Synthesis of dihydro-[1,3]oxazino[4,3-a] isoindole and tetrahydroisoquinoline through Cu(OTf)2-catalyzed reactions of N-acyliminium ions with ynamides

Article abstract of DOI:10.1016/j.tetlet.2021.152873

An efficient approach to access functionalized dihydro-[1,3]oxazino[4,3-a] isoindole and tetrahydroisoquinoline skeletons has been developed through the addition-cyclization process of ynamides 8 with N-acyliminium ions generated from N,O-acetals 6,7. The reactions were conducted under the catalysis of Cu(OTf)₂, and a number of functionalized dihydro-[1,3]oxazino[4,3-a] isoindoles 9a-9y and tetrahydroisoquinolines 10a-10g, 11a-11p were generated in 48–98% yields. When chiral ynamides 8n-8u were used, optically pure products 11a-11p could be obtained with good to excellent yields and diastereoselectivities.

Full text of DOI:10.1016/j.tetlet.2021.152873

Tetrahedron Letters 67 (2021) 152873
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of dihydro-[1,3]oxazino[4,3-a] isoindole and
tetrahydroisoquinoline through Cu(OTf)2-catalyzed reactions
of N-acyliminium ions with ynamides
a
Rui-Jun Ma a,b, Wen-Ke Xu a, Jian-Ting Sun a, Ling Chen a, Chang-Mei Si a, , Bang-Guo Wei
⇑
a Department of Natural Medicine, School of Pharmacy, Fudan University, 826 Zhangheng Road, Shanghai 201203, China
b Center for Gastrointestinal Endoscopy, Shanxi Provincial People’s Hospital, 29 ShuangTa Road, TaiYuan 030012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient approach to access functionalized dihydro-[1,3]oxazino[4,3-a] isoindole and tetrahydroisoquino-
line skeletons has been developed through the addition-cyclization process of ynamides 8 with N-acylim-
inium ions generated from N,O-acetals 6,7. The reactions were conducted under the catalysis of Cu(OTf)2,
and a number of functionalized dihydro-[1,3]oxazino[4,3-a] isoindoles 9a-9y and tetrahydroisoquinolines
10a-10g, 11a-11p were generated in 48–98% yields. When chiral ynamides 8n-8u were used, optically pure
products 11a-11p could be obtained with good to excellent yields and diastereoselectivities.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 31 December 2020
Revised 20 January 2021
Accepted 22 January 2021
Available online 4 February 2021
Keywords:
Dihydro-[1,3]oxazino[4,3-a] isoindole
Dihydro-[1,3]oxazino[4,3-a]
tetrahydroisoquinoline
N,O-acetals
Cu(OTf)2-Catalyzed
Discovery of efficient methodology to access skeletons for phar-
maceutical use is one of the most important research areas in syn-
thetic chemistry [1]. Nitrogen-containing heterocycles, due to wide
existence in nature, are valuable subunits in these fields [2]. As a
prime instance (Fig. 1), substituted isoindoline and tetrahydroiso-
quinoline (THIQ) skeletons 1 and 2 [3,4], the key frameworks of
several pharmacologically interesting molecules and clinical drugs,
have received intensive attention in recent years. Emetine (3) exhi-
bits antiprotozoic activity [5] and is used in the treatment of lym-
phatic leukemia [6]. In addition, its close structural analogue,
tubulosine, shows potent antitumor activity [7]. So far, tremendous
efforts have been devoted to the construction of functionalized
isoindoline skeleton 1 or tetrahydroisoquinoline scaffold 2, and
several important methods have been established. However,
drug-like fragments like dihydro-[1,3]oxazino[4,3-a] isoindole 4
and tetrahydroisoquinoline skeletons 5 are rarely investigated [8].
N-Acyliminium ions [9], acting as important organic synthetic
intermediates, are widely used in the formation of CAC and C–het-
eroatom bonds [10], mostly through intermolecular addition and
intramolecular cyclization [11] with various nucleophilic reagents.
For examples, Hiemstra and Speckamp group achieved a synthetic
method to oxazinones through the intermolecular reactions of
N-alkoxycarbonyliminium ions with propargyltrimethylsilane
(Fig. 2, 1) [12a]. Recently, Maruoka group established a BF3-cat-
alyzed process to form the same skeleton through the reactions
of Boc-protected aminals with alkynes (Fig. 2, 2) [12b].
Ynamides, known as nitrogen-substituted alkynes with an
electron-withdrawing group at the nitrogen atom, have
undoubtedly become one of the most popular synthons due to
their high reactivity and high regio- and stereoselectivity [13]. As
a result, they have been successfully applied in the syntheses of
many important skeletons in the past decade [14–21]. In recent
years, we studied various nucleophilic reactions of N-acyliminium
ions, and established facile approaches to pyrido or pyrrolo[1,2-c]
[1,3]oxazin-1-ones and 3,4-dihydro-1,3-oxazin-2-ones by reacting
with alkynes or ynamides (Fig. 2, 3-4) [22]. On the basis of our con-
tinuous efforts in N, O-acetals and ynamides, we envisioned that
the iminium ions derived from the N, O-acetals 6 and 7 [22f] could
undergo a nucleophilic addition-cyclization process with ynamides
8 [22a,b] to give substituted dihydro-[1,3]oxazino[4,3-a] isoindole
and tetrahydroisoquinoline skeletons, respectively (Fig. 2, 5).
Our investigation started with the reaction of N,O-acetal 6a
with ynamide 8a. When the mixture of 6a and 8a was stirred with-
out any Lewis acid at room temperature, no desired product was
afforded (Table 1, entry 1). The treatment with TMSOTf or BF.3OEt2
at À78 °C ~ À45 °C for 2 h could lead to the desired product 9a in
45% and 70% yield, respectively (Table 1, entries 2–3). Unfortu-
nately, the reaction became complicated at room temperature. In
⇑
Corresponding author.
E-mail address: sicm@fudan.edu.cn (C.-M. Si).
https://doi.org/10.1016/j.tetlet.2021.152873
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

