TMSOTf-mediated approach to 1,3-oxazin-2-one skeleton through one-pot successive reduction-[4 + 2] cyclization process of imides with ynamides

Article abstract of DOI:10.1016/j.tetlet.2021.152946

A one-pot approach to access functionalized 1,3-oxazin-2-one skeleton has been developed through successive reduction and subsequent [4 + 2] cyclization process of N-Boc lactams with ynamides by TMSOTf. As a result, a number of five to seven membered ring fused bicyclic [1,2-c][1,3]oxazin-1-ones 12a-m and tricyclic derivatives 13a-f were obtained in moderate to excellent yields with excellent regioselectivities. Moreover, linear N-Boc amides 9a-e were also amenable to this transformation, and the desired 3,4-dihydro-1,3-oxazin-2-ones 14a-m were readily achieved in moderate yields with excellent regioselectivities.

Full text of DOI:10.1016/j.tetlet.2021.152946

Tetrahedron Letters 68 (2021) 152946
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Tetrahedron Letters
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TMSOTf-mediated approach to 1,3-oxazin-2-one skeleton through one-
pot successive reduction-[4 + 2] cyclization process of imides with
ynamides
a,b,
Chen-Chen Zhang a, Zhi-Peng Huo a, Mei-Lin Tang a, Yong-Xi Liang a, , Xun Sun
⇑
⇑
a Department of Natural Medicine, School of Pharmacy, Fudan University, 826 Zhangheng Road, Shanghai 201203, China
b The Institutes of Integrative Medicine of Fudan University, 12 Wulumuqi Zhong Road, Shanghai 200040, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-pot approach to access functionalized 1,3-oxazin-2-one skeleton has been developed through suc-
cessive reduction and subsequent [4 + 2] cyclization process of N-Boc lactams with ynamides by TMSOTf.
As a result, a number of five to seven membered ring fused bicyclic [1,2-c][1,3]oxazin-1-ones 12a-m and
tricyclic derivatives 13a-f were obtained in moderate to excellent yields with excellent regioselectivities.
Moreover, linear N-Boc amides 9a-e were also amenable to this transformation, and the desired 3,4-dihy-
dro-1,3-oxazin-2-ones 14a-m were readily achieved in moderate yields with excellent regioselectivities.
Ó 2021 Published by Elsevier Ltd.
Received 19 January 2021
Revised 14 February 2021
Accepted 16 February 2021
Available online 23 February 2021
Keywords:
1,3-Oxazin-2-one Skeleton
Imides
Ynamides
Reduction-[4+2] cyclization process
Introduction
ades and thereby several synthetic methods have been developed
[9]. However, efficient synthetic approach to access functionalized
The discovery of efficient synthetic methodology to access
divergent skeletons is one of the most important tasks in the field
of organic and pharmaceutical chemistry [1]. Nitrogen-containing
heterocyclic small molecules not only widely exist in nature [2],
but also serve as important subunits of many clinical drugs discov-
ered in the past decades [3]. As a prime instance (Fig. 1), function-
alized 3,4-dihydro-1,3-oxazin-2-one skeletons 1–3 are the key
framework of several pharmacologically interesting molecules
and clinical drugs [4]. Moreover, these skeletons could be easily
converted to functionalized 1,3-amino alcohol fragment 4, an
important unit of many natural products and pharmacologically
interesting molecules [5]. One example is geissolosimine 5, which
was isolated from the geissospermum vellosii in the Amazonian
forest [6], showed antiparasitic activities and it may be a lead
molecular structure for possible antimalarial drug development
[7]. Another is PD172938 6 which was identified as a potent dopa-
mine D4 ligand and indicated possible antipsychotic activity [8].
Due to the important application of 3,4-dihydro-1,3-oxazin-2-one
skeleton in natural products and potential clinical drugs, the
related intermediates have attracted great interest in the past dec-
3,4-dihydro-1,3-oxazin-2-one skeleton is still in great demand
[10].
N-Acyliminium ions [11], an important class of organic syn-
thetic intermediates, are widely used in the formation of CAC
and C-heteroatom bonds [12] through intermolecular addition
[13] and intramolecular cyclization [14] with various nucleophilic
reagents. As a prime instance (Fig. 2), N-acyliminium ions have
been successfully used to synthesize functionalized 1,3-oxazin-2-
ones. In 1992, Hiemstra and Speckamp’s group reported the syn-
thesis of oxazinone skeleton through intermolecular reactions of
N-alkoxycarbonyliminium ions with aryl acetylenes (Fig. 2, a)
[4c]. Maruoka’s group achieved various 1,3-oxazin-2-ones
through the BF3ÁOEt2-mediated cyclization process of Boc-
protected aminals with different alkynes (Fig. 2, b) [4d]. Very
recently, Wei and Si successfully converted N-acyliminium ions
to pyrido or pyrrolo[1,2-c][1,3]oxazin-1-ones[9a-c] and 3,4-
dihydro-1,3-oxazin-2-ones (Fig. 2, c) [10b]. To our best
knowledge, all these important transformations to 1,3-oxazin-2-
ones require the use of isolated precursors N, O- acetals or N-Boc
aminals, which are relatively unstable in most cases.
Consequently, the application of these methods is limited in the
preparation of functionalized 3,4-dihydro-1,3-oxazin-2-one
skeleton[4d-e]. Ynamides, a class of popular nucleophiles, have
been widely used in organic synthesis [15]. On the basis of our con-
⇑
Corresponding authors at: Department of Natural Medicine, School of Phar-
macy, Fudan University, 826 Zhangheng Road, Shanghai 201203, China.
E-mail addresses: yongxi_liang@fudan.edu.cn (Y.-X. Liang), sunxunf@shmu.edu.
cn (X. Sun).
https://doi.org/10.1016/j.tetlet.2021.152946
0040-4039/Ó 2021 Published by Elsevier Ltd.

