 mol%)
I2 (50 mol%)
I2 (50 mol%)
I2 (50 mol%)
EtOH
DMF
a
Reaction condition: 2-phenyl substituted 4H-pyrido[1,2-a]pyrimidin-4-ones (0.125 mmol, 1 equiv.), benzene thiol (0.1875 mmol, 1.5 equiv.),
inducer (equiv./mol%), solvent (2 ml), oxidant (3 equiv.). b Isolated yields based on the reactants 1a, the reaction was run for 12–24 h.
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Having assimilated the robust reaction conditions for the corresponding chloro derivative (2c). 2,5-Dimethylbenzene thiol
C–S coupling of 4H-pyrido[1,2-a]pyrimidin-4-one, we sought to was endured under the current reaction conditions to provide
explore the scope and general applicability of this protocol 88% yield of 2e. Importantly, the bulkier naphthalene thiol also
(Table 2). A variety of 2-substituted-4H-pyrido[1,2-a]pyrimidin-4- effectively participated in this transformation to give 82% yield
one was treated with a broad range of thiols, and the corre- of the C–S coupled product (2g). For adorning the synthetic
sponding results are represented in Table 2. Satisfyingly, both potentiality further, we investigated the reactivity of various
electron-rich (–OMe, –Me) and electron-decient (–F, –Cl, –Br) thiols with diverse 4H-pyrido[1,2-a]pyrimidin-4-one. Employ-
groups bearing benzene thiols reacted smoothly with 2-phenyl- ment of both electron-neutral (–Me) and electron-decient (–Cl)
4H-pyrido[1,2-a]pyrimidin-4-one, affording the desired 3-sulfe- functional group substituted parent scaffold provided synthet-
nylated derivatives in good to excellent yields (Scheme 1; entries ically useful yields of the desired sulfenylated products with
2b–2h). Noticeably, a crucial effect in the product yield was a wide spectrum of benzene thiols (entries 2i–2p). In this
surveyed with the substituents present at the benzene thiol. 4- context, a suitable choice of benzene thiols is also important,
Methoxybenzene thiol furnished much lower yield of the cor- since electronic bias plays a pivotal role in this transformation
responding coupled product (2b), compared to electron- (entries 2i–2p). Comparatively, a higher yield of the desired ArS
withdrawing group, probably due to the generation of a more derivatives was always obtained in the presence of an electron-
stable dimer [disulphide]. However, the yield of the anticipated withdrawing group (–F, –Cl) at the para position of benzene
product [2b] could further be enhanced upon/on using stoi- thiol (entries 2k, 2m, 2n and 2p). Exposure of 2-alkyl substituted
chiometric amount of catalyst (I2). To our delight, maximum 4H-pyrido[1,2-a]pyrimidin-4-one with thiophenol and 4-chlor-
productivity of the product was obtained in the case of F- othiophenol was also fruitful to give intended products in
substituted benzenethiol compare to the other halogens. acceptable yields (entries 2q and 2r). Unfortunately, benzyl
Notably, ortho bromo-substituted benzene thiol delivered in thiol, 1-pentane thiol and heterocyclic congener of thiol (2-
a higher yield of the corresponding product (2h) than the mercapto benzimidazole) did not respond under the optimal
reaction conditions (entries 2s, 2t and 2u). Especially, upscale
synthesis of 2a was also achieved, illuminating potential capa-
bilities to assemble specialized 3-ArS substituted 4H-pyrido-
[1,2-a]pyrimidin-4-ones. It was remarkable that PhSSPh was also
Table 2 Scope of different substituted 4H-pyrido[1,2-a]pyrimidin-4-
ones and thiol derivatives for I2 mediated sulfenylationa
amenable instead of PhSH with this catalytic system.
Table 3 Scope of different substituted 4H-pyrido[1,2-a]pyrimidin-4-
ones and diselenides derivatives for I2 mediated selenylationa
a
Reaction condition: substituted 4H-pyrido[1,2-a]pyrimidin-4-ones
a
Reaction condition: various 4H-pyrido[1,2-a]pyrimidin-4-ones
(0.125 mmol, 1 equiv.), thiol (0.1875 mmol, 1.5 equiv.), I2 (50 mol%),
b
(0.125 mmol, 1 equiv.), organo diselenides (0.1875 mmol, 1.5 equiv.),
I2 (50 mol%), MeCN (2 ml), K2S2O8 (2 equiv.). Isolated yields based
on the reactants 1, the reaction was run for 12 h. 1 equiv. of I2 was
used.
MeCN (2 ml), K2S2O8 (2 equiv.). Isolated yields based on the
b
c
reactants 1, the reaction was run for 12 h. Yield at 1 g scale.
c
d
PhSSPh was used instead of PhSH. e 1 equiv. of I2 was used.
