Three-component reactions of 2-alkynylbenzaldoximes and α,β-unsaturated carbonyl compounds with bromine or iodine monochloride

Article abstract of DOI:10.1002/adsc.201000080

The three-component reaction of 2-alky-nylbenzaldoximes and α,β-unsaturated carbonyl compounds with bromine or iodine monochloride is described, which generates the unexpected 2-(4-halo-isoquinolin- 1-yl)ethanol derivatives in good to excellent yields

Full text of DOI:10.1002/adsc.201000080

FULL PAPERS
DOI: 10.1002/adsc.201000080
Three-Component Reactions of 2-Alkynylbenzaldoximes and
a,b-Unsaturated Carbonyl Compounds with Bromine or Iodine
Monochloride
Shengqing Ye,a Ke Gao,a and Jie Wua,b,*
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-216-564-1740; 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: January 29, 2010; Revised: May 24, 2010; Published online: July 2, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000080.
Abstract: The three-component reaction of 2-alky-
ACHTUNGTRENiNUGN soquinolin-1-yl)ethanol derivatives in good to excel-
nylbenzaldoximes and a,b-unsaturated carbonyl lent yields.
compounds with bromine or iodine monochloride is
described, which generates the unexpected 2-(4-halo- Keywords: 2-alkynylbenzaldoximes; bromine; 2-(4-
haloisoquinolin-1-yl)ethanols; iodine monochloride;
a,b-unsaturated carbonyl compounds
Introduction
plex molecules via Au-catalyzed internal redox/dipo-
lar cycloaddition tandem reactions of 2-alkynylbenz-
Diversity-oriented synthesis has been used as an
aldoximes.[9c] We also developed several efficient
ACHTUNGTRENNUNG
engine to efficiently generate diverse small molecules methods for nitrogen-heterocycle formation starting
in the field of chemical genetics.[1] Among the strat- from alkynylbenzaldoxime.[7] During the reaction pro-
egies utilized in diversity-oriented synthesis, multi- cess, isoquinoline N-oxide was identified as the key
component reactions have emerged as a powerful tool intermediate in the reaction of 2-alkynylbenzaldoxime
for delivering the molecular diversity needed in com- in the presence of suitable Lewis acids or electro-
binatorial approaches for the preparation of bioactive philes. Meanwhile, we have described silver triflate
compounds.[2] As a “priviliged” scaffold, isoquinoline and triphenylphosphine co-catalyzed reactions of 2-al-
shows interesting biological properties[3,4] and the kynylbenzaldehyde, amine, and a,b-unsaturated
prominence of isoquinolines in natural products and ketone.[10] In this transformation, an isoquinolinium
biologically active molecules has promoted considera- intermediate was generated for further elaboration.
ble efforts toward their synthesis.[5–7] For example, Prompted by these results and the structural similarity
Larock and co-workers reported the synthesis of iso- between isoquinoline N-oxide and isoquinolinium
quinoline derivatives via transition metal-catalyzed salt, we conceived that 2-alkynylbenzaldoxime might
cyclization of ortho-alkynylarylaldimines.[5] Recently, be employed as substrate as well for a similar trans-
we are also interested in the methodology develop- formation. The synthetic plan is depicted in Scheme 1.
ment for the construction of isoquinoline and related In the pathway shown in Scheme 1, an enolate would
compounds.[7,8] In order to build up a diverse library be formed via reaction of a,b-unsaturated carbonyl
of isoquinolines for our subsequent biological assays, compound 2 with Lewis base. Meanwhile, 2-alkynyl-
the development of novel and efficient routes for a benzaldoxime 1 reacted with the electrophile leading
rapid access to functionalized isoquinolines under to isoquinoline N-oxide A, which underwent nucleo-
mild conditions is of high interest.
philic addition to form intermediate B. After intramo-
Recently, 2-alkynylbenzaldoxime has been recog- lecular rearrangement, the expected product C would
nized as a versatile building block for the construction be afforded.
of nitrogen-containg heterocycles.[7,9] For instance,
Shin and co-workers reported the generation of com-
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Three-Component Reactions of 2-Alkynylbenzaldoximes and a,b-Unsaturated Carbonyl Compounds
Scheme 1. Propose route for the reaction of 2-alkynylbenzaldoxime and an a,b-unsaturated carbonyl compound with bro-
mine or iodine.
