Catalyst-Free Electrophilic Ring Expansion of N-Unprotected Aziridines with α-Oxoketenes to Efficient Access 2-Alkylidene-1,3-Oxazolidines

Article abstract of DOI:10.1002/adsc.202100320

2-(2-Oxoalkylidene)-1,3-oxazolidine derivatives were synthesized in good to excellent yields regiospecifically through the catalyst-free electrophilic ring expansion of N-unprotected aziridines and the ketene C=O double bond of α-oxoketenes, in situ generated from the microwave-assisted Wolff rearrangement of 2-diazo-1,3-diketones. The ring expansion predominantly underwent an S_N1 process and the hydrogen bond decides the (E)-configuration of products. (Figure presented.).

Full text of DOI:10.1002/adsc.202100320

RESEARCH ARTICLE
DOI: 10.1002/adsc.202100320
Catalyst-Free Electrophilic Ring Expansion of N-Unprotected
Aziridines with α-Oxoketenes to Efficient Access 2-Alkylidene-1,3-
Oxazolidines
Xingpeng Chen,a Zhengshuo Huang,a and Jiaxi Xua,*
a
State Key Laboratory of Chemical Resource Engineering, Department of Organic Chemistry, College of Chemistry, Beijing
University of Chemical Technology, Beijing 100029, People’s Republic of China
Fax/Tel: +86 10 64435565
E-mail: jxxu@mail.buct.edu.cn
Manuscript received: March 13, 2021; Revised manuscript received: April 15, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100320
Abstract: 2-(2-Oxoalkylidene)-1,3-oxazolidine derivatives were synthesized in good to excellent yields
regiospecifically through the catalyst-free electrophilic ring expansion of N-unprotected aziridines and the
ketene C=O double bond of α-oxoketenes, in situ generated from the microwave-assisted Wolff rearrangement
of 2-diazo-1,3-diketones. The ring expansion predominantly underwent an SN1 process and the hydrogen bond
decides the (E)-configuration of products.
Keywords: aziridine; ring expansion; oxoketene; oxazolidine; catalyst-free
Introduction
Oxazolidine derivatives are one class of very important
heterocyclic compounds and widely present in many
biologically active medicines and natural products.[1] 2-
(2-Oxoalkylidene-1,3-oxazolidines (also known as cy-
clic α-oxoketene-N,O-ketals) have received widespread
attention due to their special structural feature during
recent three decades.[2] They have been applied as
synthetic intermediates to construct more complex
biologically active and medicinal molecules, such as
antibacterial naphthyridinecarboxylic acids,[2a] antiviral
Figure 1. Construction of biologically active molecules from 2-
(2-oxoalkylidene)-1,3-oxazolidines.
and
antibacterial
isoquinoline-1(2H)-imine
derivatives,[2c,e] anticancer 2,4-diaminopyrimidine
derivatives,[2h] due to their more reactive sites. They
have also been utilized in the synthesis of functional-
ized fused indole derivatives[2b,d] (Figure 1).
reacted with vicinal amino alcohols (Scheme 1a);[2f,g,3]
Although
2-(2-oxoalkylidene)-1,3-oxazolidines (b) the active methylene compounds with two electron-
have been widely applied as synthetic intermediates, withdrawing substituents reacted with β-chloroethyl
their synthetic methods are very limited to date. The isocyanate in the presence of Et3N or Na metal in dry
most common methods for synthesis of 2-(2-oxoalkyli- DMF. In this method, alkyl isocyanates should have a
dene)-1,3-oxazolidines include: (a) the active meth- leaving group on the β position of the alkyl group
ylene compounds with one or two electron-withdraw- (Scheme 1b).[4] Thus, it is in high demand to develop a
ing substituents reacted with carbon disulfide in the new efficient and simple synthetic method for 2-(2-
presence of strong bases followed by the methylation oxoalkylidene)-1,3-oxazolidine derivatives.
with methyl iodide or dimethyl sulfate to form α-
α-Oxoketenes are highly reactive and electrophilic
oxoketene-S,S-ketals as intermediates, which further reaction intermediates, which could be generated from
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Results and Discussion
At the outset of this study, 2-diazo-5,5-dimeth-
ylcyclohexane-1,3-dione (1a) and 2-phenylaziridine
(2a) were employed as the model substrates to
optimize reaction conditions (Table 1). Initially, the
reaction was refluxed in DCE as solvent under nitrogen
atmosphere. As a result, the reaction system was messy
and some raw materials remained, only a small amount
of product 3aa was observed by GC-MS analysis.
Considering that the microwave irradiation could
greatly promote organic reactions, the microwave-
assisted reaction was further attempted. Product 3aa
1
was obtained in 63% H-NMR yield when 1a was
1.2 equivalents in DCE under microwave irradiation at
°
100 C for 30 mins in a sealed vessel (Table 1, entry 1).
The structure of product 3aa was determined by one-
and two-dimensional NMR spectra (see SI) and finally
confirmed by X-ray single crystal diffraction analysis
(CCDC-1911078).[16] The yield of product 3aa was
greatly increased to 90% with the isolated yield 88%
°
by further raising the reaction temperature to 120 C
(Table 1, entry 2). It was worth noting that product
3aa underwent slight isomerization from its 2-meth-
yleneoxazolidine form to its isomeric oxazoline form
on ordinary silica gel, which required Et3N-neutralized
silica gel for the column chromatography. Increasing
Scheme 1. Synthesis of 2-(2-oxoalkylidene)-1,3-oxazolidine de-
rivatives and reactions of aziridines with 2-diazo-1,3-dicarbon-
yls.
°
the reaction temperature to 130 C, the yield of product
the Wolff rearrangement of readily available 2-diazo- 3aa did not increase further, but decreased slightly
1,3-dicarbonyl compounds.[5] They normally react with
unsaturated compounds in inverse electron-demand [4
Table 1. Optimization of the reaction conditions.[a]
+2] hetero-Diels-Alder cycloadditions as 4π reaction
partners of 1-oxadienes.[6] Their C=C bond as dien-
ophiles can react with electron-rich 1-azadienes in
normal electron-demand [2+4] aza-Diels-Alder
cycloadditions.[7] In addition, α-oxoketenes exhibit
occasional applications in 1,3-dipolar cycloaddition,[8]
Entry 1a/2a Solvent
Temp. Time
1H NMR
(4+1) annulations[9] and (2+2) annulation.[10]
(min.) Yield (%)[b]
°
( C)
Nucleophilic ring expansions of three-membered
heterocycles have been well investigated.[11] However,
to the best of our knowledge, limited electrophilic ring
expansions of them have been reported to date,[12,13]
they are most transition-metal-catalyzed reactions.[12]
The AgOTf and Cu(OAc)2 synergetically catalyzed
reaction of activated N-tosylaziridines and 2-diazo-1,3-
dicarbonyls generated 2H-1,4-oxazines (Scheme 1c).[14]
Recently, we realized the electrophilic ring opening of
nonactivated aziridines for the direct and efficient
synthesis of chiral tridentate ligands under neutral
conditions.[15] Herein, we present an unprecedented
synthesis of 2-(2-oxoalkylidene)-1,3-oxazolidine deriv-
atives via the catalyst-free reaction of nonactivated N-
unprotected aziridines and 2-diazo-1,3-diketones under
microwave-assisted neutral conditions. It is an electro-
philic ring expansion of N-unprotected aziridines with
α-oxoketenes derived from Wolff rearrangement of 2-
diazo-1,3-diketones (Scheme 1d).
1
2
3
4
1.2/1
1.2/1
1.2/1
1.2/1
1/1
DCE
DCE
DCE
DCE
100
120
130
120
120
120
120
120
120
30
30
30
20
30
30
30
30
30
30
63
90 (88)[c]
80
82
80
86
76
85
5
DCE
6
7
8
9
1.5/1
1.2/1
1.2/1
1.2/1
1.2/1
DCE
MeCN
Toluene
THF
99 (95)[c]
89
10
1,4-dioxane 120
[a] All reactions were conducted with 1a and 2a (0.30 mmol) in
indicated solvent (1.0 mL) and were stirred under microwave
irradiation in a sealed 10 mL microwave tube.
1
[b] The yield was determined by H NMR with 1,3,5-trimeth-
oxybenzene as an internal standard.
