Electrochemically induced N-acryloylation of chiral oxazolidin-2-ones

Article abstract of DOI:10.1002/1099-0690(200107)2001:14<2765::aid-ejoc2765>3.0.co;2-c

A new method for N-acryloylation of Evans' chiral auxiliaries (oxazolidin-2-ones) with α,α′-dichloro ketones in the presence of the electrogenerated base 2-pyrrolidone anion is described. N-Enoyloxazolidin-2-ones are obtained, under mild reaction conditions, in good to high yields.

Full text of DOI:10.1002/1099-0690(200107)2001:14<2765::aid-ejoc2765>3.0.co;2-c

FULL PAPER
Electrochemically Induced -Acryloylation of Chiral Oxazolidin-2-ones
Marta Feroci,*[a] Achille Inesi,*[b] Leucio Rossi,[b] and Giovanni Sotgiu[c]
Keywords: Electrogenerated base / Halo ketones / N-Enoyloxazolidin-2-ones / Favorskii rearrangement / Electroorganic
synthesis
A new method for N-acryloylation of Evans’ chiral auxiliaries
(oxazolidin-2-ones) with α,αЈ-dichloro ketones in the pres-
ence of the electrogenerated base 2-pyrrolidone anion is de-
scribed. N-Enoyloxazolidin-2-ones are obtained, under mild
reaction conditions, in good to high yields.
Introduction
panded this method to the N-acryloylation of Evans’ chiral
auxiliaries, in particular, we have studied the reactivity of
α,αЈ-dihalo ketones with oxazolidin-2-ones in aprotic solv-
In the last two decades, chiral oxazolidin-2-ones have
been used as chiral auxiliaries in a wide range of
transformations.[1Ϫ4] High diastereoselectivity induced by
N-acyloxazolidin-2-ones in alkylation, acylation, and aldol
reactions has frequently been observed.[1,5,6] N-Enoyloxazo-
lidin-2-ones have been successfully utilized as dienophiles in
the asymmetric DielsϪAlder reaction.[7]
ents
containing
the
EGB
pyrrolidone
anion
(Scheme 1).[12Ϫ15]
Results and Discussion
The synthesis of acyloxazolidinones,[8] requires a coup-
ling of the lithium salt of the oxazolidinone with the acid
chloride or mixed anhydride of the acidic substrate. As the
preparation and isolation of acid chlorides or mixed anhyd-
rides requires operations which are undesirable for large-
scale preparations, direct N-acylation of chiral oxazolidin-
2-one auxiliaries with acids has been proposed.[9,10] How-
ever, α,β-unsaturated acids, such as 3,3-dimethylacrylic
acid, gave N-acryloyloxazolidin-2-ones in poor or modest
yields (14Ϫ57%).
The EGB was obtained by electrochemical reduction of
the probase (PB) 2-pyrrolidone (Scheme 2, reaction 1). Ox-
azolidin-2-one 1a and α,αЈ-dihalo ketone 2k were chosen as
reference compounds.
In aprotic solvents and in the presence of Et4Nϩ as coun-
ter ion, the EGB 2-pyrrolidone anion shows a remarkable
deprotonating power toward some NϪH acids.[16] In fact,
primary and secondary aliphatic and aromatic amines were
deprotonated by this EGB to yield the corresponding tetra-
ethylammonium amides and anilides. According to these re-
actions, the EGB 2-pyrrolidone anion should be able to de-
protonate oxazolidinone 1a as well as dichloro ketone 2k
to yield the anion 3a and cyclopropanone 4k intermediates,
respectively (Scheme 2, reactions 2 and 3).
