Sc(OTf)3-catalyzed intramolecular diastereoselective cyclization from tert-butoxycarbonyl to acyliminium ion

Article abstract of DOI:10.1016/j.tet.2021.131959

An effective approach to access dyadic 1,3-oxazinan-2-ones 8a-8c and 4,4a,5,6-tetrahydro-[1,3]oxazino[3,4-a]quinolin-1(3H)-ones 8d-8h was developed through Sc(OTf)₃-catalyzed intramolecular cyclization from tert-butoxycarbonyl to acyliminium io

Full text of DOI:10.1016/j.tet.2021.131959

Tetrahedron 83 (2021) 131959
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Sc(OTf)3-catalyzed intramolecular diastereoselective cyclization from
tert-butoxycarbonyl to acyliminium ion
,
,
, 2
Zhuo-Ya Mao a 1, Chen Wang a 1, Xiao-Di Nie a, Xiao-Li Han a, Han-Qing Dong b
,
,
, **
Chang-Mei Si a *, Bang-Guo Wei a
a School of Pharmacy, Institutes of Biomedical Sciences, Fudan University, 220 Handan Road, Shanghai, 200433, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An effective approach to access dyadic 1,3-oxazinan-2-ones 8a-8c and 4,4a,5,6-tetrahydro-[1,3]oxazino
[3,4-a]quinolin-1(3H)-ones 8d-8h was developed through Sc(OTf)3-catalyzed intramolecular cyclization
from tert-butoxycarbonyl to acyliminium ion 7a-7j. A variety of substituted N,O-acetals, with different
ring size, proved to be suitable substrates for this transformation, and a series of (4aS,6S,7R)-6-OTBS-7-
substituted-hexahydropyrrolo[1,2-c][1,3]oxazin-1-ones 11a-11j and other chiral dyadic 1,3-oxazinan-2-
ones 8i, 8j, 11k were synthesized in moderate yields with excellent diastereoselectivities (dr > 99:1).
Moreover, 2,5-trans-products 11a-11j were obtained through this interesting Lewis acid-catalyzed
intramolecular cyclization process.
Received 26 December 2020
Received in revised form
13 January 2021
Accepted 17 January 2021
Available online 22 January 2021
Keywords:
N-Acyliminium ions
Pyrrolidine
Sc(OTf)3
© 2021 Elsevier Ltd. All rights reserved.
Cyclization
Alkaloid
1. Introduction
skeleton. Some important approaches include the nucleophilic
substitution of N,O-acetals with organoboron [8], organosilicone [9]
Substituted 2,5-cis or 2,5-trans 3-hydroxyl-pyrrolidin-3-ol
skeleton 1 (Fig. 1) is a valuable structural unit existing in a variety
of natural products [1] and numerous biologically active pharma-
ceuticals [2]. Particularly, these heterocyclic natural products
featuring a 2,5-cis or 2,5-trans substituted chiral pyrrolidine [3]
usually exhibit potent biological and medicinal activities. For
example, (þ)-preussin [4] (2), a pyrrolidinol alkaloid containing
2,5-cis chiral pyrrolidine unit, can induce apoptosis in human tu-
mor cells. A considerable number of natural products contain 2,5-
trans pyrrolidine unit. Australine [5] (3) and casuarine [6] (4) are
not only effective inhibitors of glucosidase I, but also display a va-
riety of biological activities including antiviral and anti-HIV.
Another example is epohelmin A [7] (5), which demonstrated in-
and organometallic reagents [10], as well as the addition-
cyclization process [11], mainly forming 2,5-cis disubstituted pyr-
rolidines. The examples to generate 2,5-trans products are rela-
tively uncommon. Huang’s group developed a SmI2-mediated
radical cross-coupling reaction of azahemiacetals with ethyl acry-
late [12], and our group recently achieved gold-catalyzed inter-
molecular addition cyclization of N,O-acetals with ynamides [13].
Therefore, a direct process to 2,5-trans pyrrolidine unit is still in
demand.
N-Acyliminium ions (NAIs) [14], a type of extremely important
reactive intermediates, are widely used in chemical trans-
formations in past decades [15]. From the reaction point of view,
NAIs can act as highly electron-deficient carbocations to react with
weak nucleophiles [16], forming carbon-carbon (CeC) and carbon-
heteroatom bond through both intermolecular and intramolecular
reactions [17]. Therefore, NAIs chemistry is highly recognized in the
divergent synthesis and synthetic applications [18]. The reaction of
NAIs with sp3 hybridized carbon is very rare [19]. For examples,
Sugihara’s group [20] observed such a by-product while studying
Lewis acid-catalyzed allylation of N,N-acetal with allyl trime-
hibition of recombinant lanosterol synthase (IC50 ¼ 10
mM). In the
past decades, tremendous efforts have been devoted to the devel-
opment of stereospecific approach to 2,5- disubstituted pyrrolidine
* Corresponding author.
** Corresponding author.
E-mail address: bgwei1974@fudan.edu.cn (B.-G. Wei).
These authors contributed equally to this work.
Present address: Arvinas, Inc., 5 Science Park, New Haven, CT 06511, USA.
