Copper(I)-catalyzed [3+2] cycloaddition/ring-opening rearrangement/[4+2] cycloaddition/aromatization cascade: An unprecedented chemo- and stereoselective three component coupling of sulfonyl azide, alkyne and N-arylidenepyridin-2- amine to pyrido[1,2-a]pyrimidin-4-imine

Article abstract of DOI:10.1002/adsc.201200654

A novel synthetic protocol for the one-pot chemo- and stereoselective construction of diversely functionalized pyrido[1,2-a]pyrimidin-4-imines via copper(I)-catalyzed [3+2] cycloaddition/ring-opening rearrangement/[4+2] cycloaddition/aromatization cascade of sulfonyl azides, alkynes and N-arylidenepyridin-2-amines under mild reaction conditions is reported. In addition, the catalytic activity of copper(I)-modified zeolite, a recyclable, heterogeneous catalyst is also investigated, which gives improved yield compared to its homogeneous equivalents. Copyright

Full text of DOI:10.1002/adsc.201200654

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DOI: 10.1002/adsc.201200654
Copper(I)-Catalyzed [3+2]Cycloaddition/Ring-Opening
Rearrangement/[4+2]Cycloaddition/Aromatization Cascade: An
G
Unprecedented Chemo- and Stereoselective Three Component
Coupling of Sulfonyl Azide, Alkyne and N-Arylidenepyridin-2-
amine to Pyrido[1,2-a]pyrimidin-4-imine
A
Kayambu Namitharana and Kasi Pitchumania,*
a
School of Chemistry, Madurai Kamaraj University, Madurai – 625021, India
E-mail: pit12399@yahoo.com
Received: July 25, 2012; Revised: October 1, 2012; Published online: December 23, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200654.
Abstract: A novel synthetic protocol for the one-
pot chemo- and stereoselective construction of di-
no report on the direct synthesis of N-substituted
pyrido[1,2-a]pyrimidin-4-imines. Consequently, new
ACHTUNGTRENNUNG
versely functionalized pyrido
imines via copper(I)-catalyzed [3+2]cycloaddition/
ring-opening rearrangement/[4+2]cycloaddition/ar-
omatization cascade of sulfonyl azides, alkynes and
N-arylidenepyridin-2-amines under mild reaction
conditions is reported. In addition, the catalytic ac-
tivity of copper(I)-modified zeolite, a recyclable,
heterogeneous catalyst is also investigated, which
gives improved yield compared to its homogeneous
equivalents.
[1,2-a]pyrimidin-4-
synthetic methods that allow an efficient and easy
access to more structurally diverse new derivatives of
this fused heterocyclic system are highly warranted,
especially in the early stages of drug discovery.
AHCTUNGTRENNUNG
Cascade multicomponent reactions (MCRs),[12] ena-
bling multiple carbon-carbon and carbon-heteroatom
bond formations in one pot have become increasingly
popular as attractive tools for the rapid generation of
complex molecular scaffolds. These reactions can
serve as the basis for both targeted organic synthesis
and combinatorial chemistry, which allows one to syn-
thesize simultaneously several hundred samples for
biological screening with the use of automated effi-
cient approaches. It has been recognized that MCRs
have not only been sought after for their academic in-
terest but also for their industrial relevance, especially
owing to the presence of many elements of an ideal
Keywords: fused-ring systems; heterogeneous catal-
ysis; multicomponent reactions; stereoselectivity
The pyridoACHTUNGTRENNUNG[1,2-a]pyrimidine skeleton is a privileged synthesis, such as operational simplicity, atom econo-
scaffold in medicinal chemistry for facile access to my, bond-forming efficiency, access to molecular com-
“drug-like” small molecules.[1–5] Notably, the pyrido- plexity from simple starting materials, and low levels
ACHTUNGTRENNUNG[1,2-a]pyrimidin-4-one pharmacophore displays di- of by-product generation. Such reactions are thus eco-
verse biological activities and considerable progress nomically and environmentally attractive and have
has been made in the synthesis and expanding the become an important area of research in synthetic or-
scope of pyridoACHTUNGTRENNUNG[1,2-a]pyrimidin-4-ones in the field of ganic chemistry. However, to ensure sufficient molec-
medicinal chemistry. Some outstanding representa- ular diversity and complexity, there is a continuous
tives have been introduced into human therapy as need for novel reactions with rational design.