Recently,[11] we have shown that α-halo or α,αЈ-dihalo ke-
tones, after cathodic reduction of the carbonϪhalogen
bond or in the presence of electrogenerated bases (EGBs),
react with amines and phenols to give the corresponding
α,β-unsaturated amides and esters according to an electro-
chemically induced Favorskii rearrangement. We have ex-
Both anion 3a and EGB react with the chlorocyclopro-
panone 4k, according to the Favorskii rearrangement, to
yield N-enoyloxazolidinone 5a and N-enoylpyrrolidone 6
(Scheme 2, reactions 3Ј and 3ЈЈ). Since the reduction of the
probase (reaction 1) is monoelectronic, the flow of 1.0
F·molϪ1 of probase was used in the electrolyses. Moderate
increases (5Ϫ10%) of the above value did not significantly
improve the yield of 5a. The yield of 5a is affected by the
pyrrolidone/dichloro ketone/oxazolidinone molar ratio, it
becomes greater with an increase in probase and ketone
with respect to oxazolidinone (Table 1, Entries 1Ϫ3, 5, 6).
However, a strong excess of probase produces a drastic de-
crease in the yield of 5a (Table 1, Entry 4). In all cases, 6 is
isolated in low yields.
To simplify the synthesis of 5a (i.e., without EGB and
the formation of the by-product 6), we have performed the
deprotonation of 1a by direct electrochemical reduction
(Scheme 2, reaction 1Ј) instead of acid-base reaction with
the EGB (reaction 2). A comparison of the molecular struc-
ture of 2-pyrrolidone with oxazolidin-2-one suggests that
Scheme 1
[a]
`
Dip. ICMMPM, Universita ‘‘La Sapienza’’,
Via Castro Laurenziano 7, 00161 Roma, Italy
Fax: (internat.) ϩ 36-06/49766749
E-mail: marta.feroci@uniroma1.it
Dip. CICM, Universita degli Studi,
[b]
[c]
`
Monteluco di Roio, 67040 L’Aquila, Italy
Fax: (internat.) ϩ 36-0862/434203
E-mail: inesi@ing.univaq.it
`
Dip. IE, Universita di Roma Tre,
Via Vasca Navale 84, 00146 Roma, Italy
Eur. J. Org. Chem. 2001, 2765Ϫ2769
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0714Ϫ2765 $ 17.50ϩ.50/0
2765

M. Feroci, A. Inesi, L. Rossi, G. Sotgiu
FULL PAPER
direct electrochemical reduction of 1a could produce the
nitrogen anion 3a (Scheme 2, reaction 1Ј). When 2k or 2l
were treated with electrogenerated 3a, compound 5a was
isolated from the reaction mixture, and the starting halo
ketones were completely absent. However, with the direct
electrochemical reduction the yields of 5a (from 1a) were
reduced (Table 2, Entries 1 and 2). Under all these condi-
tions, the oxazolidin-2-one anion 3a reacts both as a nucle-
ophile (yielding 5a, reaction 3Ј) and as a base [yielding 4
and thus regenerating 1a; reaction 3 (ϩ3), theoretical max-
imum yield in 5a: 50%]. The trouble of recovery of 1a from
the reaction mixture should also be taken into account.
Table 2. Reaction of the oxazolidin-2-one anion (3a) and of the
pyrrolidone anion (EGB) with α,αЈ-dihalo ketones 2k and 2l
(MeCN/0.1 mol·dmϪ1 TEAP as solvent, n[a] ϭ 1.0, ρ[b] ϭ 1.0)
Entry
Anion
Halo ketone
Products and yields[c] (%)
1
2
3
4
3a
3a
EGB
EGB
2k
2l
2k
2l
5a (38)[d]
5a (43)[e]
6 (24)
6 (32)
[a]
Number of Faradays per mol of oxazolidin-2-one 1a or of 2-
[b]
pyrrolidone supplied to the electrodes. Ϫ Molar ratio oxazolidi-
none/α,αЈ-dichloro ketone or 2-pyrrolidone/α,αЈ-dichloro ketone. Ϫ
[c] Yields of isolated 5a based on starting oxazolidin-2-ones 1a (Ent-
ries 1, 2) or yields of isolated 6 based on starting 2-pyrrolidone
[d]
(Entries 3, 4). Ϫ Oxazolidin-2-one 1a (61%) was also recovered.
[e]
Ϫ
Oxazolidin-2-one 1a (39%) was also recovered.
