Synthesis of 1-substituted 5-aminoimidazole-4-carbaldehydes and 8-amino-1-(2-fluorobenzyl)imidazo[4′,5′:5,6]pyrido[2,3-d] pyrimidine

Article abstract of DOI:10.1055/s-2001-18705

The syntheses of a number of 1-substituted 5-aminoimidazole-4-carbaldehydes 3, by the reduction of the corresponding 5-amino-4-cyanoimidazole derivatives is reported using LiAlH(OEt)₃, prepared in situ from LiAlH₄ and ethyl acetate. The compounds of type 1 are useful intermediates for the synthesis of elongated adenine derivatives.

Full text of DOI:10.1055/s-2001-18705

PAPER
2393
Synthesis of 1-Substituted 5-Aminoimidazole-4-carbaldehydes and 8-Amino-
1-(2-fluorobenzyl)imidazo[4’,5’:5,6]pyrido[2,3-d]pyrimidine
S
ynthesis of 1-Su
r
bstituted
i
5-Am
a
inoimidazo
n
le-4-carbaldehydesL. Booth,* Robert A. Carpenter, Gaby Morlock, Zahid Mahmood, Robin B Pritchard
Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.
Fax +44(161)2004484; E-mail: brian.booth@umist.ac.uk
Received 27 July 2001; revised 25 September 2001
reducing agent. They report that reduction is favoured in
Abstract: The syntheses of a number of 1-substituted 5-aminoimi-
dazole-4-carbaldehydes 3, by the reduction of the corresponding 5-
amino-4-cyanoimidazole derivatives is reported using Li-
AlH(OEt)3, prepared in situ from LiAlH4 and ethyl acetate. The
diethyl ether, rather than THF or diglyme, for aldehyde
formation and they imply that the strong co-ordination of
the Li+ ion to the nitrogen atom of the initially formed imi-
compounds of type 1 are useful intermediates for the synthesis of ne intermediate protects it from further reduction to the
elongated adenine derivatives.
aminomethyl substituent.
Key words: heterocycles, imidazoles, cyanides, aldehydes, imida-
zo[4’,5’:5,6]pyrido[2,3-d]pyrimidines
As compounds 2a–f are completely insoluble in diethyl
ether, our reactions had to be carried out in THF by adding
2 to a pre-prepared solution of LiAl(OEt)3 (from LiAlH4
and EtOAc) in anhydrous THF. In all cases it was found
necessary to use a large excess (10 equiv) of the reducing
Although a variety of conditions have been reported 1–3 for
the reduction of heterocyclic nitriles to aldehydes, to our
knowledge, the only reported synthesis of 5-aminoimida-
zole-4-carbaldehydes 3 (R1 = R2 = Me; R1 = Me, R2 = Ph)
is by the catalytic hydrogenation of the corresponding 4-
aminoimidazole-5-carbonitriles using 10% Pd/C in dilute
sulfuric acid.4 These compounds have been shown to be
useful reagents for the synthesis of imidazo[4,5-b]pyri-
dine derivatives.4
agent to maximise the yield of the carbaldehydes 3a–f
(Table 1). Under these conditions reaction was complete
in around 10 minutes at 0 °C and moderate to high yields
of the carbaldehydes were isolated after a simple dry flash
chromatography (Scheme). The products were all fully
1
characterised by H, 13C NMR spectroscopy and mass
spectrometry (see Tables 2– 4) and a single crystal X-ray
structure was carried out on compound 3f (Figure).
Within our research group5 we have developed a versatile
method for the synthesis of 5-amino-4-cyanoimidazoles 2
from diaminomaleodinitrile via (Z)-N-(2-amino-1,2-dicy-
anovinyl)formamidines 1 and it was of some interest to
explore the reduction of the 5-cyano substituent in 2 to an
aminomethylene group. Apart from an isolated report by
Butala6 of the parent compound 4 (R1 = H), there appears
to be no general synthetic method available for this trans-
formation, perhaps because the amines are expected to be
unstable. Starting from 2a a large number of reducing
agents were tried under various conditions, but all gave
tars and complex mixtures of products. However, the use
of lithium aluminium hydride in anhydrous THF gave a
very low yield (<1%) of a product identified by NMR
spectroscopy as the carboxaldehyde 3a. The yield of this
product improved considerably when small amounts of
water were added to the THF solvent, but it was difficult
to control the exothermic reaction and the major product
was a black tar. These preliminary results did suggest that
LiAlH(OEt)3 might be a more suitable reducing agent.