[c] Yield of the isolated product 3aa through the Et3N-
neutralized silica gel column chromatography with petroleum
ether and ethyl acetate (2:1, v/v) as eluent.
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Table 2. Scope of 2-diazo-1,3-diketones 1.[a]
(Table 1, entry 3). The yield dropped to 82% when the
reaction time was shortened to 20 minutes (Table 1,
entry 4). Next, the ratio of the reactants was simply
optimized. The yield of product 3aa decreased to 80%
when 1a was reduced to 1.0 equivalent, but the yield
did not further increase when 1a was increased to
1.5 equivalents (Table 1, entries 5 and 6). Finally,
solvents MeCN, toluene, THF, and 1,4-dioxane were
screened and THF was found to be the best choice
1
(99% H-NMR yield and 95% isolated yield) (Table 1,
entries 7–10). The optimum reaction conditions were
finally determined as: 2-diazo-1,3-diketone 1a
(0.36 mmol) and aziridine 2a (0.30 mmol) were stirred
°
in THF (1 mL) at 120 C for 30 mins under microwave
irradiation in a sealed vessel (Table 1, entry 9).
With the optimal reaction conditions in hand, the
reaction scope was then evaluated (Table 2). First, the
reactions of different 2-diazo-1,3-diketones 1 with 2-
phenylaziridine (2a) were examined. 2-Diazo-5,5-
dimethylcyclohexane-1,3-dione (1a) gave product 3aa
with 99% 1H-NMR yield and an isolated yield of 95%.
The gram-scale reaction of 1a and 2a was also tried,
affording product 3aa in an excellent yield of 91%
from 6 mmol of aziridine 2a. Similarly, the reaction of
2-diazocyclohexane-1,3-dione (1b) with 2a gave prod-
uct 3ba in a good isolated yield of 86%. Furthermore,
product 3ca was obtained in an excellent yield of 93%
from seven-membered cyclic 2-diazo-1,3-diketone 1c
and aziridine 2a as raw materials. 2-Diazo-5-methyl-
and 5-phenylcyclohexane-1,3-diones (1d and 1e) were
reacted with aziridine 2a to give products 3da and
3ea in 84% and 80% yields, respectively. Although
six- and seven-membered cyclic 2-diazo-1,3-diketones
gave target products 3aa–3ea with excellent isolated
yields, five-membered 2-diazocyclopentane-1,3-dione
(1f) was tried and gave a trace amount of product 3fa,
the reaction yield could not be increased by raising the
[a] All reactions were conducted with 1 (0.36 mmol) and 2a
°
(0.30 mmol) in THF (1.0 mL) and were stirred at 120 C for
30 mins under microwave irradiation in a sealed 10 mL
microwave tube unless otherwise noted. Isolated yield by the
Et3N-neutralized silica gel column chromatography with
petroleum ether and ethyl acetate (2:1, v/v) as eluent.
1
[b] The yield was determined by H NMR with 1,3,5-trimeth-
oxybenzene as an internal standard in parentheses.
[c] Gram-scale reaction (6 mmol of 2a) was conducted in
10.0 mL of THF in a 35 mL microwave tube under standard
conditions.
[d] Detected by GC-MS.
[e] With an inseparable byproduct 3-(5-phenyl-4,5-dihydroox-
azol-2-yl)butan-2-one (3la’) in the same yield of 14%.
°
microwave reaction temperature to 150 C, even to
°
180 C. The four-membered α-oxoketenes derived
from a Wolff rearrangement have been generated and
used previously in the preparation of ketoamides.[17]
Thus, the product 3fa was obtained in very low yield yields of products 3ja and 3ka were 65% and 67%,
possibly due to its instability. The simplest 2-diazo- respectively, when X are bromo and chloro. From the
1,3-diketone, 3-diazopentane-2,4-dione (1l), gave an above results, it can be concluded that the electronic
equal amount of the desired product 3la and its effect of the substituent on the aromatic ring of 2-
isomerized byproduct 3-(5-phenyl-4,5-dihydrooxazol- diazo-1,3-diarylpropane-1,3-diones has no significant
2-yl)butan-2-one (3la’).
effect on the product yield.
The reactions of 2-diazo-1,3-diarylpropane-1,3-dio-
Considering the reactions involving unsymmetric 2-
nes 1g–k with 2-phenylaziridine (2a) were further diazo-1,3-diketones, it can be predicted that two
expanded (Table 2). Product 3ga was obtained in an isomeric products would be obtained due to the
excellent yield of 89% from 2-diazo-1,3-diphenylpro- different substituent shifts in the Wolff rearrangements
pane-1,3-dione (1g) and aziridine 2a. The effect of for the 2-diazo-1,3-diketones with two different alkyl
substituents on the aromatic ring on the reaction yield or aryl groups. It would be dificult to separate a pair of
was further investigated when R1=R2 =4-XC6H4. The similar isomeric products, showing inapplicable value
yield of product 3ha was 80% when X was a strong in synthetic application. However, the aryl groups are
electron-donating methoxy group, while the yield of more predominant groups than the alkyl ones in the
product 3ia dropped to 67% when X is methyl. The Wollf rearrangement.[6f] Thus, the reaction of represen-
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tative unsymmetric 2-diazo-1-phenylbutane-1,3-dione of unsymmetric 2-diazo-1,3-diketones in the synthetic
(1m) and 2-phenylaziridine (2a) was conducted, chemistry (Scheme 2).
giving the phenyl shift product 3ma in 37% and the
Different 2-arylaziridines 2 were further attempted
methyl shift product 3na in 5% yield with its (Table 3). Products 3ab, 3cb, and 3gb were obtained
isomerized product 3 na’ in 19% yield. The results in good to excellent yields from 2-(4-bromophenyl)
indicate that the phenyl group does not show an aziridine (2b) with six- or seven-membered cyclic 2-
obvious superiority compared with the methyl group in diazo-1,3-diketones 1a and 1c and 2-diazo-1,3-diphe-
the Wolff rearrangement, revealing poor applicability nylpropane-1,3-dione (1g), respectively. Next, the
reactions of two representative 2-diazo-1,3-diketones
1a and 1g with different 2-arylaziridines 2c–2h were
performed, giving the desired products 3 in good to
excellent yields (73%–92%).
The reactions of two representative 2-alkylaziri-
dines, 2-benzylaziridine (2i) and 2-isopropylaziridine
(2j), with two representative 2-diazo-1,3-diketones 1a
and 1g were also attempted, respectively. However,
the reaction systems were messy and no corresponding
desired alkyl products were obtained (Scheme 3).
It is noteworthy mentioning that most products
clearly show the intramolecular hydrogen bond in their
1H-NMR spectra. Due to the presence of the intra-
molecular hydrogen bond, all the double bonds in the
products have only the E configuration and no other
isomer exists.
Scheme 2. Reactions of 2-phenylaziridine and unsymmetric 2-
diazo-1-phenylbutane-1,3-dione.
Table 3. Scope of different aziridines 2.[a]
The stereoselectivity of the current reaction was
studied in order to explore the reaction mechanism.
The reaction of diazo compound 1a with (S)-2-phenyl-
aziridine [(S)-2a] was performed under optimal con-
ditions, affording product 3aa* in 93% yield with only
17% ee. Different solvents, such as toluene and DCE,
were also tried for the reaction of 1a with (S)-2a, but
unfortunately, the ee of the final product 3aa* did not
increase obviously in the tested solvents (Scheme 4).
To determine the configuration of major product 3aa*,
Scheme 3. Reactions of representative 2-alkylaziridines and 2-
diazo-1,3-diketones.
[a] All reactions were conducted with 1 (0.36 mmol) and 2
°
(0.3 mmol) in THF (1.0 mL) and were stirred at 120 C for
30 mins under microwave irradiation in a sealed 10 mL
microwave tube. Isolated yield by the Et3N-neutralized silica
gel column chromatography with petroleum ether and ethyl
acetate (2:1, v/v) as eluent.
Scheme 4. Reaction of enantiopure aziridine (S)-2a with diazo
compound 1a.
