Silver-catalyzed asymmetric amination of silyl enol ethers

Article abstract of DOI:10.1139/v00-023

Catalytic amination of silyl enol ethers with azo diester compounds was investigated. It was shown that Cu(OTf)₂ or AgOTf had high catalytic activity and that AgOTf was the most efficient among the catalysts tested in the reactions. In asymmetr

Full text of DOI:10.1139/v00-023

666
Silver-catalyzed asymmetric amination of
silyl enol ethers
Yasuhiro Yamashita, Haruro Ishitani, and Shu Kobayashi
Abstract: Catalytic amination of silyl enol ethers with azo diester compounds was investigated. It was shown that
Cu(OTf)2 or AgOTf had high catalytic activity and that AgOTf was the most efficient among the catalysts tested in the
reactions. In asymmetric reactions, the AgClO4–BINAP system showed high enantioselectivity. In a mixture of toluene
or mesitylene and THF, silyl enol ethers reacted with dibenzyl azodicarboxylate (DBnAD) smoothly to afford the
corresponding amination adducts in excellent yields with up to 86% ee.
Key words: amination, amino acid, asymmetric synthesis, chiral catalyst, silver.
Résumé : On a étudié l’amination catalytique d’éthers énoliques silylés par des composés azodiesters. Il a été
démontré que le Cu(OTf)2 et le AgOTf possèdent une activité catalytique élevée et que le AgOTf est l’un des
catalyseurs les plus efficaces pour ces réactions. Dans les réactions asymétriques, le système AgClO4·BINAP donne
lieu à une énantiosélectivité élevée. Dans un mélange de toluène ou de mésitylène avec du THF, les éthers énoliques
silylés réagissent facilement avec l’azodicarboxylate de dibenzyle (ADDBn) pour conduire à la formation des adduits
d’amination correspondants, avec d’excellents rendements et un ee pouvant aller jusqu’à 86%.
Mots clés : amination, acide aminé, synthèse asymétrique, catalyseur chiral, argent.
[Traduit par la Rédaction] Yamashita et al. 672
Introduction
amination, including the first finding of efficient achiral cata-
lysts. (Quite recently, Evans and Johnson reported chiral cop-
per-catalyzed enantioselective amination of enolsilanes with
azodicarboxylate derivatives. See ref. (8)).
Amination at the α-position of carbonyls provides one of
the most efficient methods for synthesis of α-amino acids
and their derivatives (for a review see ref. (1)). Several
electrophilic aminations have been reported (1) and among
them, the reactions of silyl enol ethers with azo diesters are
promising because the starting materials are stable and
readily available, and the hydrazine groups produced are
easily converted to amino groups by reductive N—N bond
cleavage (Scheme 1) (2, 3). In the previous reports, however,
thermal conditions (4) or stoichiometric amounts of Lewis
acids (2a, d–f) were required for the reaction of silyl enol
ethers with azo diesters. Quite recently, we have reported
our preliminary results on silver and copper-catalyzed
amination of silyl enol ethers (5). This was the first example
of catalytic amination of silyl enol ethers, and we have since
continued to investigate asymmetric versions of these reac-
tions. As for asymmetric amination, although many diastereo-
selective approaches using lithium enolates or silyl enol
ethers and azo compounds were reported, (2a–d, 6) exam-
ples of catalytic enantioselective reactions have been limited.
In 1997, Evans and Nelson (7) reported enantioselective
amination of N-acyloxazolines with an azodicarboxylate us-
ing a chiral magnesium bis(sulfonamide). In this paper, we
report full details of our results on catalytic asymmetric
Results and discussion
Recently, we have found that metal trifluoromethanesulfo-
nates (triflates) are excellent catalysts in several carbon–car-
bon bond-forming reactions. Based on these results, we first
examined the reaction of the silyl enol ether derived from S-
ethyl propanethioate with dibenzyl azodicarboxylate (DBnAD)
using catalytic amounts of several metal triflates (Table 1)
(for recent reports on reactions using metal triflates as cata-
lysts see ref. (9)). When Zn(OTf)2, Fe(OTf)3, Yb(OTf)3,
Zr(OTf)4, or Hf(OTf)4 was used, little rate enhancement was
observed. On the other hand, while the reaction proceeded
moderately in the presence of Sn(OTf)2 or Sc(OTf)3, the de-
sired products were obtained in high yields when AgOTf or
Cu(OTf)2 was employed as the catalyst.