The pioneering work of Brown and Garg7 showed that
this, and related reagents, are capable of reducing cyano
groups to either an aldehyde (via the imine) or an ami-
nomethyl group, depending upon the concentration of the
Table 1 Compounds 3a–f Prepared
Imidazole
(mmol)
LiAlH4/ EtOAc THF Temp (°C), Prod- Yield
(mmol)
(mL) Time (min) uct
(%)
88
88
76
67
61
70
2a (7.15)
2b (9.75)
2c (1.37)
2d (1.87)
2e (8.43)
2f (7.15)
71.5:53.6
71.5:53.6
71.5:53.6
71.5:53.6
84.3:63.3
23.7:9.66
45
45
45
45
45
25
0–r.t., 15
0–r.t., 15
0–r.t., 15
0–r.t., 15
0–r.t., 30
0–r.t., 30
3a
3b
3c
3d
3e
3f
Synthesis 2001, No. 16, 30 11 2001. Article Identifier:
1437-210X,E;2001,0,16,2393,2396,ftx,en;E15001SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881
Figure X-ray crystal structure of 5-amino-1-benzyloxyimidazole-
4-carbaldehyde (3f)

2394
B. L. Booth et al.
PAPER
Table 3 1H NMR Data for Compounds 1e, 2e, 3a–f, 5–7
R2
NHR1
R1
N
NH2
CN
1H NMR (300 MHz, DMSO-d6)
, J (Hz)
i
Prod-
uct
NC
NC
N
R2
2
N
4.56 (d, 2 H, 3J6,NH = 5.0, H-6), 6.15 (s, 2 H, NH2), 7.18 (m,
2 H, ArH), 7.32 (qd, 1 H, J = 13.3, 7.5, 2, ArH), 7.44 (td, 1
H, J = 8, 2, ArH), 7.73 (d, 1 H, 3J5,NH = 4, H-5), 8.17 (m, 1
H, NH)
NH2
1e
1
ii
R1
N
R1
NH2
2e
3a
5.16 (s, 2 H, H-7), 6.31 (s, 2 H, NH2), 7.05 (td, 1 H, J = 5, 1
Hz, ArH), 7.20 (m, 2 H, ArH), 7.37 (m, 2 H, overlapping
bands, ArH and H-2)
NH2
N
R2
R2
N
N
CH2NH2
CHO
3.83 (s, 3 H, OCH3), 3.87 (s, 3 H, OCH3), 4.90 (s, 2 H, CH2),
5.32 (br s, 2 H, NH2), 6.64 (d, 1 H, J = 2, ArH), 6.70 (dd, 1
H, J = 2, 8, ArH), 6.74 (d, 1 H, J = 8 Hz, ArH), 7.06 (s, 1 H,
H-2), 9.70 (s, 1 H, CHO)
3a R1 = 3,4-(MeO)2C6H3CH2, R2=H
3b R1 = 3,4-(MeO)2C6H3, R2= H
3c R1 = 4-ClC6H5, R2= H
3d R1 = 4-MeOC6H5, R2= H
3e R1 = 2-FC6H4CH2, R2= H
3f R1 = C6H5CH2O, R2= H
4
3b
3.89 (s, 3 H, OCH3), 3.92 (s, 3 H, OCH3), 5.57 (br s, 2 H,
NH2), 6.83 (d, 1 H, J = 2, ArH), 6.91 (dd, 1 H, J = 2, 8, ArH),
6.98 (d, 1 H, J = 8, ArH), 7.11 (s, 1 H, H-2), 9.72 (s, 1 H,
CHO)
iii
3c
5.56 (br s, 2 H, NH2), 7.12 (s, 1 H, H-2), 7.34 (d, 2 H, J =
8.6, ArH), 7.55 (d, 2 H, J = 8.6, ArH), 9.75 (s, 1 H, CHO)
R1
H
OEt
N
NH2
CN
N
3d
3.86 (s, 1 H, OCH3), 5.52 (br s, 2 H, NH2), 7.03 (d, 2 H, J =
9.1, ArH), 7.31 (s, 1 H, H-2), 7.36 (d, 2 H, J = 9, ArH), 9.75
(s, 1 H, CHO)
R1
iv
N
N
N
N
5 R1 = 2-FC6H4CH2
N
CN
6 R1 = 2-FC6H4CH2
3e
3fa
5
5.18 (s, 2 H, H-7), 6.85 (br s, 2 H, NH2), 7.05 (td, 1 H, J =