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it was further hydrolyzed into 1-phenylglycinol (4). selected as an example to illustrate the proposed
However, unfortunately, racemic 1-phenylglycinol (4) mechanism (Scheme 5). Initially, diazo compound 1a
was always obtained after many attempts. Product loses one molecule of nitrogen under microwave
°
3aa* was stable in acidic condition below 100 C and heating and then undergoes a Wolff rearrangement
it could be hydrolyzed in the presence of 0.5 mol/L reaction to form α-oxoketene intermediate A. When
°
HCl aqueous solution at 100 C, but generating racemic (S)-2a approaches to A, its NÀ H forms a hydrogen
1-phenylglycinol (4). It was impossible to hydrolyze it bond with the ketone oxygen of A. Then the nitrogen
without racemization. Previously, we realized the atom of (S)-2a nucleophilically attacks on the middle
carbene-induced ring expansion of 2-phenyloxirane carbon atom of the ketene in A to generate zwitterionic
with ethyl 2-diazo-3-oxobutanoate. The reaction was intermediate B, which undergoes a SN1 process to
an intimate ion-pair process.[18] Thus, it should be generate racemic 3aa through zwitterionic intermedi-
reasonable to assume that the ketene-induced ring ate C, in which the phenyl group can stabilize the
expansion of (S)-2-phenylaziridine [(S)-2a] was an carbocation through the p-π conjugation.[19] 2-Alkyla-
intimate ion-pair process partially as well, affording ziridines 2i and 2j do not work, supporting the SN1
(S)-3aa as major product. The low enantiomeric excess process because the alkyl groups cannot provide
of the product revealed that the mechanism of the enough stabilization to the carbocation with only
reaction was not stereospecific. The results indicated hyperconjugation. Intermediate B can also undergo an
that the reaction process was largely racemic SN1 intimate ion-pair process to yield product (S)-3aa,
mechanism because the aryl groups could stabilize the resulting in (S)-3aa as the major enantiomeric product
generated carbocation intermediates,[19] with partial in the reaction, similay as that in the reaction of 2-
intimate ion-pair process.
phenyloxirane with ethyl 2-diazo-3-oxobutanoate.
Interestingly, in our previous ring expansions of However, the intimate ion-pair process is not predom-
thiiranes and oxiranes with 2-diazo-1,3-dicarbonyl inant. Thus, only lower enantioselectivity is observed
compounds,[18,20] aliphatic thiirane undergoes an SN2 for product 3aa from the enantiopure starting aziridine
process in its ring expansion because alkyl group (S)-2a. The intramolecular hydrogen bond decides the
cannot stabilize the generated carbocation, not favour- (E)-configuration of products 3.
ing the formation of the carbocation. Enantiopure 2-
For further application of synthesized 2-(2-oxoalky-
phenylthiirane is not applied in the comparison lidene)-1,3-oxazolidine derivatives 3, 1,3-oxazolidine
because it is instable and decomposes under the 3ia was reacted with boron trifluoride in the presence
reaction conditions due to its weak benzylic CÀ S of triethylamine, affording the corresponding difluor-
bond.[18,20] (S)-2-Phenyloxirane shows a higher stereo- oboron compound 1,1-difluoro-6-phenyl-3,4-di(4-
selectivity than (S)-2-phenylaziridine via their intimate methylphenyl)-6,7-dihydro-1H-1λ4,8λ4-oxazolo[3,2-
ion-pair intermediates. The difference in the stereo- c][1,3,2]oxazaborinine (5) in 42% yield (Scheme 6).[21]
selectivities is rationalized to the the different hard and The similar difluoroboron complexes were mentioned
soft acidities of the two different carbocations in the as potential insecticides.[21a]
intermediates II and III. The vinyloxymeth-
ylcarbocation
should
be
harder
than
the
vinylaminomethylcarbocation because the electronega-
tivity of oxygen is greater than that of nitrogen. Thus,
the vinyloxymethylcarbocation forms a more tightened
intimate ion-pair with the hard enolate oxyanion than
the vinylaminomethylcarbocation does, leading to a
higher enantioselectivity in its ring expansion (Fig-
ure 2).
The reaction of 2-diazo-5,5-dimethylcyclohexane-
1,3-dione (1a) and (S)-2-phenylaziridine ((S)-2a) is
Figure 2. Comparison on the reaction intermediates in the ring
expansions of cis-thiirane, enantiopure oxirane and aziridine
with 2-diazo-1,3-dicarbonyl compounds.
Scheme 5. Proposed reaction mechanism.
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obtained on a Yanaco MP-500 melting point apparatus and are
uncorrected. Single crystal X-ray diffraction analysis (3aa) was
performed on an Agilent Gemini E single crystal X-ray
diffractometer. The enantiomeric excesses of 3aa* were
determined by chiral HPLC analysis using an Agilent 1260 LC
instrument with Daicel Chiralpak ASÀ H column with a mixture
of isopropyl alcohol and hexane as eluents.
Scheme 6. Application.
Preparations and analytical data of 2-diazo-1,3-diketones 1[6d,11]
and aziridines 2,[15,22] and hydrolysis of product 3aa* are
provided in SI.
Conclusion
General Procedure for the Preparation of
2-(2-Oxoalkylidene)-1,3-Oxazolidine Derivatives 3
In conclusion, we successfully achieved the catalyst-
free and efficient synthesis of synthetically important
2-(2-oxoalkylidene)-1,3-oxazolidine derivatives in
good to excellent yields from N-unprotected aziridines
and 2-diazo-1,3-diketones. The reaction is a domino
thermal Wolff rearrangement of 2-diazo-1,3-diketones
and electrophilic ring expansion of aziridines with the
in situ generated α-oxoketenes through an intramolecu-
lar SN1 as a predominant process. It is accelerated by
microwave irradiation and released only one molecule
of nitrogen as byproduct, showing high efficiency,
atom economy, and ecofriendly features. The intra-
molecular hydrogen bonding controls the C=C bond
configuration. 2-(2-Oxoalkylidene)-1,3-oxazolidines
are first obtained efficiently by the (2+3) annulation
Diazo compound 1 (0.36 mmol) and aziridine 2 (0.30 mmol)
were added in THF (1.0 mL) in a 10 mL microwave tube. The
°
resulting solution was stirred at 120 C for 30 min under
microwave irradiation in a sealed vessel. After the reaction was
completed, resulting mixture was evaporated in vacuo. The
crude residue was purified by the Et3N-neutralized silica gel
column chromatography (PE/EA 2:1, v/v) to give product 3.
(E)-4,4-Dimethyl-2-(5-Phenyloxazolidin-2-Ylidene)
cyclopentan-1-One (3aa)
Light purple crystals, 73 mg, yield 95%. 1.41 g, yield 91%
°
(6 mmol of aziridine 2a). m.p. 154–156 C, Rf =0.20 (PE/EA=
2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.13 (brs, 1H), 7.44–
reaction of the C=O double bond of α-oxoketenes with 7.32 (m, 5H), 5.60 (t, J=8.1 Hz, 1H), 4.07 (t, J=8.8 Hz, 1H),
1
3.62 (t, J=8.6 Hz, 1H), 2.42 (d, J=13.4 Hz, 1H), 2.37 (d, J=
13.4 Hz, 1H), 2.20 (d, J=17.1 Hz, 1H), 2.15 (d, J=17.0 Hz,
1H), 1.12 (s, 3H), 1.11 (s, 3H). 13C NMR (101 MHz, CDCl3) δ
202.2, 164.1, 137.7, 129.1, 128.9, 126.1, 85.6, 80.9, 53.5, 50.3,
40.1, 34.9, 29.2. IR (KBr) v 3289, 2951, 2901, 2866, 1666,
1564, 1479, 1475, 1243, 1027, 996, 908, 855, 760, 699 cmÀ 1.
2-arylaziridines through electrophilic ring expansion
mode.
Experimental Section
Unless otherwise noted, all materials were purchased from
commercial suppliers without further purification. THF was
refluxed over LiAlH4; DCE, MeCN, and 1,4-dioxane were
refluxed over CaH2; toluene was refluxed over Na with
benzophenone as an indicator, and all solvents were freshly
distilled prior to use. Flash column chromatography was
performed using silica gel (normal phase, 200–300 mesh) from
Branch of Qingdao Haiyang Chemical. Petroleum ether (PE)
+
HRMS-ESI (m/z): calcd for C16H20NO2 [M+H]+: 258.1489,
found 258.1482.
