Microwave-assisted facile synthesis of furo[3,2-b]indole derivatives via silver(I)-mediated [3+2] cycloaddition

Article abstract of DOI:10.1002/adsc.201100767

A robust one-pot process has been developed for the synthesis of a variety of furo[3,2-b]indoles from readily available o-propargylaminoacetophenones via silver(I)-mediated [3+2] cycloaddition. Copyright

Full text of DOI:10.1002/adsc.201100767

UPDATES
DOI: 10.1002/adsc.201100767
Microwave-Assisted Facile Synthesis of Furo[3,2-b]indole
G
Derivatives via Silver(I)-Mediated [3+2]Cycloaddition
Zhiguo Zhang,a,* Shiliang Fang,a Qingfeng Liu,a and Guisheng Zhanga,*
a
College of Chemistry and Environmental Science, Key Laboratory of Green Chemical Media and Reactions, Ministry of
Education, Henan Key Laboratory for Environmental Pollution Control, Henan Normal University, Xinxiang, Henan
453007, Peopleꢀs Republic of China
Fax : (+86)-373-332-5250; e-mail: zhangzhiguo3030@yahoo.com.cn or zgs@htu.cn
Received: October 3, 2011; Revised: December 27, 2011; Published online: March 7, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100767.
Abstract: A robust one-pot process has been devel-
oped for the synthesis of a variety of furo[3,2-b]in-
doles from readily available o-propargylaminoace-
tophenones via silver(I)-mediated [3+2]cycloaddi-
tion.
a retro-Mannich intermediate.[3d] Thus it can be seen
that the mild and efficient simultaneous preparation
of these cyclic systems, particularly via new strategies,
remains an important objective. Herein we wish to
AHCTUNGTRENNUNG
report a new method for the synthesis of furoACHTUNGTRENNUNG[3,2-
b]indoles from a series of o-propargylaminoacetophe-
nones via intramolecular cycloaddition reactions
under mild conditions.
Keywords: cycloaddition; furoACTHUNTGRNEUNG[3,2-b]indoles; furo-
heterocyclic compounds; microwave-assisted syn-
thesis
Intramolecular 1,3-dipolar cycloadditions are pow-
erful tools for the efficient synthesis of various cyclic
molecules.[7] Although various C3 1,3-dipolar reagents
or their synthetic equivalents have been developed,
the generation of all carbon 1,3-dipoles in an efficient
Synthetic compounds with the structural motif of approach is a much more challenging task.[8,9] Recent-
furo
show pronounced pharmacological effects such as reoselective synthesis of furo
anti-allergy,[1a] potent analgesic[1b–e,2] anti-inflamma- a novel Ag(I)-mediated intramolecular 1,3-dipolar cy-
tory[1b,d–i] and anti-pyretic activities.[1d,j] Not only that, cloaddition of oxo-N-propargylamides with the prop-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
they are also key intermediates for the synthesis of argyl moiety as a potential 1,3-dipoles. On the bases
several natural products.[3] Although they are impor- of our recent research,[10] 1-[2-(prop-2-yn-1-ylamino)-
tant, relatively little research has been conducted on phenyl]ethanone 1a was synthesized (see the Support-
their syntheses.[2,4] The two most common synthetic ing Information for details), further ring-cyclization
strategies are the cyclization of functionalized indole experiments for the constuction of furoACHTNUTRGNE[NUG 3,2-b]indole
rings[2,4a–f] and ring-closing reaction of nitrogen-con- 2a were started immediately. Unfortunately, the
taining benzene derivatives to functionalized fur- target compound 2a was not observed in the presence
ACHTUNGTRENNUNG
ans.[4g–i] To date, just few reprots focus on the con- of Ag2O (0.5 equiv.)/KOH (1.0 equiv.) as reagent in
struction of tricyclic compounds from an open-chain acetonitrile under common conditions or microwave
substrate. Parsons and co-workers disclosed a facile irradiation (MWI), even when 2.0 equivalents of t-
method to get spirocyclic furoACTHNUGRTENUNG[3,2-b]indole derivates BuOK or high temperature (reflux in acetonitrile)
via a novel cascade sequence of base-catalyzed addi- were used, and 95% of 1a was recovered (Scheme 1).