A comparison between the reactivity of the EGB and of
3a with the α,αЈ-dihalo ketones could be helpful for a better
insight into the overall synthetic process. Therefore, we have
performed the reaction between the EGB and 2k or 2l (in
the absence of oxazolidinone). From the reaction mixture,
6 has been isolated in moderate yields (yields of 6: 24 and
32% respectively, based on the starting PB; Table 2, Entries
3 and 4).
Scheme 2
According to the results reported in Table 2, both EGB
and 3a are strong enough bases to deprotonate α,αЈ-dihalo
ketones 2k and 2l. Nevertheless, the higher yields of 5a than
6, show that 3a is a stronger nucleophile than the EGB
(Table 2, Entries 1 vs. 3 and 2 vs. 4). In addition, the results
in Table 2 suggest a higher reactivity of bromocyclopro-
panone, than chlorocyclopropanone, with the nucleophilic
agent (i.e., 3a or EGB), according to the assumption that
bromide is a better leaving group than chloride. In fact,
both 5a and 6 were obtained with higher yields from α,αЈ-
dibromo ketone 2l than from α,αЈ-dichloro ketone 2k.
To ascertain whether this method for N-acylation of 2-
oxazolidinones could be generalized, the reactivity of differ-
ent dihalo ketones 2 as well as of different oxazolidinones 1,
under optimized reaction conditions (i.e. ρ ϭ 2:2:1, MeCN,
0.1 mol dmϪ3 TEAP as solvent) has been studied (Table 3).
The reactivity is affected by the nature of the substituents
on the carbon atoms in positions 4 and 5 on the oxazolidi-
Table 1. Reaction of 2-pyrrolidone EGB with oxazolidin-2-one 1a
and dichloro ketone 2k under different conditions (MeCN/0.1
mol·dmϪ3 TEAP as solvent, n ϭ 1.0[a]
)
Entry
ρ[b]
Yield of 5a (%)[c]
Yield of 6 (%)[d]
1
2
3
4
5
6
1.0:1.0:1.0
1.5:1.0:1.0
2.0:1.0:1.0
3.0:1.0:1.0
2.0:1.5:1.0
2.0:2.0:1.0
33
42
64
17
75
81
5
4
4
14
3
traces
[a]
Number of Faradays per mol of 2-pyrrolidone supplied to the
[b]
electrodes. Ϫ
Mol ratio 2-pyrrolidone/α,αЈ-dichloro ketone/ox-
azolidinone. Ϫ [c] Yields of isolated 5a based on starting oxazolidin-
[d]
2-one 1a. Ϫ
tone 2k.
Yields of isolated 6 based on starting dichloro ke-
2766
Eur. J. Org. Chem. 2001, 2765Ϫ2769

Electrochemically Induced N-Acryloylation of Chiral Oxazolidin-2-ones
FULL PAPER
Table 3. Reaction of 2-pyrrolidone EGB with oxazolidin-2-ones 1aϪg and dihaloketones 2k (Entries 1Ϫ6, 9) or 2h (Entry 7) or 2l (Entry
8); MeCN/0.1 mol·dmϪ3 TEAP as solvent, n ϭ 1.0[a], ρ ϭ2:2:1;[b] yields[c] and stereochemistry of reaction products
[b]
[a] Number of Faradays per mol of 2-pyrrolidone supplied to the electrodes. Ϫ Molar ratio 2-pyrrolidone/α,αЈ-dichloro ketone/oxazolidi-
[c]
[d]
none. Ϫ Yields of isolated 5aϪf based on starting oxazolidin-2-ones 1aϪf. Ϫ 6 was also recovered at the end of the reaction (Entries
[e]
1, 4, 5: traces; Entry 2: 5%; Entry 3: 9%; Entry 8: 15%; yields based on starting dihalo ketones). Ϫ A mixture of (Z) and (E) isomers in
a 1.25 ratio was obtained.
none and in the α-position to the carbonyl group for the lished, namely the reaction of α,αЈ-dihalo ketones with ox-
dihalo ketone as well as by the nature of the halogen atom. azolidin-2-ones in aprotic solvent with the EGB 2-pyrroli-
For the oxazolidin-2-one, the steric hindrance and elec- done anion. N-Enoyloxazolidin-2-ones have been obtained
tronic structure of the substituent groups at both 4 (Table 3, under mild reaction conditions in good to high yields.