(R/S, E)-4,4-Dimethyl-2-(5-Phenyloxazolidin-
2-Ylidene)cyclopentan-1-One (3aa*)
72 mg, yield 93% (THF as solvent). HPLC analysis: chiralpak
ASÀ H (i-PrOH/hexane=10:90, v/v, 0.9 mL/min, 220 nm),
minor tR =21.34 min and major tR =25.71 min, ee=17%.
°
used for column chromatography was the 60–90 C fraction,
and the removal of residue solvent was accomplished under
rotovap. Reactions were monitored by thin-layer chromatogra-
phy on silica gel GF254 coated 0.2 mm plates from Institute of
Yantai Chemical Industry. Microwave-assisted reactions were
conducted on a CEM discovery SP microwave reactor. The
plates were visualized under UV light, as well as other TLC
65 mg, yield 84% (toluene as solvent), minor tR =21.14 min
and major tR =25.45 min, ee=17%.
68 mg, yield 88% (DCE as solvent), minor tR =21.13 min and
major tR =25.55 min, ee=21%.
1
stains. H and 13C NMR spectra were recorded on a Bruker
400 MHz spectrometer in CDCl3 or DMSO-d6 with solvent
peaks as internal standards, and the chemical shifts (δ) are
reported in parts per million (ppm). All coupling constants (J)
(E)-2-(5-Phenyloxazolidin-2-Ylidene)cyclopentan-
1-One (3ba)
1
in H NMR are absolute values given in hertz (Hz) with peaks
°
Light purple solid, 59 mg, yield 86%, m.p. 141–143 C, Rf =
0.20 (PE/EA=1:1, v/v). H NMR (400 MHz, CDCl3) δ 8.52
(brs, 1H), 7.44–7.33 (m, 5H), 5.61 (t, J=8.2 Hz, 1H), 4.08 (t,
J=8.9 Hz, 1H), 3.63 (t, J=8.6 Hz, 1H), 2.67–2.53 (m, 2H),
2.33 (t, J=7.9 Hz, 2H), 1.90 (p, J=7.4 Hz, 2H). 13C NMR
(101 MHz, CDCl3) δ 203.0, 164.1, 137.6, 129.1, 128.9, 126.1,
1
labeled as single (s), broad singlet (brs), doublet (d), triplet (t),
quartet (q), and multiplet (m). The IR spectra (KBr pellets, v
[cmÀ 1]) were taken on a Bruker Tensor 27 FTIR spectrometer.
HRMS measurements were carried out on a Waters Acquilty
UPLC/Quattro Premier mass spectrometer. Melting points were
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6
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+
85.5, 80.9, 50.3, 38.5, 25.0, 20.8. IR (KBr) v 3280, 2953, 2925,
759, 699 cmÀ 1. HRMS-ESI (m/z): calcd for C20H20NO2 [M+
1665, 1560, 1481, 1450, 1273, 1115, 1025, 993, 907, 858, 759,
H]+: 306.1489, found 306.1498.
+
699 cmÀ 1. HRMS-ESI (m/z): calcd for C14H16NO2 [M+H]+:
230.1176, found 230.1184.
(E)-1,2-Diphenyl-2-(5-Phenyloxazolidin-2-Ylidene)
ethan-1-One (3ga)
(E)-2-(5-Phenyloxazolidin-2-Ylidene)cyclohexan-
1-One (3ca)
°
Light pink crystals, 91 mg, yield 89%, m.p. 191–192 C, Rf =
0.35 (PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 10.98
1
°
Light purple crystal, 68 mg, yield 93%, m.p. 128–130 C, Rf =
0.30 (PE/EA=1:1, v/v). H NMR (400 MHz, CDCl3) δ 10.94
(s, 1H), 7.43–7.31 (m, 5H), 7.27–7.05 (m, 10H), 5.64 (t, J=
8.3 Hz, 1H), 4.24 (t, J=9.4 Hz, 1H), 3.80 (dd, J=9.7, 8.0 Hz,
1H). 13C NMR (101 MHz, CDCl3) δ 190.6, 169.0, 141.5, 138.1,
136.6, 132.2, 129.1, 129.0, 128.9, 128.7, 127.8, 127.3, 126.0,
125.7, 93.6, 81.1, 51.1. IR (KBr) v 3433, 3400, 3019, 2971,
2925, 1664, 1621, 1598, 1448, 1246, 1046, 931, 908, 751,
1
(s, 1H), 7.43–7.32 (m, 5H), 5.58 (t, J=8.4 Hz, 1H), 4.17 (t, J=
9.7 Hz, 1H), 3.71 (dd, J=10.3, 8.1 Hz, 1H), 2.39 (dd, J=9.8,
5.9 Hz, 2H), 2.32 (t, J=6.4 Hz, 2H), 1.79–1.71 (m, 2H), 1.69–
1.62 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 188.1, 167.9,
138.7, 128.9, 126.0, 87.7, 80.5, 53.1, 35.3, 23.1, 22.9. IR (KBr)
v 3234, 2929, 2857, 1633, 1524, 1473, 1451, 1286, 1032, 921,
698 cmÀ 1. HRMS-ESI (m/z): calcd for C23H20NO2 [M+H]+:
+
342.1489, found 342.1495.
822, 761, 699 cmÀ 1. HRMS-ESI (m/z): calcd for C15H18NO2
+
[M+H]+: 244.1332, found 244.1341.
(E)-1,2-Bis(4-Methoxyphenyl)-
2-(5-Phenyloxazolidin-2-Ylidene)ethan-1-One (3ha)
(E)-4-Methyl-2-(5-Phenyloxazolidin-2-Ylidene)
cyclopentan-1-One (3da)
°
Light yellow solid, 96 mg, yield 80%, m.p. 157–161 C, Rf =
0.22 (PE/EA=1:1, v/v). 1H NMR (400 MHz, DMSO-d6) δ
10.51 (s, 1H), 7.48–7.34 (m, 5H), 7.12 (d, J=8.8 Hz, 2H), 6.95
(d, J=8.7 Hz, 2H), 6.71 (d, J=8.7 Hz, 2H), 6.68 (d, J=
8.8 Hz, 2H), 5.77 (t, J=8.1 Hz, 1H), 4.24 (t, J=9.5 Hz, 1H),
3.73 (t, J=8.1 Hz, 1H), 3.69 (s, 3H), 3.66 (s, 3H). 13C NMR
(101 MHz, DMSO-d6) δ 187.4, 167.7, 159.4, 157.0, 138.6,
134.2, 133.0, 130.1, 129.5, 128.8, 126.2, 113.1, 112.4, 91.2,
80.0, 55.0, 54.8, 50.7. IR (KBr) v 3256, 2956, 2836, 1601,
1575, 1511, 1431, 1147, 1174, 981, 936, 776, 699 cmÀ 1.
°
Light purple solid, 61 mg, yield 84%, m.p. 118–120 C, Rf =
0.30 (PE/EA=1:1, v/v). A mixture of diastereoisomers (cis-
isomer: trans-isomer=1:1). 1H NMR (400 MHz, CDCl3) δ 8.41
(brs, 1H), 7.44–7.33 (m, 5H), 5.61 (t, J=8.2 Hz, 1H), 4.08 (t,
J=8.8 Hz, 1H), 3.63 (t, J=8.6 Hz, 1H), 2.82–2.78 (m, 1H, for
isomer 1), 2.76 (dd, J=8.5, 4.6 Hz, 1H, for isomer 2), 2.52–
2.48 (m, 1H, for isomer 1), 2.47–2.43 (m, 1H, for isomer 2),
2.38–2.27 (m, 1H), 2.21 (dd, J=8.5, 4.8 Hz, 1H, for isomer 1),
2.17 (dd, J=8.5, 4.8 Hz, 1H, for isomer 2), 2.02 (dd, J=7.5,
2.8 Hz, 1H, for isomer 1), 1.98 (dd, J=7.5, 2.8 Hz, 1H, for
isomer 2), 1.10 (d, J=6.6 Hz, 3H, for isomer 1), 1.09 (d, J=
6.6 Hz, 3H, for isomer 2). 13C NMR (101 MHz, CDCl3) δ
202.6, 164.1, 137.8 (137.7), 129.2, 129.0, 126.2, 85.7 (85.6),
81.0, 50.4, 47.0, 33.60 (33.57), 29.7 (29.6), 21.4 (21.3). IR
(KBr) v 3287, 2953, 2926, 2868, 1735, 1667, 1562, 1482, 1442,
1372, 1258, 1118, 1026, 999, 916, 758, 699 cmÀ 1. HRMS-ESI
+
HRMS-ESI (m/z): calcd for C25H24NO4 [M+H]+: 402.1700,
found 402.1701.