tion of amines to alkynes.[5] MuÇiz described a palladi-
um-catalyzed, sequential transfer of two sulfonamides
to internal alkenes that affords the bisindolines via
construction of vicinal diamines from the readily
available acyclic precursors, which included one case
of a benzofuro
borated a novel rearrangement of 3-multi-substituted
pyrrolidine derivatives to a prototype benzofuro[3,2-
b]indole precursor on the way to phalarine via struct the furoAHCTUNGTRENNUNG
ACHTUNGTRENNUNG
[3,2-b]indole.[6] Danishefsky et al. ela-
AHCTUNGTRENNUNG
Scheme 1. N-Propargyl-o-aminoacetophenone cannot con-
[3,2-b]indole system.
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UPDATES
Table 1. Survey of reaction conditions.[a]
Entry
Solvent
Ag(I) (equiv.)
Base (equiv.)
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
MeOH
THF
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (0.1)
KOH (0)
DBU (1.0)
DABCO (1.0)
DMAP (1.0)
K2CO3 (1.0)
DIPEA (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
KOH (1.0)
24
1.5
0.5
24
5
1.5
6
2
2
3
4
5.5
3
3
6
2
6
6
6
6
5
75[c]
76
70[d]
0
Ag2O (0.25)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
Ag2O (0.5)
AgSbF6 (1.0)
AgBF4 (1.0)
AgOTf (1.0)
AgNO3 (1.0)
43
65
63
76
68
72
73
70
62
65
42
0
trace
trace
0
0
65
63
9
10
11
12
13
14
15
16
17
18
19
20
21
22
DMF
PhH
DMSO
MeCN
MeCN
MeCN
MeCN
KOH
N
KOH (1.0)
KOH (1.0)
KOH (1.0)
1.5
[a]
All microwave irradiation experiments were carried out using microwaves at 258C and the power range was up to 800 W,
unless otherwise indicated.
Isolated yields.
Reactions were performed without MWI.
Reflux.
[b]
[c]
[d]
We suspected that the acidity of the methylene of
propargyl group was not strong enough,[10a] so the N-
acyl-o-aminoacetophenone derivative 1-[2-(prop-2-yn-
1-ylamino)phenyl]ethanone 1b was designed and syn-
thesized and then tested for the cyclization. We treat-
ed 1b with Ag2O (0.5 equiv.)/KOH (1.0 equiv.) in ace-
tonitrile for 1.5 h under microwave irradiation (MWI)
at 258C, the yield of 1-(8b-methyl-3aH-furo
ACHTUNGTNER[NUNG 3,2-
b]indol-4(8bH)-yl)ethanone 2b reached 76% (Table 1,
ACHTUNGTRENNUNG
entry 2), while the reaction was finished in 24 h with-
out MWI (Table 1, entry 1), and the yield of 2b could
not be raised by heating at reflux under MWI
(Table 1, entry 3). The structure of 2 was further con-
firmed by a single-crystal X-ray diffraction analysis of
2b (Figure 1).[11] The crystal structure showed that
compound 2b was one of a pair of diastereomers.[12]
With this encouraging result in hand, the optimiza-
tion of the reaction conditions, including solvents and
the feed ratio of 1b, Ag(I) source, base reagent, was Figure 1. ORTEP drawing of 2b.
