Sulfamic acid as an effective catalyst in solvent-free synthesis of β-enaminoketone derivatives and X-ray crystallography of their representatives

Article abstract of DOI:10.1080/00397910701873250

Two types of β-enaminoketone derivatives of 3-(2-oxo-2-arylethylidene) -3,4-dihydro-1H-quinoxalin-2-ones and 3-(2-oxo-2-arylethylidene)-3,4-dihydro- benzo[1,4]oxazin-2- ones were effectively and conveniently prepared in good to excellent yields under solv

Full text of DOI:10.1080/00397910701873250

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Sulfamic Acid as an Effective Catalyst in
Solvent‐Free Synthesis of β‐Enaminoketone
Derivatives and X‐ray Crystallography of Their
Representatives
Min Xia a , Bin Wu a & Guo‐Feng Xiang a
a Department of Chemistry, Zhejiang Sci‐Tech University, Hangzhou, China
Available online: 17 Apr 2008
To cite this article: Min Xia, Bin Wu & Guo‐Feng Xiang (2008): Sulfamic Acid as an Effective Catalyst in
Solvent‐Free Synthesis of β‐Enaminoketone Derivatives and X‐ray Crystallography of Their Representatives,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
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Synthetic Communicationsw, 38: 1268–1278, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701873250
Sulfamic Acid as an Effective Catalyst
in Solvent-Free Synthesis
of b-Enaminoketone Derivatives and X-ray
Crystallography of Their Representatives
Min Xia, Bin Wu, and Guo-Feng Xiang
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou,
China
Abstract: Two types of b-enaminoketone derivatives of 3-(2-oxo-2-arylethylidene)-
3,4-dihydro-1H-quinoxalin-2-ones and 3-(2-oxo-2-arylethylidene)-3,4-dihydro-ben-
zo[1,4]oxazin-2- ones were effectively and conveniently prepared in good to
excellent yields under solvent-free conditions via the catalysis of sulfamic acid in
the corresponding condensations of o-phenylenediamine and o-aminophenol with
ethyl 2,4-dioxo-4-arylbutyrate respectively. The compounds were confirmed by IR,
1H NMR, and 13C NMR, and a representative was further determined by X-ray
crystallography.
Keywords: Catalysis, b-enaminoketone, solvent free, sulfamic acid, X-ray
crystallography
It is well known that b-enolic ketones are O,O-double dentates with powerful
capacity to form of various complexes with many metal or nonmetal species.
However, their isosteres. b-enaminoketones, are not as familiar as the
N,O-double dentates: the complicated and difficult preparations as well as
their chemical instability can be attributed to a few reports[1–4] in the literature
on their utilizations as ligands. To overcome those drawbacks, b-enaminoke-
tones are designed and arranged into some heterocycles.
Received in Japan October 12, 2007
Address correspondence to Min Xia, Department of Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, China. E-mail: xiamin@zstu.edu.cn
1268