8, 1.5, H-10), 7.26 (m, 4 H, ArH + H-2), 9.47 (s, 1 H, CHO)
v
R1
5.05 (s, 2 H, CH2), 5.20 (br s, 2 H, NH2), 7.42 (s, 1 H, H-2),
7.25–7.45(m, 5 H, C6H5), 9.55 (s, 1 H, CHO)
N
N
N
N
N
5.37 (s, 2 H, H-11), 6.76 (s, 2 H, H-7), 7.22 (m, 1 H, ArH),
8.25 (d, 2 H, overlapping bands for H-2 and H-5)
NH2
7 R1 = 2-FC6H4CH2
6a
1.45 (t, 3 H, J = 7.1, H-10), 4.55 (q, 2 H, J = 7.1, H-9), 5.44
(s, 2 H, H-11), 7.24 (m, 4 H, ArH), 8.13 (s, 1 H, H-3), 8.28
(s, 1 H, H-1) 8.32 (s, 1 H, H-8)
Scheme Reagents and conditions: (i) 1 M aq KOH, r.t.; (ii) LiAlH4/
EtOAc in THF (anhyd); (iii) CH2(CN)2, NaOMe in MeOH, reflux, 2.5
h; (iv) HC(OEt)3, Ac2O, reflux, 24 h; (v) NH3, MeOH, –78 °C, 1 h
7
5.60 (s, 2 H, H-9), 7.20 (m, 4 H, ArH), 8.04 (br s, 2 H, H-5)
8.46 (s, 1 H, H-3), 8.77 (s, 1 H, H-1), 9.07 (s, 1 H, H-6)
a Measured in CDCl3.
Table 2 Analytical and Mass Spectroscopic Data for Compounds
1e, 2e, 3a–f, and 5–7
Perandones and Soto4 have previously demonstrated the
versatility of compounds of type 3 for the synthesis of im-
idazo[4,5-b]pyridine derivatives by condensation with a
variety of carbonitriles, ketones and polyfunctional carbo-
nyl compounds. We were particularly interested in the
synthesis of elongated purine derivatives related to known
anti-epileptic agents.8 6-Amino-9-benzylpurines have
been reported8 to be potent anti-epileptic agents, as well as
showing anti-anginal and anti-inflammatory activity.
From structure-activity relationship studies it has been es-
tablished that both the 9-benzyl group (preferably 2-fluo-
robenzyl) and a basic group (preferably methylamino) in
the 6-position are essential for activity. We are interested
in exploring the dimensional restrictions of the binding
sites of such drugs by extending the spatial separation
of these two groups. For this reason we were interested
in the synthesis of 8-amino-1-(2-fluorobenzyl)imidazo-
[4’,5’:5,6]pyrido[2,3-d]pyrimidine (7). Such pyrido[2,3-
d]pyrimidines have been described previously by Rams-
Product
1e
Mp (°C)
145
MS (FAB) m/z (%)
244 [(M + 1)+, 100]
217 (M + 1)+, 81]
262 [(M + 1)+, 100]
248 [(M + 1)+, 100]
222 [(M + 1)+, 100]
218 [(M + 1)+, 100]
220 [(M + 1)+, 100]
2e
172 (dec.)
171 (dec.)
98 (dec.)
170 (dec.)
>200 (dec.)
165 (dec.)
110–112
3a
3b
3c
3d
3e
3f
218.0926 [(M + 1)+, 100];
reqd. 218.0928
5
6
7
214 (dec.)
122 (dec.)
150 (dec.)