Entries 1Ϫ3 and 4Ϫ6) and 5 position (Table 3, Entries 1
and 5) seems to play a significant role. As for the α,αЈ-di-
halo ketones, the replacement of chlorine with bromine in-
creases the reactivity of the intermediate halocyclopro-
Experimental Section
General Remarks: The electrochemical apparatus, the cells, and the
reference electrode[17] as well as the method of purification of ace-
tonitrile (MeCN) and tetraethylammonium perchlorate (TEAP)
have already been described.[18] Ϫ 2-Pyrrolidone and all oxazolidin-
2-ones were commercially available and used as received. Dihalo
panone 4 (see above) and consequently decreases the select-
ivity [chloro ketone: 5a (81%), 6 (traces); bromo ketone: 5a
(63%), 6 (15%); Table 3, Entries 1, 8, footnote[d]]. Thus, N-
enoyloxazolidin-2-ones have been obtained in good to high
yields. In addition, we have ascertained that the absolute
configurations of all the chiral atoms of 5aϪf remained un-
changed with respect to 1aϪf (Table 3, Entries 1Ϫ5 and
7Ϫ9).
1
ketones were prepared as described in the literature.[19] Ϫ H and
13C NMR spectra were recorded using a Bruker AC 200 spectro-
meter with CDCl3 as internal standard. Ϫ GC-MS measurements
were carried out with an SE 54 capillary column using a Fisons
8000 gas chromatograph coupled with a Fisons MD 800 quadru-
pole mass-selective detector. Ϫ Optical rotations were measured
with a PerkinϪElmer 241 polarimeter.
Conclusion
Reduction of 2-Pyrrolidone (PB): MeCN (30 mL, 0.1 mol·LϪ1) in
In conclusion, a new method for the N-enoylation of Ev-
ans’ chiral oxazolidin-2-ones auxiliaries has been estab- TEAP with 2-pyrrolidone (1.0 mmol) was electrolyzed under N2 at
Eur. J. Org. Chem. 2001, 2765Ϫ2769 2767

M. Feroci, A. Inesi, L. Rossi, G. Sotgiu
FULL PAPER
room temperature in a divided cell (Pt anode and cathode) under ar), 6.95 (s, 1 H, COCHϭC), 5.46 (dd, J ϭ 8.7, J ϭ 4.0 Hz, 1 H,
galvanostatic control (I ϭ 25 mA·cmϪ2). After the flow of 1.0
PhCH), 4.64 (t, J ϭ 8.7 Hz, 1 H, OCH2), 4.24Ϫ4.18 (m, 1 H,
OCH2), 2.22 (q, J ϭ 7.3 Hz, 2 H, CH3CH2), 2.07 (s, 3 H, CH3Cϭ),
F·molϪ1 of PB, the cathodic solution was added to a mixture of
the dihalo ketone (0.5 or 1.0 mmol) and/or oxazolidin-2-one 1.08 (t, J ϭ 7.3 Hz, 3 H, CH3CH2). Ϫ 13C NMR (CDCl3,
(0.5 mmol) and the resulting solution was stirred overnight at room 50.3 MHz): δ ϭ 164.65, 163.95, 153.66, 139.44, 129.06, 128.41,
temperature. After the usual workup, the electrolysis products were
purified by flash chromatography using a mixture of n-hexane and
ethyl acetate as eluent in a 9:1 to 7:3 ratio. The products and yields
are reported in Tables 1, 2, and 3.