(E)-2-(5-Phenyloxazolidin-2-Ylidene)-1,2-Di-
(4-Methphenyl)ethan-1-One (3ia)
°
White solid, 74 mg, yield 67%, m.p. 218–221 C, Rf =0.40 (PE/
EA=1:1, v/v). H NMR (400 MHz, CDCl3) δ 11.03 (s, 1H),
1
+
(m/z): calcd for C15H18NO2 [M+H]+: 244.1332, found
7.44–7.34 (m, 5H), 7.20 (d, J=8.1 Hz, 2H), 7.03 (d, J=8.1 Hz,
2H), 7.00–6.93 (m, 4H), 5.66 (t, J=8.3 Hz, 1H), 4.26 (t, J=
9.4 Hz, 1H), 3.81 (dd, J=9.8, 7.7 Hz, 1H), 2.28 (s, 6H). 13C
NMR (101 MHz, CDCl3) δ 190.1, 169.0, 138.9, 138.6, 138.3,
135.1, 133.7, 132.0, 128.99, 128.95, 128.85, 128.5, 128.0,
126.0, 93.3, 81.0, 51.3, 21.4, 21.2. IR (KBr) v 3247, 3027,
2921, 1609, 1571, 1513, 1432, 1304, 1181, 984, 936, 895, 766,
244.1330.
(E)-4-Phenyl-2-(5-Phenyloxazolidin-2-Ylidene)
cyclopentan-1-One (3ea)
°
Light purple solid, 73 mg, yield 80%, m.p. 145–147 C, Rf =
+
0.30 (PE/EA=1:1, v/v). A mixtures of diastereoisomers (cis-
699 cmÀ 1. HRMS-ESI (m/z): calcd for C25H24NO2 [M+H]+:
1
isomer:trans-isomer=1:1). H NMR (400 MHz, CDCl3) δ 8.49
370.1802, found 370.1804.
(brs, 1H), 7.46–7.35 (m, 5H), 7.35–7.28 (m, 4H), 7.25–7.19 (m,
1H), 5.66 (t, J=8.1 Hz, 1H, for isomer 1), 5.65 (t, J=8.0 Hz,
1H, for isomer 2), 4.12 (t, J=8.9 Hz, 1H), 3.68 (t, J=8.5 Hz,
1H, for isomer 1), 3.68 (t, J=8.6 Hz, 1H, for isomer 2), 3.57–
3.43 (m, 1H), 3.12 (dd, J=12.4, 8.2 Hz, 1H, for isomer 1), 3.09
(dd, J=13.0, 8.2 Hz, 1H, for isomer 2), 2.76 (dd, J=17.6,
8.3 Hz, 1H), 2.69 (dd, J=13.9, 8.2 Hz, 1H), 2.59 (dd, J=16.9,
9.9 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 201.1, 164.0, 145.6
(145.4), 137.7 (137.6), 129.3, 129.0, 128.5, 126.9, 126.22,
126.18, 85.4 (85.3), 81.2, 50.40 (50.38), 46.1 (46.0), 40.6
(40.5), 33.63 (33.55). IR (KBr) v 3478, 3373, 3028, 2941, 1737,
1668, 1563, 1481, 1444, 1369, 1286, 1120, 1027, 987, 922,
(E)-1,2-Bis(4-Bromophenyl)-2-(5-Phenyloxazolidin-
2-Ylidene)ethan-1-One (3ja)
°
Yellow solid, 98 mg, yield 65%, m.p. 246–248 C, Rf =0.30
(PE/EA=1:1, v/v). H NMR (400 MHz, DMSO-d6) δ 10.47 (s,
1
1H), 7.47–7.36 (m, 7H), 7.30 (d, J=8.4 Hz, 2H), 7.07 (d, J=
8.4 Hz, 2H), 6.96 (d, J=8.4 Hz, 2H), 5.83 (t, J=8.4 Hz, 1H),
4.27 (t, J=9.7 Hz, 1H), 3.73 (dd, J=10.1, 8.1 Hz, 1H). 13C
NMR (101 MHz, DMSO-d6) δ 187.2, 167.4 141.0, 138.1, 136.3,
134.0, 130.5, 130.4, 130.2, 129.0, 128.9, 126.3, 122.1, 118.5,
90.8, 80.6, 50.6. IR (KBr) v 3248, 2927, 1617, 1582, 1475,
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1436, 1324, 1069, 1010, 933, 765, 698 cmÀ 1. HRMS-ESI (m/z):
(dt, J=10.2, 7.7 Hz, 1H), 4.59–4.43 (m, 1H), 4.24 (t, J=
10.0 Hz, 1H), 3.78 (dd, J=13.9, 7.9 Hz, 1H), 1.59 (dd, J=7.0,
4.6 Hz, 3H). 3na+3 na’: 13C NMR (101 MHz, CDCl3) δ 196.3,
169.5, 166.3 (166.2), 141.4, 140.8 (140.7), 138.3, 136.0 (135.9),
133.41 (133.39), 129.1, 129.0, 128.8, 128.73, 128.71, 128.3,
128.2, 127.9, 127.4, 126.1, 125.8, 125.6, 84.31, 81.29 (81.27),
80.8, 62.80 (62.78), 52.3, 42.2 (42.1), 14.6 (14.3), 12.6. IR
(KBr) v 3245, 2923, 1609, 1506, 1487, 1396, 1010, 923, 843,
+
calcd for C23H18Br2NO2 [M+H]+: 499.9678, found 499.9673.
(E)-1,2-Bis(4-Chlorophenyl)-2-(5-Phenyloxazolidin-
2-Ylidene)ethan-1-One (3ka)
°
Yellow solid, 82 mg, yield 67%, m.p. 248–250 C, Rf =0.40
(PE/EA=1:1, v/v). 1H NMR (400 MHz, CDCl3) δ 10.89 (s,
1H), 7.46–7.39 (m, 3H), 7.34 (d, J=7.5 Hz, 2H), 7.20 (d, J=
8.4 Hz, 2H), 7.13 (dd, J=8.5, 3.4 Hz, 4H), 7.01 (d, J=8.3 Hz,
2H), 5.70 (t, J=8.4 Hz, 1H), 4.29 (t, J=9.5 Hz, 1H), 3.85 (t,
J=9.4 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 189.1, 168.9,
139.5, 137.6, 135.0, 134.8, 133.3, 131.6, 130.1, 129.3, 129.1,
128.0, 127.7, 126.0, 92.3, 81.4, 51.0. IR (KBr) v 3248, 2926,
+
755, 693 cmÀ 1. HRMS-ESI (m/z): calcd for C18H18NO2 [M+
H]+: 280.1332, found 280.1336.
(E)-2-(5-(4-Bromophenyl)oxazolidin-2-Ylidene)-
4,4-Dimethylcyclopentan-1-One (3ab)
1610, 1509, 1491, 1361, 1012, 926, 846, 757, 692 cmÀ 1.
Light purple solid, 98 mg, yield 97%, m.p. 131–134 C, Rf =
0.10 (PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.37
°
+
1
HRMS-ESI (m/z): calcd for C23H18Cl2NO2
[M+H]+:
410.0709, found 410.0706.
(brs, 1H), 7.53 (d, J=8.3 Hz, 2H), 7.23 (d, J=8.3 Hz, 2H),
5.56 (t, J=8.0 Hz, 1H), 4.07 (t, J=8.8 Hz, 1H), 3.58 (t, J=
8.5 Hz, 1H), 2.41 (d, J=13.4 Hz, 1H), 2.37 (d, J=13.4 Hz,
1H), 2.19 (d, J=17.1 Hz, 1H), 2.14 (d, J=17.0 Hz, 1H), 1.12
(s, 3H), 1.10 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 202.5,
164.0, 136.9, 132.1, 132.0, 127.9, 123.2, 85.9, 80.2, 53.5, 50.3,
40.2, 35.1, 29.3. IR (KBr) v 3294, 2953, 2866, 1670, 1560,
1485, 1297, 1032, 1006, 927, 821 cmÀ 1. HRMS-ESI (m/z):
(E)-3-(5-Phenyloxazolidin-2-Ylidene)butan-2-One
(3la) and 3-(5-Phenyl-4,5-Dihydrooxazol-2-yl)
butan-2-One (3la’)
Colorless liquid, inseparable mixture of 3la and 3la’ in the ratio
of 1:1 on the basis of 1H NMR analysis, 30 mg, yield 28%. 3la:
1H NMR (400 MHz, CDCl3) δ 10.22 (s, 1H), 7.44–7.24 (m,
5H), 5.58 (t, J=8.2 Hz, 1H), 4.11 (t, J=9.2 Hz, 1H), 3.65 (dd,
+
calcd for C16H19BrNO2 [M+H]+: 336.0594, found 336.0595.