a neuroleptic (risperidone),[6] a tranquilizer (pirenper-
Recently, copper-catalyzed MCRs involving the cy-
one),[7] an antiallergic (ramastine),[8] an antidepressant cloaddition of sulfonyl azides and terminal alkynes
(lusaperidone)[9] and a non-narcotic analgesic (rima- have attracted enormous attention from various re-
ACTHNUTRGNEUNG
zolium)[10] agents. Wu et al., synthesized a pyrido[1,2- search groups.[13] This copper-catalyzed MCR has
a]pyrimidin-4-imine, namely (E)-3-(benzenesulfonyl)- been utilized for the synthesis of variety of heterocy-
2-(methylsulfanyl)pyrido[1,2-a]pyrimidin-4-ylidenam-
clic systems.[14] Despite the significant advances made
ine, and screened it as a potent and selective 5-HT6 in this area, there still exists ample scope for explor-
antagonist.[11] To the best of our knowledge, there is ing this reaction towards fine molecules synthesis. Re-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
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93

Kayambu Namitharan and Kasi Pitchumani
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cently, we have developed cascade syntheses of imida-
zolidin-4-ones[15] and bis-N-sulfonylcyclobutenes[16]
from N-sulfonylketenimine, generated in situ from
copper-catalyzed cycloaddition of sulfonyl azides and
terminal alkynes. Encouraged by this straightforward
access to valuable chemical motifs, we became inter-
ested in exploiting the synthetic potential offered by
this unique intermediate towards the synthesis of
other novel and pharmacologically relevant heterocy-
clic systems.
In the present study, we have utilized a 1,3-diaza-
diene skeleton, namely, N-arylidenepyridin-2-amine
for a facile [4+2]cycloaddition with N-sulfonylketeni-
mine, generated in situ from the copper(I)-catalyzed
[3+2]cycloaddition of sulfonyl azides and terminal
alkynes. To our surprise, the cycloaddition proceeds
exclusively across the C=C double bond of N-sulfo-
nylketenimine and subsequent aromatization leading
to pyridoACHTUNGTRENNUNG[1,2-a]pyrimidin-4-imines (Scheme 1), in con-
Scheme 2. Stereoselective
[4+2]cycloaddition
towards
trast to the previous report[14d] with a,b-unsaturated
imine. The presence of two electron-rich nitrogen
atoms in diazadiene (Scheme 1 and Scheme 2) in-
creases the electron density and hence the reactivity
of diene component towards Diels–Alder reactions.
pyrido[1,2-a]pyrimidin-4-imines.
AHCTUNGTRENNUNG
Herein, we report our results on this unprecedented
Consequently the more electron-deficient side of di- multicomponent reaction cascade involving sulfonyl
enophile (namely the C=C bond, but not the C=N azide, alkyne and N-arylidenepyridin-2-amine for the
bond) is the more reactive. The resultant reorganiza- synthesis of medicinally relevant pyridoACTHNUTRGNE[NUG 1,2-a]pyrimi-
tion in HOMO (of diene) and LUMO (of deneno- din-4-imines via an one-pot 1,3-dipolar [3+2]cycload-
phile) energy levels may be responsible for the stereo- dition/ring-opening rearrangement/Diels–Alder [4+
selective addition to the C=C bond but not to theC= 2]cycloaddition/aromatization sequence. The attrac-
N bond of the ketenimine (Scheme 1). Moreover, this tive features of this protocol are the ready aromatiza-
[4+2]cycloaddition is highly stereoselective towards tion without any oxidizing agents, the reversal of che-
pyrido
N-arylidene-pyridin-2-amine) over the other possible a new method for the synthesis of an important phar-
product 1,8-naphthyridine (azadiene centre) macophore, pyrido[1,2-a]pyrimidine-4-imine with a di-
(Scheme 2). The structure of pyrido[1,2-a]pyrimidine verse range of biological activities and applications at
ACHTUNGTRENNUNG[1,2-a]pyrimidin-4-imine (diazadiene centre of moselectivity, the excellent stereoselectivity and
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
was unambiguously established by NMR and single- room temperature.
crystal X-ray analyses (Figure 1).[17]
Scheme 1. Reversal of chemoselectivity with diazadiene.
Figure 1. X-ray crystal structure of 4d.
94
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Copper(I)-Catalyzed [3+2]Cycloaddition/Ring-Opening Rearrangement/
Table 1. Optimization of reaction conditions.[a]
ACHTUNGTRENN[UNG 4+2]Cycloaddition/Aromatization
yield was observed (Table 1, entries 9–12). Thus, the
optimal reaction conditions for this copper-catalyzed
cascade process are use of Cu(I)-Y as a catalyst and
TEA as base in DCM in air for 12 h (Table 1,
entry 5).