~
thylsilane, with isolated yields of 16e38% (Fig. 2a). Mancheno also
1
observed a similar cyclization by-product during the oxidative
2
https://doi.org/10.1016/j.tet.2021.131959
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
cyclization product 8a was obtained in 21% yield (Table 1, entry 1). A
variety of metallic Lewis acids were examined. Ni(OTf)2, In(OTf)3
and Er(OTf)3 led to no desired product (Table 1, entries 2e4). A
small amount of 8a was obtained when AgSbF6 was used (10%,
Table 1, entry 5). Fortunately, Zn(OTf)2, Cu(OTf)2 and Y(OTf)3 could
significantly increase the yield of 8a (up to 78%, Table 1, entries
6e8). It was worth noting that Sc(OTf)3 could lead to the desired 8a
in 90% yield (Table 1, entry 9). In this case, when the reaction was
conducted at room temperature or with lower loading of Sc(OTf)3,
the desired product 8a could still be produced, albeit in moderate
yields (51% and 77%, Table 1, entries 10e11). Different solvents were
also screened for this transformation. The reaction could also afford
the desired product in DCM, but the corresponding yield of 8a was
only 30% (Table 1, entry 12). Other solvents, such as THF, PhMe and
MeCN, resulted in complex reaction mixtures (Table 1, entries
13e15), and no desired 8a was isolated.
Fig. 1. Some 2,5- disubstituted pyrrolidines.
Next, we turned to investigate the scope and limitation of such
intramolecular addition cyclization from tert-butoxycarbonyl group
to acyliminium ion (Scheme 1). A variety of N-Boc lactams 6 were
prepared according to typical procedure of reduction and subse-
quent etherification, and the resulting cyclization precursors 7
were used in the following step without further purification. When
the crude pyrrolidine substrate 7b was subjected to the optimized
Sc(OTf)3 conditions, the yield of 8b was much lower compared with
8a. However, 4,4-disubstituted pyrrolidine substrate 7c could
afford the desired cyclization product 8c in 87% yield. Several tet-
rahydroquinoline substrates were also surveyed under the opti-
mized conditions. As a result, the desired cyclization products 8d-
8h were obtained in moderate to excellent yields. Notably, chiral
substrates 7i and 7j led to the desired products 8i and 8j with
excellent stereoselectivities and yields. The chemical structures and
relative stereochemistry of 8i and 8j were unambiguously
confirmed based on the X-ray crystallographic analysis of com-
pound 8j (see the Supporting Information).
Next, we extend this method to synthesize 2,5-disubstituted
pyrrolidines and piperidines (Scheme 2). Due to the potential
Table 1
Optimization of the reaction conditions.
Fig. 2. The intramolecular cyclization of NAIs.
.
Entry[a]
Lewis Acid
Solvent
T (oC)
Yield %[b]
alkylation-cyclization process [21] (Fig. 2b). On the basis of our
continuous efforts in exploring chemical transformations of NAIs
[11e,13,22a,22b], we envisioned that such intramolecular cycliza-
tion from methyl of tert-butoxycarbonyl group [23] could be ach-
ieved. Herein we present this interesting Lewis acid-catalyzed
intramolecular cyclization from tert- butoxycarbonyl group to NAIs
(Fig. 2c).
1
2
3
4
5
6
7
8
BF3.Et2O (2.0 eq.)
Ni(OTf)2 (0.5 eq.)
In(OTf)3 (0.5 eq.)
Er(OTf)3 (0.5 eq.)
AgSbF6 (0.5 eq.)
Zn(OTf)2 (0.5 eq.)
Cu(OTf)2 (0.5 eq.)
Y(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.2 eq.)
Sc(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.5 eq.)
Sc(OTf)3 (0.5 eq.)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
THF
r.t.-60
60
60
60
60
60
60
60
60
r.t.
60
40
60
60
21
e
e
e
10
50
67
78
90
51
77
30
9
10
11
12
13
14
15
2. Results and discussion
Complex
Complex
Complex
Our investigation started with the seven-membered cyclic
imide 6a, which underwent the reduction with lithium trie-
thylborohydride (LiEt3BH) and subsequent treatment with mont-
morillonite K-10 in methanol to give the cyclization precursor, N,O-
acetal 7a. To avoid potential elimination, the newly prepared crude
7a was directly employed in next step without further purification.
When BF.3Et2O was used for this transformation, the desired
PhMe
MeCN
60
a
The reactions were performed with 6a (2.0 mmol), LiEt3BH (2.1 mmol) in THF
(8 mL) at ꢀ78 ꢁC for 30 min, the crude product was stirred in MeOH (8 mL) with K-
10 (200 mg) at rt overnight. Then crude 7a (1.0 mmol) was treated with Lewis acid
in solvent (4 mL)
b
Isolated yield.
2

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
Scheme 1. [a] The reactions were performed with 6 (2.0 mmol), LiEt3BH (2.1 mmol) in
THF (8 mL) at ꢀ78 ꢁC for 30 min, crude product was stirred in MeOH (8 mL) with K-10
(200 mg) at rt overnight. Then, the crude 7 (1.0 mmol) was treated with Sc(OTf)3 in
DCE (4 mL) at 60 ꢁC for 1e2 h; [b] Isolated yield for 3 steps, [c] dr were determined by 1
H
NMR of crude products.
elimination issue during silica gel column chromatography, the
newly prepared N,O-acetals 10a-10k were used immediately in
next step without further purification. All of them afforded the
desired cyclization products with excellent diastereoselectivities
(dr > 99:1). Substrates with different substituted phenyl groups led
to 2,5-trans-disubstituted pyrrolidines 11a-11f in 45e78% yields.