125.67, 114.14, 69.64, 57.55, 34.29, 19.88, 12.29. Ϫ GC-MS m/z:
260 [Mϩ ϩ 1] (3), 259 (27) [Mϩ], 164 (15), 162 (9), 104 (27)
1
[PhCHCH2ϩ], 97 (87), 96 (100). Ϫ ( ) Isomer: H NMR (CDCl3,
200 MHz): δ ϭ 7.40Ϫ7.24 (m, 5 H, ar), 6.89 (s, 1 H, COCHϭC),
5.46 (dd, J ϭ 8.7 Hz, J ϭ 4.02 Hz, 1 H, PhCH), 4.64 (t, J ϭ 8.7 Hz,
1 H, OCH2), 4.24Ϫ4.18 (m, 1 H, OCH2), 2.53Ϫ2.45 (m, 2 H,
CH3CH2), 1.94 (s, 3 H, CH3Cϭ), 0.99 (t, J ϭ 7.5 Hz, 3 H,
CH3CH2). Ϫ 13C NMR (CDCl3, 50.3 MHz): δ ϭ 165.02, 163.95,
153.66, 139.41, 129.06, 125.77, 125.67, 115.04, 69.64, 57.57, 27.46,
25.18, 11.86. Ϫ GC-MS m/z: 260 (3) [Mϩ ϩ 1], 259 (27) [Mϩ], 164
(15), 162 (9), 104 (27) [PhCHCH2ϩ], 97 (87), 96 (100).
Reduction of Oxazolidin-2-one 1a: MeCN (30 mL, 0.1·mol LϪ1) in
TEAP with oxazolidin-2-one 1a (1.0 mmol) was electrolyzed under
N2 at room temperature in a divided cell (Pt anode and cathode)
under galvanostatic control (I ϭ 25 mA·cmϪ2). After the flow of
1.0 F molϪ1 of substrate, the cathodic solution was added to the
dihalo ketone (0.5 mmol) and the resulting solution was stirred
overnight at room temperature. After the usual workup, the electro-
lysis products were purified by flash chromatography using a mix-
ture of n-hexane and ethyl acetate as eluent in an 8:2 ratio. The
products and yields are reported in Table 2.
-(3-Methylbut-2-enoyl)-2-pyrrolidone (6): 1H NMR (CDCl3,
200 MHz): δ ϭ 6.91 (s, 1 H, HCϭC), 3.81 (t, J ϭ 7.1 Hz, 2 H,
CH2N), 2.58 (t, J ϭ 8.1 Hz, 2 H, CH2CϭO), 2.14 (s, 3 H, Z H3C),
2.03Ϫ1.95 (m, 2 H, CH2CH2CH2), 1.95 (s, 3 H, E H3C). Ϫ 13C
NMR (CDCl3, 50.3 MHz): δ ϭ 175.08, 166.31, 157.63, 117.61,
45.58, 34.03, 27.94, 21.34, 17.12. Ϫ GC-MS m/z: 167 (9) [Mϩ], 84
(14), 83 (100).
( )-(؊)-4-Benzyl- -(3-methylbut-2-enoyl)oxazolidin-2-one (5b),[20]
( )-(؊)-4-Isopropyl- -(3-methylbut-2-enoyl)oxazolidin-2-one
(5c),[21] (4 ,5 )-(؉)-4-Methyl- -(3-methylbut-2-enoyl)-5-phenylox-
azolidin-2-one (5d):[22] These electrolysis products gave spectro-
scopic data according to the literature.
( )-(؊)- -(3-Methylbut-2-enoyl)-4-phenyloxazolidin-2-one (5a): 1H
NMR (CDCl3, 200 MHz): δ ϭ 7.36Ϫ7.27 (m, 5 H, ar), 6.94Ϫ6.93
(m, 1 H, HCϭC), 5.46 (dd, J ϭ 8.6, J ϭ 4.3 Hz, 1 H,CH2O), 4.65
(t, J ϭ 8.6 Hz, 1 H, PhCHN), 4.20 (dd, J ϭ 8.6, J ϭ 4.3 Hz, 1 H,
CH2O), 2.07 (d, J ϭ 1.0 Hz, 3 H, Z H3C), 1.95 (d, J ϭ 1.0 Hz, 3
H, E H3C). Ϫ 13C NMR (CDCl3, 50.3 MHz): δ ϭ 164.37, 159.63,
153.67, 139.43, 129.07, 128.43, 125.76, 115.70, 69.65, 57.58, 27.98,
21.28. Ϫ GC-MS m/z: 245 (8) [Mϩ], 162 (24), 83 (100). Ϫ [α]2D0ϭ
Ϫ123.1 (c ϭ 0.52, CHCl3).
