Asymmetric Synthesis of N-Fused 1,3-Oxazolidines via Pd-Catalyzed Decarboxylative (3+2) Cycloaddition

Article abstract of DOI:10.1002/adsc.201901497

Efficient synthesis of optically active N-fused 1,3-oxazolidines containing quaternary and tertiary stereocenters was achieved via Pd-catalyzed asymmetric (3+2) cycloadditions of sulfamate-derived cyclic imines and vinylethylene carbonates. Using a chiral phosphoramidite ligand, the cycloadditions proceeded effectively providing sulfamidate-fused 1,3-oxazolidines in high yields (up to 96%) with stereoselectivities (up to 25:1 dr; >99% ee). Additionally, the scale-up reaction and further transformations of the product were also achieved demonstrating the synthetic utility toward the construction of useful heterocycles such as chiral oxazoline bearing a quaternary stereocenter. (Figure presented.).

Full text of DOI:10.1002/adsc.201901497

Title: Asymmetric Synthesis of N-Fused 1,3-Oxazolidines via Pd-
Catalyzed Decarboxylative [3+2] Cycloaddition
Authors: Ju Hyun Kim, Jong-Un Park, Hye-In Ahn, Ho-Jun Cho, and
Xuan Zi
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901497
Link to VoR: http://dx.doi.org/10.1002/adsc.201901497

