Gold(I)-catalyzed regio- and stereoselective 1,3-dipolar cycloaddition reactions of 1-(1-Alkynyl)cyclopropyl oximes with nitrones: A modular entry to highly substituted pyrrolo[3,4-d][1,2]oxazepines

Article abstract of DOI:10.1002/adsc.201200288

An efficient approach for the synthesis of highly substituted pyrrolo[3,4-d][1,2]oxazepines has been achieved by gold(I)-catalyzed 1,3-dipolar cycloaddition reactions of 1-(1-alkynyl)cyclopropyl oximes with nitrones in good to excellent yields as a single diastereomer. A complete chirality transfer was observed in this transformation. Copyright

Full text of DOI:10.1002/adsc.201200288

UPDATES
DOI: 10.1002/adsc.201200288
Gold(I)-Catalyzed Regio- and Stereoselective 1,3-Dipolar
Cycloaddition Reactions of 1-(1-Alkynyl)cyclopropyl Oximes
with Nitrones: A Modular Entry to Highly Substituted
Pyrrolo
N
U
Yanqing Zhanga and Junliang Zhanga,b,*
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal
University, 3663 N. Zhongshan Road, Shanghai 200062, Peopleꢀs Republic of China
Fax : (+86)-021-6223-5039; e-mail: jlzhang@chem.ecnu.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, Shanghai 200032, Peopleꢀs Republic of China
Received: April 6, 2012; Revised: July 9, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200288.
Abstract: An efficient approach for the synthesis of
highly substituted pyrrolo[3,4-d][1,2]oxazepines has
synthetically challenging fused heterobicyclic pyrroles
is still highly desirable.
N
ACHTUNGTRENNUNG
been achieved by gold(I)-catalyzed 1,3-dipolar cy-
cloaddition reactions of 1-(1-alkynyl)cyclopropyl
oximes with nitrones in good to excellent yields as
a single diastereomer. A complete chirality transfer
was observed in this transformation.
Recently, 1-(1-alkynyl)-cyclopropyl ketones have
been used in gold-catalyzed tandem reactions to effi-
ciently construct highly substituted furans.[11] Very re-
cently, Wang[12] and our group[13] independently re-
ported copper- or gold-catalyzed highly diastereose-
lective 1,3-dipolar cycloaddition reactions[14] of 1-(1-
alkynyl)-cyclopropyl ketones with nitrones leading to
Keywords: cycloaddition; diastereoselectivity; enan-
tioselectivity; gold; pyrroles
heterobicyclic furoACTHUNTGRNENUG[3,4-d]CAHTUNGTRENN[UGN 1,2]oxazepines [Eq.(2)]. We
envisaged that 1-(1-alkynyl)-cyclopropyl oximes,
which could be easily prepared from the correspond-
ing 1-(1-alkynyl)-cyclopropyl ketones, may be used to
construct
fused
heterobicyclic
pyrroloACHTUNGTRENNUNG[3,4-d]-
After pyrrole was discovered in the 1830s and finally
ACHTUNGTREN[NUGN 1,2]oxazepines through 1,3-dipolar cycloaddition with
isolated in 1857 by T. Anderson, it has attracted con- nitrones under the metal catalysis.
