Facile assembly of H-pyrazolo[5,1-a]isoquinolines via silver triflate-catalyzed one-pot tandem reaction of 2-alkynylbenzaldehyde, sulfonohydrazide, and ketone or aldehyde

Article abstract of DOI:10.1002/adsc.201000176

A novel and efficient route for the generation of H-pyrazolo[5,1-a] isoquinolines via silver triflate-catalyzed one-pot tandem reaction of 2-alkynylbenzaldehyde, sulfonohydrazide, and ketone or aldehyde is described. This reaction proceeds with good functional group tolerance under mild conditions with high efficiency and excellent selectivity.

Full text of DOI:10.1002/adsc.201000176

FULL PAPERS
DOI: 10.1002/adsc.201000176
Facile Assembly of H-Pyrazolo[5,1-a]isoquinolines via Silver
G
Triflate-Catalyzed One-Pot Tandem Reaction of 2-Alkynyl-
benzaldehyde, Sulfonohydrazide, and Ketone or Aldehyde
Xingxin Yu,a Shengqing Ye,a and Jie Wua,b,*
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-216-510-2412; e-mail: jie_wu@fudan.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, 354 Fenglin Road, Shanghai 200032, Peopleꢀs Republic of China
Received: March 7, 2010; Revised: June 3, 2010; Published online: July 28, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000176.
Abstract: A novel and efficient route for the genera- functional group tolerance under mild conditions
tion of H-pyrazoloACTHNUTRGNE[NUG 5,1-a]isoquinolines via silver tri- with high efficiency and excellent selectivity.
flate-catalyzed one-pot tandem reaction of 2-alkynyl-
benzaldehyde, sulfonohydrazide, and ketone or alde- Keywords: aldehydes; 2-alkynylbenzaldehydes; ke-
hyde is described. This reaction proceeds with good tones; H-pyrazoloACTHNURGTNE[UNG 5,1-a]isoquinolines; silver triflate;
sulfonohydrazide
Introduction
lective inhibition against HIV-1 integrase in vitro.[11]
Thus, the development of novel and efficient routes
The availability of practical routes for the generation for rapid access to functionalized isoquinolines under
of small molecules-based natural products is of the mild conditions is of high demand.
utmost urgency and importance in biomedical re-
Recently, we have witnessed the important progress
search.[1] In the last decade, diversity-oriented synthe- of cascade reactions[12] using 2-alkynylbenzaldehyde
sis has been widely used to efficiently generate di- as a versatile building block for construction of het-
verse small molecules. Among the strategies em- erocycles.[13] Based on these results, we have devel-
ployed in diversity-oriented chemical synthesis, multi- oped efficient routes for the construction of nitrogen-
component reactions are very attractive processes containing heterocycles starting from 2-alkynylbenzal-
that push the limits of synthetic efficiency by using doxime or N’-(2-alkynylbenzylidene)hydrazide.[14] In
more than two reactants to create novel products with order to build up a diverse library of fused isoquino-
an optimal number of new bonds and functionalities.[2] lines for our subsequent biological assays, we are in-
These reactions are ideally suited for the construction terested in the methodology development for the gen-
of natural product-like libraries prone to display bio- eration of fused isoquinoline compounds. Prompted
logical activity.[3] As a privileged fragment, isoquino- by the advancement of multi-component reactions
line is known to show a broad spectrum of biological and our efforts in N’-(2-alkynylbenzylidene)hydrazide
activities[4,5] and the prominence of isoquinolines in chemistry, we conceived that H-pyrazolo
ACHTNUTRGNEUNG[5,1-a]isoqui-
natural products and biologically active molecules has nolines might be easily accessible via a one-pot
prompted considerable efforts toward their prepara- tandem reaction of 2-alkynylbenzaldehyde, sulfonohy-
tion.[6–8] Among the family of isoquinolines, the fused drazide, and ketone or aldehyde (Scheme 1). 2-Alky-
isoquinolines have attracted much attention. For in- nylbenzaldehydes 1 could be easily obtained via a So-
stance, the lamellarin alkaloids which constitute a nogashira reaction of 2-bromobenzaldehyde with
family of novel marine natural products, contain a alkyne. After condensation with sulfonohydrazide 2,
highly substituted fused 1,2-dihydroisoquinoline N’-(2-alkynylbenzylidene)hydrazide A would be af-
core.[9] Among these compounds, lamellarin D has forded. Subsequently, the 6-endo-cyclization would
been discovered as a potent inhibitor of human topo- occur to generate the isoquinolinium-2-yl amide B in
ACHTUNGTRENNUNG
isomerase I[10] and lamellarin a-20-sulfate displays se- the presence of suitable Lewis acid. In this step, the
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Facile Assembly of H-PyrazoloACHTNUTRGNE[NUG 5,1-a]isoquinolines via Silver Triflate-Catalyzed One-Pot Tandem Reaction
Scheme 1. Proposed route for one-pot tandem reaction of 2-alkynylbenzaldehyde, sulfonohydrazide, and ketone or aldehyde.
