Facile synthesis of substituted 3-aminofurans through a tandem reaction of N-sulfonyl-1,2,3-triazoles with propargyl alcohols

Article abstract of DOI:10.1039/c6ob00377j

A relay catalysis strategy for substituted 3-aminofurans synthesis has been developed. This transformation involves a tandem reaction sequence through aza-vinyl-rhodium(ii) carbene O-H bond insertion, thermal propargyl-Claisen rearrangement and gold(i)-catalyzed intramolecular cyclization. More importantly, the current strategy employs simple feedstocks as starting materials, providing substituted 3-aminofurans in a highly efficient manner.

Full text of DOI:10.1039/c6ob00377j

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DOI: 10.1039/C6OB00377J
Organic & Biomolecular Chemistry
ARTICLE
Facile Synthesis of Substituted 3‐Aminofurans Through Tandem
Reaction of N‐Sulfonyl‐1,2,3‐triazoles with Propargyl Alcohols
Xing Cheng,a Yinghua Yu,b Zhifeng Mao,b Jianxin Chen,*a and Xueliang Huang*b
Received 00th January 20xx,
Accepted 00th January 20xx
A relay catalysis strategy on substituted 3‐aminofurans synthesis has been developed. This transformation involving
tandem reaction sequence through aza‐vinyl‐rhodium(II) carbene O‐H bond insertion, thermal propargyl‐Claisen
rearrangement and gold(I)‐catalyzed intramolecular cyclization. More importantly, the current strategy employs simple
feedstocks as starting materials, providing substituted 3‐aminofurans in highly efficient manner.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Polysubstituted furan and its derivatives are ubiquitous
heterocycles. They often serve as core structural motifs in
natural products, pharmaceuticals, and other functional
molecules.1 Thus, the development of efficient methods for
the synthesis of tetra‐substituted furans is of continuing
interest in the field of organic chemistry. Besides the classic
methods, such as Paal−Knorr condensation and Feist‐Benary
synthesis,2 significant advances have been achieved in the past
decades.3 Among these, transition‐metal catalyzed
cycloisomerization of allenyl ketones4 and intermolecular
reaction of alkynes with carbonyl compounds5 have been
proved to be efficient approaches to build up furan cores.
Furans bearing amino functionality at the 3‐position are
important scaffolds that embedded in many molecules which
show potential biological activities (Scheme 1).6 However,
general and modular approach to access 3‐aminofurans is not
well explored.7
Scheme 2. Working hypothesis on 2,3‐dihydropyrrole
synthesis.
Aza‐vinyl rhodium carbene
A generated from 1‐sulfonyl‐
1,2,3‐triazole is a useful intermediate, which has been widely
applied for the expedient synthesis of N‐heterocyclic
compounds in recent years.8,9 Cationic gold(I) complexes have
been proved to be active catalysts for Meyer‐Schuster
rearrangement.10 As our continuing interests in heterocycle
synthesis via aza‐vinyl carbene intermediates,11 we speculated
that the intermediate
may be captured by aza‐vinyl rhodium carbene intermediated
, thus affording the formal [2+3] cycloaddition product in a
B generated in situ via gold(I) catalysis
A
synergistic fashion (Scheme 2).12 Initial attempt on the
reaction of propargyl alcohol 1a with triazole 2a in presence of
Ph3PAuNTf2 and Rh2(Piv)4 (piv = pivaloyl), giving dihydropyrrole
4a in 41% yield, and speculated regioisomer 3a was not
observed (Scheme 3). The structure of 4a was unambiguously
confirmed by X‐ray crystallographic analysis. Thus the pathway
Scheme 1. Selected examples of molecules with potential
biological activities containing 3‐aminofuran scaffold.
a. College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou,
Fujian 350007, P. R. China. E‐mail: jxchen@fjnu.edu.cn.
b. Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian
Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, P. R. China. E‐mail: huangxl@fjirsm.ac.cn. Web:
http://www.fjirsm.cas.cn/yjxt/yyhxyjzs/hxlktz/.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 3. Initial attempt on the reaction of 1a with 2a
.
for the generation of 4a may be similar to one‐pot synthesis of
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2,5‐dihydropyrroles through relayed rhodium(II)/gold(I) aminofurans were obtained in moderate to high yields.
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catalysis reported by Murakami and co‐workers.9j
DOI: 10.1039/C6OB00377J
Electron donating groups (including methyl, n‐propyl and
Surprisingly, when simple propargyl alcohol 1b was tested methoxyl) sitting at the para position of the phenyl ring are
for the reaction, a switch of the reaction mode was observed, tolerated under current conditions (cf. 5b 5d). Similarly,
‐
and 3‐aminofuran 5a was obtained as the major product triazoles bearing mild electron withdrawing groups (chloro,
(Scheme 4a). Further study revealed that the anion in the bromo and fluoro) at the para position could also react with 1b
cationic gold catalyst had great impacts on the selectivity for smoothly, giving 5e to 5g in good yields. Moving the chloro
the cycloisomerization (Scheme 4b). This observation is very group to the meta position had no obvious influence on the
interesting. As by simple altering the gold catalyst, another reaction outcome (cf. 5h). Interestingly, the reaction of triazole
important heterocycle 3‐aminofuran which is not easily possessing a chloro group at the ortho position on the phenyl