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Microwave-Assisted Facile Synthesis of FuroCAHTUNGTRENNNUG
Table 2. Cyclization of compounds 1 to the furoCAHTUNGTRENNNUG
2b: 76%, dr>95:5
2c: 65%, dr>95:5
2d: 72%, dr>95:5
2e: 66%, dr>95:5
2h: 75%, dr>95:5
2f: 65%, dr>95:5
2i: 69%, dr>95:5
2g: 68%, dr>95:5
2j: 67%, dr>95:5
2k: 60%, dr>95:5
2l: 72%, dr>95:5
2m; 68%, dr>95:5
2p: 70%, dr>95:5
2n: 40%,[d] dr>95:5
2o: 75%, dr>95:5
2r: 25%,[f] dr>95:5
2q: 0%[e]
[a]
Unless otherwise indicated, all reactions were carried out with 1 (0.5 mmol), Ag2O (0.5 equiv.), KOH (1.0 equiv.) in
CH3CN (3 mL) at 258C and the microwave power range was up to 800 W.
Isolated yield.
The dr value was determined by H NMR analysis.
N-(2-Benzoylphenyl)acetamide and 4-phenylquinolin-2(1H)-one were obtained in 12% and 40%, respectively.
90% of 1q was recovered.
[b]
[c]
[d]
[e]
[f]
1
2.0 equiv. of t-BuOK were used and 65% of 1r was recovered.
then investigated. The experiments showed that most diisopropylethyl
of the bases like the inorganic base t-BuOK and the for this fascinating base-dependent [3+2]annulation
organic bases 1,8-diazabicyclo[5.4.0]undec-7-ene and the yield of 2b amounted to 68–76% (Table 1, en-
[2.2.2]octane (DABCO), 4-di- tries 8–12). The solvents including CH2Cl2, THF,
(DMAP), and N,N- DMF, MeOH, benzene, DMSO were also screened.
ACHTUNGTRNEaNUNG mine (DIPEA) were also efficient
AHCTUNGTRENNUNG
(DBU), 1,4-diazabicyclo
methylaminopyridine
ACHTUNGTRENNUNG
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Zhiguo Zhang et al.
UPDATES
Among them, CH2Cl2, MeOH and THF were not as
In order to understand the reaction mechanism in
effective as acetonitrile and gave 2b in relatively more detail, several control reactions were done to
lower yields of 62%, 65% and 42%, respectively (en- test the effect of the amount of catalyst on this [3+
tries 13–15). Other solvents, such as DMF, benzene 2]cycloaddition reaction. We increased the amount of
and DMSO were unable to afford to the target com- Ag2O from 0 to 0.5 equivalent, during these attempts
pound (entries 16–18). AgSbF6, AgBF4, AgOTf, 1.0 equivalent of KOH was used, the yield of 2b in-
AgNO3 were proved to be ineffective or less effective creased from 0% to 76% accordingly (Table 1, en-
for the reaction (entries 19–22; please see the Sup- tries 2, 4, 5). But the yield of 2b was slightly lower on
porting Information for a more detailed survey of re- lowering the amount of KOH from 1.0 to 0 equiva-
action conditions).