The scope of this one-pot, three-component cascade
reaction was further extended to various substituted
analogues. As depicted in Table 2, the reaction pro-
ceeded efficiently with different combinations of
azides, alkynes and imines. In many cases, the pyrido-
Entry
Catalyst
Base
Solvent
Yield [%][b]
AHCTUNGTREG[NNNU 1,2-a]pyrimidin-4-imines were isolated in good yields
by column chromatography on silica gel. Both elec-
tron-rich and electron-deficient sulfonyl azides pro-
vided higher yields (Table 2, entries 1–4). Similarly,
employment of different alkynes, bearing aryl or alkyl
groups (Table 2, entries 1, 5–7), produced pyridines in
good to excellent yields. To further widen the applica-
bility of the procedure, we also tested a variety of
imines. The imines derived from aryl aldehydes with
electron-withdrawing as well as electron-donating
groups are generally well tolerated in the cascade re-
actions (entries 8–13). Control experiments towards
1
2
3
4
5
6
7
8
CuI
CuI
CuBr
CuCl
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
Cu(I)-Y
TEA
TEA
TEA
TEA
TEA
DIPEA
K2CO3
K3PO4
TEA
TEA
TEA
TEA
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
MeCN
toluene
DMF
THF
21[c]
74
69
66
82
68
71
75
54
51
77
65
9
10
11
12
an one-pot sequential synthesis of pyridoACTHNURTGNEU[GN 1,2-a]pyri-
[a]
Reaction conditions: tosyl azide (1 mmol), phenylacety-
lene (1 mmol), N-(4-chlorobenzylidene)pyridin-2-amine
midin-4-imine from 2-aminopyridine, 4-chlorobenzal-
dehyde, tosyl azide, and phenylacetylene however re-
sulted in poor yields.
The observed preference of 1,3-diazadiene over
azadiene as the diene compartment in stereoselective
(1 mmol), TEA (2 mmol), catalyst (20 mg, 0.13 mmol)[18]
solvent (2 mL), room temperature, 12 h.
Isolated yield.
,
[b]
[c]
Under an N2 atmosphere
[4+2]cycloaddition in synthesis of pyrido
midines prompted us to verify this significant result
Preliminary studies were carried out with tosyl with a terminal azadiene 3h (Scheme 3). Interestingly,
ACHTUNGTREN[NUNG 1,2-a]pyri-
azide 1a, phenylacetylene 2a and (E)-N-(4-chloroben- here also the cycloaddition is highly stereoselective
zylidene)pyridin-2-amine 3a (prepared by the conden- towards the 1,3-diazadiene skeleton and the corre-
sation of 2-aminopyridine with 4-chlorobenzalde- sponding pyrido
hyde), in the presence of CuI and triethylamine good yield (see the Supporting Information).
(TEA) under an N2 atmosphere in dichloromethane The recyclability of Cu(I)-Y was also investigated.
(DCM). The pyrido[1,2-a]pyrimidin-4-imine 4a was After completion of the reaction, the catalyst was sep-
ACHTUNGTRNE[NUNG 1,2-a]pyrimidine was obtained in
AHCTUNGTRENNUNG
obtained in only 21% yield (Table 1, entry 1). On the arated by simple filtration and washed with ethyl ace-
other hand, in the presence of air a dramatic improve- tate (2 mLꢁ2). After air-drying, it was reused directly
ment in the yield (74%; Table 1, entry 2) was noticed. without any further purification. Almost consistent
Other copper sources like CuCl and CuBr were also activity, with only a marginal decrease in yield, was
effective in catalyzing this reaction with marginal de- observed upto 4 consecutive cycles (Table 3). The
creases in yields (Table 1, entries 3 and 4). With in- UV-Vis DRS of the reused Cu(I)-Y zeolite shows
creasing environmental legislation, more ecofriendly that Cu(I) is largely intact, within the zeolite frame-
methodologies for organic reactions employing solid work without any significant oxidation.