Scheme 2. [a] The reactions were performed with 9 (2.0 mmol), LiEt3BH (2.1 mmol) in
THF (8 mL) at ꢀ78 ꢁC for 30 min, crude product was stirred in MeOH (8 mL) with K-10
(200 mg) at rt overnight. Then, the crude 10 (1.0 mmol) was treated with Sc(OTf)3 in
DCE (4 mL) at 60 ꢁC for 1e2 h; [b] Isolated yield for 3 steps; [c] dr were determined by
1H NMR of crude products.
When a-naphthyl substituted N,O-acetal 10g was investigated, the
desired 2,5-trans-11g was obtained in 60% yield with excellent
diastereoselectivities (dr > 99:1). Aliphatic substituted substrates
also worked well under the optimal conditions. n-Pentyl, cyclo-
propyl and allyl N,O-acetals 10h-10j could afford 2,5-trans-11h-
11j in moderate yields, along with outstanding diaster-
regenerate isobutene. In the half-chair conformation of acylimi-
nium ion A, both OTBS and R groups is positioned in the pseudo
equatorial orientation. Presumably due to the steric effect of axial
hydrogen at 3-position, the nucleophilic isobutene would attack
the iminium A from the bottom to form a trans-product.
eoselectivities (dr
>
99:1). When n-butyl substituted
piperidine N,O-acetal 10k was examined, the yield of 2,6-trans -
11k slightly reduced, but the diastereoselectivity was still excellent
(dr > 99:1). The stereochemistry of the products 11a-11k were
unambiguously confirmed based on X-ray crystallographic analysis
of compound 11i (see Supporting Information).
On the basis of literature reports and our experimental results, a
mechanism for this reaction is tentatively proposed (Fig. 3). First,
Sc(OTf)3 led to partial N-Boc deprotection of N,O-acetal to release
isobutene. Upon the formation of acyliminium ion A through the
elimination of MeOH assisted by Sc(OTf)3, isobutene could add to it
to give a stable tertiary carbon cation B. Further intramolecular
attack by carbonyl group could give the product 8 or 11, and
3. Conclusions
In summary, we established a novel and interesting approach
for the synthesis of dyadic 1,3-oxazinan-2-ones 8a-8c, 4,4a,5,6-
tetrahydro-[1,3]oxazino[3,4-a]quinolin-1(3H)-ones 8de8h, chiral
piperidine, chiral dyadic 1,3-oxazinan-2-ones 8i, 8j, 11k and
(4aS,6S,7R)-6-OTBS-7-substituted-hexahydropyrrolo[1,2-c][1,3]
oxazin-1-ones 11a-11j. The Lewis acid Sc(OTf)3 could catalyze the
intramolecular cyclization from tert-butoxycarbonyl to acylimi-
nium ion 7 and 10, and the corresponding products 8 and 11 were
obtained in moderate to excellent yields with excellent diaster-
eoselectivities (dr > 99:1). In addition, 2,5-trans-products 11a-11j
3

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
(m, 2H), 1.80-1.75 (m, 2H), 1.75-1.62 (m, 4H), 1.50-1.41 (m, 1H), 1.38
(s, 3H), 1.37 (s, 3H), 1.35-1.25 (m, 1H) ppm; 13C{1H} NMR (100 MHz,
CDCl3)
d 153.9, 76.3, 51.7, 46.4, 40.1, 34.4, 29.8, 29.7, 28.2, 25.2,
22.4 ppm. HRMS (ESI) m/z calcd for C11H20NOþ2 (M þ H)þ 198.1489,
found 198.1489.
4.3. 3,3-Dimethylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one
(8b)
White solid (48 mg, 28%), m.p. 73e75 ꢁC, IR (film): nmax 2971,
1647, 1430, 1369, 1624 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d 3.63-3.52
(m, 2H), 3.51-3.44 (m, 1H), 2.19-2.11 (m, 1H), 2.06 (dd, J ¼ 13.6,
4.4 Hz, 1H), 2.03-1.97 (m, 1H), 1.89-1.76 (m, 1H), 1.54-1.43 (m, 2H),
1.43 (s, 3H), 1.38 (s, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
d
153.0, 78.6, 53.7, 46.4, 39.2, 33.4, 29.8, 25.8, 22.9 ppm. HRMS (ESI)
m/z calcd for C9H16NOþ2 (M þ H)þ 170.1176, found 170.1176.
4.4. 3,3,6,6-Tetramethylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-
one (8c)
Fig. 3. Proposed mechanism.
White solid (172 mg, 87%), m.p. 99e101 ꢁC, IR (film): nmax 2969,
2081, 1637, 1453, 1369, 1646 cmꢀ1 1H NMR (400 MHz, CDCl3)
;
were obtained using this interesting Lewis acid-catalyzed intra-
molecular cyclization process.
d
3.87-3.77 (m, 1H), 3.37 (d, J ¼ 11.2 Hz, 1H), 3.22 (d, J ¼ 11.2 Hz, 1H),
1.52-1.45 (m, 1H), 1.43 (s, 3H), 1.39 (s, 3H), 1.16 (s, 3H), 1.15 (s, 3H)
ppm; 13C{1H} NMR (100 MHz, CDCl3)
153.0, 78.9, 59.9, 52.7, 47.5,
d
4. Experimental section
39.7, 36.5, 30.0, 27.7, 27.6, 26.1 ppm. HRMS (ESI) m/z calcd for
C
11H20NOþ2 (M þ H)þ 198.1489, found 198.1489.
