Samarium diiodide as an efficient catalyst for the conversion of N-acyloxazolidinones into esters

Article abstract of DOI:10.1055/s-2008-1072587

The transformation of N-acyloxazolidinones into esters is readily performed using catalytic amounts of samarium diiodide in tetrahydrofuran at room temperature. This method allows the isolation of various esters without racemization in the case of scalemi

Full text of DOI:10.1055/s-2008-1072587

LETTER
1211
Samarium Diiodide as an Efficient Catalyst for the Conversion of
N-Acyloxazolidinones into Esters
S
amarium-Cataly
a
zedConve
r
rsionof
o
N
-Acyloxazo
l
lidinon
i
es into
n
Esters e Magnier-Bouvier, Iréna Reboule, Richard Gil, Jacqueline Collin*
Equipe de Catalyse Moléculaire, ICMMO, UMR 8182, Université Paris-Sud, 91405 Orsay, France
Fax +33(1)69154680; E-mail: jacollin@icmo.u-psud.fr
Received 27 November 2007
the highest activities were observed for [t-
Bu2SnCl(OH)]2, MgBr2, and Sc(OTf)3.10 These three de-
rivatives were further used for the catalysis of various re-
actions performed in toluene at 85 °C in most cases. The
Lewis acids described so far as promoters or catalysts for
the transformation of N-acyloxazolidinones into esters
suffer from drawbacks such as their difficulty of prepara-
tion or handling, their toxicity, or the large amounts of re-
agents required to achieve the reactions.
Abstract: The transformation of N-acyloxazolidinones into esters
is readily performed using catalytic amounts of samarium diiodide
in tetrahydrofuran at room temperature. This method allows the iso-
lation of various esters without racemization in the case of scalemic
substrates and the recovery of oxazolidinones in good yields.
Key words: samarium diiodide, catalysis, deprotection, N-acylox-
azolidinones, esters
Following the pioneering work of Evans, chiral oxazolid-
inones have been widely employed as auxiliaries for the
synthesis of enantioenriched compounds.1 Another effi-
cient strategy is the use of N-acyloxazolidinones as chelat-
ing achiral templates for enantioselective catalysis.2 A
variety of methods has been studied for the subsequent re-
moval of the oxazolidinone moiety, which involves en-
docyclic or exocyclic cleavage.3,4 The transformation of
N-acyloxazolidinones into carboxylic acids was realized
in basic conditions. Exocyclic or endocyclic cleavage or
both reactions were observed depending on the reaction
conditions and on the nature of the substrates.3a,b The re-
ductive cleavage of oxazolidinones promoted by metallic
hydrides afforded alcohols without racemization of sub-
strates.4 The transformation of N-acyloxazolidinones into
esters by an exocyclic cleavage of oxazolidinone is a
highly attractive process since it allows both an easy sep-
aration of the reaction products and the recovery of the
auxiliary. Esterifications have been performed in basic
conditions such as with magnesium methylate in metha-
nol which led to the opening of the oxazolidinone in some
cases,3c,5 or in acid conditions by HCl–EtOH under re-
flux.6 Several promoters have been described for the
cleavage of oxazolidinone moiety used as a chiral auxilia-
ry. Titanium(IV) isopropoxide [Ti(i-OPr)4] in a large ex-
cess allowed the formation of isopropyl esters in refluxing
toluene.7 Samarium(III) triflate [Sm(OTf)3] has been em-
ployed for the transformation of an oxazolidinone into es-
ter in the course of the synthesis of altohyrtin.8 Lanthanum
triiodide was found to be more efficient than other lan-
thanide triiodides for the reactions of N-acyl oxazolidino-
nes with benzyl alcohol or methanol but this reagent had
to be used in stoichiometric amount.9 Various Lewis acids
were screened by Otera et al. as catalysts for the transfor-
mation of 3-cinnamoyl-1,3-oxazolidinone into esters and
In the course of our previous studies we have investigated
the activity of samarium diiodide as an efficient Lewis
acid catalyst for a wide range of carbon–carbon bond-
forming reactions, such as Mukaiyama aldol reactions,
Diels–Alder reactions, or tandem Mukaiyama–Michael–
aldol reactions.11 Recently, we have found that samarium
diiodide is also an efficient catalyst for the Michael addi-
tion of aromatic amines to a,b-unsaturated N-acyloxazoli-
dinones leading to b-amino acid precursors.12 According
to the nature of the substrates aza-Michael reactions can
be followed by an in situ amidation with the aromatic
amine, leading to b-amino amide. The use of an excess of
amine allowed to cleave the N-acyloxazolidinone bond
and to prepare the amide in most cases. This result led us
to consider the use of alcohols for similar cleavage reac-
tions and we now report a new method for the transforma-
tion of N-acyloxazolidinones into esters with catalytic
amounts of samarium diiodide under mild conditions.