lents (Table 1, entries 2, 6, 7). So it seems that 1 equiv-
The optimal cycloaddition conditions thus far de- alent of the silver (I) catalyst was necessary. To gain
veloped employ the substrate 1b (0.5 mmol) and further insights into the mechanism, the black
Ag2O (0.5 equiv.)/KOH (1.0 equiv.) in CH3CN (3 mL) powder, filtered at the end of the reaction, was ana-
under MWI at 258C (Table 1, entry 2). By employing lyzed by X-ray powder diffraction (XRPD), and it re-
this protocol, a number of o-propargylaminoaceto- vealed that the black powder was elemental silver,
phenones 1b–1p were cyclized to give the correspond- not silver oxide. Therefore, we speculated that Ag2O
ing furoACHTUNGTRENNUNG[3,2-b]indoles 2b–2p in 40–76% yields plays at least two roles in the reaction, including the
(Table 2). It was apparent that diverse substituents on following: (i) base, (ii) oxidant. Also according to pre-
the aryl, whether electron-donating group or electron- vious reports[17] and our work,[10a] we deduce the fol-
withdrawing group, had no significant effect on the lowing cycloaddition reaction pathway (Scheme 2) for
yield of furoACHTUNGTRENNUNG[3,2-b]indoles. The ꢂsteric hindrance the Ag(I)-mediated transformation from 1 to 2. In the
effectꢀ of N-protecting groups was not observed presence of KOH and Ag2O, the in-situ generated
either, because even the substrate with a bulky pivalo- carbanion species A attacks the carbonyl carbon to
yl group could be cyclized in satisfactory yield. Fur- give a mixture of cis- and trans-B in equilibrium,[18]
thermore, the N-chloroacetyl group was also stable and the silver-containing cis-B intermediate is gener-
during the annulation process. But the 2-benzoyl ated regiospecifically,[19] which may lead to the forma-
group of 1n significantly affected the cyclization pro- tion of dihydrofuro
ACHTUNGTNER[NUNG 3,2-b]indole products and the
cess and yielded the furo
AHCTUNGTRENNUNG
zoylphenyl)acetamide and 4-phenylquinolin-2(1H)- a single electron-transfer (SET) of oxygen anion to
one in 40%, 12% and 40%, respectively. The 2-alde- Ag(I) and subsequent intramolecular cycloaddition
hyde-type reactants 1o and 1p employed in the pro- leads to intermediate C which generates 2 via back
cess can smoothly give the desired tricyclic com- electron-transfer (BET) and protonation. The SET
pounds 2o and 2p. Of note was that the reaction and BET processes of silver-catalyzed reactions are
could not proceed when non-terminal alkyne reagent well known.[21]
1q was employed, and 90% 1q was recovered. This in-
triguing novel cyclization also allows an expeditious Ag2O-mediated tandem cyclization reaction of readily
entry to the unusual dihydrofuro[3,2-b]benzofuran available o-propargylaminoacetophenones/o-propar-
skeleton,[13] a ring system present in natural products gylaminobenzaldehydes to give the corresponding
In conclusion, we have developed an efficient
AHCTUNGTRENNUNG
such as panacene, a shark antifeedant,[14] psorofebrin, dihydrofuro
ACTHUNTGRENNG[U 3,2-b]indole derivatives in moderate
an antileukemic xanthone,[15] and other bioactive sub- yields under mild conditions. All products were con-
stances.[16] But we were a little disappointed that the structed efficiently in a single step in a regiospecific
o-hydroxybenzaldehyde derivatives 5-chloro-2-(prop- and highly diastereoselective manner from the readily
2-yn-1-yloxy)benzaldehyde 1r underwent the Ag-cata- available acyclic precursors. A further detailed inves-
lyzed intramolecular [3+2]annulation in poor yield.
tigation of the reaction mechanism is in progress in
our laboratory.
Scheme 2. Proposed mechanism.
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Adv. Synth. Catal. 2012, 354, 927 – 932

Microwave-Assisted Facile Synthesis of FuroACTHNUTRGNE[UNG 3,2-b]indole Derivatives
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Experimental Section
General Procedure for Synthesis of DihydrofuroACTHUNTGRNEUNG[3,2-
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Details)
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The mixture of N-(2-acetylphenyl)-N-(prop-2-yn-1-yl)acet-
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and KOH (0.028 g, 0.5 mmol) was well stirred for 1.5 h
under microwaves in CH3CN (3.0 mL), then to the mixture
was added water (10 mL) and HCl (0.5M, 1 mL), followed
by extraction with CH2Cl2 (15 mLꢃ3). The solvent was re-
moved under reduced pressure, and the residue was purified
by a short flash silica gel column chromatography (eluent:
diethyl ether/petroleum ether=1/2) to give compound 2b;
yield: 0.084 g (76%).
Acknowledgements
Financial support of this research by NSFC (21002051,
21172056), PCSIRT (IRT1061) and the Program of Founda-
tion and Frontier Technology Research of Henan Province
(112300410312) is gratefully acknowledged. We thank Prof.
Qian Zhang for helpful discussions.
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932
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