4.1. General methods
4.5. 3,3-Dimethyl-4,4a,5,6-tetrahydro-1H,3H-[1,3]oxazino[3,4-a]
quinolin-1-one (8d)
THF was distilled from sodium/benzophenone. Reactions were
monitored by thin layer chromatography (TLC) on glass plates
coated with silica gel with fluorescent indicator. Flash chromatog-
raphy was performed on silica gel (300e400) with PE/EA as eluent.
Optical rotations were measured on a polarimeter with a sodium
lamp. HRMS were measured on LTQ-Orbitrap. IR spectra were
recorded using film on a Fourier Transform Infrared Spectrometer.
NMR spectra were recorded at 400 or 600 MHz, and chemical shifts
are reported in (ppm) referenced to an internal TMS standard for 1H
NMR and CDCl3 (77.16 ppm) for 13C NMR.
General procedure for synthesis of 8 and 11. To a solution of 6 or
9 (2.0 mmol) in dry THF (8 mL) at ꢀ78 ꢁC was added LiEt3BH
(2.1 mL, 2.1 mmol, 1M in THF). After being stirred at ꢀ78 ꢁC for
0.5 h, the reaction was quenched with saturated sodium bicar-
bonate aqueous solution then the mixture was extracted with
EtOAc (30 mL*3). The combined organic layers were washed with
brine. Dried, filtrated and concentrated to give the crude product
without further purification. The residue was dissolved in MeOH
(8 mL) and montmorillonite K-10 (200 mg) was added, after being
stirred at rt for overnight, the mixture was filtrated through a short
silica gel and concentrated to give the N,O-acetal 7 or 10.
White solid (213 mg, 92%), m.p. 113e115 ꢁC, IR (film): nmax 2955,
2932, 1692, 1418, 1329, 1126, 872, 838 cmꢀ1 1H NMR (400 MHz,
;
CDCl3)
d
7.52-7.48 (m, 2H), 7.47-7.40 (m, 2H), 4.82 (d, J ¼ 6.4 Hz,1H),
4.16-4.10 (m, 1H), 4.05-3.96 (m, 1H), 2.36-2.28 (m, 1H), 2.09 (dd,
J ¼ 13.6, 4.8 Hz, 1H), 1.78-1.70 (m, 2H), 1.49 (s, 3H), 1.46 (s, 3H), 0.86
(s, 9H), ꢀ0.07 (s, 3H), ꢀ0.14 (s, 3H) ppm; 13C{1H} NMR (100 MHz,
CDCl3)
d 152.7, 142.2, 129.2, 128.9, 124.3, 124.3, 122.6, 122.6, 79.0,
78.7, 69.3, 53.1, 41.6, 39.5, 29.9, 25.7, 25.5, 17.9, ꢀ4.7, ꢀ5.1 ppm.
HRMS (ESI) m/z calcd for C14H18NOþ2 (M þ H)þ 232.1332, found
232.1334.
4.6. 8-Methoxy-3,3-dimethyl-4,4a,5,6-tetrahydro-1H,3H-[1,3]
oxazino[3,4-a]quinolin-1-one (8e)
White solid (253 mg, 97%), m.p. 110e112 ꢁC, IR (film): nmax 3473,
2920, 1690, 1500, 1404, 1311, 1250 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
7.80-7.76 (m, 1H), 6.78-6.73 (m, 1H), 6.63-6.60 (m, 1H), 3.77 (s,
3H), 3.72-3.66 (m, 1H), 2.89-2.84 (m, 2H), 2.15-2.08 (m, 2H), 1.89-
1.78 (m, 2H), 1.47 (s, 3H), 1.46 (s, 3H) ppm; 13C{1H} NMR (125 MHz,
To a solution of 7 or 10 (1.0 mmol) in DCE (4 mL) was added
Sc(OTf)3 (0.5 mmol), then the mixture was heated at 60 ꢁC for
1e2 h. After being cooled to rt, the reaction was quenched with
saturated sodium bicarbonate aqueous solution then the mixture
was extracted with EtOAc (30 mL*3). The combined organic layers
were washed with brine. Dried, filtrated and concentrated. The
residue was purified by flash chromatography on silica gel to give 8
or 11.
CDCl3) d 155.8, 148.7, 131.0, 129.4, 125.0, 113.5, 112.4, 55.6, 51.3, 46.9,
41.1, 30.7, 29.7, 26.6, 24.6 ppm. HRMS (ESI) m/z calcd for C15H20NO3þ
(M þ H)þ 262.1438, found 262.1438.