accessible via conventional methods could be obtained in a ring proceeded well, leading to the corresponding aminofuran
straightforward manner.
in 59% yield. At the meantime, 2,5‐dihydropyrrole 4i was also
isolated in 23% yield. Triazoles derived from 2‐
ethynylthiophene and 2‐ethynylnaphthalene were viable
substrates, the corresponding furans were isolated in 70% and
75% yields, respectively. Because of the competing 1,2‐
alkyl/hydride migration, the reactions of 4‐alkyl substituted
triazoles failed to give desired aminofuran (results not
shown).13
With Triazole 2a as model substrate, various propargyl
alcohols were then examined to explore the substrate scope
(Table 2). As depicted, high yields of the substituted 3‐
aminofurans were obtained for most of the substrates with a
substituted phenyl group at the terminal position of the triple
bond. In detail, propargyl alcohols bearing mildly electron‐
donating or withdrawing groups at the para postion of the
Scheme 4. Synthesis of 3‐aminofurans 5a from the
reaction of 1b and 2a
.
After establishing an effective one‐pot procedure for
substituted 3‐aminofuran synthesis, the generality and
limitation refers to N‐sulfonyl 1,2,3‐triazoles
2 were examined,
and results were compiled in Table 1. As can be seen, a variety
of aryl substituted sulfonyl triazoles could react with 1b, after
a sequential transformations, the corresponding substituted 3‐
phenyl participated well in these cascade reactions (cf. 5l‐5p).
Introduction of a chloro group to the meta position on the
Table 2. Substrates scope with respects to a variety of
propargyl alcohol 1 [a]
.
Table 1. Substrates scope with respects to a variety of
triazoles 2.
R
[a]
1) 1 mol% Rh2(Piv)4
TsHN
Ph
N
toluene, 100 oC, 40 min
N
OH
N
Ts
+
2) 3 mol% Ph3PAuNTf2
toluene, 100 oC, 4 h
Me
R
Ph
O
5
1, 0.5 mmol
2a, 0.55 mmol
Me
OMe
TsHN
Ph
TsHN
Ph
TsHN
Me
O
Me
Me
Ph
O
O
5n, 87%
5m, 86%
5l, 88%
Cl
Br
TsHN
Ph
TsHN
Ph
Me
O
Me
O
5o, 92%
5p, 92%
X-ray of 5n
Cl
TsHN
Ph
TsHN
Ph
TsHN
Ph
Cl
Me
Me
O
Me
O
O
5q, 91%
5r, 75%
5s, 80%
Me
TsHN
Ph
Me
TsHN
Ph
TsHN
Ph
Me
Me
5v, 72%[b]
Me
O
O
O
[a] [
2] = 0.2 M , yields were referred to pure products
5
u, 88
%
5t, 80%
upon isolation after column chromatography on silica gel.
[b] Yield given in parentheses was referred to product for
5 mmol scale reaction. [c] Dihydropyrrole 4i was isolated
in 23% yield.
[a] [1] = 0.2 M , yields were referred to pure products
upon isolation after column chromatography on silica gel.
[b] 2,5‐Dihydropyrrole 4v was isolated in 27% yield.
2 | J. Name., 2012, 00, 1‐3
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phenyl ring, had no influence on the reaction, and the place would lead to the formation of 4.9j Replacing the counter
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corresponding furan (cf. 5q) was obtained in 91% yield. anion SbF6 by NTf2 (for reaction detDaiOlsI: 1s0e.1e0:39S/Cch6OemB0e03747)J,
Pleasingly, propargyl alcohols incorporated with sterically resulted in a switch of the reaction site (from N‐cyclization to
hindered groups (2‐chlorophenyl and 1‐naphthalenyl) were O‐cyclization). Although the exact way on counter anion
tolerated in the current protocol, and furans 5r and 5s were regulating the cyclization mode remains unclear at current
isolated in 75% and 80% yields, respectively. Moreover, the moment, this work represents a facile protocol for poly‐
alkyl substituted propargyl alcohols (with methyl, n‐butyl and substituted furan synthesis (Scheme 6).4m,4n
cyclopropyl groups on the alkyne terminal) were also suitable
for this cyclization reactions, thus providing the desired 5a was firstly reacted with benzoyl chloride, and then the tosyl
To avoid the hydrolysis of the unprotected 3‐aminofuran,15
products 5t‐5v in 72‐88% overall yields. It is worth pointing out group could be easily removed via a photoredox approach
that when 3‐cyclopropylprop‐2‐yn‐1‐ol was examined, besides (Scheme 7).16
the desired amino furan 5v, 2,5‐dihydropyrrole 4v was
obtained in 27% yield. Unambiguous proof of the structures
was achieved by single‐crystal X‐ray analysis of furans 5j and
5n 14
.
Scheme 7. Removal of tosyl group.
Conclusions
In summary, we have demonstrated a new protocol for
substituted 3‐aminofuran synthesis by a relayed rhodium(II)
and gold(I) catalysis strategy. Compared with Murakami’s 2,5‐
dihydropyrrole synthesis, this work has uncovered an
intriguing phenomenon that gold(I) catalyst could alter the
chemoselectivity for the cycloisomerization, thus leading to
different heterocycles. This procedure features high efficiency,
broad substrate scope. Further study on heterocycle synthesis
via dual catalysis is underway.
Scheme 5. Reaction of propargyl alcohols 1m or 1a with
triazole 2a
.
Encouraged by above results, we then set our sights on
more challenging substrates, polysubstituted propargyl
alcohols. Interestingly, substituted dihydropyrrole was
obtained instead of the expected 3‐aminofuran, when 2‐
methylbut‐3‐yn‐2‐ol 1m was subjected to our standard
conditions, and 4ma was isolated in 70% yield (Scheme 5a).
Similarly, the reaction of 1,3‐diphenylprop‐2‐yn‐1‐ol 1a with
triazole 2a took place, and the yield of dihydropyrrole 4a could
be enhanced to 72% yield (Scheme 5b).
Acknowledgements
We thank NSFC (Grant No. 21402197, 21502190), NSF of Fujian
(Grant No. 2015J05043), Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, and
Hundred‐Talent Program (Chinese Academy of Sciences) for
financial support.
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