Rui-Jun Ma, Wen-Ke Xu, Jian-Ting Sun et al.
Tetrahedron Letters 67 (2021) 152873
Table 1
Optimization of reaction conditions.
b
Entriesa
LA (equiv.)
Temp/°C
Solvent
Y%(9a)
1
2
3
4
5
6
7
8
–
rt
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
NR
45
70
23
<10
34
37
26
31
54
73
40
NR
22
20
62
18
Trace
TMSOTf (1.0)
BF.3OEt2 (1.0)
AuCl3 (0.1)
À78 to À45
À78 to À45
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
Fig. 1. Several structures of privileged azacycles and corresponding benzo
heterocycles.
AgSbF6 (0.1)
SIPrAuCl (0.1)
SIPrAuCl/AgSbF6 (0.1)
PPh3AuCl/AgNTf2 (0.1)
PPh3AuCl/AgSbF6 (0.1)
Cu(OTf)2 (0.1)
Cu(OTf)2 (0.2)
Cu(OTf)2 (0.5)
CuSO4 (0.2)
CuCl2 (0.2)
CuBr2 (0.2)
Cu(OTf)2 (0.2)
Cu(OTf)2 (0.2)
Cu(OTf)2 (0.2)
9
10
11
12
13
14
15
16
17
18
THF
Toluene
a
The reaction was performed with 6a (0.5 mmol), 8a (0.6 mmol) and catalyst in
dry solvent (3 mL) at assigned reaction temperature for 2 h under Ar atmosphere.
b
Isolated yield; NR = no reaction.
order to improve the yield under mild reaction conditions, various
metal Lewis acids were screened. AuCl3, AgSbF6, SIPrAuCl,
SIPrAuCl/AgSbF6, PPh3AuCl/AgNTf2, PPh3AuCl/AgSbF6 could afford
the desired product, albeit with low yield (<10~31%, Table 1,
entries 4–9). Delightfully, Cu(OTf)2 could lead to the desired pro-
duct 9a in 54% yield (Table 1, entry 10). When the use of Cu
(OTf)2 was increased to 0.2 equivalent, the yield could be signifi-
cantly improved to 73% (Table 1, entry 11). Further increasing
the use of Cu(OTf)2 to 0.5 equivalent led to the erosion of yield
to 40% (Table 1, entry 12). Other copper salts, like CuSO4, CuCl2
and CuBr2 did not result in any satisfactory yield. Different reaction
solvents including DCE, THF, toluene were also examined. Among
them, DCE led to moderate yield of 9a (62%, Table 1, entry 16),
while THF gave much lower yield of 9a and toluene was not suit-
able for this transformation at all (Table 1, entries 17 and 18).
Next, we turned to investigate the scope and limitation of such
cyclization of N,O-acetals 6a-6e with ynamides 8a-8l (Table 2). A
variety of substitutions at the acetylene phenyl ring and nitrogen
of ynamides 8a-8l were investigated. TsNBn-type ynamides with
different para-substituted (methyl, methoxyl and halogen) aryls
were surveyed under the optimal conditions, as summarized in
Table 2. In general, all these substituted aryl TsNBn ynamides
(8b-8g) could smoothly react with N, O-acetals 6a, affording the
desired products 9b-9g in moderate yields. Thiophene substituted
TsNBn ynamide 8h was also a suitable substrate for this cyclization
reaction, leading to the desired products 9h in 62% yield. When Bn
group in ynamides was replaced with different aryl, alkyl, allyl
groups, the corresponding derivatives 8i-8l also worked well to
give the desired 9i-9l in moderate yields. Different substitutions
at the isoindole ring were also investigated. 5-Chloro-substituted
isoindole N, O-acetal 6b could smoothly react with ynamides 8a-
8l, affording the desired products 9m-9u in 66–87% yields. 6-
bromo, 4-bromo, 6-methoxy isoindole N, O-acetals 6c-6e were also
Fig. 2. Lewis Acid-promoted nucleophilic addition-cyclization of N, O-acetals with
ynamides.
2