We then investigated the effect of solvents using AgOTf
as a catalyst (Table 2). While the amination reaction pro-
ceeded cleanly in dichloromethane, lower yields were ob-
tained in toluene, THF, and propionitrile. For azo diester
compounds, not only DBnAD but also diethyl azodicarboxylate
and tert-butyl azodicarboxylate gave the corresponding amination
Received September 17, 1999. Published on the NRC Research Press website on May 19, 2000.
Dedicated to Professor Stephen Hanessian on the occasion of his 65th birthday.
Y. Yamashita, H. Ishitani, and S. Kobayashi.1 Graduate School of Pharmaceutical Sciences, The University of Tokyo, CREST,
Japan Science and Technology Corporation (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
1Author to whom correspondence may be addressed.
Can. J. Chem. 78: 666–672 (2000)
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Yamashita et al.
667
Scheme 1.
adducts in high yields (Table 3). The catalytic activities of
AgOTf and Cu(OTf)2 in the reactions of several silyl enol
ethers with DBnAD were studied, and the results are sum-
marized in Table 4. When the amount of AgOTf or
Cu(OTf)2 was decreased in the reaction with the
trimethylsilyl enol ether derived from S-ethyl propanethio-
ate, the catalytic activity was maintained and the adduct was
obtained in a high yield even when 1 mol% of AgOTf was
used. In the reaction of the ketene silyl acetal derived from
methyl isobutyrate, AgOTf gave a better yield of the desired
adduct.2 Moreover, the tert-butyldimethylsilyl group also
2 Side reactions predominated when the amination was performed without the catalyst.
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Can. J. Chem. Vol. 78, 2000
BINAP system, (10) the silyl enol ether of propiophenone
was treated with DBnAD in THF. The reaction proceeded
smoothly at –45°C to afford the corresponding amination
adduct in 83% yield with 29% ee. Several reaction condi-
tions were then examined to improve the selectivity. The ef-
fect of silver salts is shown in Table 5. When AgClO4 was
used as a catalyst, the ee was improved to 68%. The effects
of temperature, azo compounds, and solvents are summa-
rised in Tables 6, 7, and 8, respectively. The ee obtained at
–45°C was not improved even when the reaction was carried
out at lower temperatures such as –60°C and –78°C. As for
azo compounds, DBnAD gave the best result. We observed
interesting effects of the solvents. When a single solvent of
THF or toluene was used, 68% ee (THF) or 67% ee (tolu-
ene) was obtained. On the other hand, greater than 74% ee
were observed when a mixture of THF and toluene, m-xylene,
or mesitylene was used. The best ee (86%) was obtained
when a 5:1 mixture of mesitylene and THF was employed as
the solvent. It is interesting to note that no chiral induction
was observed when dichloromethane was used as a solvent,
albeit the reaction proceeded cleanly to give the desired
adduct in 90% yield.
Several examples of catalytic asymmetric amination are
shown in Table 9. The silyl enol ethers derived from
α-tetralone and phenyl propyl ketone reacted with DBnAD
in the presence of a catalytic amount of AgClO4–BINAP to
afford the corresponding amination adducts in high yields
with good ee. In the reaction of the silyl enolate of phenyl
propionate, 51% ee of the amination adduct was obtained
using the same system. As for the absolute configuration of
the amination product, the product was converted to the lit-
erature known compound (1) (8)3 according to standard pro-
cedures (Scheme 2).
worked well as a silyl moiety instead of the trimethylsilyl
group. When less reactive silyl enol ethers derived from
ketones were examined, it was found that the reactions also
proceeded smoothly in the presence of a catalytic amount of
AgOTf or Cu(OTf)2. Also in these cases, AgOTf showed
higher activity than Cu(OTf)2.