268 [(M + 1)+, 60]
324 [(M + 1)+, 74]
295 [(M + 1)+, 100]
Synthesis 2001, No. 16, 2393–2396 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Synthesis of 1-Substituted 5-Aminoimidazole-4-carbaldehydes
2395
den et al.9 via 5-amino-2,3-dimethyl[4,5-b]pyridine-6- Compounds 2a–d,5 and 5-amino-4-cyano-1-benzyloxyimidazole
(2f),11 were prepared by procedures previously reported in the liter-
carbonitrile, obtained in three steps from 1,2-dimethyl-5-
ature. 1H and 13C NMR spectra were recorded on a Brucker AC-300
nitroimidazole. More recently, Harris and Pendergast10
instrument and mass spectra on a Kratos Concept 1-S instrument. IR
spectra were recorded on a Perkin-Elmer 1710 FT-IR instrument.
have also prepared
a number of 8-aminoimida-
zo[4’,5’:5,6]pyrido[2,3-d]pyrimidines from 5-aminoimi-
dazo[4,5-b]pyridine-6-carbonitrile, obtained in high yield
from purine by condensation with malononitrile. In our
work, reaction of 3e with malonodinitrile gave the 5-ami-
noimidazo[4,5-b]pyrimidine-6-carbonitrile (5) in 75%
yield and this was converted by standard methods via 6
into 7, which was fully characterised by spectroscopic
methods (Table 2 4).
(Z) –N1-(2-Fluorobenzyl)-N2-(amino-1,2-dicyanovinyl)forma-
midine (1e); Typical Procedure
2-Fluorobenzylamine (3.81 g, 30.48 mmol) was added dropwise to
a
suspension of pure ethyl (Z)-N-(2-amino-1,2-dicyanovi-
nyl)formimidate (5.0 g, 30.48 mmol) in anhyd EtOH (20 mL) con-
taining a catalytic amount of anilinium hydrochloride (0.02 g).
After approximately 1 h, a pale orange solid precipitated and stir-
ring was continued for a further 3 h when TLC confirmed that all
the starting material had disappeared. The precipitate was filtered,
washed with Et2O and dried under vacuum to give 6.18 g (83%) of
1e.
In conclusion, we have developed a new procedure for the
synthesis of previously unreported 1-substituted 4-ami-
noimidazole-5-carbaldehydes and have shown that these
can be used to prepare 5-aminoimidazo[4,5-b]pyrimidine-
6-carbonitriles and extended adenine analogues. Evalua-
tion of 7 and related compounds as anti-epileptic drugs is
in progress.
IR (KBr): 3480s (N–H), 3360s (N–H), 2240m (C N), 2200m
(C N), 1640s cm–1 (C=N).
5-Amino-N1-(2-fluorobenzyl)-4-cyanoimidazole (2e); Typical
Procedure
A suspension of 1e (4.0g, 16.46 mmol) in 1 M aq KOH solution (96
mL) was stirred at r.t. overnight. The white precipitate formed was
filtered, washed with H2O (5 mL) and then with Et2O (5 mL) and
dried under vacuum to give 2h as a white solid (3.08g, 87%).
IR (KBr): 3440s (N–H), 3200s (N–H), 2220m (C N), 1660s cm–1
(C=N).
Table 4 13C NMR Data for Compounds 1e, 2e, 3a–f, 5–7
Prod-
uct
13C NMR (300 MHz, DMSO-d6) , J (Hz)
1e
164.5 (d, 1JC-F = 242.9, Ar), 154.5 (s, C-5), 134.4 (d, 3JC,F
=
7.8, Ar), 133.3 (d, 3JC,F = 4.1, Ar), 129.4 (d, 4JC,F = 3.2, Ar),
128.3 (d, 2JC,F = 14.1, Ar), 121.5 (s, C-3), 120.3 (s, C-4),
119.1 (d, 2JC,F = 23.2, Ar), 119.1 (s, C-2), 110.1 (s, C-1),
42.1 (d, 3JC,F = 3.8, C-6)
1-Substituted 5-Aminoimidazole-4-carbaldehydes 3a–f;
General Procedure
A suspension of LiAlH4 in anhyd THF was cooled to 0 °C. Anhyd
EtOAc was added to the suspension over a period of 15 min. The
mixture was then allowed to warm to r.t. and was stirred at this tem-
perature for 1 h. Then the appropriate 5-amino-1-substituted-4-cy-
anoimidazole 2a–f was added to the suspension over 20 min, and
the mixture was stirred for a further 30 min before carefully quench-
ing with H2O. The mixture was extracted with CHCl3 and the CHCl3
layer was filtered through Celite, dried (MgSO4), filtered and the
solvent was evaporated to yield the title compounds, which were
further purified by dry flash chromatography (CHCl3–EtOH, 1:1) as
eluent (Tables 1– 4).