Rui-Jun Ma, Wen-Ke Xu, Jian-Ting Sun et al.
Tetrahedron Letters 67 (2021) 152873
Table 2
The reactions of N,O-acetals 6a-6e with Ynamides 8a-8l.a-b
a
The reaction was performed with 6 (0.5 mmol), 8 (0.6 mmol) and Cu(OTf)2 (0.1 mmol) in dry DCM
(3 mL) under Ar atmosphere at room temperature for 2 h.
b
Isolated yield.
suitable substrates for this cyclization reaction, and the desired
products 9v-9x could be obtained in 64–86% yields. In addition,
the replacement of Ts group in ynamides with Ms also gave posi-
tive results under the optimal conditions, and the desired products
9y was obtained in 61% yield. The chemical structures of 9a-9y
were unambiguously confirmed based on the X-ray crystallo-
graphic analysis of compound 9l (see the Supporting Information)
[23].
After the successful cyclization of isoindole N, O-acetals 6a-6e
with ynamides 8a-8l to produce a variety of substituted dihydro-
[1,3]oxazino[4,3-a] isoindoles, we turned our attention to investi-
gate the reaction of tetrahydroisoquinoline N, O-acetal 7a with
3

Rui-Jun Ma, Wen-Ke Xu, Jian-Ting Sun et al.
Tetrahedron Letters 67 (2021) 152873
Table 3
Table 4
The reactions of N,O-acetal 7a with Ynamides 8a-8g.a-b
The reactions of N,O-acetals 7 with Ynamides 8n-8u.a-b
a
The reaction was performed with 7a (0.5 mmol), 8 (0.6 mmol) and Cu(OTf)2
(0.1 mmol) in dry DCM (3 mL) under Ar atmosphere at room temperature for 2 h.
b
Isolated yield.
ynamides in order to achieve dihydro-[1,3]oxazino[4,3-a] tetrahy-
droisoquinoline skeleton (Table 3). To our delight, Bn, Me, n-C4H9-
NTs ynamides 8a-8c could react with N, O-acetal 7a, affording the
desired dihydro-[1,3]oxazino[4,3-a] tetrahydroisoquinolines 10a-
10c in 86%, 64%, 93% yields, respectively. The replacement of Ts
group in ynamides with Ms and 4-ClC6H4SO2 also gave positive
results under the optimal conditions, and the desired products
10d and 10f were obtained in 98% and 87% yields, respectively.
Notably, oxazolidinone substituted ynamide 8m could also afford
the desired product 10e in moderate yield. In addition, the replace-
ment of phenyl group in ynamide 10a with 4-Me-phenyl also
worked well to give the desired products 10g in 96% yield. The
chemical structures of 10a-10g were unambiguously confirmed
based on the X-ray crystallographic analysis of compound 10g
(see the Supporting Information) [24].
Motivated by the successful reaction with oxazolidinone yna-
mide 8m, we further explored the use of several chiral oxazolidi-
none ynamides. As summarized in Table 4, when optically pure
phenyloxazolidinone and isopropyloxazolidinone ynamides were
used, the desired products 11a-11k were successfully obtained in
68%–97% yields, albeit with moderatate diastereoselectivities.
Interestingly, chiral benzyloxazolidinone and (4R,5S)-4-methyl-5-
phenyloxazolidinone ynamides could afford the desired products