We then examined asymmetric versions of these amin-
ations. In the presence of a catalytic amount of a AgOTf–
3[α ]3D1 = +8.9 (c 1.69, CHCl3) (63% ee) (lit. (8) [α ]2D4 = –17.9 (c 1.15, CHCl3); [α ]D20 = –15.9 (c 0.38, CHCl3) (4R,5R)) (11).
© 2000 NRC Canada

Yamashita et al.
669
most efficient methods for the preparation of chiral and
achiral α-amino carbonyl compounds.
Experimental
General
1
Melting points were uncorrected. H and 13C NMR spec-
tra were recorded on a JEOL JNM-LA300, JNM-LA400, or
JNM-LA500 spectrometer in (CD3)2SO. (CD3)2SO was used
as internal standard (δ = 2.49 for H NMR and δ = 39.5 for
In summary, amination reactions of silyl enol ethers with
azo diester compounds proceeded smoothly in the presence
of a catalytic amount of AgOTf or Cu(OTf)2. AgOTf gave
better results than Cu(OTf)2 in most cases. Various kinds of
silyl enol ethers including those derived from esters,
thioesters, and ketones worked well. It is noted that all the
examined reactions proceeded cleanly using a small amount
of the catalyst, and that the procedures are very simple. As
for the catalytic asymmetric version of these aminations,
AgClO4–BINAP worked well to afford the corresponding
adducts in excellent yields with moderate to high ee. Inter-
esting solvent effects were observed, and these silver-
catalyzed asymmetric aminations would provide one of the
1
13C NMR). IR spectra were measured with JASCO FT/IR-
610 spectrometers. High-resolution mass spectra (HRMS)
were measured with JEOL JMX-SX-102 mass spectrometer.
High-performance liquid chromatography was carried out
using the following apparatuses: SHIMADZU LC-10AT
(liquid chromatograph), SHIMADZU SPD-10A (UV detec-
tor), and SHIMADZU C-R6A Chromatopac. Column chro-
matography was conducted on Silica gel 60 (Merck) and
preparative thin-layer chromatography was carried out using
Wakogel B-5F. All chemical compounds were purified based
on standard procedures.
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Can. J. Chem. Vol. 78, 2000
Scheme 2.
ter stirring for 30 min at room temperature, the mixture was
cooled to the desired temperatures. In cases of the reactions
in toluene or toluene–THF, THF of the catalyst solution was
removed under reduced pressure (1.0 mmHg, 30 min) first,
and the desired solvent was added. To the cooled catalyst
solution was added DBnAD (0.44 mmol) in the solvent
(0.5 mL). After stirring for 10 min, a silyl enol ether
(0.4 mmol) in the solvent (0.5 mL) was added dropwise, and
the mixture was stirred at the same temperature. An aqueous
20% HF–THF (1:1) solution was then added and the mixture
was stirred for 1h at room temperature. After basified with
saturated NaHCO3, the aqueous layer was extracted with di-
chloromethane. The organic layers were combined and dried
over anhyd Na2SO4. After filtration and concentration, the
residue was purified by preparative thin layer chromatogra-
phy on silica gel to afford the desired adduct. The
enantiomeric excess was determined by HPLC analysis
(DAICEL CHIRALCEL OD or AS).
S-Ethyl 2-(N,N′-dibenzyloxycarbonylhydrazino)propanethioate:
IR (neat) (cm–1): 3304, 1759, 1724, 1687. 1H NMR
((CD3)2SO, 70°C) δ: 1.16 (t, 3H, J = 7.0 Hz), 1.31 (d, 3H,
J = 7.1 Hz), 2.81 (q, 2H, J = 7.0 Hz), 4.75 (m, 1H), 5.10 (s,
2H), 5.12 (s, 2H), 7.2–7.4 (m, 10H), 9.48 (br, 1H). 13C NMR
((CD3)2SO, 70°C) δ: 13.6, 13.9, 22.2, 63.2, 66.0, 67.1,
127.0, 127.2, 127.6, 127.9, 128.0, 135.7, 136.0, 154.8,
156.1, 198.2. MS (m/z): 417 (M++1). HRMS (m/z) calcd. for
C21H25N2O5S (M++1): 417.1484; found: 417.1472.