With the aim to find mild and easily applicable reaction
conditions we investigated the conversion of N-acylox-
azolidinones into esters by catalysis with a solution of sa-
marium diiodide in THF. We were pleased to find that
cleavage of oxazolidinone from N-benzoyloxazolidinone
(1a) could be realized using 10 mol% samarium diiodide
in THF and 100 equivalents of ethanol. Ethyl benzoate
(2a) was cleanly formed either when N-benzoyloxazolid-
inone (1a) was added to the mixture of ethanol and samar-
ium diiodide or when samarium diiodide was added to N-
benzoyloxazolidinone in a solution of THF–EtOH. The
addition of ethanol to a mixture of samarium diiodide and
N-benzoyloxazolidinone led to the formation of byprod-
ucts. Whatever procedure is used reaction mixture turns
from blue to light yellow after a short time and color does
not change during the reaction indicating that the catalytic
species is trivalent.15 The reaction was further studied
with various substrates and the results are gathered in
Table 1. For the reaction of N-benzoyloxazolidinone (1a)
with ethanol a total conversion was observed, the moder-
SYNLETT 2008, No. 8, pp 1211–1215
x
x.
x
x.
2
0
0
8
Advanced online publication: 16.04.2008
DOI: 10.1055/s-2008-1072587; Art ID: D38107ST
© Georg Thieme Verlag Stuttgart · New York

1212
C. Magnier-Bouvier et al.
LETTER
entries 1 and 2). For the reaction of N-dodecanoyloxazo-
lidinone (1b) with ethanol the amount of alcohol could be
decreased to 10 equivalents without diminution of the
yield of ester 2c and oxazolidinone 3a could be recovered
in good yield (entries 3 and 4). This reaction could be
achieved with a lower amount of catalyst (5 mol% SmI2)
but reaction time was increased (entry 5). The reaction of
N-dodecanoyloxazolidinone (1b) with 10 equivalents of
benzyl alcohol led to the quantitative formation of ester
2d whereas with isopropanol low conversion was ob-
served (entries 6 and 7). By performing the esterification
of N-hexanoyloxazolidinone 1c with 50 equivalents of
benzyl alcohol the ester 2f was isolated in high yield after
16 hours (entry 8). The reaction could be extended to a
substrate disubstituted in a-position to the carbonyl group
1d and to N-dihydrocinnamoyloxazolidinone (1e) afford-
ing ethyl esters in good yields after the same reaction time
(entries 9 and 10). The cleavage of oxazolidinone from an
aminoester 1f or an amine 1g prepared by aza-Michael re-
actions was next examined. Ethyl esters 2i and 2j were
isolated in satisfying yields in both cases (entries 11–13)
and the amount of samarium diiodide could be decreased
to 5 mol%. Samarium diiodide catalyzes the cleavage of
the N-carbonyl bond of acyloxazolidinone in THF with
ethanol or benzyl alcohol and reaction can be realized in
the presence of bulky or functionalized groups.
Table 1 Transformation of Scalemic N-Acyloxazolidinones into
Esters Catalyzed by Samarium Diiodide
O
O
O
R2 = H 3a
i-Pr 3b
SmI2 (10 mol%)
R1OH, THF, r.t.