1
J=9.4, 8.2 Hz, 1H), 2.13 (s, 3H), 1.88 (s, 3H). 3la’: H NMR
(E)-2-(5-(4-Bromophenyl)oxazolidin-2-Ylidene)
(400 MHz, CDCl3) δ 7.43–7.25 (m, 5H), 5.54–5.47 (m, 1H),
4.30 (dd, J=14.2, 10.4 Hz, 1H), 3.83 (dd, J=14.5, 8.0 Hz,
1H), 3.58 (q, J=7.1 Hz, 1H), 2.27 (s, 1H), 1.44 (dd, J=7.1,
2.9 Hz, 1H). 3al+3al’: 13C NMR (101 MHz, Acetone-d6) δ
189.5, 167.2, 165.1, 139.4, 128.75, 128.70, 128.6, 128.1, 126.1,
125.7, 82.8, 80.7, 80.1, 62.8, 62.7, 52.1, 46.8, 24.8, 12.8, 10.7.
IR (KBr) v 3233, 2929, 2856, 1632, 1522, 1471, 1450, 1285,
1030, 923, 824, 762, 697 cmÀ 1. HRMS-ESI (m/z): calcd for
cyclohexan-1-One (3cb)
1
Red oil, 94 mg, yield 97%, Rf =0.10 (PE/EA=2:1, v/v). H
NMR (400 MHz, DMSO-d6) δ 10.65 (s, 1H), 7.62 (d, J=
8.3 Hz, 2H), 7.33 (d, J=8.3 Hz, 2H), 5.73 (t, J=8.0 Hz, 1H),
4.16 (dd, J=10.7, 9.2 Hz, 1H), 3.59 (dd, J=10.9, 7.2 Hz, 1H),
2.30 (t, J=6.0 Hz, 2H), 2.14 (t, J=6.3 Hz, 2H), 1.70–1.62 (m,
2H), 1.61–1.52 (m, 2H). 13C NMR (101 MHz, DMSO-d6) δ
185.9, 166.5, 138.5, 131.7, 128.2, 121.7, 86.0, 78.8, 52.5, 34.9,
22.8, 22.7, 22.6. IR (KBr) v 3247, 2933, 2860, 1637, 1599,
1521, 1480, 1321, 1038, 1011, 941, 825 cmÀ 1. HRMS-ESI (m/
+
C13H16NO2 [M+H]+: 218.1176, found 218.1170.
z): calcd for C15H17BrNO2 [M+H]+: 322.0437, found
322.0444.
+
(E)-1-Phenyl-1-(5-Phenyloxazolidin-2-Ylidene)
propan-2-One (3ma)
1
Yellow oil, 31 mg, yield 37%. H NMR (400 MHz, CDCl3) δ
10.50 (s, 1H), 7.39–7.30 (m, 5H), 7.28–7.21 (m, 5H), 5.57 (t,
J=8.1 Hz, 1H), 4.18 (t, J=9.2 Hz, 1H), 3.72 (dd, J=9.6,
7.5 Hz, 1H), 1.98 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 193.8
167.4, 138.3, 136.9, 132.0, 128.9, 128.9, 128.1, 126.4, 125.9,
94.0, 80.7, 51.1, 27.2. IR (KBr) v 3245, 2922, 1608, 1506,
1490, 1360, 1010, 924, 844, 756, 691 cmÀ 1. HRMS-ESI (m/z):
(E)-2-(5-(4-Bromophenyl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3gb)
°
Light purple solid, 107 mg, yield 85%, m.p. 65–68 C, Rf =0.15
(PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 10.99 (s,
1H), 7.52 (d, J=8.3 Hz, 2H), 7.24 (d, J=6.9 Hz, 2H), 7.22–
7.16 (m, 3H), 7.16–7.05 (m, 7H), 5.59 (t, J=8.2 Hz, 1H), 4.24
(t, J=9.4 Hz, 1H), 3.76 (dd, J=9.7, 7.8 Hz, 1H). 13C NMR
(101 MHz, CDCl3) δ 190.4, 168.6, 141.2, 137.0, 136.3, 132.11,
132.07, 128.9, 128.6, 127.64, 127.60, 127.3, 125.7, 123.1, 93.7,
80.2, 51.0. IR (KBr) v 3248, 3055, 2962, 1617, 1597, 1515,
1428, 1323, 1010, 982, 937, 755, 737, 700 cmÀ 1. HRMS-ESI
+
calcd for C18H18NO2 [M+H]+: 280.1332, found 280.1338.
(E)-1-Phenyl-2-(5-Phenyloxazolidin-2-Ylidene)
propan-1-One (3na) and 1-Phenyl-2-(5-Phenyl-
4,5-Dihydrooxazol-2-yl)propan-1-One (3 na’)
+
(m/z): calcd for C23H19BrNO2 [M+H]+: 420.0594, found
Yellow oil, inseparable mixture of 3na and 3 na’ in the ratio of
420.0595.
1
5:19 on the basis of H NMR analysis, 20 mg, yield 24%. 3na:
1H NMR (400 MHz, CDCl3) δ 10.96 (s, 1H), 8.11–8.00 (m,
1H), 7.66–7.55 (m, 1H), 7.50–7.19 (m, 7H), 7.08–6.99 (m, 1H),
5.68 (t, J=8.5 Hz, 1H), 4.28 (t, J=10.0 Hz, 1H), 3.73 (t, J=
1
8.0 Hz, 1H), 1.89 (s, 3H). 3 na’: H NMR (400 MHz, CDCl3) δ
8.10–8.00 (m, 1H), 7.50–7.19 (m, 8H), 7.06–7.01 (m, 1H), 5.44
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RESEARCH ARTICLE
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(E)-2-(5-(4-Chlorophenyl)oxazolidin-2-Ylidene)-
4,4-Dimethylcyclopentan-1-One (3ac)
(E)-2-(5-(3-Chlorophenyl)oxazolidin-2-Ylidene)-
4,4-Dimethylcyclopentan-1-One (3ae)°
°
Light purple solid, 80 mg, yield 91%, m.p. 125–128 C, Rf =
0.10 (PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.57
Purple solid, 69 mg, yield 79%, m.p. 102–105 C, Rf =0.15 (PE/
1
1
EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.34 (brs, 1H),
(brs, 1H), 7.39 (d, J=8.5 Hz, 2H), 7.29 (d, J=8.4 Hz, 2H),
5.58 (t, J=8.1 Hz, 1H), 4.08 (t, J=8.8 Hz, 1H), 3.59 (dd, J=
9.1, 7.9 Hz, 1H), 2.42 (d, J=13.4 Hz, 1H), 2.37 (d, J=13.4 Hz,
1H), 2.20 (d, J=16.9 Hz, 1H), 2.15 (d, J=16.8 Hz, 1H), 1.13
(s, 3H), 1.11 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 202.5,
163.9, 136.3, 135.1, 129.2, 127.6, 85.9, 80.1, 53.5, 50.3, 40.2,
35.1, 29.32, 29.31. IR (KBr) v 3325, 2955, 1662, 1541, 1492,
1299, 1091, 1013, 825 cmÀ 1. HRMS-ESI (m/z): calcd for
7.38–7.33 (m, 3H), 7.25–7.21 (m, 1H), 5.58 (t, J=8.0 Hz, 1H),
4.09 (t, J=8.8 Hz, 1H), 3.60 (dd, J=9.0, 8.0 Hz, 1H), 2.44 (d,
J=13.4 Hz, 1H), 2.38 (d, J=13.4 Hz, 1H), 2.20 (d, J=
16.9 Hz, 1H), 2.15 (d, J=16.8 Hz, 1H), 1.14 (s, 3H), 1.11 (s,
3H). 13C NMR (101 MHz, CDCl3) δ 202.6, 163.9, 139.9, 135.0,
130.3, 129.3, 126.2, 124.2, 85.9, 80.0, 53.6, 50.4, 40.2, 35.1,
29.32, 29.31. IR (KBr) v 3292, 2952, 2897, 1669, 1561, 1481,
1032, 1031, 1000, 786, 693 cmÀ 1. HRMS-ESI (m/z): calcd for
+
+
C16H19ClNO2 [M+H]+: 292.1099, found 292.1100.