4.7. 8-Chloro-3,3-dimethyl-4,4a,5,6-tetrahydro-1H,3H-[1,3]oxazino
[3,4-a]quinolin-1-one (8f)
White solid (188 mg, 71%), m.p. 100e102 ꢁC, IR (film): nmax 3473,
2920, 1690, 1500, 1404, 1311, 1250 cmꢀ1; 1H NMR (600 MHz, CDCl3)
4.2. 3,3-Dimethyloctahydro-1H-[1,3]oxazino[3,4-a]azepin-1-one
d 7.83-7.81 (m, 1H), 7.14-7.11 (m, 1H), 7.09-7.07 (m, 1H), 3.71-3.65
(8a)
(m, 1H), 2.88-2.83 (m, 2H), 2.15-2.09 (m, 2H), 1.89-1.83 (m, 1H),
1.81-1.75 (m, 1H), 1.47 (s, 3H), 1.46 (s, 3H) ppm; 13C{1H} NMR
White solid (177 mg, 90%), m.p. 93e95 ꢁC, IR (film): nmax 2919,
(150 MHz, CDCl3) d 151.7, 136.3, 129.8, 128.8, 126.5, 125.2, 51.4, 40.9,
1684, 1504, 1431, 1370, 1288 cmꢀ1
;
1H NMR (400 MHz, CDCl3)
30.2, 29.7, 26.3, 24.6 ppm. HRMS (ESI) m/z calcd for C14H16ClNO2þ
(M þ H)þ 266.0948, found 266.0949.
d
4.01-3.93 (m, 1H), 3.80-3.72 (m, 1H), 2.98-2.89 (m, 1H), 1.92-1.80
4

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
4.8. 9-Bromo-2,2-dimethyl-1,6,7,11b-tetrahydro-2H,4H-[1,3]
4.13. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
oxazino[4,3-a]isoquinolin-4-one (8g)
(o-tolyl)hexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11b)
21
White solid (253 mg, 82%), m.p. 107e109 ꢁC, IR (film): nmax 3473,
White solid (276 mg, 71%), m.p. 68e70 ꢁC, [
a]
¼ þ30.0 (c 0.10,
D
2920, 1690, 1500, 1404, 1311, 1250 cmꢀ1; 1H NMR (400 MHz, CDCl3)
CHCl3); IR (film): nmax 2924, 1636, 1417, 1220, 1110, 773 cmꢀ1
;
1H
d
7.80-7.74 (m, 1H), 7.29-7.22 (m, 2H), 3.72-3.64 (m, 1H), 2.90-2.83
(m, 2H), 2.17-2.07 (m, 2H), 1.91-1.83 (m, 1H), 1.82-1.72 (m, 1H), 1.47
(s, 3H), 1.46 (s, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
1151.7,
NMR (400 MHz, CDCl3) 7.15-7.10 (m, 3H), 7.10-7.05 (m,1H), 5.10 (d,
d
J ¼ 5.2 Hz, 1H), 4.26-4.20 (m, 1H), 4.12-4.04 (m, 1H), 2.36 (s, 3H),
2.35-2.29 (m, 1H), 2.04 (dd, J ¼ 13.6, 4.4 Hz, 1H), 1.77-1.69 (m, 2H),
1.46 (s, 3H), 1.45 (s, 3H), 0.84 (s, 9H), ꢀ0.10 (s, 3H), ꢀ0.14 (s, 3H)
d
136.9, 131.8, 130.2, 129.3, 125.5, 116.9, 51.4, 40.9, 30.1, 29.7, 26.2,
24.7 ppm. HRMS (ESI) m/z calcd for C14H17BrNOþ2 (M þ H)þ
310.0437, found 310.0437.
ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 152.7, 139.7, 136.1, 130.6,
127.2, 126.4, 125.1, 78.9, 78.7, 67.0, 53.4, 41.2, 40.0, 30.1, 29.9, 25.8,
19.9, 18.0, ꢀ4.9, ꢀ5.0 ppm. HRMS (ESI) m/z calcd for C22H36NO3Siþ
(M þ H)þ 390.2459, found 390.2460.
4.9. 9-Methoxy-2,2-dimethyl-1,6,7,11b-tetrahydro-2H,4H-[1,3]
oxazino[4,3-a]isoquinolin-4-one (8h)
4.14. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
(m-tolyl)hexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11c)
White solid (223 mg, 85%), m.p. 110e112 ꢁC, IR (film): nmax 2919,
1684, 1431, 1288, 1036, 754 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d 7.05-
21
White solid (249 mg, 64%), m.p. 69e72 ꢁC, [
a]
¼ þ9.4 (c 0.50,
D
CHCl3); IR (film): nmax 2855, 1693, 1415, 1255, 862, 837 cmꢀ1
;
1H
7.00 (m,1H), 6.81-6.76 (m,1H), 6.70-6.67 (m,1H), 4.78-4.72 (m,1H),
4.59-4.53 (m, 1H), 3.80 (s, 3H), 3.07-2.97 (m, 2H), 2.73-2.67 (m, 1H),
2.40 (dd, J ¼ 13.6, 5.2 Hz, 1H), 1.88-1.80 (m, 1H), 1.50 (s, 3H), 1.43 (s,
NMR (400 MHz, CDCl3) 7.21-7.18 (m, 1H), 7.06-7.03 (m, 2H), 7.02-
d
6.99 (m,1H), 4.83 (d, J ¼ 5.2 Hz,1H), 4.22-4.15 (m,1H), 4.03-3.94 (m,
1H), 2.33 (s, 3H), 2.05 (dd, J ¼ 13.6, 4.4 Hz, 1H), 1.80-1.74 (m, 1H),
1.73-1.67 (m, 1H), 1.49 (s, 3H), 1.46 (s, 3H), 0.87 (s, 9H), ꢀ0.06 (s,
3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 167.9,158.5,153.0,136.3,
125.9, 113.8, 113.2, 55.5, 51.3, 42.1, 41.0, 29.8, 29.2, 25.4 ppm. HRMS
(ESI) m/z calcd for C15H20NO3þ (M þ H)þ 262.1438, found 262.1438.