Chen-Chen Zhang, Zhi-Peng Huo, Mei-Lin Tang et al.
Tetrahedron Letters 68 (2021) 152946
ica gel column (Table 1, entries 1 and 2). When Cu(OTf)2 was used,
the desired product 12a was obtained in 26% yield[15e] (Table 1
entry 3). Other triflates, such as Sc(OTf)3, Zn(OTf)2, Yb(OTf)3, In
(OTf)3 and Ce(OTf)3, could also afford the desired product 12a with
36%-51% yields (Table 1, entries 4–8) because of incomplete reac-
tion. Similarly, strong Lewis acid TiCl4 could only afford trace
amount of desired product 12a (Table 1 entry 9). Delightfully, BF.3-
OEt2 (1.0 equiv.) and TMSOTf (1.0 equiv.) could lead to the desired
12a in moderate yields (Table 1, entries 10–11). When the TMSOTf
increased to 2 equiv., the desired product 12a was achieved in 80%
yield (Table 1, entry 12). Further warming the reaction mixture to
room temperature could not improve the yield of 12a (Table 1,
entry 13). In addition, other solvents like toluene, THF and Hexanes
were also screened, and none of them led to the better yields
(Table 1, entries 14–16). Especially in Hexanes, the reaction
became complex with great amount of side reaction.
With the above optimized reaction conditions in hand, different
N-Boc lactams 7a-e and ynamides 10a-f were examined for this
one-pot transformation, and the results were summarized in
Fig. 1. Natural products containing [1,2-c][1,3]oxazine skeleton.
Scheme 1. N-Boc c-lactam 7a could react with different ynamides
10a-e, affording the desired 12a-e in moderate yields (68%-80%).
N-Boc d-lactam 7b also worked well, albeit leading to the desired
products 12f and 12g in slightly lower yields (56% and 61%, respec-
tively). Notably, the reactions of N-Boc e-lactam 7c with N-(Ts)
alkyl or Bn ynamides could afford the desired products 12h, 12i
and 12j in excellent yields (70%–87%). Importantly, 7c could react
with oxazolidone-substituted ynamide 10f to afford 12k in 45%
yield. Substituted N-Boc
c-lactams were also investigated. The
reactions of 2-OTBS substituted 7d with ynamides 10a-b could
afford the desired 12l and 12m with excellent diastereoselectivi-
ties (dr > 99:1) and in moderate yields [9a]. It was worth
mentioning that N-Boc b-lactam 7f could not work for this
transformation, probably due to the small ring tension effect
during the reduction-[4 + 2] cycloaddition process.
Next, the benzoimides 8a-c were examined under optimal reac-
tion conditions, and the results are shown in Scheme 2. tert-Butyl
2-oxo-3,4-dihydroquinoline-1(2H)-carboxylate 8a (n = 1) could
react with ynamides 10a-d to give the desired product 13a-d in
moderate yields (43%-70%), whereas the reaction of tert-butyl 2-
oxoindoline-1-carboxylate 8b (n = 0) and ynamide 10a could not
lead to any desired product. Notably, tert-butyl 2-oxo-2,3,4,5-
tetrahydrobenzo[b]azepine-1-carboxylate 8c (n = 2) could react
with ynamides 10a and 10b to afford the desired 13e and 13f in
54% and 67% yields respectively. The structures of 13a-f were
unambiguously confirmed by the X-ray crystallographic analysis
of compound 13a (see Supporting Information).
Then we turned our attention to investigate the application of
linear imides for this one-pot transformation, and the results are
summarized in Scheme 3. The imide substrate methylamine
(tert-butyl acetyl(methyl)carbamate) 9a worked well with yna-
mides 10a-c, 10e, 10g under the optimized reaction conditions,
giving the desired 3,4-dihydro-1,3-oxazin-2-ones skeleton 14a-e
in moderate yields (57%–80%). Similar results were obtained for
the imide substrates 9b and 9c, and the desired products 14f-m
were afforded in moderate yields (57%-87%). Unfortunately, the
imide substrates 9d and 9e with phenyl group for either R1 or R2
led to no desired products.
Fig. 2. The reactions of N-acyliminium ions.
tinuous efforts in exploring novel heterocyclic compounds as
potential innovative drugs [16], we envisioned that functionalized
1,3-oxazin-2-one skeletons 1, 2 and 3 could be conveniently pre-
pared in a one-pot fashion from N-Boc protected lactams. Herein
we present our one-pot approach to access 1,3-oxazin-2-ones
through TMSOTf-mediated successive reduction-[4 + 2] cyclization
process (Fig. 2, d).
Finally, two chiral ynamides 11a and 11b were selected to
investigate the potential stereochemical control of this one-pot
transformation in Scheme 4. The achiral oxazolidone-substituted
ynamide 10f could react with linear imide 9a to give the desired
15a in 44% yield. When chiral oxazolidone-substituted ynamide
11a (R4 = Bn, R5 = H) was used, the desired 15b was produced in
59% yield and with moderate diastereoselectivity (dr = 78:22). Sim-
ilar diastereoselectivity (dr = 75:25) was observed for the desired
product 15c when another chiral oxazolidone-substituted ynamide
Our investigation started with the reaction of imide 7a with
ynamide 10a. The general one-pot transformation was conducted
as followed: After the imide 7a reacted with lithium triethylboro-
hydride [17] (1.1 equiv.) at À78 °C for 0.5 h, the resulting mixture,
without quenching, was treated with the ynamide 10a and Lewis
acid in turn. A variety of Lewis acids were screened for this process.
Initially, AgOTf and AgNTf2 both afforded a lot of by-products, and
trace amount of desired product which was hardly isolated by sil-
2