b-Enaminoketone Derivatives
1269
Figure 1. Structures of compound 2 and 3.
3-(2-Oxo-2-arylethylidene)-3,4-dihydro-1H-quinoxalin-2-ones (2) and
3-(2-oxo-2-arylethylidene)-3,4-dihydro-benzo[1,4]oxazin-2-ones (3) are two
types of heterocycles in which b-enaminoketone structures can be skillfully
inserted (Fig. 1). To best of our knowledge, there has been no report in the lit-
erature on their applications as N,O-double dentates in formation of any
complexes.
In the literature, several methods have described the preparations of 2 and
3, such as 5-aryl-2,3-furandione reacting with a nucleophile and subsequent
treatment of o-phenylenediamine (o-PDA)/o-aminophenol (o-AP),[5]
arylethanones treated with 2-oxo-1,2,3,4-4H-quinoxaline in the presence of
copper(I) chloride,[6] nucleophilic recyclization of pyrrolo[5,1-c][1,4] benzox-
azine-1,2,4-triones with o-PDA/o-AP,[7] a-keto-b-substituted-g-butyrolac-
tones treated with o-PDA/o-AP,[8] 4-arylsulfonyl-2-hydroxy-3-aryl-2-
cyclobutan-1-ones treated with bisnucleophiles such as o-PDA/o-AP,[9]
thermolysis of 3-bromo-4-aryl-2,4-dioxobutanones with diphenyldiaza-
methane and o-PDA/o-AP,[10] recyclization of 3-aroyl-4-methoxycarbonyl-
3H-1,5-benzodiazepine-2-carboxylates in acidic media,[11] and regioselective
synthesis involving 1,3,4,6-tetracarbonyl compounds with o-PDA/o-AP.[12]
However, these methodologies are associated with several drawbacks such
as complicated routes, prolonged reaction times, harsh conditions, low
product yields, occurrence of some side reactions, very expensive and not
easily available substrates, heavy environmental pollution, and so on.
In the past years, owning to its prominent physical and chemical proper-
ties such as outstanding stability, nontoxicity, nonvolatility, odorlessness,
ready availability, and extremely inexpensive price, sulfamic acid (SA) has
increasingly attracted attention as an alternative acidic catalyst in many het-
erogeneously catalytic reactions.[13–20] Moreover, solid-state reactions have
become an intriguing strategy in organic synthesis because of their general
merits such as improved yields, shortened time, mild conditions, and
avoidance of organic solvents.[21–26] In connection with our research on util-
ization of SA and development of solvent-free reactions,[27–29] herein we
report that 3-(2-oxo-2-arylethylidene)-3,4-dihydro-1H-quinoxalin-2-ones (2)
and 3-(2-oxo-2-arylethylidene)-3,4-dihydro-benzo[1,4]oxazin-2-ones (3) can
be efficiently and readily generated via the catalysis of SA in the solid-state
condensation of aroylpyruvates with o-PDA/o-AP.

1270
M. Xia, B. Wu, and G.-F. Xiang
Scheme 1.
First, ethyl benzoylpyruvate was selected as the case to investigate the
catalysis of SA (Scheme 1), and the results are indicated in Table 1. In
methanol, a yellow solid was precipitated and isolated in 38% yield when
ethyl benzoylpyruvate and o-phenylenediamine were mixed and stirred at
room temperature for 30 min. Instead, when the two substrates were mixed
and ground at room temperature, they turned syrupy. The melted mixture
was kept for 5 min and gradually solidified. Through washing with
methanol, 2a was collected in 61% yields. The yield of 3-(2-oxo-2-pheny-
lethylidene)-3,4-dihydro-1H-quinoxalin-2-one 2a could be improved dramati-
cally to 82% when 10 mol% SA was added to the mixture under solvent-free
conditions. It seemed that 10 mol% SA was sufficient to drive this solid-state
condensation, because of more than that amount of catalyst could not signifi-
cantly increase the yield of 2a. In the case of the condensation of ethyl ben-
zoylpyruvate with o-aminophenol at room temperature, the solution-phase
reaction afforded just product 3a in 31% yield even if the cyclization was
carried out in a much prolonged time. Because of the weak nucleophility of
o-aminophenol, the solid-state reaction involved with it was difficult to
execute at room temperature, and 3a was produced in only 18% yield. It
was apparent that increased reaction temperature would be beneficial to
Table 1. Reaction conditions in the generation of compounds 2a and 3a
Entry Compound
Conditions
Temp (8C) Time (min) Yield (%)
1
2
2a
2a
2a
2a
2a
3a
3a
3a
3a
3a
3a
MeOH
No solvent
rt
rt
30
5
38
61
73
81
83
31
18
47
62
73
75
3
No solvent/SA (5 mol%)
No solvent/SA (10 mol%)
No solvent/SA (15 mol%)
MeOH
rt
5
4
rt
5
5
rt
5
6
rt
60
8
7
No solvent
No solvent
rt
8
70
70
70
70
8
9
No solvent/SA (5 mol%)
No solvent/SA (10 mol%)
No solvent/SA (15 mol%)
8
10
11
8
8