2e
164.0 (d, 1JC-F =243.8, Ar), 152.1 (s, C-5), 136.6 (s, C-2),
134.1 (d, 3JC,F = 7.9, Ar), 132.6 (d, 3JC,F = 4.0, Ar), 128.9 (d,
4JC,F = 3.3, Ar), 127.4 (d, 2JC,F = 14.6, Ar), 121.6 (s, C-6),
119.5 (d, 2JC,F = 20.3, Ar), 94.5 (s, C-4), 44.9 (d, 3JC,F = 4.3,
C-7)
3a
3b
47.2 (C-7), 56.1 (2 OCH3), 110.3 (C-13), 111.7 (C-10),
119.8 (C-9), 123.3 (C-4), 126.1 (C-8), 132.9 (C-2), 146.1
(C-5), 149.8, 149.5 (C-11,12), 185.4 (C-6)
56.1, 56.0 (2 OCH3), 108.2 (C-12), 111.6 (C-9), 117.6 (C-
8), 122.1 (C-4), 125.6 (C-7), 132.2 (C-2), 145.3 (C-5),
149.7, 149.9 (C-10.11), 185.2 (C-6)
Crystal Structure of 3f
Crystal data and refinement details for the compound 3f are depos-
ited with the Cambridge Crystallographic Database (CCDC
171415). The crystal was mounted on a glass fibre. All measure-
ments were made on a Siemens R3m/v diffractometer with graph-
ite-monochromated Mo-K X-radiation. The data were collected at
a temperature of 23 1 °C using the /2 scanning technique to a
maximum of 2 values of 50.0o. The structures were solved by di-
rect methods using MITHRIL12 and refined using DIRDIF.13 The
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were refined isotropically or were included in the structure factor
calculation in ideal positions, and were assigned isotropic thermal
parameters which were 20% greater then the equivalent B value of
the atom to which they were bonded.
3c
132.2 (C-2), 144.6 (C-5), 154.5 (Ar) 185.6 (C-6)
3d
55.6 (OCH3), 115.3 (Ar), 122.1 (C-4), 125.5 (C-7), 126.5
(Ar), 132.1 (C-2), 145.4 (C-5), 160.2 (Ar), 185.2 (C-6)
3e
44.2 (d, 3JC,F = 4.3, C-7), 119.7 (d, 2JC,F = 20.1, C-12), 125.6
(s, C-4), 127.6 (d, 2JC,F = 14.6, C-8), 128.9 ((d, 4JC,F = 3.2,
C-10), 132.9 ((d, 3JC,F = 3.8, C-9), 134.1(d, 3JC,F = 8.0, C-
11), 137.3 (s, C-2 confirmed by DEPT), 149.1 (s, C-5 con-
firmed by DEPT), 164.0 (d, JC,F = 243.9, C-13), 186.9 (s,
C-6)
3fa
6a
81.2 (C-7), 101.8 (C-4), 128.2, 128.4, 129.1, 129.8 (Ar),
130.1 (C-2), 141.2 (C-5), 185.6 (C-6)
5-Amino-N1-(2-fluorobenzyl)-3H-imidazo[4,5-b]pyridine-6-
carbonitrile (5)
A mixture of 3e (0.48 g, 2.19 mmol) and malononitrile (0.14g, 2.19
mmol) in NaOMe solution (40 mg sodium in 30 mL of MeOH) was
refluxed. After approximately 1 h a white solid precipitated and re-
fluxing was continued until TLC confirmed that all the aldehyde
had been consumed. The white solid was collected by filtration,
18.1 (s, C-10), 42.7 (d, 3JC,F = 4.0, C-11), 67.8 (s, C-9),
101.8 (s, C-3), 102.0 (s, C-4), 119.6 (d, 2JC,F = 20.7, C-16),
121.4 (s, C-5), 127.4 (d, 2JC,F = 14.6, C-12), 128.9 ((d, 4JC,F
= 3.2, C-1-4), 134.6 (m, overlapping bands for C-13 and C-
15), 135.9 (s, C-2), 151.4 (s, C-1), 151.9 (s, C-7), 160.3 (s,
C-6), 164.3 (d, JC,F = 246.0, C-17), 164.9 (s, C-8)
a Measured in CDCl3.
Synthesis 2001, No. 16, 2393–2396 ISSN 0039-7881 © Thieme Stuttgart · New York

2396
B. L. Booth et al.
PAPER
washed with H2O followed by EtOH and dried under vacuum to
(2) (a) Reiman, E. Liebigs Ann. Chem. 1978, 1963.
give 5 (0.44g, 75%).