Results and Discussion
The result could not be improved when 2.0 equiv. of
DABCO were added. Since DMF has been well rec-
To verify the practicability of the proposed route as ognized as a Lewis base,[11] thus the reaction was car-
shown in Scheme 1, we decided to pursue this three- ried out in DMF directly without the addition of
component reaction. At the outset, the reaction of 2- DABCO in the presence of different inorganic bases.
alkynylbenzaldoxime 1a, methyl acrylate 2a, and bro- To our delight, the product 3b was isolated in 65%
mine was performed in the presence of NaOAc yield when LiOH·H2O (1.1 equiv.) was employed in
(1.1 equiv.) and DABCO (20 mol%) in THF the reaction. Further screening of solvents displayed
(Scheme 2). The addition of an inorganic base was es- inferior results. With this result in hand, we re-sur-
sential as HBr scavenger in the reaction process. For- veyed the three-component reaction of 2-alkynylbenz-
tunately, a product was observed with 10% isolated
ACHTUNGTRENaNUNG ldoxime 1a, methyl acrylate 2a, and bromine in
yield. Structural identification revealed that the com- DMF in the presence of LiOH·H2O. As expected, the
pound obtained was the unexpected 2-(4-bromoiso- corresponding product 3a was furnished in 80% yield.
quinolin-1-yl)ethanol 3a instead of the compound C
Using the mild conditions (1.1 equiv. of LiOH·H2O,
as proposed in Scheme 1. When triphenylphosphine DMF), the scope of this three-component reaction
was utilized as a replacement of DABCO as nucleo- was examined, and the results are shown in Table 1.
philic catalyst, the reaction was complicated. No For the reactions of 2-alkynylbenzaldoxime 1a, not
better results were obtained when other Lewis bases only acrylates were found to be workable with bro-
were examined in the reaction. Blank experiments in- mine or iodine monochloride, but also a,b-unsaturat-
dicated that no reaction occurred without the addition ed ketones were good partners in the transformation.
of DABCO. Subsequently, but-3-en-2-one 2b was For instance, reaction of 2-alkynylbenzaldoxime 1a, n-
tested in this reaction as well in the presence of butyl acrylate 2d with bromine afforded the corre-
DABCO and NaOAc in THF. Again, 2-(4-bromoiso- sponding isoquinoline 3e in 76% yield (entry 5).
quinolin-1-yl)ethanol 3b was generated in 27% yield. When pent-1-en-3-one 2e was utilized as a replace-
ment in the reaction, the product 3f was generated in
70% yield (entry 6). Besides bromine, iodine mono-
chloride was a good substrate as well in this transfor-
mation, which furnished the product 3g in 65% yield
(entry 7). However, diminished reactivity was ob-
served when iodine was used (data not shown in
Table 1). Other 2-alkynylbenzaldoximes were tested
meanwhile. Compound 1b reacted with ethyl acrylate
2c and bromine leading to the product 3h in 70%
yield (entry 8). A lower yield was observed when
iodine monochloride was used in the reaction
(entry 9). Three-component reactions of fluoro-substi-
Scheme 2. Initial studies for the reaction of 2-alkynylbenzal-
doxime 1a and a,b-unsaturated carbonyl compound 2a or 2b
with bromine.
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Table 1. Three-component reactions of 2-alkynylbenzaldoxime 1 and a,b-unsaturated carbonyl compound 2 with bromine or
iodine monochloride.
Entry
1
Compound 1
R3
X2
Yield [%][a]
1a
OMe (2a)
Br2
80 (3a)
2
3
4
5
6
7
1a
1a
1a
1a
1a
1a
Me (2b)
OEt (2c)
OEt (2c)
O-n-Bu (2d)
Et (2e)
Br2
Br2
ICl
Br2
Br2
ICl
65 (3b)
80 (3c)
86 (3d)
76 (3e)
70 (3f)
65 (3g)
Et (2e)
8
1b
OEt (2c)
Br2
70 (3h)
9
10
1b
1b
OEt (2c)
Et (2e)
ICl
Br2
48 (3i)
81 (3j)
11
1c
OEt (2c)
Br2
80 (3k)
12
13
1c
1c
OEt (2c)
Et (2e)
ICl
Br2
60 (3l)
71 (3m)
14
1d
OEt (2c)
Br2
63 (3n)
15
16
1d
1d
OEt (2c)
Et (2e)
ICl
Br2
47 (3o)
61 (3p)
17
1e
OEt (2c)
Br2
58 (3q)
[a]
Isolated yield based on 2-alkynylbenzaldoxime 1. PMP=p-methoxyphenyl.