Chen-Chen Zhang, Zhi-Peng Huo, Mei-Lin Tang et al.
Tetrahedron Letters 68 (2021) 152946
Table 1
Optimization of reaction conditions.
Entrya
Lewis acid (equiv.)
T(°C)
Solvent
Yield(%)b
1
2
3
4
5
6
7
8
AgOTf (0.2)
rt ~ 45
rt ~ 45
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Toluene
THF
trace
trace
26
36
51
45
38
46
trace
53
58
80
78
31
24
complex
AgNTf2 (0.2)
Cu(OTf)2 (0.2)
Sc(OTf)3 (0.2)
Zn(OTf)2 (0.2)
Yb(OTf)3 (0.2)
In(OTf)3 (0.2)
Ce(OTf)3 (0.2)
TiCl4 (1.0)
À78 ~ rt
À78 ~ rt
À78 ~ rt
À78 ~ 45
rt ~ 45
rt ~ 45
9
À78 ~ -45
À78 ~ -45
À78 ~ -45
À78 ~ -45
À78 ~ rt
À78 ~ -45
À78 ~ -45
À78 ~ -45
10
11
12
13
14
15
16
BF.3OEt2 (1.0)
TMSOTf (1.0)
TMSOTf (2.0)
TMSOTf (2.0)
TMSOTf (2.0)
TMSOTf (2.0)
TMSOTf (2.0)
Hexane
a
7a (0.5 mmol) reacted with LiBHEt3 (0.55 mmol) in dry DCM (2 mL) under N2 atmosphere at À78 °C for 0.5 h, then the mixture was treated with the 10a (0.6 mmol) and
Lewis acid.
b
Isolated yield.
11b (R4 = Me, R5 = Ph) was employed. The cyclic imides 7a and 7c
were subsequently examined for the purpose of futher improving
the stereoselectivity. Unfortunately, such efforts turned out to be
fruitless, and the desired products 15d-f were obtained in moder-
ate diastereoselectivities. The stereochemistry of the products 15a-
f was unambiguously confirmed by X-ray crystallographic analysis
of compound 2S-15d (see Supporting Information).
The proposed mechanism of the TMSOTf-mediated [4 + 2] pro-
cess was illustrated in Fig. 3. Firstly, TMSOTf activated the reduc-
tion product 7a’-e’/8a’-c’ to form an iminonium int-1 [18]. Then,
the [4 + 2] cyclization with the triple bond in ynamides 10a-
f/11a-b would take place regioselectively with int-1 to give a six-
membered intermediate int-2 via the transition state TS-1. Finally,
the int-2 was subjected to the cleavage of the tert-butyl group to
afford the desired product 12a-m/13a-f/15a-f, together with
simultaneous generation of 2-methylprop-1-ene and triflic acid
[10a].
Scheme 1. a7a-e (0.5 mmol) reacted with LiBHEt3 (0.55 mmol) in dry DCM (2 mL)
under N2 atmosphere at À78 °C for 0.5 h, then the mixture was treated with the
10a-f (0.6 mmol) and TMSOTf (1.0 mmol); bIsolated yield. cdr values were
determined by 1H NMR of crude products.
Scheme 2. a8a-c (0.5 mmol) reacted with LiBHEt3 (0.55 mmol) in dry DCM (2 mL)
under N2 atmosphere at À78 °C for 0.5 h, then the mixture was treated with the
10a-d (0.6 mmol) and TMSOTf (1.0 mmol); bIsolated yield.
3