tinuing interest over the years in natural products,[1]
pharmaceuticals,[2] materials science[3] and supra-
molecular chemistry[4] due to its unique structure and
properties . As a consequence, various methods for
the synthesis of highly substituted pyrroles have been
developed.[5] For example, in the past years, many cy-
ACHTUNGTRENNUNG
clization reactions[6,7] and multi-component reactions[8]
have been well established for the synthesis of pyr-
roles. Meanwhile, N-alkoxypyrroles could be used as
insecticides in crop protection,[9] which makes it rea-
sonable to develope new methods for synthesis of N-
alkoxypyrroles. Recently, Park[10] explored a copper-
catalyzed [3+2] cycloaddition of a-diazo oxime
ethers with alkenes leading to highly substituted pyr-
This hypothesis was initially tested by reacting of
roles in good to excellent yields [Eq.(1)]. However, (E)-1a with nitrone 2a in the presence of
many of the methods developed so far have several Ph3PAuOTf[11a] (generated by 1:1 reaction of
drawbacks, such as narrow substrate scope, low yield, Ph3PAuCl/AgOTf), affording the desired product 3a
a significant amount of by-products formed or low re- in 99% yield with moderate diastereoselectivity (dr=
gioselectivity. Furthermore, the efficient synthesis of 8:1), which inspired us to explore better conditions to
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Yanqing Zhang and Junliang Zhang
UPDATES
improve the diastereoselectivities. Changing the silver gate the effect of the ligand of the gold catalyst. To
salts did not bring any obvious improvement (Table 1, our delight, the diastereoselectivity could be effective-
entries 1–4). The reaction proceeded very slowly ly improved without loss of the reactivity by the use
when Ph3PAuCl was used as catalyst (Table 1, of a series of monophosphane ligands[15] (Table 1, en-
entry 5). AgOTf could catalyze the reaction with tries 10, and 12–16). Finally, it was found that the cy-
a little improvement in the diastereoselectivity, but cloaddition reaction proceeds very smoothly in DCE
AgOMs could not catalyze the reaction effectively at room temperature under the catalysis of 2 mol% of
(Table 1, entries 6 and 7). Next, we turned to investi- (2-furyl)3PAuOTf (standard conditions) to give cyclo-
adduct 3a in 99% yield with excellent diastereoselec-
tivity.
Table 1. Optimization studies on cycloaddition of 1a and
2a.[a]
Under the optimized conditions, the scope and limi-
tations of this transformation were next explored by
variation of the 1-(1-alkynyl)-cyclopropyl O-methyl-
oximes (E)-1. Firstly, the substituent effect of R2 was
studied by introduction of different alkynyl groups
(Table 2, entries 1–7), affording the corresponding 3
in high yields with excellent diastereoselectivities, and
there is no obvious substituent effect for those substi-
tuted alkynes. Moreover, not only (E)-1i and (E)-1j
with aryl-substituted R3 but also (E)-1k with n-butyl-
substituted R3 are applicable to these reaction condi-
tions, affording the corresponding cycloadducts 3i–k
in 94–99% yields as single stereoisomer (Table 2, en-
tries 8–10). However, a relatively low yield of 3l was
obtained with R3 =H, indicating that the R3 substitu-
ent has a slight effect on yield (Table 2, entry 11).
Afterwards, we turned our attention to the effect of
R1. Using the same method, E and Z-isomers of 1m
were isolated from the reaction of corresponding
ketone with methoxyamine. When they were added
Table 2. The cycloaddition of 1 with 2a.
Entry Cat.
Time [h] Yield [%] dr[e]
1[b]
2[b]
3[b]
4[b]
5[b,c]
6[b]
7[b]
8[b]
9[b]
10
Ph3PAuCl/AgOTf
0.3
0.3
0.3
5
12
0.5
21
0.3
2.5
11
22.5
11
6
99
99
94
99
9
87
trace
82
94
94
8
99
99
87
99
99
8:1
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgNTf2
Ph3PAuCl/AgOMs
Ph3PAuCl
AgOTf
AgOMs
10:1
10:1
29:1
>50:1
9:1
–
8:1
Entry R1/R2/R3 (1)
Time
[h]
3
Yield[a]
[%]
IPrAuCl/AgOTf
IPrAuCl/AgOMs
(L1)AuCl/AgOTf
29:1
42:1
46:1
>50:1
23:1
>50:1
43:1
>50:1
1
2
3
4
Me/4-MeOC6H4/Ph (1b)
2.5
15
11
4
3b 99
3c 99
3d 92
3e 95
11[d] (L2)AuCl/AgOTf
Me/4-MeC6H4/Ph (1c)
Me/4-BrC6H4/Ph (1d)
Me/1-cyclohexenyl/Ph
(1e)
12
13
14
15
16
(L3)AuCl/AgOTf
(L4)AuCl/AgOTf
(L5)AuCl/AgOTf
6
11
A
5
6
7
8
9
10
Me/cyclopropyl/Ph (1f)
Me/AcOC2H4/Ph (1g)
Me/n-C4H9/Ph (1h)
Me/Ph/4-MeOC6H4 (1i)
Me/Ph/4-MeC6H4 (1j)
Me/Ph/n-C4H9 (1k)
4
3f 98
3g 99
3h 95
3i 99
3j 99
3k 94
3l 84
A
10
15
10
4.5
2.5
22
[a]
All reactions were carried out using 1a (0.3 mmol), 2a
(0.36 mmol), catalyst (2 mol%) in 3 mL of DCE with
60 mg 4 ꢂ MS at room temperature unless otherwise
specified.