and easy recyclable catalysts are desired. Consequent-
A plausible mechanistic pathway for the present re-
ly, a great deal of effort has been made in the devel- action cascade is depicted in Scheme 4. Sulfonyl azide
opment of organic reactions catalyzed by solid-sup- 1 reacts readily with the terminal alkyne 2 in the pres-
ported catalysts.[19] As part of our ongoing interest in ence of Cu(I)-Y and Et3N to give the ketenimine in-
the use of cation exchanged zeolites and clays in or- termediate B via ring-opening rearrangement of the
ganic synthesis,[15,16,20] Cu(I)-exchanged zeolites initially formed N-sulfonyl copper triazole species A,
[Cu(I)-Y][21] was used as a reusable catalyst in the which then underwent a formal [4+2]cycloaddition
present study and this cascade reaction proceeds with imine 3 to offer the dihydro intermediate C. Sub-
quite efficiently with higher yields (82%; Table 1, sequent aromatization by air oxidation generated the
entry 5). TEA generally gave better yields. Besides pyridoACTHNUGTRNEUNG[1,2-a]pyrimidin-4-imines (4). Efforts to isolate
DCM, other solvents such as MeCN, THF and toluene this dihydro intermediate were unsuccessful, which is
were also used and no significant improvement in attributed to the vulnerability of C towards oxidation
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Kayambu Namitharan and Kasi Pitchumani
ACHTUNGTRENNUNG
[1,2-a]pyrimidine-4-imines.[a]
COMMUNICATIONS
Table 2. Copper(I)-Y zeolite-catalyzed cascade synthesis of pyrido
Entry
R1
R2
R3
Yield [%][b]
1
2
3
4
5
6
7
8
4-MeC6H4 (1a)
3-CF3C6H4 (1b)
2-naphthyl (1c)
Ph (1d)
1a
1a
1a
1a
1a
1a
1a
1a
1a
Ph (2a)
2a
2a
2a
4-MeC6H4 (2b)
4-n-pentylC6H4 (2c)
cyclopropyl (2d)
2a
2a
2a
2a
2a
2a
4-ClC6H4 (3a)
3a
3a
3a
3a
3a
3a
3-ClC6H4 (3b)
4-iPrC6H4 (3c)
Ph (3d)
4-MeC6H4 (3e)
4-MeOC6H4 (3f)
3-MeOC6H4 (3g)
4a, 82
4b, 73
4c, 66
4d, 85
4e, 70
4f, 58
4g, 63
4h, 82
4i, 77
4j, 78
4k, 71
4l, 64
4m, 76
9
10
11
12
13
[a]
Reaction conditions: sulfonyl azide (1 mmol), alkyne (1 mmol), N-arylidenepyridin-2-amine (1 mmol), TEA (2 mmol),
catalyst (20 mg, 0.13 mmol), solvent (2 mL), room temperature, 12 h.
Isolated yield.
[b]
Scheme 3.
to yield the more stable aromatized product 4. This uct 1,8-naphthyridine (Scheme 2), via an azadiene
[4+2]cycloaddition is chemoselective at the C=C skeleton, is attributed to (i) the increased electron
double bond of the ketenimine intermediate A. It is density, and (ii) the faster nucleophilic attack initiated
also found to be highly stereoselective (with only the via nitrogen atom in the case of the diazadiene unit.
diazadiene component reacting) to yield pyrido
G
In conclusion, a novel copper(I)-catalyzed three-
a]pyrimidin-4-imine. Absence of the alternative prod- component reaction of sulfonyl azide, alkyne and
imine to generate the fused heterocycles having the
pyridoACHTNUTRGNE[UNG 1,2-a]pyrimidine pharmacophore has been suc-
Table 3. Reusability of Cu(I)-Y zeolite in the cascade syn-
thesis of pyrido
cessfully developed. This reaction proceeds via a cas-
cade azide/alkyne cycloaddition–ring opening rear-
rangement–ketenimine/N-arylidinepyridin-2-amine
cycloaddition–aromatization sequence. This method-
ology demonstrates for the first time, an efficient one-
ACHTUNGTRENNUNG
[1,2-a]pyrimidin-4-imines.[a]
Reuse
1st
82
2nd
79
3rd
75
4th
70
Yield[b]
pot synthesis of N-substituted pyridoACTHNUGRTENUNG[1,2-a]pyrimi-
[a]
Reaction conditions: tosyl azide (5 mmol), phenylacety-
lene (5 mmol), N-(4-chlorobenzylidene)pyridin-2-amine
(5 mmol), TEA (10 mmol), catalyst (100 mg, 0.65 mmol),
solvent (8 mL), room temperature, 12 h.
dines and offers advantages in terms of simplicity of
the procedure, ready variation in building blocks, and
mild reaction conditions. The much simpler experi-
mental protocol and product isolation procedure,
[b]
96
Isolated yield.
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Copper(I)-Catalyzed [3+2]Cycloaddition/Ring-Opening Rearrangement/ACTHNUGTRNEUNG[4+2]Cycloaddition/Aromatization
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General Procedure for the Cu(I)-Zeolite-Catalyzed
One-Pot Synthesis of PyridoACTHNUTRGENUGN[1,2-a]pyrimidin-4-imines
A solution of alkyne (1 mmol) in one mL of DCM was
added slowly to a mixture of Cu(I)-zeolite (20 mg), sulfonyl
azide (1 mmol), N-arylidenepyridin-2-amine (1 mmol) and
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New Delhi, India for financial support.
Adv. Synth. Catal. 2013, 355, 93 – 98
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Adv. Synth. Catal. 2013, 355, 93 – 98