Chen-Chen Zhang, Zhi-Peng Huo, Mei-Lin Tang et al.
Tetrahedron Letters 68 (2021) 152946
Fig. 3. The proposed mechanism of the TMSOTf-mediated [4 + 2] process.
In summary, we established a one-pot procedure from imides
and ynamides to generate a variety of cyclic structures containing
[1,3]oxazin-1-one unit. The reactions proceeded through succes-
sive reduction and subsequent [4 + 2] cyclization process, and
the desired products 12a-m and 13a-f were obtained in moderate
yields and with excellent regioselectivities. In addition, linear imi-
des 9a-e were also suitable for this transformation, leading to the
desired 3,4-dihydro-1,3-oxazin-2-ones 14a-m in moderate yields.
Chiral oxazolidone-substituted ynamides were also examined,
and the desired products 15b-f were generated with moderate
diastereoselectivities.
Declaration of Competing Interest
Scheme 3. a9a-e (0.5 mmol) reacted with LiBHEt3 (0.55 mmol) in dry DCM (2 mL)
under N2 atmosphere at À78 °C for 0.5 h, then the mixture was treated with the
10a-c, 10e, 10g (0.6 mmol) and TMSOTf (1.0 mmol); bIsolated yield.
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.
Acknowledgements
This project was supported financially by the National Natural
Science Foundation of China (82073688), Shanghai Municipal
Commission of Economy and Informatization (ZJ6005065), and
Special Project for Clinical Research, Shanghai Municipal Health
Commission (20194Y0120).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152946.
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