tuted 2-alkynylbenzaldoxime 1c proceeded smoothly electrophile (bromine or iodine monochloride). The
as well, which generated the isoquinoline products in subsequent nucleophilic addition of Lewis base to iso-
good yield (entries 11–13). In addition, the structure quinoline N-oxide A would afford intermediate D. In-
of 3m was verified by X-ray diffraction analysis tramolecular removal of benzylic hydrogen resulted
(Figure 1). Moreover, substrates 1 with alkyl groups in a carbon anion, which then attacked the a,b-unsa-
(cyclopropyl, n-butyl) attached on the triple bond turated carbonyl compound 2 to generate the inter-
were examined, and all reactions worked well to give mediate E. Subsequently, an oxaziridine F would be
rise to the products in moderate yields (entries 14– formed with release of the Lewis base. After intramo-
17).
lecular rearrangement, the 2-(4-haloisoquinolin-1-
The possible mechanism is proposed in Scheme 3. yl)ethanol 3 would be obtained. In order to provide
We reasoned that 2-alkynylbenzaldoxime 1 would be the evidence that base played an important role in
cyclized to isoquinoline N-oxide A in the presence of this transformation, reaction of isoquinoline N-oxide
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Three-Component Reactions of 2-Alkynylbenzaldoximes and a,b-Unsaturated Carbonyl Compounds
Conclusions
In conclusion, we have described an efficient three-
component reaction of 2-alkynylbenzaldoxime and
a,b-unsaturated carbonyl compound with bromine or
iodine monochloride under mild conditions. This reac-
tion proceeded smoothly to generate the unexpected
2-(4-haloisoquinolin-1-yl)ethanol derivatives in good
to excellent yields. Further studies by adaptation of
the methodology for isoquinoline alkaloids synthesis
are currently in progress, and the results will be re-
ported in due course.
Experimental Section
General Procedure for Three-Component Reactions
of 2-Alkynylbenzaldoxime 1, a,b-Unsaturated
Carbonyl Compound 2 with Br2 or ICl
Figure 1. ORTEP Illustration of compound 3m (30% proba-
bility ellipsoids).
2-Alkynylbenzaldoxime 1 (0.2 mmol) was added to a solu-
tion of Br2 or ICl in dichloromethane (0.4 mmolmLÀ1,
0.5 mL). The solution was stirred at room temperature for
10 min, then LiOH·H2O (1.1 equiv.) was added. Subsequent-
ly, DMF (1.5 mL) and a,b-unsaturated carbonyl compound
2 (4.0 equiv.) were added and the mixture was stirred at
room temperature. After completion of reaction as indicated
by TLC, the mixture was diluted with ethyl acetate (5.0 mL)
and quenched with water (5.0 mL). The organic layer was
washed with brine, dried over Na2SO4, and concentrated
under reduced pressure. The residue obtained was purified
A1 (R1 =H, R2 =Ph, X=Br) with but-3-en-2-one 2b
was tested (for details, please see Supporting Informa-
tion). The corresponding product 3b could be afford-
ed when the solvent acted as a Lewis base in the reac-
tion (DMF: 75%, DMA: 68%, NMP: 48%). In addi-
tion, inferior results were obtained when DABCO
was added in the reaction as a base in different sol-
vents.
Scheme 3. Possible mechanism for reaction of 2-alkynylbenzaldoxime 1 and a,b-unsaturated carbonyl compound 2 with elec-
trophile.
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Shengqing Ye et al.
FULL PAPERS
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desired product 3.
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hydroxypropanoate (3a): 1H NMR (400 MHz, CDCl3): d=
3.69 (s, 3H), 3.77 (dd, J=5.3, 16.0 Hz, 1H), 3.83 (dd, J=5.3,
16 Hz, 1H), 4.86 (dd, J=4.1, 5.5 Hz, 1H), 5.07 (br, 1H),
7.43–7.50 (m, 3H), 7.66–7.72 (m, 3H), 7.83 (t, J=7.3 Hz,
1H), 8.14 (d, J=8.3 Hz, 1H), 8.37 (d, J=8.3 Hz, 1H);
13C NMR (100 MHz, CDCl3): d=36.4, 52.2, 69.7, 117.2,
124.9, 127.3, 127.8, 127.9, 128.1, 128.4, 129.9, 131.8, 136.1,
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C19H16BrNO3 (M+H+): 386.0392. (For details, please see
Supporting Information)
Crystallographic data for the structure 3m has been de-
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supplementary publication no. CCDC 757863. These data
can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_re-
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Acknowledgements
Financial support from National Natural Science Foundation
of China (20972030), the Science and Technology Commis-
sion of Shanghai Municipality (09JC1404902), and Program
for New Century Excellent Talents in University (NCET-07-
0208) is gratefully acknowledged.
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