b-Enaminoketone Derivatives
1271
Scheme 2.
such solvent-free condensation, because the yield of 3a was remarkably
improved to 47% when the reaction took place at 70 8C. In the presence of
10 mol% SA, the production of 3a could be further improved to 73% yield.
Clearly, the combination of the maximum substrate concentration in the
solid-state reactions with the positive catalyst effect resulted in better yields
and shorter reaction times than those of the same reactions in solution phase.
According to these facts, the scope of such protocol was tested
(Scheme 2), and the results are shown in Table 2. Obviously, SA played
the role of an effective catalyst in the solvent-free generations of
3-(2-oxo-2-arylethylidene)-3,4-dihydro-1H-quinoxalin-2-ones (2) and 3-(2-
oxo-2-arylethylidene)-3,4-dihydro-benzo[1,4]oxazin-2-ones (3), because all
the involved transformations could be carried out within 5–20 min.
Besides, ethyl aroylpyruvates were also efficient candidates for the cycliza-
tions with o-phenylenediamine or o-aminophenol in good to excellent
yields. Apparently, an outstanding electronic effect existed in production of
compounds 2 and 3, in which aroylpyruvates containing electron-withdrawing
group like NO2 on aromatic rings afforded only the corresponding products
of 2d and 3d in 61% and 50% yields respectively, even if their reactions
consumed more time, whereas those aroylpyruvates having electron-
Table 2. SA-promoted solid-state synthesis of b-enaminoketone derivatives
Entry
Compound
X
R
Temp (8C) Time (min)
Yield (%)
1
2
2a
2b
2c
2d
2e
2f
NH2
NH2
NH2
NH2
NH2
NH2
OH
H
rt
rt
5
5
81
86
77
61
81
88
73
76
69
50
4-CH3O
4-Cl
3
rt
10
15
5
4
4-NO2
4-NH2
4-OH
H
rt
5
rt
6
rt
5
7
3a
3b
3c
3d
70
70
70
70
8
8
OH
OH
4-CH3O
4-Cl
4-NO2
8
9
10
15
20
OH

1272
M. Xia, B. Wu, and G.-F. Xiang
Figure 2. Enaminoketone-enolimine tautomeric equilibrium of compounds 2 and 3.
donating groups such as methoxyl on their aromatic rings could execute the
condensations with improved yields. Because of the weaker nucleophility of
hydroxyl than of amino, the yields of products 3 were universally poorer
than those of products 2, but the reaction time was prolonged and it was
necessary to raise the reaction temperature for the generation of compounds 3.
For products 2 and 3, there are two types of theoretical tautomers (i.e., eno-
limines and enaminoketones, Fig. 2). In spite of their potential both as
N,O-double-dentate ligands, the different structures have distinctively dissimi-
lar behaviors in generation of complexes. Therefore, it is necessary to determine
which tautomeric type these important compounds dominantly adopted in their
solid and solution states through some more convincing evidence.
Although IR spectra can afford some information about a molecule’s
structure in its solid state, they are not sufficient and forceful enough, even
unidentifiable in many cases. X-ray crystallography can supply more reliable
information about the actual structure for a given molecule. Therefore,
compound 3b was screened as an example for X-ray crystallographic measure-
ment. By examining the bond lengths in 3b (Fig. 3), it was found that C10-O3
˚
˚
(1.25 A) and C8-C9 (1.36 A) distances were slightly more stretched than the
standard[30] C55O bond (1.20 A) and C55C bond (1.34 A), whereas the
˚
˚
Figure 3. The molecular structure of compound 3b.