A general procedure of asymmetric reactions
Achiral reactions
A typical experimental procedure is as follows: To a
stirred suspension of AgOTf (0.040 mmol) in CH2Cl2
(1.0 mL) was added DBnAD (0.44 mmol) in CH2Cl2
(0.5 mL) at the temperature shown in Table 2. After stirring
for 10 min, a silyl enol ether (0.40 mmol) in CH2Cl2
(0.5 mL) was added dropwise, and the mixture was stirred at
the same temperature. A 20% HF(aq)–THF (1:1) solution
was then added and the mixture was stirred for 1 h at room
temperature. After being basified with saturated aq NaHCO3,
the aqueous layer was extracted with CH2Cl2. The organic
layers were combined and dried over anhyd Na2SO4. After
filtration and concentration, the residue was purified by pre-
parative thin layer chromatography on silica gel to afford the
desired amination adduct.
Methyl 2-(N,N′-dibenzyloxycarbonylhydrazino)-2-methylpro-
pionate: mp 104.0°C. IR (KBr) (cm–1): 3317, 1739, 1696.
1H NMR ((CD3)2SO, 70°C) δ: 1.09 (s, 3H), 1.30 (s, 3H),
3.54 (s, 3H), 4.9–5.2 (m, 4H), 7.2–7.4 (m, 10H), 9.59 (br,
1H). 13C NMR ((CD3)2SO, 70°C) δ: 22.8, 23.6, 51.6, 63.3,
66.0, 66.7, 127.0, 127.2, 127.5, 127.6, 128.0, 128.0, 135.9,
136.2, 154.3, 156.2, 173.0. MS (m/z): 401 (M++1). Anal.
calcd for C21H24N2O6: C 62.99, H 6.04, N 7.00; found: C
63.06, H 5.96, N 6.90.
Dibenzyl N-(2-oxo-2-phenylethyl)-N,N′-hydrazinedicarboxylate:
1
mp 103.5°C. IR (KBr) (cm–1): 3285, 1739, 1708, 1679. H
NMR ((CD3)2SO, 70°C) δ: 4.98 (s, 2H), 5.11 (s, 2H), 5.13
(s, 2H), 7.2–7.5 (m, 10H), 7.52 (m, 2H), 7.65 (m, 1H), 7.96
(d, 2H, J = 7.7 Hz), 9.70 (br, 1H). 13C NMR ((CD3)2SO,
70°C) δ: 56.7, 65.9, 66.9, 126.9, 127.2, 127.5, 127.5, 127.6,
Asymmetric reactions
AgClO4 (0.040 mmol, 10 mol%) and (R)-BINAP
(0.048 mmol, 12 mol%) were dissolved in THF (1 mL). Af-
© 2000 NRC Canada

Yamashita et al.
671
Dibenzyl N-(1,2,3,4-tetrahydro-1-oxonaphthalen-2-yl)-N,N′-
hydrazinedicarboxylate: mp 141.0°C. IR (KBr) (cm–1):
3295, 1737, 1704, 1685. H NMR ((CD3)2SO, 70°C) δ: 2.2–
127.9, 128.0, 128.4, 133.2, 134.6, 135.9, 136.1, 155.4,
193.5. MS (m/z): 418 (M++1). Anal. calcd. for C24H22N2O5:
C 68.89, H 5.30, N 6.69; found: C 69.03, H 5.46, N 6.55.
1
2.4 (m, 2H), 2.9–3.5 (m, 2H), 4.92 (br, 1H), 5.09 (s, 2H),
5.13 (s, 2H), 7.2–7.5 (m, 12H), 7.56 (t, 1H, J = 7.5 Hz), 7.89
(d, 1H, J = 7.9 Hz), 9.35 (br, 1H). 13C NMR ((CD3)2SO,
70°C) δ: 26.7, 27.6, 64.7, 65.9, 66.8, 126.3, 126.6, 126.9,
127.2, 127.5, 127.5, 128.0, 128.0, 128.6, 131.3, 133.5,
136.0, 136.2, 143.8, 155.3, 156.3, 192.6. MS (m/z): 444
(M+). Anal. calcd for C26H24N2O5: C 70.26, H 5.44, N 6.30;
found: C 70.52, H 5.57, N 6.13.