C16H19ClNO2 [M+H]+: 292.1099, found 292.1105.
(E)-2-(5-(4-Chlorophenyl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3gc)
(E)-2-(5-(3-Chlorophenyl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3ge)
°
°
Light pink solid, 104 mg, yield 92%, m.p. 159–162 C, Rf =
0.20 (PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 10.99
Light yellow solid, 96 mg, yield 85%, m.p. 153–156 C, Rf =
0.22 (PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 11.00 (s,
1H), 7.35–7.30 (m, 3H), 7.28–7.16 (m, 5H), 7.16–7.08 (m, 6H),
5.61 (t, J=8.0 Hz, 1H), 4.25 (t, J=9.6 Hz, 1H), 3.78 (dd, J=
10.0, 7.6 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 190.3, 168.6,
141.12, 140.07, 136.3, 134.9, 132.1, 130.3, 129.2, 128.9, 128.6,
127.7, 127.3, 126.1, 125.7, 124.0, 93.7, 80.0, 51.1. IR (KBr) v
3247, 3055, 2925, 1619, 1598, 1517, 1418, 1356, 1126, 983,
950, 755, 738, 700 cmÀ 1. HRMS-ESI (m/z): calcd for
1
(s, 1H), 7.39–7.34 (m, 2H), 7.27–7.22 (m, 4H), 7.20–7.06 (m,
8H), 5.60 (t, J=8.0 Hz, 1H), 4.23 (t, J=10.0 Hz, 1H), 3.75
(dd, J=10.0, 7.6 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ
190.4, 168.6, 141.2, 136.5, 136.3, 134.9, 132.1, 129.2, 128.9,
128.6, 127.6, 127.33, 127.25, 125.7, 93.6, 80.2, 51.0. IR (KBr)
v 3246, 3055, 2956, 1618, 1597, 1518, 1428, 1323, 1014, 981,
937, 755, 700 cmÀ 1. HRMS-ESI (m/z): calcd for C23H19ClNO2
+
[M+H]+: 376.1099, found 376.1103.
C23H19ClNO2 [M+H]+: 376.1099, found 376.1102.
+
(E)-2-(5-(2-Chlorophenyl)oxazolidin-2-Ylidene)-
4,4-Dimethylcyclopentan-1-One (3ad)
(E)-4,4-Dimethyl-2-(5-(p-Tolyl)oxazolidin-
2-Ylidene)cyclopentan-1-One (3af)
°
°
Light purple solid, 76 mg, yield 87%, m.p. 138–142 C, Rf =
0.20 (PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.21
Purple solid, 73 mg, yield 90%, m.p. 108–111 C. Rf =0.15 (PE/
EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.56 (brs, 1H),
1
1
(brs, 1H), 7.42–7.38 (m, 2H), 7.34–7.30 (m, 2H), 5.96 (dd, J=
8.4, 7.1 Hz, 1H), 4.24 (t, J=9.0 Hz, 1H), 3.53 (dd, J=9.3,
6.9 Hz, 1H), 2.49 (d, J=13.4 Hz, 1H), 2.44 (d, J=13.4 Hz,
1H), 2.21 (d, J=16.8 Hz, 1H), 2.17 (d, J=16.8 Hz, 1H), 1.16
(s, 3H), 1.13 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 202.2,
164.0, 136.3, 131.3, 129.8, 129.6, 127.3, 126.2, 85.7, 77.7,
53.5, 49.6, 40.2, 35.1, 29.3. IR (KBr) v 3291„ 2951, 2895,
1673, 1569, 1481, 1301, 1029, 1009, 941, 754 cmÀ 1. HRMS-
7.25 (d, J=8.7 Hz, 2H), 7.21 (d, J=8.2 Hz, 2H), 5.57 (t, J=
8.2 Hz, 1H), 4.05 (t, J=8.8 Hz, 1H), 3.63 (t, J=8.4 Hz, 1H),
2.41 (d, J=13.4 Hz, 1H), 2.37 (s, 3H), 2.36 (d, J=13.4 Hz,
1H), 2.19 (d, J=16.8 Hz, 1H), 2.15 (d, J=16.8 Hz, 1H), 1.12
(s, 3H), 1.10 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 202.2,
164.3, 139.2, 134.7, 129.6, 126.3, 85.7, 81.1, 53.6, 50.3, 40.2,
35.1, 29.4, 21.2. IR (KBr) v 3288, 2951, 1669, 1568, 1483,
1301, 1031, 1000, 912, 814 cmÀ 1. HRMS-ESI (m/z): calcd for
ESI (m/z): calcd for C16H19ClNO2 [M+H]+: 292.1099, found
C17H22NO2 [M+H]+: 272.1645, found 272.1652.
+
+
292.1102.
(E)-1,2-Diphenyl-2-(5-(p-Tolyl)oxazolidin-2-Ylidene)
ethan-1-One (3gf)
(E)-2-(5-(2-Chlorophenyl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3gd)
°
Yellow solid, 81 mg, yield 76%, m.p. 114–116 C. Rf =0.22
°
Light yellow solid, 99 mg, yield 88%, m.p. 201–203 C, Rf =
(PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 10.97 (s,
1H), 7.27–7.16 (m, 7H), 7.14–7.05 (m, 7H), 5.62 (t, J=8.3 Hz,
1H), 4.23 (t, J=9.4 Hz, 1H), 3.80 (dd, J=9.8, 7.9 Hz, 1H),
2.36 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 190.6, 169.0,
141.5, 139.1, 136.6, 135.0, 132.2, 129.6, 128.8, 128.7, 127.6,
127.3, 126.1, 125.6, 93.6, 81.2, 51.0, 21.4. IR (KBr) v 3270,
3054, 2975, 1616, 1598, 1516, 1475, 1306, 983, 932, 755,
0.50 (PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 11.02 (s,
1H), 7.42–7.38 (m, 2H), 7.35–7.27 (m, 4H), 7.22–7.12 (m, 8H),
6.01 (dd, J=9.0, 7.3 Hz, 1H), 4.45 (t, J=9.6 Hz, 1H), 3.74 (dd,
J=10.0, 7.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 190.2,
168.7, 141.2, 136.5, 136.4, 132.1, 131.1, 129.74, 129.68, 128.9,
128.6, 127.7, 127.4, 127.3, 126.3, 125.8, 93.7, 78.1, 50.6. IR
(KBr) v 3247, 3058, 2954, 1621, 1597, 1422, 1355, 1127, 985,
+
700 cmÀ 1. HRMS-ESI (m/z): calcd for C24H22NO2 [M+H]+:
+
754, 699 cmÀ 1. HRMS-ESI (m/z): calcd for C23H19ClNO2 [M
356.1645, found 356.1649.
+H]+: 376.1099, found 376.1105.
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126.8, 126.7, 125.8, 125.6, 123.0, 93.7, 81.3, 51.0. IR (KBr) v
(E)-4,4-Dimethyl-2-(5-(naphthalen-1-Yl)oxazolidin-
2-Ylidene)cyclopentan-1-One (3ag)
3246, 3055, 2955, 1616, 1513, 1423, 1306, 1180, 1125, 931,
+
753, 700 cmÀ 1. HRMS-ESI (m/z): calcd for C27H22NO2 [M+
H]+: 392.1645, found 392.1650.