RCOOR1
R
N
O
+
HN
R2
O
Bn 3c
1
2
R2
Entry Substrate
Alcohola
Time Yield
(h)c
16
2
of 2 (%)
O
O
O
1
EtOH (100)
BnOH (10)
EtOH (100)
2a 46
2b 98
2c 98d
Ph
N
1a
1a
2
3
O
O
2
H23
C11
N
O
1b
1b
4
5
6
7
EtOH (10)
EtOH (10)b
BnOH (10)
2
96
2c 99
2c 86
2d 99
2e 13e
1b
1b
1b
168
i-PrOH (10) 168
O
O
O
The selectivity of the cleavage of an oxazolidinone in the
presence of acid-sensitive groups was checked by per-
forming the samarium-catalyzed reaction with ethanol on
mixtures of substrates (Scheme 1). Gratifyingly, we found
that N-dodecanoyloxazolidinone (1b) was quantitatively
converted into ethyl dodecanoate (2c) in the presence of a
tert-butyldimethyl silyl ether, a trimethylsilyl enol ether,
or an acetal without cleavage of these molecules. Samari-
um diiodide acts as an efficient, mild, and selective cata-
lyst for the deprotection of oxazolidinone groups which
should be useful for sophisticated molecules with various
acid-sensitive functionalities.
8
9
BnOH (50)
EtOH (100)
EtOH (100)
16
16
16
2f 83
2g 99
2h 81
H
11C5
N
O
1c
O
Ph
N
O
O
O
Ph
1d
10
N
O
Ph
1e
O
O
4-MeOC6H4HN
O
O
SmI2 (10 mol%)
11
12
13
EtOH (20)
7
48
48
2i 65
2i 52
2j 72
+
X
N
O
EtO2C
H23C11COOEt
H
23C11
N
O
+
X
EtOH (10 equiv), THF
2c
1b
1f
1f
EtOH (100)b
EtOH (100)
OSiMe3
XA
=
O
H22C11OSiMe2t-Bu
XB =
2-MeOC6H4HN
O
N
O
O
O
XC
=
1g
a Ratio alcohol/substrate.
Scheme 1
b Reactions performed with 5 mol% SmI2.
c For experimental details see ref. 14.
d Oxazolidinone was recovered in 67% yield.
e Recovered substrate: 53%.
The application of samarium-catalyzed reactions to non-
racemic substrates was next studied and the results are
gathered in Table 2. We first examined the esterification
reactions with substrates containing a chiral oxazolidino-
ne moiety. Using 10 mol% SmI2 in THF the reaction of
(S)-N-dodecanoyl-4-isopropyloxazolidinone (1h) with 10
equivalents ethanol afforded the expected ester 2c within
ate yield is due to the low boiling point of ethyl benzoate
(2a), while using 10 equivalents of benzyl alcohol, benzyl
benzoate (2b) was isolated in quantitative yield (Table 1,
Synlett 2008, No. 8, 1211–1215 © Thieme Stuttgart · New York

LETTER
Samarium-Catalyst Conversion of N-Acyloxazolidinones into Esters
1213
one hour and the chiral oxazolidinone 3b was recovered in ne (3b, entries 2 and 3). In cleavage reactions involving
quantitative yield (Table 2, entry 1). When the reaction (S)-N-dodecanoyl-4-benzyloxazolidinone (1i) and etha-
was performed in the same conditions with benzyl alco- nol or benzyl alcohol, 20 mol% SmI2 were employed to
hol, total conversion was not observed but a higher obtain total conversion into esters and high yields of the
amount of catalyst (20 mol% SmI2) allowed to obtain high recovered chiral auxiliary 3c (entries 4–7). The cleavage
yields of benzyl ester 2d and (S)-4-isopropyloxazolidino- of N-acyl bonds of adducts resulting of aza-Michael reac-
Table 2 Transformation of Scalemic N-Acyloxazolidinones into Esters Catalyzed by Samarium Diiodide14
O
O
O
R2 = H 3a
i-Pr 3b
SmI2 (10 mol%)
R1OH, THF, r.t.
RCOOR1
+
R
N
O
HN
R2
O
Bn 3c
2
1
R2
Entry
1
Substrate
SmI2 (mol%) Alcohola
Time (h)
Yield of 2 (%)b
Yield of 3 (%)
3b 99
O
O
H
23C11
N
O
10
EtOH (10)
1
2c 76
1h
1h
2
3
10
20
BnOH (10)
BnOH (10)
72
16
2d 57 (20)
2d 88
3b 35
3b 99
1h
O
O
H
23C11
N
Bn
O
4
10
EtOH (10)
72
2c 70 (30)
3c 61
1i
1i
5
6
7
20
10
20
EtOH (10)
BnOH (10)
BnOH (10)
168
72
2c 99
3c 99
3c 68
3c 99
1i
1i
2d 75 (21)
2d 83
168
O
2-MeOC6H4HN
O
O
2j 84
N
N
O
O
8
9
10
20
10
EtOH (100)
EtOH (100)
MeOH (100)
48
72
72
28% eec
1g
28% ee
O
4-MeOC6H4HN
2k 67
72% eed
1j
72% ee
O
O
2l 98
N
O
10
3c 99
ee>99%e
Bn
Bn
1k
a Ratio Alcohol/substrate.