b-Enaminoketone Derivatives
1273
˚
˚
lengths of C8-N1 (1.35 A) and C9-C10 (1.43 A) were remarkably shorter than
˚
those of standard[30] C-N (1.47 A) and C-C (1.54 A) bonds, respectively; they
˚
˚
˚
were closer to heteroaromatic C55N (1.34 A) and aromatic C55C (1.40 A)
distances. This indicated that there was obvious delocalization of electrons on
the bonds, enolimine structure existed in the tautomeric equilibrium to some
degree, but the molecular structure should be dominantly presented as an enam-
inoketone form in its solid state.
In solution states, it seemed that the 13C NMR spectrum was more favorable
than 1H NMR as an effective tool to distinguish unsaturated carbonyl from enolic
carbon atoms in such an enolimine- enaminoketone equilibrium. It could be
clearly deduced by the 13C NMR spectra of these samples that carbonyls
instead of enolic double bonds existed in their molecular structures, because
the chemical shifts in the lowest field were located at 185–191 ppm, which cor-
responded to the approximately standard unsaturated ketones. For enolic carbon
atoms attached to hydroxyl, their signals should appear at 150–160 ppm. This
phenomenon could account for the fact that the amount of enaminoketone-type
structures in solution state was the overwhelming majority in above tautomeric
equilibrium. Therefore, such equilibrium between enolimine and enaminoketone
was largely shifted to the side of the latter in both solution and solid state.
Herein we describe an effective and convenient approach to 3-(2-oxo-2-
arylethylidene)-3,4-dihydroquinoxalin-2(1H)-ones and 3-(2-oxo-2-arylethyli-
dene)-3,4-dihydrobenzo[b][1,4]oxazin-2-ones via the solid-state condensations
of aroylpyruvates with o-phenylenediamine or o-aminophenol in the catalysis of
sulfamic acid. The mild conditions and brief reaction time in addition with good
yields and simple operations make our protocol an attractive method for the
rapid and efficient preparation of such compounds. The work concerned with
their applications as the b-enaminoketone ligands in formation of some novel
complexes are in the progress.
EXPERIMENTAL
All the compounds used were analytical reagents, and some chemicals were
further purified by recrystallization or distillation. Melting points were deter-
mined on an X-4 micromelting instrument, and the thermometer was uncor-
1
rected. H NMR and 13C NMR spectra were obtained on Bruker Avance
DMX 400-MHz and 100-MHz instruments respectively using TMS as
internal standard in DMSO-d6. FT-IR spectra were recorded on a Nicolet
Avatar 700 spectrophotometer in KBr pellets. X-ray crystallographic data
was collected on Rigaku Raxis-Rapid.
General Procedure for the Solvent-Free Synthesis of 2 or 3
At room temperature, aroylpyruvate (1.0 mmol), o-phenylenediamine
(1.2 mmol) or o-aminophenol (1.2 mmol), and SA (10% mol) were mixed

1274
M. Xia, B. Wu, and G.-F. Xiang
and ground completely. The mixture turned syrupy and was kept at room
temperature for o-phenylenediamine or at 70 8C for o-aminophenol for the
appropriate time. The solidified lumps were crushed and washed with water,
MeOH, and ether successively two or three times. The obtained solid was
pure enough for further spectroscopic analysis.
Data
Compound 2a: Mp 259–260 8C (lit.[31] 266–267 8C); FT-IR (KBr) n 1685,
1607, 1377, 1262, 1110, 1023, 833, 805, 754 cm21 1H NMR(400 MHz,
;
DMSO-d6) d 6.82 (s, 1H, CH55C), 7.15 (s, 3H, Ar-Hs), 7.52–7.61 (m, 4H,
Ar-H), 7.99 (d, J ¼ 7.6 Hz, 2H, Ar-H), 12.06 (s, 1H, NHCO), 13.65 (s, 1H,
NHC55CH); 13 C NMR (100 MHz, DMSO-d6) d 89.55, 115.74, 115.81, 116.90,
117.05, 124.17, 124.43, 127.47, 129.23, 132.41, 139.12, 145.69, 156.10, 188.95.
Compound 2b: Mp 241–243 8C (lit.[31] 245–246 8C); FT-IR(KBr) n 1683,
1
1610, 1377, 1266, 1035, 840 cm21; H NMR (400 MHz, DMSO-d6) d 3.84
(s, 3H, OCH3), 6.78 (s, 1H, C55CH), 7.05 (d, J ¼ 8.4 Hz, 2H, Ar-Hs),
7.11–7.13 (m, 3H, Ar-Hs), 7.45–7.47 (m 1H, Ar-H), 7.97 (d, J ¼ 8.4 Hz,
2H, Ar-Hs), 12.00 (s, 1H, NHCO), 13.59 (s, 1H, NHC55CH); 13C NMR
(100 MHz, DMSO-d6) d 55.35, 88.83, 113.89, 115.24, 116.18, 123.59,
124.16, 126.34, 129.09, 131.29, 144.53, 144.89, 155.80, 162.26, 187.68.
Compound 2c: Mp 266–267 8C (lit.[31] 270–271 8C); FT-IR (KBr) n 1688,
1
1602, 1378, 1114, 1033, 844, 759 cm21; H NMR (400 MHz, DMSO-d6) d
6.69 (s, 1H, CH55C), 7.15 (s, 3H, Ar-Hs), 7.51–7.53 (m, 1H, Ar-H), 7.58
(d, J ¼ 8.0 Hz, 2H, Ar-Hs), 7.99 (d, J ¼ 8.0 Hz, 2H, Ar-Hs), 12.06 (s, 1H,
NHCO), 13.65 (s, 1H, NHC55CH); 13C NMR (100 MHz, DMSO-d6) d
88.88, 115.28, 115.35, 116.49, 116.63, 123.67, 124.15, 128.77, 128.85,
136.68, 137.27, 145.84, 155.55, 186.92.
Compound 2d: Mp 298–299 8C (lit.[31] 304–305 8C); FT-IR (KBr) n 1689,
1
1611, 1379, 1244, 1026, 845 cm21; H NMR (400 MHz, DMSO-d6) d 6.85
(s, 1H, CH55C), 7.13–7.21 (m, 3H, Ar-Hs), 8.21 (d, J ¼ 8.8 Hz, 2H, Ar-Hs),
8.33 (d, J ¼ 8.8 Hz, 2H, Ar-Hs), 12.15 (s, 1H, NHCO), 13.79 (s, 1H,
NHC55CH); 13C NMR (100 MHz, DMSO-d6) d 89.46, 115.41, 117.12, 123.79,
123.90, 12.69, 127.11, 128.29, 137.44, 143.72, 146.66, 149.08, 155.34, 185.93.
Compound 2e: Mp 301–302 8C; FT-IR (KBr) n 3230, 1686, 1615, 1344,
1234, 1188, 846, 754, 690 cm21 1H NMR (400 MHz, DMSO-d6) 6.79
;
(s, 1H, CH55C), 7.07 (d, J ¼ 8.0 Hz, 2H, Ar-Hs), 7.17–7.23 (m, 4H,
Ar-Hs), 7.90 (d, 2H, J ¼ 8.0 Hz, Ar-Hs), 8.01 (s, br, 2H, NH2), 11.97
(s, 1H, NHCO), 13.51 (s, 1H, NHC55CH); 13C NMR (100 MHz, DMSO-d6)
d 88.74, 113.34, 115.33, 118.71, 121.22, 124.79, 124.91, 126.77, 130.58,
138.63, 148.11, 150.55, 155.82, 187.01.