3H), ꢀ0.08 (s, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 152.8,
140.8, 138.2, 128.6, 128.1, 126.7, 122.4, 78.8, 78.7, 69.9, 52.9, 43.0,
41.3, 39.8, 30.1, 25.8, 21.7, 18.0, ꢀ4.7, ꢀ4.9 ppm. HRMS (ESI) m/z
calcd for C22H36NO3Siþ (M þ H)þ 390.2459, found 390.2458.
4.10. (4aR,5S)-5-((tert-Butyldimethylsilyl)oxy)-3,3-
dimethylhexahydro-1H,3H-pyrido[1,2-c][1,3]oxazin-1-one (8i)
4.15. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
(p-tolyl)hexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11d)
21
White solid (260 mg, 83%), m.p. 78e80 ꢁC, [
a]
¼ ꢀ23.3 (c 1.00,
D
CHCl3); IR (film): nmax 2931, 2856, 1689, 1432, 1132, 871, 832 cmꢀ1
;
1H NMR (400 MHz, CDCl3)
d
4.25 (d, J ¼ 13.3 Hz, 1H), 3.78-3.73 (m,
21
White solid (284 mg, 73%), m.p. 73e75 ꢁC, [
a]
¼ þ41 (c 0.50,
D
1H), 3.48-3.40 (m, 1H), 2.80-2.68 (m, 1H), 2.10-1.87 (m, 2H), 1.85-
1.74 (m, 2H), 1.47-1.37 (m, 1H), 1.30 (s, 3H), 1.27 (s, 3H), 0.87 (s, 9H),
0.05 (s, 3H), 0.04 (s, 3H) ppm; 13C{1H} NMR (125 MHz, CDCl3)
CHCl3); IR (film): nmax 2927, 1613, 1388, 1247, 1109 cmꢀ1
(400 MHz, CDCl3)
;
1H NMR
d
7.13-7.11 (m, 4H), 4.85 (d, J ¼ 5.2 Hz, 1H),
4.17e4.26 (m, 1H), 4.03-3.95 (m, 1H), 2.33-2.25 (m, 1H), 2.31 (s, 3H),
2.04 (dd, J ¼ 13.2, 4.4 Hz, 1H), 1.79-1.73 (m, 1H), 1.72-1.67 (m, 1H),
1.47 (s, 3H), 1.45 (s, 3H), 0.86 (s, 9H), ꢀ0.06 (s, 3H), ꢀ0.07 (s, 3H)
d
154.0, 75.8, 66.8, 55.4, 44.4, 35.8, 31.9, 29.7, 25.8, 24.9, 18.7,
18.2, ꢀ4.2, ꢀ5.0 ppm. HRMS (ESI) m/z calcd for C16H32NO3Siþ
(M þ H)þ 314.2146, found 314.2145.
ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 152.8, 137.9, 136.9, 129.4,
125.7, 78.7, 78.6, 69.9, 52.9, 41.2, 39.9, 30.1, 25.8, 25.7, 21.2,
18.0, ꢀ4.7, ꢀ4.8 ppm. HRMS (ESI) m/z calcd for C22H36NO3Siþ
(M þ H)þ 390.2459, found 390.2459.
4.11. (4aR,5S)-5-((tert-Butyldimethylsilyl)oxy)-3,3-
dimethylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (8j)
21
White solid (242 mg, 81%), m.p. 66e68 ꢁC, [
a]
¼ ꢀ23.3 (c 1.00,
4.16. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
(3-(trifluoromethyl)phenyl)hexahydro-1H-pyrrolo[1,2-c][1,3]
oxazin-1-one (11e)
D
CHCl3); IR (film): nmax 2954, 2930, 1703, 1386, 1420, 1131, 862,
838 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
3.82-3.75 (m, 1H), 3.60-3.50
(m, 2H), 3.36-3.29 (m, 1H), 2.21-2.13 (m, 1H), 2.07 (dd, J ¼ 13.2,
21
White solid (199 mg, 45%), m.p. 80e82 ꢁC, [
a]
¼ þ4.8 (c 0.25,
D
4.4 Hz, 1H), 1.84-1.73 (m, 1H), 1.43 (s, 3H), 1.37 (s, 3H), 0.90 (s, 9H),
CHCl3); IR (film): nmax 2928, 1690, 1329, 1127, 872, 839 cmꢀ1
NMR (400 MHz, CDCl3) 7.53-7.49 (m, 2H), 7.47-7.40 (m, 2H), 4.83
;
1H
0.80 (s, 6H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 153.0, 79.0, 58.8,
d
43.8, 37.5, 32.1, 29.9, 25.8, 25.8, 18.1, ꢀ4.4, ꢀ4.6 ppm. HRMS (ESI) m/
z calcd for C15H30NO3Siþ (M þ H)þ 300.1989, found 300.1989.