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Scheme 3 Plausible mechanism (radical pathway).
the reactivity, refuting the involvement of non-radical pathway
in the reaction mechanism (Scheme 2; eqn (3) and (4)). In
addition, we have trapped the in situ generated radical inter-
mediate (PhSec) and isolated the compound 4 in reasonable
yield (Scheme 2; eqn (4)).
Scheme 2 Mechanistic studies.
With the successful establishment of a straightforward and
practical protocol for the C–S coupling reaction, we next investigated
the broadness of the selenylation reaction between 2-substituted-4H-
pyrido[1,2-a]pyrimidin-4-one and organodiselenides for the selective
installation of the –SeAr group at the C-3 position of parent
precursor (Table 3). Generally, diphenyl diselenide was smoothly
coupled with diverse functionalized 2-substituted 4H-pyrido[1,2-a]
pyrimidin-4-one, resulting in excellent yields of the nal C–Se
coupled products 3. Various organo diselenides bearing electron-
donating (–OMe) and electron-withdrawing groups (–CF3 and –F)
were found compatible, howbeit electron-decient substrates
showed better reactivity to render higher yields (entries 3b–3c, 3e).
The methodology was successfully applied to bulkier naphthyl dis-
elenide, delivering the desired product (3d) in 67% yield. Notably,
halogen substituent (–F, –Br, –Cl) at the C-6 position of 4H-pyrido
[1,2-a]pyrimidin-4-one were tolerable, which are useful synthetic
handle for late stage functionalization (entries 3h–3j). Substrates
having methyl functionality in the 4H-pyrido[1,2-a]pyrimidin-4-one
was also amenable (3f–3g) and smoothly produced the anticipated
products in high yields (81ꢁ93%). Furthermore, reaction efficiency
of C-2 alkyl substituted parent molecule was also evaluated,
providing an excellent yield (80%) of the targeted 3-SeAr product 3k.
Besides to a broad range of aromatic diselenides, dibutyl diselenide
also proved to be an efficient selenylating agent, offering the desired
3l in 71% isolated yield.
To comprehend the plausible reaction mechanism, we
executed the sulfenylation reaction under inert atmosphere (N2)
and isolated 76% yield of the desired product 2a (Scheme 2; eqn
(1)). This observation revealed that aerial oxygen was not only
the sole oxidant for this transformation. Additionally, the
reaction of 2-phenyl-4H-pyrido[1,2-a]pyrimidin-4-one and
iodine in presence potassium persulfate did not afford the
corresponding iodo derivative (Scheme 2; eqn (2)). The result
unambiguously conrmed that iodinated derivative of 4H-pyr-
ido[1,2-a]pyrimidin-4-one was not involved in the catalytic cycle.
Furthermore, the presence of stoichiometric amount of radical
scavengers (TEMPO, BHT and 1,1-diphenyl ethylene) inhibited
On the basis of these ndings and previous literature
reports,24 a plausible mechanistic pathway is elaborated in
Scheme 3. Presumably, this sulfenylation/selenylation strategy
involve an initial generation of the thiyl radical or selenyl
radical species A (cSY/cSeY, Y ¼ R) in presence of persulfate
(S2O82ꢁ) or sulfate radical anion (SO4cꢁ). Subsequently, the
reactive sulphur/selenyl radical intermediates A coupled with 2-
phenyl-4H-pyrido[1,2-a]pyrimidin-4-one substrate leading to
a formation of next intermediate 1ab. Then, it underwent
further oxidation by sulfate radical anion via a SET mechanism
to generate a cationic intermediate 1ac which could be stabi-
lized by resonance to 1ac0. Lastly, the nal coupled product (2a/
3a) was formed with the liberation of H2 species.
In summary, we have developed an efficient and straightfor-
ward transformative tool for regioselective chalcogenation of 4H-
pyrido[1,2-a]pyrimidin-4-one under mild conditions. The protocol
tolerated diverse common organic functional groups and resulted
in good to excellent yields of the desired sulfenylated/selenylated
products. Our methodology is operationally simple, regiose-
lective, scalable and avoid the use any expensive metal catalyst.
This present protocol opens a new avenue for the direct and
convenient chalcogenation of 4H-pyrido[1,2-a]pyrimidin-4-one.
Further, C–H bond functionalization reactions on 4H-pyrido[1,2-
a]pyrimidin-4-one are currently underway in our laboratory and
these observations will be forthcoming.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Financial support (EMR/2016/001250) SERB, New Delhi is
gratefully acknowledged. We also acknowledge DST-FIST for
providing infrastructure facilities.
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