b-Enaminoketone Derivatives
1275
Compound 2f: Mp 327–328 8C (lit.[32] 323–324 8C); FT-IR (KBr) n 3340,
1
1688, 1608, 1525, 1264, 1182, 833, 750, 699 cm21; H NMR (400 MHz,
DMSO-d6) 6.81 (s, 1H, CH55C), 7.01 (d, J ¼ 8.0 Hz, 2H, Ar-Hs), 7.11–
7.17 (m, 3H, Ar-Hs), 7.43–7.47 (m, m, 1H, Ar-H), 7.93 (d, 2H, J ¼ 8.0 Hz,
Ar-Hs), 8.88 (s, 1H, OH), 12.01 (s, 1H, NHCO), 13.53 (s, 1H, NHC55CH);
13C NMR (100 MHz, DMSO-d6) d 88.94, 113.97, 115.33, 117.73, 121.23,
124.26, 126.47, 129.13, 131.17, 138.66, 146.35, 156.19, 163.07, 188.13.
Compoudn 3a: Mp 200–201 8C (lit.[33] 203–204 8C); FT-IR (KBr) n 1761,
1
1653, 1602, 1562, 1535, 1363, 1049, 753, 692 cm21; H NMR (400 MHz,
DMSO-d6) d 7.06 (s, 1H, CH55C), 7.09–7.13 (m, 2H, Ar-Hs), 7.18–7.22
(m, 2H, Ar-Hs), 7.47–7.51 (m, 2H, Ar-Hs), 7.54–7.61 (m, 2H, Ar-Hs),
8.01 (d, J ¼ 7.6 Hz, 2H, Ar-Hs), 13.07 (s, 1H, NHC55C); 13C NMR
(100 MHz, DMSO-d6) d 94.58, 115.94, 117.12, 123.72, 123.96, 125.87,
127.64, 128.72, 132.67, 138.19, 139.02, 141.21, 156.24, 191.51.
Compound 3b: Mp 219–220 8C (lit.[34] 215–216 8C); FT-IR (KBr) n 1757,
1
1622, 1604, 1595, 1584, 1288, 1256, 1174, 755 cm21; H NMR (400 MHz,
DMSO-d6) d 3.88 (s, 3H, OCH3), 6.96 (d, J ¼ 8.8 Hz, 2H, Ar-Hs), 7.01
(s, 1H, CH55C), 7.03–7.09 (m, 2H, Ar-Hs), 7.16–7.19 (m, 2H, Ar-Hs),
7.99 (d, J ¼ 8.8 Hz, 2H, Ar-Hs), 12.98(s, 1H, NHC55C); 13C NMR
(100 MHz, DMSO-d6) d 55.51, 94.54, 113.93, 115.73, 117.06, 123.59,
123.91, 125.81, 129.87, 131.09, 138.43, 141.04, 156.47, 163.34, 190.33.
Compound 3c: Mp 186–187 8C (lit.[32] 180 8C); FT-IR (KBr) n 1757, 1624,
1591, 1287, 751 cm21 1H NMR (400 MHz, DMSO-d6) d 6.83 (s, 1H,
;
CH55C), 7.13 (d, J ¼ 6.8 Hz, 1H, Ar-H), 7.19 (d, J ¼ 6.8 Hz, 1H, Ar-H),
7.24 (d, J ¼ 7.6 Hz, 1H, Ar-H), 7.57 (m, 3H, Ar-Hs), 8.00 (d, J ¼ 7.6 Hz,
2H, Ar-Hs), 12.78 (s, 1H, NHC55C); 13C NMR (100 MHz, DMSO-d6) d
92.29, 116.36, 116.86, 123.80, 123.83, 125.27, 128.90, 129.10, 136.72,
137.30, 140.23, 141.19, 155.85, 188.06.
Compound 3d: Mp 240–241 8C (lit.[34] 236–237 8C); FT-IR (KBr) n 1750,
1623, 1603, 1565, 1518, 1347, 1272, 759 cm21; 1H NMR (400 MHz, DMSO-
d6) d 6.88 (s, 1H, CH55C), 7.13–7.26 (m, 3H, Ar-Hs), 7.63 (d, J ¼ 8.0 Hz,
1H, Ar-H), 8.21 (d, J ¼ 8.0 Hz, 2H, Ar-Hs), 8.31 (d, J ¼ 8.0 Hz, 2H, Ar-
Hs), 12.88 (s, 1H, NHC55C); 13C NMR (100 MHz, DMSO-d6) d 93.02,
116.90, 117.71, 124.18, 124.47, 124.78, 125.79, 129.07, 141.59, 141.98,
146.66, 149.88, 156.36, 187.63.
X-ray Crystallographic Data Collection, Structure Determination,
and Refinement
A yellow single crystal (0.62 mm ꢀ 0.28 mm ꢀ 0.20 mm) was attached to a
glass fiber and mounted on a Rigaku Raxis-Rapid equipped with a graphite