b Isolated yield% (yield of recovered N-acyloxazolidinone, %).
c Enantiomeric excesses determined by HPLC: 1g [Chiralcel OD-H, hexane–i-PrOH (85:15), 0.7 mL min–1, l = 254 nm, tR (minor) = 30.83
min, tR (major) = 33.73 min] and 2j [Whelk-O1, hexane–i-PrOH (75:25), 0.8 mL min–1, l = 254 nm, tR (major) = 5.20 min, tR (minor) = 6.32
min].
d Enantiomeric excesses determined by HPLC: 1j [Chiralcel OD-H, hexane–i-PrOH (75:25), 0.7 mL min–1, tR (minor) = 41.03 min and tR
(major) = 65.17 min] and 2k [Chiralcel IA, hexane–i-PrOH (75:25), 0.7 mL min–1, l = 254 nm, tR (minor) = 7.18 min and tR (major) = 8.05
min].
e Enantiomeric excess determined by HPLC: 2l [Chiralcel OD-H, hexane–i-PrOH (99.5:0.5), 0.5 mL min–1, l = 254 nm, tR (major) = 16.72 min
and tR (minor) = 19.73 min].
Synlett 2008, No. 8, 1211–1215 © Thieme Stuttgart · New York

1214
C. Magnier-Bouvier et al.
LETTER
(3) For endocyclic cleavage of N-acyloxazolidinones, see:
tions has been described above and we wanted to study
this reaction with scalemic products. We found indeed
that for the N-acyloxazolidinone 1g resulting from
enantioselective aza-Michael reactions with an amino
group in b-position of the carbonyl,13 samarium diiodide
catalyzes the reactions with ethanol leading to the corre-
sponding ester without racemization (entry 8). Similarly,
enantioenriched N-acyloxazolidinones 1j and 1k with a
chiral center in a-position of the carbonyl were trans-
formed into methyl esters with high yields and without ra-
cemization (entries 9 and 10). In the case of substrate 1k
the chiral auxiliary could be recovered quantitatively.
(a) Evans, D. A.; Britton, T. C.; Ellman, J. A. Tetrahedron
Lett. 1987, 28, 6141. (b) Davies, S. G.; Hermann, G. J.;
Sweet, M. J.; Smith, A. D. Chem. Commun. 2004, 1128.
(c) Kanomata, N.; Maruyama, S.; Tomono, K.; Anada, S.
Tetrahedron Lett. 2003, 44, 3599.
(4) For exocyclic reductive cleavage of N-acyloxazolidinones,
see: (a) Evans, D. A.; Ennis, M. D.; Mathre, D. R. J. Am.
Chem. Soc. 1982, 104, 1737. (b) Barnett, C. J.; Wilson, T.
M.; Evans, D. A.; Somers, T. C. Tetrahedron Lett. 1997, 38,
735. (c) Prashad, M.; Har, D.; Kim, H.-Y.; Repic, O.
Tetrahedron Lett. 1998, 39, 7067.
(5) (a) Evans, D. A.; Morrissey, M. M.; Dorow, R. L. J. Am.
Chem. Soc. 1985, 107, 4346. (b) Evans, D. A.; Britton, T.
C.; Dorow, R. L.; Dellaria, J. F. J. Am. Chem. Soc. 1986,
108, 6395. (c) Hintermann, T.; Seebach, D. Helv. Chim.
Acta 1998, 81, 2093.