Dibenzyl N-(1-methyl-2-oxo-2-phenylethyl)-N,N′-hydrazine-
30
dicarboxylate: [α]D = +27.8 (c 1.29, CHCl3) (86% ee). IR
1
(neat) (cm–1): 3301, 1714. H NMR ((CD3)2SO, 70°C) δ:
1.32 (d, 3H, J = 7.0 Hz), 5.08 (br, 4H), 5.62 (q, 1H, J =
7.0 Hz), 7.2–7.4 (m, 10H), 7.46 (t, 2H, J = 7.3 Hz), 7.60 (t,
1H, J = 7.3 Hz), 7.94 (d, 2H, J = 8.9 Hz), 9.49 (br, 1H). 13C
NMR ((CD3)2SO, 70°C) δ: 13.3, 57.7, 65.9, 67.0, 127.0,
127.2, 127.5, 127.5, 127.9, 128.0, 128.0, 128.2, 132.7,
135.0, 135.7, 136.1, 154.8, 156.1, 196.7. HRMS (m/z) calcd.
for C25H24N2O5 (M+): 432.1685; found: 432.1685.
Dibenzyl N-(1-ethyl-2-oxo-2-phenylethyl)-N,N′-hydrazinedicar-
1
boxylate: mp 119.5°C. H NMR ((CD3)2SO, 70°C) δ: 0.89
(t, 3H, J = 7.3 Hz), 1.79 (dq, 2H, J = 7.3 Hz, 7.3 Hz), 4.8–
5.2 (m, 4H), 5.50 (br, 1H), 7.2–7.7 (m, 13H), 9.35 (br, 1H).
13C NMR ((CD3)2SO, 70°C) δ: 9.9, 20.5, 61.7, 65.8, 67.1,
127.1, 127.4, 127.9, 128.1, 132.7, 135.7, 136.0, 155.1,
155.8, 195.9. IR (KBr) (cm–1): 3275, 1746, 1693, 1671. MS
(m/z): 446 (M+). Anal. calcd for C26H26N2O5: C 69.94, H
5.87, N 6.27; found: C 69.65, H 5.90, N, 6.25.
Dibenzyl N-(2-oxocyclohexyl)-N,N′-hydrazinedicarboxylate:
1
IR (neat) (cm–1): 3303, 1729, 1713. H NMR ((CD3)2SO,
70°C) δ: 1.4–1.6 (m, 1H), 1.7–1.9 (m, 3H), 1.9–2.0 (m, 1H),
2.17 (br, 1H), 2.3–2.4 (m, 1H), 2.4–2.5 (m, 1H), 4.70 (br,
1H), 5.0–5.2 (m, 4H), 7.3–7.5 (m, 10H), 9.14 (br, 1H). 13C
NMR ((CD3)2SO, 70°C) δ: 23.1, 25.7, 29.6, 40.1, 65.8, 65.9,
66.7, 126.9, 127.1, 127.4, 127.5, 127.9, 136.0, 136.2, 155.2,
156.2, 204.7. MS (m/z): 396 (M+). HRMS (m/z) calcd. for
C22H24N2O5 (M+): 396.1685; found: 396.1690.
Phenyl 2-(N,N′-dibenzyloxycarbonylhydrazino)propionate:
1
IR (neat) (cm–1): 3299, 1758, 1716. H NMR ((CD3)2SO,
70°C) δ: 1.48 (d, 3H, J = 7.1 Hz), 5.0 (m, 1H), 5.16 (s, 2H),
5.20 (s, 2H), 7.0–7.5 (m, 15H), 9.68 (br, 1H). 13C NMR
((CD3)2SO, 70°C) δ: 13.6, 56.7, 66.1, 67.1, 121.2, 125.5,
127.1, 127.3, 127.6, 128.0, 128.0, 129.0, 135.8, 136.1,
150.3, 154.9, 156.2, 168.9. MS (m/z): 449 (M++1). HRMS
(m/z) calcd. for C25H25N2O6 (M++1): 449.1713; found:
449.1725.