Acknowledgments
This work was supported by research grants from MURST (2000,
40%) and CNR, Roma, Italy.
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1
one (5e): H NMR (CDCl3, 200 MHz): δ ϭ 7.12Ϫ6.84 (m, 11 H,
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13C NMR (CDCl3, 50.3 MHz): δ ϭ 164.18, 160.05, 153.63, 134.84,
133.08, 128.26, 128.18, 127.93, 126.54, 126.13, 115.54, 80.01, 62.82,
28.06, 22.58. Ϫ GC-MS m/z: 321 (3) [Mϩ], 83 (100). Ϫ [α]2D0ϭ
Ϫ43.8 (c ϭ 0.53, CHCl3).
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(3a )-(؊)- -(3-Methylbut-2-enoyl)-3,3a,8,8a-tetrahydro- -2
-
indeno[1,2- ]oxazol-2-one (5f): 1H NMR (CDCl3, 200 MHz): δ ϭ
7.67Ϫ7.64 (m, 1 H, ar), 7.31Ϫ7.23 (m, 3 H, ar) 6.91 (s, 1 H, HCϭ
C), 5.97 (d, J ϭ 6.9 Hz, 1 H, PhCHN), 5.25 (dt, J ϭ 6.9, J ϭ
3.6 Hz, 1 H, CH2CHO), 3.36 (d, J ϭ 3.6 Hz, 2 H, CH2CHO), 2.22
(s, 3 H, Z H3C), 1.96 (s, 3 H, E H3C). Ϫ 13C NMR (CDCl3,
50.3 MHz): δ ϭ 165.34, 159.11, 153.41, 139.48, 129.69, 128.06,
127.38, 125.12, 115.76, 77.73, 63.05, 38.02, 28.03, 21.41. Ϫ GC-MS
m/z: 257 (1) [Mϩ], 83 (100). Ϫ [α]2D0ϭ Ϫ189.3 (c ϭ 0.38, CHCl3).
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[14]
1
Mmixture): H NMR (CDCl3, 200 MHz): δ ϭ 7.37Ϫ7.31 (m, 5 H,
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(dd, J ϭ 11.2, J ϭ 8.2 Hz, 1 H, CH2N), 3.90 (dd, J ϭ 11.2, J ϭ
8.2 Hz, 1 H, CH2N), 2.16 (s, 3 H, Z H3C), 1.98 (s, 3 H, E H3C). Ϫ
13C NMR (CDCl3, 50.3 MHz): δ ϭ 164.99, 159.47, 152.86, 137.30,
129.19, 129.00, 125.69, 115.39, 74.55, 49.85, 28.03, 21.31. Ϫ GC-
MS m/z: 245 (16) [Mϩ], 230 (16) [Mϩ Ϫ CH3], 83 (100).
[15]
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[16]
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[18]
( )-(؊)- -(3-Methylpent-2-enoyl)-4-phenyloxazolidin-2-one (5h). ؊
1
( ) Isomer: H NMR (CDCl3, 200 MHz): δ ϭ 7.40Ϫ7.24 (m, 5 H,
eletti-Moracci, Tetrahedron 1997, 53, 167Ϫ176.
2768
Eur. J. Org. Chem. 2001, 2765Ϫ2769

Electrochemically Induced N-Acryloylation of Chiral Oxazolidin-2-ones
FULL PAPER
D. M. Walba, W. N. Thurmes, R. C. Haltiwanger, J. Org. Chem.
1988, 53, 1046Ϫ1056.
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[20]
[21]
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