Samarium diiodide is an efficient catalyst for exo cleavage
reactions of the N-carbonyl bond of N-acyloxazolidinone
by alcohols. Esters are formed in good yields and the ox-
azolidinone can be recovered quantitatively. The depro-
tection of oxazolidinone moiety employed either as a
chiral auxiliary or as a template in enantioselective cata-
lyzed reactions is realized without racemization. Worth to
remark is that the removal of an oxazolidinone auxiliary
in a complex molecule with several functionalities is often
a difficult task. This leads to various strategies such as the
replacement of oxazolidinone by more sophisticated
auxiliairies16 such as for the synthesis of (+)-brefeldin.16b
Since the transformation of N-acyl oxazolidinones into es-
ters catalyzed by samarium diiodide is chemoselective in
the presence of acid-sensitive groups and reaction condi-
tions are mild and simple (room temperature, use of com-
mercially available solution of samarium diiodide) this
procedure can be a useful tool in synthesis as an alterna-
tive method to those previously described.
(6) Tomioka, K.; Muraoka, A.; Kanai, M. J. Org. Chem. 1995,
60, 6188.
(7) Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. J. Org. Chem.
1996, 61, 346.
(8) Evans, D. A.; Coleman, P. J.; Dias, L. C. Angew. Chem., Int.
Ed. Engl. 1997, 36, 2738.
(9) Fukusawa, S.; Hongo, Y. Tetrahedron Lett. 1998, 39, 3521.
(10) Orita, A.; Nagano, Y.; Hirano, J.; Otera, J. Synlett 2001, 637.
(11) (a) Collin, J.; Giuseppone, N.; Van de Weghe, P. Coord.
Chem. Rev. 1998, 178-180, 117. (b) Giuseppone, N.;
Van de Weghe, P.; Mellah, M.; Collin, J. Tetrahedron 1998,
54, 13129. (c) Van de Weghe, P.; Collin, J. Tetrahedron
Lett. 1994, 35, 2545. (d) Giuseppone, N.; Collin, J.
Tetrahedron 2001, 57, 8989. (e) Jaber, N.; Assié, M.; Fiaud,
J.-C.; Collin, J. Tetrahedron 2004, 60, 3075.
(12) Reboule, I.; Gil, R.; Collin, J. Tetrahedron Lett. 2005, 46,
7761.
(13) (a) Reboule, I.; Gil, R.; Collin, J. Tetrahedron: Asymmetry
2005, 16, 3881. (b) Reboule, I.; Gil, R.; Collin, J. Eur. J.
Org. Chem. 2008, 532.
(14) In a typical experiment to THF (5 mL) was added a solution
of SmI2 (0.1 N) in THF (0.1 mmol, 1 mL) and EtOH (10
mmol, 600 mL) followed by a solution of N-dodecanoyl-
oxazolidinone (1b, 1 mmol, 269 mg) in THF (2 mL). The
reaction mixture turned from blue to light yellow within a
few minutes. The reaction was monitored by TLC, which
indicated total conversion of the starting product after 2 h.
The reaction was hydrolyzed with 1 N HCl, extracted by
CH2Cl2, and dried over MgSO4. The crude product was
purified by column chromatography on silica gel to afford
ethyl dodecanoyl ester [2c, heptane–EtOAc (90:10), 223 mg,
98% yield] and oxazolidinone 3a [CH2Cl2–MeOH (90:10),
58 mg, 67%]. For reactions in the presence of acid-sensitive
groups, workup was performed with H2O.
Acknowledgment
The Centre National de la Recherche Scientifique (CNRS) is ack-
nowledged for financial support and the Ministère de l’Enseigne-
ment Supérieur et de la Recherche (MESR) for a PhD grant (I.R.).
References and Notes
(1) For reviews on chiral oxazolidinones as auxiliaries, see:
(a) Ager, D. J.; Prakash, I.; Schaad, D. R. Chem. Rev. 1996,
96, 835. (b) Ager, D. J.; Prakash, I.; Schaad, D. R.
Aldrichimica Acta 1997, 30, 3. (c) Evans, D. A.
Aldrichimica Acta 1982, 15, 23. (d) Sibi, M. Aldrichimica
Acta 1999, 32, 93.
Diethyl 2-(4-Methoxyphenylamino)succinate (2i)
1H NMR (250 MHz, CDCl3): d = 6.81 (d, J = 8.8 Hz, 2 H),
6.69 (d, J = 8.8 Hz, 2 H), 4.35–4.45 (m, 1 H), 4.14–4.38 (m,
4 H), 3.77 (s, 3 H), 2.85 (d, J = 6.3 Hz, 2 H), 1.25–1.31 (m,
6 H) ppm. 13C NMR (62.9 MHz, CDCl3): d = 172.6, 170.6,
153.1, 140.4, 115.7, 114.8, 61.5, 60.9, 55.6, 55.0, 53.4, 37.6,
14.1 ppm. ESI-HRMS: m/z calcd for [C15H21NO5 + Na]+:
318.1312; found: 318.1323.