10.1002/adsc.201901497
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Asymmetric Synthesis of N-Fused 1,3-Oxazolidines via Pd-
Catalyzed Decarboxylative (3+2) Cycloaddition
Jong-Un Park,a Hye-In Ahn,a Ho-Jun Cho,a Zi Xuan,a and Ju Hyun Kima*
a
Department of Chemistry (BK21 Plus), Research Institute of Natural Science, Gyeongsang National University, 52828,
Jinju, Korea
Fax: (+82) 772-1189
E-mail: juhyun@gnu.ac.kr
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Efficient synthesis of optically active N-
fused 1,3-oxazolidines containing quaternary and
tertiary stereocenters was achieved via Pd-catalyzed
asymmetric (3+2) cycloadditions of sulfamate-derived
cyclic imines and vinylethylene carbonates. Using a
chiral phosphoramidite ligand, the cycloadditions
proceeded effectively providing sulfamidate-fused 1,3-
oxazolidines in high yields (up to 96%) with
stereoselectivities (up to 25:1 dr; >99% ee).
Additionally, the scale-up reaction and further
transformations of the product were also achieved
demonstrating the synthetic utility toward the
construction of useful heterocycles such as chiral
oxazoline bearing a quaternary stereocenter.
Figure 1. Representative alkaloids and pharmacologically
active molecules containing N-fused 1,3-oxazolidines
Keywords: Oxazolidines; Cycloaddition; Asymmetric
catalysis; Sulfamidates; Quaternary stereocenter
Cyclic sulfamidates have attracted considerable
attention not only in medicinal chemistry for
improving pharmacological properties but also in
organic chemistry for synthetic application to construct
various heterocycles.[4] Therefore, sulfamidate-fused
1,3-oxazolidines are attractive synthetic targets for
new drug designs and are useful intermediates in
organic synthesis. Catalytic asymmetric methods for
synthesizing several sulfamidate-fused N-heterocycles
such as piperidines, piperidinones, dihydropyrroles,
imidazolines, and tetrahydroquinazolines have been
developed;[5] however, enantioselective synthesis of
sulfamidates-fused 1,3-oxazolidines has not been
reported. Recently, an elegant example to access
sulfamidate-fused 1,3-oxazolidines via Pd-catalyzed
(3+2) cycloadditions of sulfamate-derived cyclic
imines and vinyloxiranes was developed by Guo et al.
(Scheme 1a).[6] In this report, vinyloxiranes acted as
three- or five-atom synthons to furnish (3+2) or (5+2)
cycloadditions depending on the substituents using the
Pd(0) catalyst and DPEphos ligands. Similarly, Pd-
catalyzed (3+2) cycloadditions of sulfamate-derived
cyclic imines and vinylaziridines have been also
Fused heterocyclic scaffolds containing an 1,3-
oxazolidine are found in a considerable number of
natural products and bioactive molecules such as
quinocarcin, tetrazomine, and jadomycin alkaloids
(Figure 1).[1] Enantioenriched 1,3-oxazolidines are also
commonly utilized as chiral auxiliaries in
pharmaceutical synthesis as well as ligands in
transition
metal
catalysis.[2]
Consequently,
stereospecific and stereoconvergent synthetic methods
toward optically active 1,3-oxazolidines have been
extensively explored over the last decades.[3] However,
the synthesis of chiral oxazolidine derivatives fused
with nitrogen heterocycles has not been studied. The
incorporation of versatile functional groups into the
oxazolidines, which can be converted into various
types of fused ring systems, such as cyclic sulfamidate,
is highly desirable.
reported,
affording
cyclic
imidazolidines.[7]
Considering the importance of quaternary centers in
bioactive compounds,[8] we attempted to construct
1
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10.1002/adsc.201901497
Advanced Synthesis & Catalysis
sulfamidate-fused 1,3-oxazolidines
bearing
a
enantioselectivity of 3a were relatively lower
quaternary stereocenter with various substituents and compared with the reaction using 2a (entry 7).
focused on the use of readily accessible vinylethylene Performing the reaction of 1a and 2a at room
carbonates (VECs) instead of vinyloxiranes. VECs temperature resulted in a considerable increase of
have emerged as stable dipole precursors in Pd- enantioselectivity to 94% ee without decrease in the
catalyzed decarboxylative cycloadditions producing yield (entry 8). Other phosphoramidite ligands L7-L10
zwitterionic -allyl palladium species that could act as were also examined at room temperature (entries 9-12).
O-nucleophilic/C-electrophilic 1,3-dipole or 1,5- Some of ligands showed better reactivity (L8) and dr.
dipole equivalents.[9] Herein, we disclose the Pd- (L7, L9), but the enantioselectivity was not improved
catalyzed asymmetric (3+2) cycloadditions of compared to ligand L6.
sulfamate-derived cyclic imines and VECs providing
enantioenriched sulfamidate-fused 1,3-oxazolidines Table 1. Optimization of Reaction Conditions.[a]
bearing a quaternary center (Scheme 1b). The
zwitterionic-allyl Pd intermediate A would be
generated from VECs under the palladium catalyst,
then the oxygen anion react with electrophilic imines
to give intermediate B. Subsequently, the
intramolecular N–C bond formation leads to the
oxazolidines 3 bearing a quaternary carbon center.[3a,7]
Yield (%)[b]
ee (%)[d]
3a or 4a
Ligand
(mol%)
2
dr[c]
3a
Entry
3a
4a
51
21
0
1
2
L1 (5.5)
L2 (5.5)
L3 (5.5)
L4 (5.5)
L5 (11)
L6 (11)
L6 (11)
L6 (11)
L7 (11)
L8 (11)
L9 (11)
L10 (11)
2a
2a
2a
2a
2a
2a
2b
2a
2a
2a
2a
2a
0
-
4a, 60
4a, 45
3a, 56
3a, 90
4a, 71
3a, 78
3a, 56
3a, 94
3a, 50
3a, 50
3a, 40
3a, 60
0
-
3
42
30
8
4:1
5:1
2:1[e]
3:1
3:1
3:1
7:1
3:1
18:1
2:1
4
0
5
25
6
94
85
89
80
99
52
29
0
0
0
0
0
0
0
Scheme 1. Pd-catalyzed asymmetric (3+2) cycloaddition
7
8[f]
9[f]
10[f]
11[f]
12[f]
Initially, the asymmetric cycloaddition of sulfate-
derived cyclic imine 1a and 4-phenyl-4-vinyl-1,3-
dioxolan-2-one (2a) was chosen as the model reaction
to produce 1,3-oxazolidine with a quaternary center
(Table 1, Table SI). Initial experiments revealed that
the selectivity of 1,3-oxazolidine 3a and 1,3-oxazepine
4a was largely affected by ligands in the presence of 5
mol% of PdCp(allyl). As previously described,[6] 1,3-
oxazepine 4a was obtained as the major compound via
(5+2) cycloaddition with several chiral bisphosphine
and monophosphine ligands such as L1, L2, and L5
(entries 1-2, 5). Changing the ligand to L3 altered the
selectivity of the product and selectively produced 1,3-
oxazolidine 3a in 42% yield with moderate
stereoselectivity (4:1 dr., 56% ee) (entry 3). After the