°
Light pink solid, 84 mg, yield 91%, m.p. 138–141 C, Rf =0.10
(PE/EA=2:1, v/v). H NMR (400 MHz, CDCl3) δ 8.56 (brs,
1
1H), 7.94–7.84 (m, 2H), 7.80–7.75 (m, 1H), 7.59–7.46 (m, 4H),
6.31 (t, J=8.1 Hz, 1H), 4.29 (t, J=8.9 Hz, 1H), 3.68 (t, J=
8.0 Hz, 1H), 2.52 (d, J=13.4 Hz, 1H), 2.46 (d, J=13.4 Hz,
1H), 2.23 (d, J=16.9 Hz, 1H), 2.19 (d, J=16.9 Hz, 1H), 1.16
(s, 3H), 1.14 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 202.3,
164.2, 133.9, 133.4, 129.8, 129.4, 129.3, 126.9, 126.1, 125.4,
123.0, 122.3, 85.9, 78.8, 53.6, 49.9, 40.3, 35.1, 29.39, 29.37. IR
(KBr) v 3292, 3053, 2951, 1670, 1563, 1481, 1445, 1302, 1030,
Large-Scaled Preparation of (E)-4,4-Dimethyl-
2-(5-Phenyloxazolidin-2-Ylidene)cyclopentan-1-One
(3aa)
2-Diazo-5,5-dimethylcyclohexane-1,3-dione
(1a)
(1.20 g,
7.2 mmol) and 2-phenylaziridine (2a) (714 mg, 6.0 mmol) were
added in THF (20 mL) in a 50 mL round bottom flask. The
°
+
1002, 799, 778 cmÀ 1. HRMS-ESI (m/z): calcd for C20H22NO2
resulting solution was stirred at 120 C for 30 min under
[M+H]+: 308.1645, found 308.1652.
microwave irradiation in a sealed vessel. After the reaction was
completed, resulting mixture was evaporated in vacuo. The
crude residue was purified by the Et3N-neutralized silica gel
column chromatography (PE/EA 2:1, v/v) to give product 3aa
as light purple crystals 1.41 g, 91% yield.
(E)-2-(5-(Naphthalen-1-Yl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3gg)
°
Light yellow solid, 99 mg, yield 84%, m.p. 100–104 C, Rf =
0.35 (PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 11.08 (s,
1H), 7.90 (dd, J=6.9, 2.6 Hz, 1H), 7.84 (d, J=8.0 Hz, 1H),
7.77–7.72 (m, 1H), 7.57–7.46 (m, 4H), 7.31–7.27 (m, 2H),
7.22–7.06 (m, 8H), 6.34 (t, J=8.0 Hz, 1H), 4.48 (t, J=9.5 Hz,
1H), 3.84 (dd, J=9.7, 7.8 Hz, 1H). 13C NMR (101 MHz,
CDCl3) δ 190.4, 169.0, 141.4, 136.6, 133.9, 133.6, 132.2, 129.6,
129.3, 129.0, 128.9, 128.7, 127.8, 127.3, 126.9, 126.1, 125.8,
125.5, 123.0, 122.3, 93.8, 79.2, 50.8. IR (KBr) v 3247, 3054,
2926, 1618, 1597, 1512, 1425, 1356, 1005, 933, 778, 755,
Application of 2-(2-Oxoalkylidene)-1,3-Oxazolidine
3ia
2-(2-Oxoalkylidene)-1,3-oxazolidine 3ia (29.5 mg, 0.08 mmol)
was dissolved in a mixture of anhydrous acetonitrile (1 mL) and
toluene (1 mL). To the solution was added triethylamine
(0.1 mL) and boron trifluoride etherate (0.1 mL). The resulting
reaction mixture was stirred at room temperature for 5 h. After
removal of solvents, the residue was dissolved in dichloro-
methane. The solution was washed with saturated sodium
bicarbonate aqueous solution and with brine, dried over
anhydrous sodium sulfate, and concentrated in vacuo. The
residue was purified by silica gel column chromatography with
699 cmÀ 1. HRMS-ESI (m/z): calcd for C27H22NO2 [M+H]+:
+
392.1645, found 392.1650.
°
petroleum ether (60–90 C) and ethyl acetate (2:1, v/v) as eluent
(E)-4,4-Dimethyl-2-(5-(naphthalen-2-Yl)oxazolidin-
2-Ylidene)cyclopentan-1-One (3ah)
to afford difluoro compound 5 as colorless crystals 14 mg, 42%
1
°
yield, m.p. 186–187 C. H NMR (400 MHz, CDCl3) (δ, ppm)=
°
Light pink solid, 67 mg, yield 73%, m.p. 149–152 C, Rf =0.40
(DCM/MEOH=20:1, v/v). H NMR (400 MHz, CDCl3) δ 8.83
7.45–7.40 (m, 3H, ArH), 7.34–7.28 (m, 4H, ArH), 7.08–6.98
(m, 6H, ArH), 5.88 (dd, J=8.0, 11.8 Hz, 1H), 4.52 (dd, J=
11.8, 12.0 Hz, 1H), 4.12 (dd, J=8.0, 12.0 Hz, 1H), 2.31 (s,
3H), 2.29 (s, 3H). 13C NMR (101 MHz, CDCl3) (δ, ppm)=
171.8, 169.7, 141.1, 137.4, 136.7, 131.6, 131.3, 130.0, 129.7,
129.3, 129.2, 129.0, 128.4, 126.2, 97.5, 84.0, 77.2, 52.4, 21.3
(d, JFÀ C =21.2 Hz). 19F NMR (377 MHz, CDCl3) (δ, ppm)=
À 138.6, À 138.7. HRMS (ESI-TOF) m/z: [M+H]+ calcd for
1
(brs, 1H), 7.93–7.81 (m, 4H), 7.56–7.51 (m, 2H), 7.45 (dd, J=
8.5, 1.7 Hz, 1H), 5.79 (t, J=8.2 Hz, 1H), 4.15 (t, J=8.9 Hz,
1H), 3.74 (dd, J=9.1, 8.1 Hz, 1H), 2.47 (d, J=13.4 Hz, 1H),
2.42 (d, J=13.4 Hz, 1H), 2.23 (d, J=16.8 Hz, 1H), 2.19 (d,
J=16.8 Hz, 1H), 1.16 (s, 3H), 1.14 (s, 3H). 13C NMR
(101 MHz, CDCl3) δ 202.4, 164.3, 135.0, 133.5, 133.0, 129.2,
128.1, 127.8, 126.80, 126.76, 125.9, 123.2, 85.8, 81.2, 53.6,
50.3, 40.3, 35.1, 29.37, 29.35. IR (KBr) v 3300, 3056, 2952,
1665, 1551, 1482, 1302, 1029, 1003, 895, 748 cmÀ 1. HRMS-
+
C25H23BF2NO2 418.1784, found 418.1788.
+
ESI (m/z): calcd for C20H22NO2 [M+H]+: 308.1645, found
Acknowledgements
308.1652.
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21572017, 21772010, and
21911530099). The authors thank Ms. Qiuyue Wu and Mr. Duo
Fu for their help in conducting hydrolysis and determination of
the configuration of product 3aa*.
(E)-2-(5-(Naphthalen-2-Yl)oxazolidin-2-Ylidene)-
1,2-Diphenylethan-1-One (3gh)
°
Light yellow solid, 92 mg, yield 78%, m.p. 134–137 C, Rf =
0.30 (PE/EA=2:1, v/v). 1H NMR (400 MHz, CDCl3) δ 11.07 (s,
1H), 7.92 (d, J=8.5 Hz, 1H), 7.89–7.84 (m, 2H), 7.80 (s, 1H), References
7.57–7.52 (m, 2H), 7.48 (dd, J=8.5, 1.6 Hz, 1H), 7.32–7.28
(m, 2H), 7.23–7.07 (m, 8H), 5.84 (t, J=8.3 Hz, 1H), 4.35 (t,
J=9.5 Hz, 1H), 3.94 (dd, J=9.9, 7.7 Hz, 1H). 13C NMR
(101 MHz, CDCl3) δ 190.5, 168.9, 141.4, 136.5, 135.2, 133.5,
132.9, 132.2, 129.2, 128.8, 128.7, 128.1, 127.8, 127.6, 127.3,
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RESEARCH ARTICLE
Catalyst-Free Electrophilic Ring Expansion of N-Unprotected
Aziridines with α-Oxoketenes to Efficient Access 2-Alkyli-
dene-1,3-Oxazolidines
Adv. Synth. Catal. 2021, 363, 1–12
X. Chen, Z. Huang, J. Xu*