S-Ethyl 2-(N,N′-diethyloxycarbonylhydrazino)propanethioate:
IR (neat) (cm–1): 3304, 1748, 1724, 1691. 1H NMR
((CD3)2SO, 70°C) δ: 1.1–1.2 (m, 9H), 1.29 (d, 3H, J =
7.1 Hz), 2.82 (q, 2H, J = 7.3 Hz), 4.0–4.2 (m, 4H), 4.69 (br,
1H), 9.14 (br, 1H). 13C NMR ((CD3)2SO, 70°C) δ: 13.6,
13.9, 14.0, 22.2, 60.6, 61.7, 63.1, 154.9, 156.1, 198.5. MS
(m/z): 293 (M++1). HRMS (m/z) calcd. for C11H21N2O5S
(M++1): 293.1171; found: 293.1161.
Acknowledgment
S-Ethyl 2-(N,N′-di-tert-butyloxycarbonylhydrazino)propanethioate:
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,
Sports, and Culture, Japan.
1
IR (neat) (cm–1): 3330, 1714, 1691. H NMR ((CD3)2SO,
70°C) δ: 1.18 (t, 3H, J = 7.3 Hz), 1.27 (d, 3H, J = 7.1 Hz),
1.40 (s, 9H), 1.41 (s, 9H), 2.81 (q, 2H, J = 7.3 Hz), 4.61 (br,
1H), 8.62 (br, 1H). 13C NMR ((CD3)2SO, 70°C) δ: 13.7,
14.1, 22.1, 27.5, 27.6, 62.4, 79.3, 80.5, 153.5, 155.1, 198.9.
MS (m/z): 349 (M++1). HRMS (m/z) calcd. for C15H29N2O5S
(M++1): 349.1797; found: 349.1810.
References
1. (a) O. Mitsunobu. In Comprehensive organic synthesis. Vol. 6.
Edited by B.M. Trost. Pergamon Press, Oxford. 1991. p. 65;
(b) R. Askani. In Comprehensive organic synthesis. Vol. 6.
Edited by B.M. Trost. Pergamon Press, Oxford. 1991. p. 103;
(c) T. Sheradsky. The chemistry of amino, nitroso, and nitro
compounds and their derivertives. Part 1. Edited by S. Patai.
John Wiley & Sons, New York. 1982. p. 395; (d) J.D. Bois,
C.S. Tomooka, J. Hong, and E.M. Carreira. Acc. Chem. Res.
30, (1997); (e) P. Phukan and A. Sudalai. Tetrahedron: Asym-
metry, 9, 1001 (1998); ( f ) S. Minakata, T. Ando, M.
Nishimura, I. Ryu, and M. Komatsu. Angew. Chem. Int. Ed.
Engl. 37, 3392 (1998).
Diethyl N-(1-methyl-2-oxo-2-phenylethyl)-N,N′-hydrazine-
dicarboxylate: IR (neat) (cm–1): 3304, 1734. 1H NMR
((CD3)2SO, 70°C) δ: 1.11 (t, 6H, J = 7.1 Hz), 1.30 (d, 3H, J
= 7.0 Hz), 3.8–4.3 (m, 4H), 5.56 (br, 1H), 7.4–7.8 (m, 3H),
7.96 (d, 2H, J = 8.6 Hz), 9.10 (br, 1H). 13C NMR
((CD3)2SO, 70°C) δ: 13.3, 13.8, 13.8, 57.5, 60.4, 61.6,
128.0, 128.2, 132.6, 135.1, 155.0, 156.0, 196.8. MS (m/z):
308 (M+). HRMS (m/z) calcd. for C15H20N2O5 (M+):
308.1372; found: 308.1360.
2. (a) C. Gennari, L. Colombo, and G. BertoLini. J. Am. Chem.