11l-11p in 57%-84% yields, along with excellent diastereoselectiv-
ities (up to dr > 20:1).
a
The reaction was performed with 7a-7c (0.5 mmol), 8n-8u (0.6 mmol) and Cu
(OTf)2 (0.1 mmol) in dry DCM (3 mL) under Ar atmosphere at room temperature for
2 h.
b
Isolated yield.
dr was determined by HPLC.
c
To determine the relative configuration by crystals, we tried to
introduce
a large group. Dihydro-[1,3]oxazino[4,3-a] tetrahy-
droisoquinolin 11f and 11m were coupled with 2-naph-
thaleneboronic acid, under standard Suzuki-Miyaura conditions
[PdCl2 (dppf)2/K3PO4], smoothly afforded products 12 and 13 in
62% and 58% yields (Scheme 1) [25]. Unfortunately, both products
obtained were foam solids and failed for crystallization.
The ECD calculations were used to assign the absolute configu-
ration of 12. As shown in Fig. 3, the experimental ECD curve of 12
was in good agreement with the calculated one for M336-S.
Therefore, the absolute configurations of the new stereogenic
centers for 11a-11p were determined as S [26].
4

Rui-Jun Ma, Wen-Ke Xu, Jian-Ting Sun et al.
Tetrahedron Letters 67 (2021) 152873
A plausible mechanism for this Cu(OTf)2-catalyzed nucleophilic
addition-cyclization process was illustrated in Fig. 4 on the basis of
our obtained experimental results and the precedent ynamide
chemistry [27]. First, the triple bond in ynamide 8 was activated
by copper (II) triflate. Then, the resulting vinyl Cu(II) intermediate
int-1 was attacked by 6/7 through the cleavage of t-butyl group to
give int-2, in which an intramolecular cyclization occurred to form
the desired product 9/10/11.
In summary, we established a novel and efficient approach for
the synthesis of functionalized dihydro-[1,3]oxazino[4,3-a] isoin-
dole 9a-9y and tetrahydroisoquinoline 10a-10g, 11a-11p. Both
isoindole N, O-acetals 6 and tetrahydroisoquinoline N, O-acetals 7
proved to be suitable dipolarophiles for such a Cu(OTf)2-catalyzed
nucleophilic addition-cyclization process. As a result, a number of
functionalized dihydro-[1,3]oxazino[4,3-a] isoindole 9a-9y and
tetrahydroisoquinoline 10a-10g, 11a-11p were achieved in moder-
ate to good yields (48–98%). In addition, the chiral ynamides 8n-8u
could lead to optically pure products 11a-11p with moderate to
excellent yields and diastereoselectivities.
Declaration of Competing Interest
Scheme 1. Suzuki-Miyaura coupling reaction of 11f and 11 m with boronic acid.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
We thank the National Natural Science Foundation of China
(21702032 to C.-M. Si and 21772027 to B.-G. Wei) for financial
support. The authors also thank Dr. Han-Qing Dong (Arvinas,
Inc.) for helpful suggestions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152873.
Fig. 3. Experimental and calculated ECD spectra of 12 in acetonitrile.
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