1276
M. Xia, B. Wu, and G.-F. Xiang
˚
monochromated Mo Ka (l ¼ 0.71073 A) radiation source and a IP detector.
Lp effects and multiscan were applied in data corrections. The structure was
solved by direct methods using SHELXS-97 and refined by full matrix
least-squares on F2 using SHELXL-97. Molecular graphics were produced
using ORTEP-3. Nonhydrogen atoms were refined anisotropically; all
hydrogen atoms were placed in geometrically idealized positions and con-
strained to ride on their parent atoms.
Crystallographic Data for Compound 3a
C17H13NO4, M ¼ 295.28, orthorhombic, a ¼ 11.628(2), b ¼ 7.3761(15),
3
˚
˚
c ¼ 32.679(7) A, U ¼ 2802.8(10) A , T ¼ 293 K, Pbca space group, Z ¼ 8,
GOF ¼ 1.096, 3.058 , u , 25.68, 21263 reflections measured, 2634 unique
[R(int) ¼ 0.0338], which were used in all calculation. The final R1 and wR2
(all data) were 0.0635 and 0.1227 respectively. CCDC 651042 contains the
more supplementary crystallographic data for this article. These data can be
obtained free of charge via www.ccdc.cam.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: þ44 1223 336033; or deposit@ccdc.cam.uk).
ACKNOWLEDGMENT
We are grateful for the financial support from the Nature Science Foundation
of Zhejiang Province (Project No. R405465).
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1. Itoh, K.; Okazaki, K.; Fujimoto, M. The structure of 1,3-enaminoketonatoboron
difluorides in solution and in the solid state. Aust. J. Chem. 2003, 56, 1209.
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