Soc. 108, 6394 (1986); (b) D.A. Evans, T.C. Britton, R.L.
Dorow, J. F. Dellaria, Jr. Tetrahedron, 44, 5525 (1988);
(c) L.A. Trimble, and J.C. Vederas. J. Am. Chem. Soc. 108,
6397 (1986); (d) W. Oppolzer, and R. Moretti. Tetrahedron 44,
5541 (1988); (e) Y. Yamamoto, S. Hatsuya, and J. Yamada.
Tetrahedron Lett. 30, 3445 (1989); ( f ) Y. Yamamoto, M.
Yumoto, and J. Yamada. Tetrahedron Lett. 32, 3079 (1991).
3. T.L. Gilchrist. In Comprehensive organic synthesis. Vol. 8.
Edited by B.M. Trost. Pergamon Press, Oxford. 1991. p. 381.
Di-tert-butyl N-(1-methyl-2-oxo-2-phenylethyl)-N,N′-hydrazine-
dicarboxylate: IR (neat) (cm–1): 3325, 1690. 1H NMR
((CD3)2SO, 70°C) δ: 1.28 (d, 3H, J = 6.9 Hz), 1.32 (s, 9H),
1.37 (br, 9H), 5.44 (br, 1H), 7.48 (m, 2H), 7.59 (m, 1H),
7.96 (d, 2H, J = 7.3 Hz), 8.65 (br, 1H). 13C NMR
((CD3)2SO, 70°C) δ: 13.3, 27.4, 27.6, 56.8, 79.1, 80.3,
128.0, 128.1, 132.5, 135.2, 153.8, 155.1, 197.1. MS (m/z):
364 (M+). HRMS (m/z) calcd. for C19H28N2O5 (M+):
364.1998; found: 364.1990.
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672
4. (a) R.M. Moriarty, and I. Prakash. Synth. Commun. 15, 649
(1985); (b) P. Beslin, and P. Marion. Tetrahedron Lett. 33, 935
(1992).
5. S. Kobayashi, Y. Yamashita, and H. Ishitani. Chem. Lett. 307
(1999).
6. (a) G. Guanti, L. Banfi, and E. Narisano. Tetrahedron, 44,
5553 (1988); (b) C. Greck, L. Bischoff, F. Ferreira, C. Pinel,
E. Piveteau, and J.P. Genêt. Synlett. 475 (1993); (c) P.C.B.
Page, S.M. Allin, E.W. Collington, and R.A.E. Carr. Tetrahe-
dron Lett. 35, 2427 (1994); (d) A.R. Sting and D. SeeBach.
Tetrahedron, 52, 279 (1996); (e) J.M. Harris, R. McDonald,
and J.C. Vederas. J. Chem. Soc. Perkin Trans. 1, 2669 (1996).
7. D.A. Evans and S.G. Nelson. J. Am. Chem. Soc. 119, 6452
(1997).
Can. J. Chem. Vol. 78, 2000
8. D.A. Evans and D.S. Johnson. Org. Lett. 1, 595 (1999).
9. (a) S. Kobayashi. Eur. J. Org. Chem. 15 (1999); (b) S.
Kobayashi and S. Iwamoto. Tetrahedron Lett. 39, 4697 (1998).
10. (a) A. Yanagisawa, H. Nakashima, A. Ishiba, and H. Yamamoto.
J. Am. Chem. Soc. 118, 4723 (1996); (b) A. Yanagisawa, Y.
Matsumoto, H. Nakashima, K. Asakawa, and H. Yamamoto. J.
Am. Chem. Soc. 119, 9319 (1997); (c) A. Yanagisawa, Y.
Matsumoto, K. Asakawa, and H. Yamamoto. J. Am. Chem.
Soc. 121, 892 (1999); (d) D. Ferraris, B. Young, T. Dudding,
and T. Lectka. J. Am. Chem. Soc. 120, 4548 (1998).
11. V.J. Jephcote, A.J. Pratt, and E.J. Thomas. J. Chem. Soc.
Perkin Trans. 1, 1529 (1989).
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