(b) Weistock, L. T.; O’Brien, D. E.; Cheng, C. C. J. Med.
Chem. 1968, 11, 1238. (c) Taurins, A.; Khnouw, V. T. Can.
J. Chem. 1973, 51, 1741.
Ethyl N-[6-Cyano-3-(2-fluorobenzyl)-3H-imidazo[4,5-b]pyri-
dine-5-yl]formimidate (6)
(3) (a) Bredereck, H.; Simchen, G.; Traut, H. Chem. Ber. 1967,
100, 366. (b) Tieckelmann, H.; Guthrie, R.; Nairn, J. G. J.
Org. Chem. 1960, 25, 1257. (c) Arpad, G.; Odon, F.;
Oszkár, F. Magy. Kem. Foly. 1955, 61, 112.
(4) Perandones, F.; Soto, J. L. J. Heterocycl. Chem. 1997, 34,
107.
(5) Alves, M. J.; Booth, B. L.; Al-Duaij, O. Kh.; Eastwood, P.;
Nezhat, L.; Proença, M. F. J. R. P.; Ramos, A. S. J. Chem.
Res. (M) 1993, 2701.
(6) Butula, I. Liebigs Ann. Chem. 1969, 729, 73.
(7) Brown, H. C.; Garg, C. P. J. Am. Chem. Soc. 1964, 86, 1085.
(8) (a) Kelley, J. L.; Krochmal, M. P.; Linn, J. A.; McLean, E.
W.; Soroko, F. E. J. Med. Chem. 1988, 31, 606. (b) Kelley,
J. L.; Krochmal, M. P.; Linn, J. A.; McLean, E. W.; Soroko,
F. E. J. Med. Chem. 1988, 31, 1005.
(9) Al-Shaar, A. H. M.; Chambers, R. K.; Gilmour, D. W.;
Lythgoe, D. J.; McClenaghan, I.; Ramsden, C. A. J. Chem.
Soc., Perkin Trans. 1 1992, 2789.
(10) Harris, P. A.; Pendergast, W. J. Heterocycl. Chem. 1996, 33,
319.
(11) Booth, B. L.; Costa, F. A. T.; Mahmood, Z.; Pritchard, R. G.;
Proença, M. F. J. R. P. J. Chem. Soc., Perkin Trans. 1 1999,
1853.
A mixture of 5 (0.2 g, 0.75 mmol), triethyl orthoformate (1.5 mL,
8.99 mmol, 12 mol equiv) and Ac2O (0.42 mL, 4.49 mmol, 6 mol
equiv) was refluxed for 24 h at 90 °C. After this period TLC con-
firmed that all the carbonitrile starting material had been consumed.
The brown solution was cooled and light petroleum (bp 30–40 °C)
was added dropwise to give a red precipitate. The mixture was
cooled to 0 °C and was held at this temperature for several hours,
before filtering off the solid, and washing it with light petroleum to
give 6 (0.19 g, 79%). This was used in the next stage without further
purification and no elemental analysis was obtained.
3-(2-Fluorobenzyl)-3H-imidazo[4’,5’:5,6]pyrido[2,3-d]pyri-
midin-8-amine (7)
Compound 6 (0.185 g, 0.57 mmol) was dissolved in MeOH (100
mL) and the solution was then cooled to –76 °C before bubbling
ammonia gas through the stirred solution for 1 h. After this period
TLC confirmed that all the starting material had been consumed.
The MeOH was removed by evaporation under reduced pressure to
give the product (0.071 g, 42%) as a white solid, which was purified
by washing with anhyd Et2O (5 mL) and dried under vacuum. There
was insufficient sample for elemental analysis.
(12) Gilmore, C. J. J. Appl. Crystallogr. 1984, 17, 42.
(13) Beurskens, P. T. DIRDIF: Direct Methods for Difference
Structures – An Automatic Procedure for Phase Extension
and Refinement of Difference Structure Factors, Technical
report 1984/1, Crystallography Laboratory: Toernooiveld,
6525 Ed, Nijmegen, 1984.
References
(1) (a) Stephen, H. J. Chem. Soc. 1925, 1874. (b) Stephen, H. J.
Chem. Soc. 1930, 2786.
Synthesis 2001, No. 16, 2393–2396 ISSN 0039-7881 © Thieme Stuttgart · New York