(d, J ¼ 6.0 Hz,1H), 4.18-4.11 (m,1H), 4.05-3.97 (m,1H), 2.37-2.29 (m,
1H), 2.09 (dd, J ¼ 13.2, 4.4 Hz, 1H), 1.78-1.70 (m, 2H), 1.50 (s, 3H),
1.47 (s, 3H), 0.86 (s, 9H), ꢀ0.07 (s, 3H), ꢀ0.13 (s, 3H) ppm; 13C{1H}
4.12. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
NMR (100 MHz, CDCl3) d 152.7,142.3,129.2,129.0,124.4, 122.7, 79.0,
phenylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11a)
78.7, 69.3, 53.1, 41.6, 39.5, 30.0, 29.9, 25.7, 25.6, 17.9,
0.1, ꢀ4.7, ꢀ5.1 ppm. HRMS (ESI) m/z calcd for C22H33F3NO3Siþ
(M þ H)þ 444.2176, found 444.2178.
21
White solid (289 mg, 77%); m.p. 89e91 ꢁC, [
a]
¼ þ5.9 (c 1.00,
D
CHCl3); IR (film): nmax 2954, 1692, 1416, 1187, 866, 837 cmꢀ1
;
1H
NMR (400 MHz, CDCl3) 7.34-7.29 (m, 2H), 7.26-7.20 (m, 3H), 4.86
d
4.17. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-7-(4-
methoxyphenyl)-3,3-dimethylhexahydro-1H-pyrrolo[1,2-c][1,3]
oxazin-1-one (11f)
(d, J ¼ 5.2 Hz, 1H), 4.24-4.18 (m, 1H), 4.04-3.94 (m, 1H), 2.34-2.25
(m, 1H), 2.05 (dd, J ¼ 13.6, 4.8 Hz,1H), 1.78-1.68 (m, 2H),1.48 (s, 3H),
1.45 (s, 3H), 0.86 (s, 9H), ꢀ0.06 (s, 3H), ꢀ0.09 (s, 3H) ppm; 13C{1H}
21
NMR (100 MHz, CDCl3)
d
152.8, 140.9, 128.7, 127.3, 125.7, 78.7, 70.0,
White solid (316 mg, 78%), m.p. 80e82 ꢁC, [
a]
¼ þ8.2 (c 0.50,
D
52.7, 41.2, 39.7, 31.5, 30.0, 25.7, 25.7, 17.9, ꢀ4.8, ꢀ4.9 ppm. HRMS
(ESI) m/z calcd for C21H34NO3Siþ (M þ H)þ 376.2302, found
376.2302.
CHCl3); IR (film): nmax 2855, 1692, 1513, 1415, 867, 836 cmꢀ1
NMR (400 MHz, CDCl3) 7.19-7.13 (m, 2H), 0.88-0.84 (m, 2H), 4.83-
4.79 (d, J ¼ 5.2 Hz, 1H), 4.23-4.17 (m, 1H), 4.02-3.93 (m, 1H), 3.79 (s,
;
1H
d
5

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
3H), 2.33-2.25 (m, 1H), 2.07-2.00 (dd, J ¼ 13.2, 4.4 Hz, 1H), 1.78-1.66
(m, 2H),1.46 (s, 3H), 1.45 (s, 3H), 0.86 (s, 9H), ꢀ0.06 (s, 3H), ꢀ0.08 (s,
71.0, 60.0, 49.8, 39.9, 39.3, 31.7, 30.6, 29.9, 27.9, 25.9,
18.2, ꢀ4.7, ꢀ4.9 ppm. HRMS (ESI) m/z calcd for C18H34NO3Siþ
(M þ H)þ 340.2303, found 340.2303.
3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
d 156.9,152.8, 133.1, 127.0,
114.2, 78.7, 78.6, 69.5, 55.4, 52.9, 41.2, 39.8, 30.1, 25.8, 25.7,
18.0, ꢀ4.7, ꢀ4.8 ppm. HRMS (ESI) m/z calcd for C22H36NO4Siþ
(M þ H)þ 406.2408, found 406.2410.
4.22. (4aS,7S,8R)-8-Butyl-7-((tert-butyldimethylsilyl)oxy)-3,3-
dimethylhexahydro-1H,3H-pyrido[1,2-c][1,3]oxazin-1-one (11k)
White solid (197 mg, 53%), m.p. 84e86 ꢁC, 1H NMR (400 MHz,
4.18. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
(naphthalen-1-yl)hexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one
(11g)
CDCl3)
d 4.16-4.10 (m, 1H), 4.04-3.93 (m, 1H), 3.85-3.76 (m, 1H),
2.01-1.90 (m, 2H), 1.80-1.72 (m, 2H), 1.70-1.54 (m, 3H), 1.38 (s, 3H),
1.36 (s, 3H), 0.87 (s, 9H), 0.07 (s, 3H), 0.06 (s, 3H) ppm; 13C{1H} NMR
21
White solid (255 mg, 60%), m.p. 100e102 ꢁC, [
a]
¼ þ4.4 (c 1.00,
(100 MHz, CDCl3) d 153.2, 67.8, 59.6, 46.5, 41.2, 33.0, 30.0, 29.8, 29.5,
D
CHCl3); IR (film): nmax 2929, 2855, 1690, 1416, 1257, 895, 869 cmꢀ1
;
27.4, 26.3, 25.9, 25.7, 23.1,17.9,14.2, ꢀ4.6, ꢀ4.8 ppm. HRMS (ESI) m/z
calcd for C20H40NO3Siþ (M þ H)þ 370.2772, found 370.2772.