Ethyl 3-(2-Methoxyphenylamino)butanoate (2j)
1H NMR (400 MHz, CDCl3): d = 6.85 (t, J = 7.6 Hz, 1 H),
6.75 (d, J = 8.1 Hz, 1 H), 6.27–6.67 (m, 2 H), 4.32 (br s, 1
H), 4.12 (q, J = 7.3 Hz, 2 H), 3.90–3.95 (m, 1 H), 3.81 (s, 3
H), 2.67 (dd, J1 = 5.1 Hz, J2 = 14.9 Hz, 1 H), 2.36 (dd, 1 H,
J1 = 7.6 Hz, J2 = 14.9 Hz, 1 H), 1.28 (d, J = 6.3 Hz, 3 H),
1.23 (t, J = 7.3 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3):
d = 171.8, 146.9, 136.6, 121.3, 116.5, 110.4, 109.6, 60.4,
(2) For applications of N-acyloxazolidinone in Diels–Alder or
Michael reactions, see: (a) Narasaka, K.; Inoue, M.; Okada,
N. Chem. Lett. 1986, 1109. (b) Corey, E. J.; Imai, N.;
Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (c) Evans,
D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988,
110, 1238. (d) Evans, D. A.; Miller, S. J.; Leckta, T.;
von Matt, P. J. Am. Chem. Soc. 1999, 121, 7559. (e) Evans,
D. A.; Willis, M. C.; Johnston, J. N. Org. Lett. 1999, 1, 865.
(f) Sibi, M. P.; Manyem, S.; Palencia, H. J. Am. Chem. Soc.
2006, 128, 13660. (g) Li, K.; Phua, P. H.; Hii, K. K.
Tetrahedron 2005, 61, 6237. (h) Hamashima, Y.; Somei,
H.; Shimura, Y.; Tamura, T.; Sodeoka, M. Org. Lett. 2004,
6, 1861. (i) Zhuang, W.; Hazell, R. G.; Jørgensen, K. A.
Chem. Commun. 2001, 1240.
Synlett 2008, No. 8, 1211–1215 © Thieme Stuttgart · New York

LETTER
Samarium-Catalyst Conversion of N-Acyloxazolidinones into Esters
1215
55.4, 45.5, 41.3, 20.7, 14.2 ppm. ESI-HRMS: m/z calcd for
[C13H19NO3 + Na]+: 260.1257; found: 260.1254.
ppm. ESI-HRMS: m/z calcd for [C13H19NO3 + Na]+:
260.1257; found: 260.1252.
Ethyl 3-(4-Methoxyphenylamino)-2-methylpropanoate
(2k)
(15) Lindsay, K. B.; Ferrando, F.; Christensen, K. L.; Overgaard,
J.; Roca, T.; Bennasar, M.-L.; Skrydstrup, T. J. Org. Chem.
2007, 72, 4181; and references cited therein.
(16) (a) Alexander (née Gillon), K.; Cook, S.; Gibson, C. L.;
Kennedy, A. R. J. Chem. Soc., Perkin Trans. 1 2001, 1538.
(b) Wu, Y.; Shen, X.; Yang, Y.-Q.; Hu, Q.; Huang, J.-H.
J. Org. Chem. 2004, 69, 3857.
1H NMR (250 MHz, CDCl3): d = 6.76 (d, J = 8.8 Hz, 2 H),
6.55 (d, J = 8.8 Hz, 2 H), 4.10 (q, J = 6.9 Hz, 2 H), 3.72 (s,
3 H), 3.35 (dd, J1 = 8.3 Hz, J2 = 12.8 Hz, 1 H), 3.14 (dd,
J1 = 5.3 Hz, J2 = 12.8 Hz, 1 H), 2.72–2.80 (m, 1 H), 1.18–
1.30 (m, 6 H) ppm. 13C NMR (62.9 MHz, CDCl3): d = 175.4,
152.3, 141.9, 114.9, 114.4, 60.5, 55.8, 48.1, 39.3, 15.0, 14.2
Synlett 2008, No. 8, 1211–1215 © Thieme Stuttgart · New York

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