formation of a p-complex via coordination of the al- drazide 2, and silver triflate was demonstrated as the
kynyl moiety of 1 to the Lewis acid would be in- most effective catalyst for the subsequent 6-endo-cy-
volved, thus activating the triple bond for further cy-
ACHTUNGTRENNUNGclization. Meanwhile, the in situ formed enolate (de- by AgOTf (10 mol%) in EtOH in the presence of dif-
clization,[13] at the outset the reactions were catalyzed
ACHTUNGTRENNUNG
rived from ketone or aldehyde 3 in the presence of ferent bases. To our delight, small amounts of product
base) would attack the isoquinolinium-2-yl amide B 4a were formed when diisopropylamine, proline, pi-
to produce intermediate C. Subsequent intramolecu- peridine, or 2,2,6,6-tetramethylpiperidine was utilized
lar condensation and aromatization would give rise to in the reaction. However, the large major product iso-
the desired H-pyrazoloACTHNUTRGNEUGN[5,1-a]isoquinoline 4. However, lated was the isoquinolinium-2-yl amide. Elevating
there are several questions associated with the pro- the reaction temperature or increasing the amount of
posed synthetic route, such as selectivity, compatibili- butyraldehyde 3a could not improve the result. We
ty, and relative rates. Thus, to verify the practicability then shifted our focus on inorganic bases. Gratifying-
of the proposed route as shown in Scheme 1, we initi- ly, the reaction proceeded smoothly in the presence of
ated the search for suitable conditions for this trans- Na2CO3 at 708C, which gave rise to the desired prod-
formation.
uct 4a in 80% yield. Further investigation revealed
that K3PO4 was the best choice with a 96% isolated
yield. Reducing the amount of silver triflate retarded
the reaction.
Results and Discussion
Using the mild conditions [AgOTf (10 mol%),
Initially, a set of experiments was carried out using 2- K3PO4 (3.0 equiv.), EtOH, 708C], the scope of this
alkynylbenzaldehyde 1a, sulfonohydrazide 2, and bu- one-pot tandem reaction was investigated, and the re-
tyraldehyde 3a as model substrates (Scheme 2). Since sults are shown in Table 1. All reactions proceeded
the condensation worked efficiently in ethanol for the smoothly leading to the desired H-pyrazoloACTHUNRTGNE[GNU 5,1-a]iso-
reaction of 2-alkynylbenzaldehyde 1a with sulfonohy- quinolines 4 in good to excellent yields. For instance,
reaction of 2-alkynylbenzaldehyde 1a, sulfonohydra-
zide 2, and pentan-3-one 3b afforded the expected
product 4b in 86% yield (Table 1, entry 2). A similar
result was obtained when cyclohexanone 3d was used
as a replacement (83% yield, Table 1, entry 4). Vari-
ous acetophenones 3e–3i were employed in the reac-
tion of 2-alkynylbenzaldehyde 1a and sulfonohydra-
zide 2 as well (Table 1, entries 5–9). This silver-cata-
lyzed H-pyrazoloACTHNUTRGNE[NUG 5,1-a]isoquinoline formation was
found to be workable for acetophenones 3e–3i with
electron-withdrawing and -donating substituents on
the aromatic backbone. Interestingly, it was found
that the hydroxy group as substitution in the substrate
3f or 3g was well tolerated under these conditions
Scheme 2. Initial studies for one-pot tandem reaction of 2-
alkynylbenzaldehyde 1a, sulfonohydrazide 2, and butyralde-
hyde 3a.
and H-pyrazoloACTHNUTRGNEUNG[5,1-a]isoquinoline 4f or 4g could be
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Table 1. Silver triflate-catalyzed one-pot tandem reaction of 2-alkynylbenzaldehyde 1, sulfonohydrazide 2, and ketone or al-
dehyde 3.