screening of chiral ligands L4-L6, the yield of 3a could
not only be increased to 94% but the enantioselectivity
of 3a could also be improved to 78% ee by using ligand
L6 (entry 6). Under the same reaction conditions, 2-
phenyl-2-vinyloxirane (2b) selectively underwent a
(3+2) cycloaddition, but its reactivity and the
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.5 mmol),
PdCp(allyl) (5 mol%), and ligand in toluene (2.0 mL) at
80°C for 8 h under Ar. [b] Isolated yield. [c] Diastereomers of
3a were not separated by the column chromatography.
Diastereomeric ratio (dr) was determined by 1H NMR
[d]
analysis. Determined by HPLC using a chiral stationary
phase. [e] 82% ee was obtained for the 3a.[f] The reaction was
performed at room temperature for 20 h.
Under the optimized reaction conditions, the
generality of these asymmetric (3+2) cycloadditions
was investigated using various sulfamate-derived
cyclic imines 1 (Table 2). The reactions of cyclic
imines 1 bearing electron donating groups, such as
methoxy and methyl substituents, on different
2
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10.1002/adsc.201901497
Advanced Synthesis & Catalysis
positions of the benzene ring with 2a were well enantioselectivity. Vinylethylene carbonate without a
tolerated to produce the desired products 3b-3f with quaternary center was also feasible in this asymmetric
yields up to 82% with excellent enantioselectivities cycloaddition producing 3t in 92% yield with slightly
(83-94%
ee)
and
moderate
to
good reduced enantioselectivity (74% ee). The reactions of
diastereoselectivities (2:1 to 17:1 dr). In general, the 1a and terminal alkyl-substituted VECs were also
stereoselectivities of these reactions were good examined to test the feasibility of olefin moiety instead
regardless of the position of the substituents; however, of vinyl group. The corresponding oxazolidines 3u and
the reactivity considerably decreased when the –OMe 3v were obtained in 93% and 15% yields, respectively.
group was at the 8-position of benzene. On the other
hands, the reactions proceeded more effectively with
substrates 1 bearing electron-withdrawing groups, Table 3. Scope of vinylethylene carbonates.[a]
such as –F and –Cl, irrespective of the substituent
positions providing 3g-3j in high yields (84%-96%)
with excellent enantioselectivities (88%-99% ee). The
naphthyl-substituted cyclic imine 1k smoothly reacted
with 2a to afford the corresponding product 3k with an
89% yield with almost a single diastereomer and 93%
ee. The structure of the major diastereomer of 3k was
unambiguously assigned by X-ray diffraction,[10] and
those of the other oxazolidines 3 were assigned by
analogy.
Table 2. Scope of sulfamate-derived cyclic imines.[a]
[a] The reactions were carried out with 1a (0.2 mmol), 2 (0.5
mmol), PdCp(allyl) (5 mol%), and L6 (11 mol%) in toluene
[b]
(2.0 mL) at room temperature for 20 h under Ar. When
reacted at 80°C using L4 ligand instead of L6, 29% yield
with 4:1 dr and 89% ee were obtained for the 3u. For more
details, see the SI. [c] The reaction was performed at 60°C.
[a] The reactions were carried out with 1 (0.2 mmol), 2a (0.5
To demonstrate the utility of sulfamidate-fused 1,3-
mmol), PdCp(allyl) (5 mol%), and L6 (11 mol%) in toluene
oxazolidines in synthesis and the potential of scaling-
[b]
(2.0 mL) at room temperature for 20 h under Ar.
reaction was performed at 60°C.
The
up the protocol, the (3+2) cycloaddition of 1a and 2a
was performed on a gram-scale under the standard
conditions. As shown in scheme 2, the desired product
3a was obtained in 86% yield with 2:1 dr and 93% ee,
regardless of the reaction scale, and it was tested for
various transformations. The selective cleavage of C–
N and C–O bonds on oxazolidine ring was achieved by
using LiAlH4, which provided cyclic sulfamidate 5 in
44% yield. Interestingly, in the presence of H2 and 30%
Pd/C, the sulfamidate 5 was also obtained in 70% yield.
In contrast, the SO2 extrusion and the formation of
C=N double bond were accomplished by treating KOt-
Bu in DMSO, and an enantiomerically enriched
Subsequently, a variety of substituted vinylethylene
carbonates 2 were tested under the standard reaction
conditions for asymmetric (3+2) cycloadditions (Table
3). Various aryl-substituted VECs having different
electronic and steric properties were reacted with 1a to
afford corresponding products 3l-3r in excellent yields
up to 96% with good stereoselectivities (up to 86% ee;
4:1 dr). The substituents at the 4-position of VECs
were not limited to aryl; a methyl substituted VEC was
also tolerated providing 3s in good yield with good
3
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10.1002/adsc.201901497
Advanced Synthesis & Catalysis
Korea Basic Institute(KBSI) National Research Facilities &
Equipment Center (NFEC) grant funded by the Korea government
(Ministry of Education) (No. 2019R1A6C1010042).
oxazoline 6, which is one of the most important
families of chiral ligands,[11] was obtained without
losing enantioselectivity. The versatile vinyl group on
3a could be converted to ethyl by the hydrogenation,
which afforded N-fused oxazolidine
maintaining the stereoselectivity.
7
while
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yield) and diastereoselectivities (up to 25:1 dr) with
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chiral phosphoramidite ligand. The cycloaddition
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Experimental Section
Typical Procedure for Synthesizing Optically Active
Sulfamidate-Fused 1,3-Oxazolidines 3: To a flame-dried
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(11.9 mg; 11 mol%) were added in a glove box. Then, vinyl
ethylene carbonate 2 (0.5 mmol) and anhydrous toluene (2.0
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mixture was stirred at room temperature for the
corresponding reaction time. After the reaction was
completed, the solvent was evaporated, and the desired
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4
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10.1002/adsc.201901497
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Asymmetric Synthesis of N-Fused 1,3-
Oxazolidines via Pd-Catalyzed Decarboxylative
(3+2) Cycloaddition
Adv. Synth. Catal. Year, Volume, Page – Page
Jong-Un Park, Hye-In Ahn, Ho-Jun Cho, Zi Xuan
and Ju Hyun Kim*
6
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