Without 4 ꢂ MS.
90% of 1a was recovered.
11[b] Me/Ph/H (1l)
[b]
[c]
[d]
[e]
[a]
Isolated yield and only one diastereomer was formed,
which was detected by NMR.
The reaction was carried out at 508C.
81% of 1a was recovered.
The dr was determined by NMR of the crude products.
[b]
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Gold(I)-Catalyzed Regio- and Stereoselective 1,3-Dipolar Cycloaddition Reactions
to the reaction system, only the (E)-isomer of 1m
could react with nitrone 2a to give the desired prod-
uct 3m in 95% yield [Eq.(3)], the (Z)-1m remained
untouched even when the reaction temperature was
raised to 508C [Eq.(4)].
Figure 1. X-ray crystal structures of compounds 3d (left) and
(E)-4 (right).
which reacted with MeI under the basic conditions
leading to the same product (E)-1a as that from the
correponding cyclopropyl ketone and CH3ONH2·HCl.
The isomer of 1m which could react with nitrone 2a,
was inferred as (E)-1m, and the other unreactive
isomer was inferred as (Z)-1m.
After studying the scope of oximes 1, the substitu-
ent effect of nitrones 2 was investigated. Gratifyingly,
both electron-deficient and electron-rich aryl substitu-
Polysubstituted 1H-pyrrol-1-ol 5 was afforded from ents R5 on nitrones 2 were compatible to the present
the corresponding unprotected oxime (E)-4 with ni- transformation, affording the corresponding cycload-
trone 2a under the catalysis of Ph3PAuCl/AgOTf in duct 3 in excellent yields as a single diastereomer,
30 min [Eq.(5)]. Moreover, optically active pyrrolo- except for 3o and 3p (Table 3, entries 1–7). In addi-
A
ACHTUNGTRENNUNG
[1,2] oxazepine (1R,4R)-3a was obtained in the tion, a benzyl group as R4 on nitrone 2i was also com-
indicating that chirality transfer takes place in this product, heterobicyclic 3a was reduced by hydrogena-
transformation. Thus, the reaction should proceed via tion in EtOH at 508C catalyzed by Pd/C under an H2
an SN2 reaction pathway.[13b]
atmosphere [Eq (7)], affording pyrrole 6 in 84%
yield. In this transformation, it is noteworthy that not
Table 3. The cycloaddition of 1a with various 2.
Entry R4/R5
Time [h] 3 (Yield [%]) dr
1
2
3
4
5
6
7
8
Ph/2-furyl (2b)
Ph/4-MeOC6H4 (2c) 2.5
Ph/styryl (2d)
Ph/1-naphthyl (2e)
2.5
3n (99)
3o (99)
3p (99)
3q (99)
3r (96)
3s (98)
3t (88)
3u (99)
>50:1
10:1
16:1
>50:1
>50:1
>50:1
>50:1
>50:1
7.5
3
The structure and relative stereochemistry of 3
were established by single-crystal X-ray diffraction
analysis of 3d (Figure 1)[16]. The structures and setero-
chemistry of 1a–1l were further confirmed by
a single-crystal X-ray diffraction analysis of (E)-4,
Ph/4-NO2C6H4 (2f) 7.5
Ph/4-BrC6H4 (2g)
Ph/3-ClC6H4 (2h)
Bn/Ph (2i)
3.5
3.5
2.5
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UPDATES
À
À
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UPDATES
6 Gold(I)-Catalyzed Regio- and Stereoselective 1,3-Dipolar
Cycloaddition Reactions of 1-(1-Alkynyl)cyclopropyl
Oximes with Nitrones: A Modular Entry to Highly
Substituted PyrroloACHTUNGTRENNUNG[3,4-d]ACHTUNGTRENN[UGN 1,2]oxazepines
Adv. Synth. Catal. 2012, 354, 1 – 6
Yanqing Zhang, Junliang Zhang*
6
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These are not the final page numbers!