1H NMR (400 MHz, CDCl3)
d
8.20-8.16 (m, 1H), 7.86-7.82 (m, 1H),
7.76-7.72 (m, 1H), 7.51-7.46 (m,2H), 7.44-7.39 (m, 1H), 7.32-7.27 (m,
1H), 4.35-4.31 (m, 1H), 4.23-4.15 (m, 1H), 2.38-2.31 (m, 1H), 2.10
(dd, J ¼ 13.6, 4.4 Hz, 1H), 1.94-1.86 (m, 1H), 1.80-1.74 (m, 1H), 1.57 (s,
Declaration of competing interest
3H), 1.50 (s, 3H), 0.89 (s, 9H), ꢀ0.10 (s, 3H), ꢀ0.16 (s, 3H) ppm; 13
C
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
{1H} NMR (100 MHz, CDCl3)
d 152.9, 136.7, 134.1, 131.0, 128.7, 128.0,
125.9, 125.8, 125.5, 124.5, 121.8, 79.0, 78.3, 68.4, 41.2, 40.4, 30.3,
26.2, 25.8, 17.9, ꢀ4.6, ꢀ4.9 ppm. HRMS (ESI) m/z calcd for
C
23H37NO3SiHþ (M þ H)þ 426.2435, found 426.2435.
Acknowledgments
4.19. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-3,3-dimethyl-7-
pentylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11h)
We thank the National Natural Science Foundation of China
(21772027 to B.-G. Wei and 21702032 to C.-M. Si) for financial
support. The authors also thank Dr. Pan Han and Dr. Yi-Wen Liu for
helpful suggestions.
21
White solid (244 mg, 66%), m.p. 91e93 ꢁC, [
a
]
D
¼ þ15.1 (c 1.00,
CHCl3); IR (film): nmax 2955, 2929, 1673, 1419, 1113, 882, 838 cmꢀ1
;
1H NMR (400 MHz, CDCl3)
d
4.40-4.31 (m, 1H), 3.87-3.79 (m,1H),
References
3.62-3.48 (m, 1H), 2.12-2.02 (m, 1H), 1.98-1.90 (m, 1H), 1.89-1.76 (m,
1H), 1.69-1.62 (m, 1H), 1.61-1.51 (m, 2H), 1.42 (s, 3H), 1.35 (s, 3H),
1.32-1.18 (m, 6H), 0.89 (s, 9H), 0.86-0.80 (m, 3H), 0.07 (s, 6H) ppm;
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4.20. (4aS,6S,7R)-6-((tert-Butyldimethylsilyl)oxy)-7-cyclopropyl-
3,3-dimethylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11i)
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21
White solid (227 mg, 67%), m.p. 78e80 ꢁC, [
a]
¼ þ0.6 (c 0.50,
D
CHCl3); IR (film): nmax 2928, 1691, 1413, 1259, 890, 837 cmꢀ1
NMR (400 MHz, CDCl3) 4.22-4.18 (m, 1H), 3.78-3.70 (m, 1H), 3.62
(dd, J ¼ 7.6, 2.8 Hz, 1H), 2.42-2.34 (m, 1H), 1.93 (dd, J ¼ 13.2, 4.4 Hz,
1H), 1.72-1.65 (m, 1H), 1.40 (s, 3H), 1.38 (s, 3H), 0.87 (s, 9H), 0.83-
0.77 (m, 1H), 0.60-0.54 (m, 1H), 0.52-0.44 (m, 2H), 0.41-0.35 (m,
1H), 0.08 (s, 3H), 0.06 (s, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3)
;
1H
d
d
153.4, 78.4, 75.9, 70.5, 51.9, 41.2, 40.1, 30.2, 25.8, 25.6, 18.0, 13.7,
3.2, 2.4, ꢀ4.5, ꢀ4.8 ppm. HRMS (ESI) m/z calcd for C18H34NO3Siþ
(M þ H)þ 340.2303, found 340.2301.
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dimethylhexahydro-1H-pyrrolo[1,2-c][1,3]oxazin-1-one (11j)
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21
White solid (197 mg, 58%), m.p. 80e82 ꢁC, [
a]
¼ ꢀ24.0 (c 0.10,
D
CHCl3); IR (film): nmax 2927, 2855, 1694, 1414, 867, 837 cmꢀ1
NMR (400 MHz, CDCl3) 5.92-5.80 (m, 1H), 5.13-4.99 (m, 2H), 4.46-
4.36 (m, 1H), 4.05-3.96 (m, 1H), 3.63-3.53 (m, 1H), 2.89-2.76 (m,
1H), 2.39-2.29 (m, 1H), 2.12-2.03 (m, 1H), 1.91 (dd, J ¼ 12.8, 2.8 Hz,
1H), 1.73-1.60 (m, 2H), 1.41 (s, 3H), 1.35 (s, 3H), 0.92 (s, 9H), 0.09 (s,
;
1H
d
3H), 0.01 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3)
d
136.4, 117.9, 79.6,
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6

Z.-Y. Mao, C. Wang, X.-D. Nie et al.
Tetrahedron 83 (2021) 131959
€
~
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