Entry
1
R1, R2
Compound 3
Product 4
Yield [%][a]
H, C6H5 (1a)
3a
3b
96 (4a)
2
3
H, C6H5 (1a)
H, C6H5 (1a)
86 (4b)
67 (4c)
3c
4
H, C6H5 (1a)
3d
83 (4d)
5
6
H, C6H5 (1a)
H, C6H5 (1a)
3e
3f
81 (4e)
77 (4f)
7
H, C6H5 (1a)
3g
70 (4g)
8
9
H, C6H5 (1a)
H, C6H5 (1a)
3h
3i
63 (4h)
68 (4i)
10
H, cyclopropyl (1b)
3a
98 (4j)
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Facile Assembly of H-Pyrazolo
Table 1. (Continued)
ACHTUNGTNER[NUNG 5,1-a]isoquinolines via Silver Triflate-Catalyzed One-Pot Tandem Reaction
Entry
R1, R2
Compound 3
Product 4
Yield [%][a]
11
H, cyclopropyl (1b)
3c
80 (4k)
12
13
14
15
H, n-Bu (1c)
H, n-Bu (1c)
5-F, C6H5 (1d)
5-F, C6H5 (1d)
3a
3c
3a
3c
91 (4l)
81 (4m)
98 (4n)
68 (4o)
16
17
5-F, cyclopropyl (1e)
5-F, cyclopropyl (1e)
3a
3c
86 (4p)
90 (4q)
18
19
4-F, C6H5 (1f)
3a
3a
95 (4r)
90 (4s)
4-F, cyclopropyl (1g)
20
21
4-F, n-Bu (1h)
3a
3a
85 (4t)
67 (4u)
4,5-(OMe)2, C6H5 (1i)
22
H, C6H5 (1a)
3j
40 (4v)
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Table 1. (Continued)
Entry
R1, R2
Compound 3
Product 4
Yield [%][a]
23
H, cyclopropyl (1b)
3j
71 (4w)
24
5-Cl, C6H5 (1j)
5-Cl, C6H5 (1j)
3j
35 (4x)
31 (4y)
25
3k
[a]
Isolated yield based on 2-alkynylbenzaldehyde 1.
isolated in 77% and 70% yield, respectively (Table 1, was also a suitable partner in this reaction, which fur-
entries 6 and 7). Other 2-alkynylbenzaldehydes were nished the expected product 4o in 68% yield (Table 1,
tested meanwhile. In addition to the phenyl group at- entry 15). A good yield was observed as well in the
ꢀ
tached to the C C triple bond, 2-alkynylbenzalde- reaction of methoxy-substituted 2-alkynylbenzalde-
hydes with a cyclopropyl or n-butyl group were found hyde 1i with sulfonohydrazide 2 and butyraldehyde 3a
to be suitable as well to generate the desired products (Table 1, entry 21). The structure of compound 4r was
in good yields (Table 1, entries 10–13). The conditions verified by X-ray diffraction analysis meanwhile
have also proven to be useful for 2-alkynylbenzalde- (Figure 1). Reactions employing 1,3-dicarbonyl com-
hydes with substituents in the aromatic ring. For ex- pounds 3j and 3k as substrates were examined as well
ample, an almost quantitative yield of compound 4n (Table 1, entries 22–25). For example, reaction of 2-al-
was isolated for the reaction of 2-alkynylbenzalde- kynylbenzaldehyde 1b, sulfonohydrazide 2, and cyclo-
hyde 1d with sulfonohydrazide 2 and butyraldehyde hexane-1,3-dione 3j gave rise to the corresponding
3a (98% yield, Table 1, entry 14). Cyclopentanone 3c product 4w in 71% yield (Table 1, entry 23). The yield
was decreased to 35% when 2-alkynylbenzaldehyde
1j was used as a replacement in the reaction (Table 1,
entry 24). A similar result was generated when ethyl
3-oxobutanoate 3k was tested (Table 1, entry 25).
Conclusions
In conclusion, we have described a novel and efficient
route for the generation of H-pyrazoloACTHNUTRGNE[NUG 5,1-a]isoquino-
lines via the silver triflate-catalyzed one-pot tandem
reaction of 2-alkynylbenzaldehyde 1, sulfonohydra-
zide 2, and ketone or aldehyde 3. This reaction pro-
ceeded with good functional group tolerance under
mild conditions with high efficiency and excellent se-
lectivity. Currently, the related library construction
and screening for biological activity of these small
molecules are ongoing, and the results will be report-
ed in due course.
Figure 1. ORTEP illustration of compound 4r (30% proba-
bility ellipsoids).
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Facile Assembly of H-PyrazoloACHTNUTRGNE[NUG 5,1-a]isoquinolines via Silver Triflate-Catalyzed One-Pot Tandem Reaction
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G
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7.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=14.2, 19.6,
112.5, 117.2, 123.3, 125.5, 127.2, 127.3, 127.4, 128.4, 129.3,
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Acknowledgements
Financial support from National Natural Science Foundation
of China (20972030) and the Science & Technology Commis-
sion of Shanghai Municipality (09JC1404902) is gratefully
acknowledged.
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