Synthetic routes to aminotriamantanes, topological analogues of the neuroprotector Memantine

Article abstract of DOI:10.1055/s-0028-1087979

The amino derivatives of diamantane and triamantane, representing close topological analogues of the neuroprotective drug Memantine, were prepared via amination of the respective carboxylic acids or alcohols. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087979

PAPER
909
Synthetic Routes to Aminotriamantanes, Topological Analogues of the
Neuroprotector Memantine“
Synthetic Routes
t
n
o Aminotriam
d
antanes rey A. Fokin,*a,b Anika Merz,b Natalie A. Fokina,b Hartmut Schwertfeger,b Shenggao L. Liu,c Jeremy E. P. Dahl,c
Robert K. M. Carlson,c Peter R. Schreiner*b
a
Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37, 03056 Kiev, Ukraine
Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
MolecularDiamond Technologies, Chevron Technology Ventures, 100 Chevron Way, Richmond, CA 94802, USA
b
c
Fax +49(641)9934309; E-mail: aaf@xtf.ntu-kpi.kiev.ua; E-mail: prs@org.chemie.uni-giessen.de
Received 22 September 2008; revised 1 December 2008
Herein, we report the preparation of hitherto unknown
amino triamantanes that are geometrically and electroni-
cally related to Memantine“ allowing variations in the to-
pological and spatial characteristics of these potential
neurodrugs. The computed geometries of 1, as well as 2-
amino- (2), 3-amino- (3), 4-amino-(4) and 9-amino tri-
amantane (5), show that all these molecules have a com-
mon maximum dimension of around 5.6 Å (Figure 1). For
comparison, we also prepared 1- and 4-amino diamantane
(6 and 7, vide infra) as higher homologues of amantadine
(1-amino adamantane), as well as diamantane diamines (8
and 9).18,19,23
Abstract: The amino derivatives of diamantane and triamantane,
representing close topological analogues of the neuroprotective
drug Memantine“, were prepared via amination of the respective
carboxylic acids or alcohols.
Key words: amines, diols, drugs, hydrocarbons, medicinal chemis-
try
Adamantane amino derivatives play an outstanding role in
the treatment of influenza and a wide spectrum of diseases
of the central nervous system. However, many recent
studies have shown an increase in resistance of common
viruses toward adamantane-containing drugs such as
amantadine and rimantadine.2 These findings have
prompted a number of studies on the preparation of the
amino derivatives of other cage compounds.3,4
Fortunately, Memantine“[1-amino-3,5-dimethyladaman-
tane; 1] is still the most effective drug for combating
Alzheimer’s disease and it was recently considered to be
the ‘new hope’ in the development of N-methyl-D-aspar-
tate-receptor antagonists.5 Memantine“ also shows pro-
nounced antiparkinsonian activity. The key mechanistic
feature in the mechanism of action involves blocking of
the NMDA-receptor channel, where 1 displays a moderate
binding affinity due to its size and hydrophobicity.5 Fur-
ther development of this class of neuroprotectors should
build on these two key features of the Memantine“
molecule.
A number of higher adamantane analogues, i.e., diamon-
doids (nanodiamonds), were recently isolated from crude
oil,6 and triamantane, [121]-, [1(2)3]-, [123]tetraman-
tanes, [1(2,3)4]pentamantane and [12312]hexamantane
(cyclohexamantane), are now available in preparative
quantities.7,8 Recently, we developed a variety of methods
for the selective preparation of functional derivatives of
these hydrocarbons involving selective C–H substitutions
as well as functional group exchanges.1,9–15 The diamon-
doid derivatives have already shown some remarkable
properties such as negative electron affinity and highly ef-
fective self-assembly.16,17
Figure 1 The key spatial dimensions of Memantine“ (1), 2-amino-
(2), 3-amino- (3), 4-amino- (4), and 9-amino triamantane (5) compu-
ted at the B3LYP/6-31+G(d) level of theory
SYNTHESIS 2009, No. 6, pp 0909–0912
Advanced online publication: 02.03.2009
DOI: 10.1055/s-0028-1087979; Art ID: T16508SS
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© Georg Thieme Verlag Stuttgart · New York

910
A. A. Fokin et al.
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For the preparation of the amino derivatives we utilized
two alternative procedures shown in Scheme 1: (a) the re-
action of the respective carboxylic acids with diphe-
nylphosphoryl azide20 followed by hydrolysis (method A)
and (b) acid-catalyzed exchange of the hydroxyl group of
the respective diamondoid alcohol with chloroacetonitrile
followed by cleavage of the chloroacetamide thus
formed21 (method B).
X
Y
X
Y
6, X = NH2, Y = H, 76% (A), 99% (B) 7, X = NH2, Y = H, 76% (A), 80% (B)
9, X = Y = NH2, 76% (B)
8, X = Y = NH2, 88% (B)
14, X = NHC(O)CH2Cl, Y = H, 94%
15, X = Y = NHC(O)CH2Cl, 84%
16, X = NHC(O)CH2Cl, Y = H, 79%
17, X = Y = NHC(O)CH2Cl, 89%
Method A
O
1. HCl
RHNCOt-Bu
(PhO)2P(O)N3
2. NaOH
RCOOH
X
t-BuOH, Et3N
X
RNH2
11, X = NHC(O)CH2Cl, 78%
3, X = NH2, 90% (B)
10, X = NHC(O)CH2Cl, 93%
2, X = NH2, 4% (B)
Method B
O
ClCH2CN
H2O, H+
thiourea
RHNCCH2Cl
ROH
AcOH, H2SO4
X
Scheme 1 The two alternative approaches used for the preparation
X
of diamondoid amino derivatives
12, X = NHC(O)CH2Cl, 72%
4, X = NH2, 80% (B)
13, X = NHC(O)CH2Cl, 91%
5, X = NH2, 96% (B)
As starting compounds for method A we used the diamon-
doid carboxylic acids prepared as described previously
through the carboxylation of the respective diamondoid
derivatives.22 Method B, applied to the readily available
alcohols,12 gave mono- (6 and 7) as well as diamines (8
and 9)23,18 in high preparative yields with complete con-
servation of the substitution pattern (Figure 2). The X-ray
crystal structures of intermediate chloroacetamides 14
and 16 are shown in Figure 3. These routes offer a conve-
nient alternative to the existing synthetic approach to ami-
no diamantanes based on the reaction of the respective
bromo-derivatives with nitrogen trichloride24 or through
the Ritter reaction, which gives hydrolytically stable acet-
amides.25 It is also safer and operationally superior to the
method of Davis and Nissan (starting from the bromides
utilizing a large excess of TMSN3 and SnCl2) because it
does not involve potentially hazardous (and expensive)
chemicals.26
Figure 2 The structures and preparative yields of compounds 2–17
As we described previously,12 all possible tertiary hy-
droxy derivatives form through the nitroxylation/hydroly-
sis of triamantane. Method B gave the 2-, 3-, 4-, and 9-
amino triamantanes (2–5) via the cleavage of the respec-
tive chloroacetamides (11–13) in high preparative yields
(Figure 2). The only exception is the most sterically hin-
dered acetamide 10, which gave amine 2 in very low
yield.
Figure 3 The X-ray crystal structures and crystal packing of chloro-
acetamides 14 and 1627
In summary, we have obtained a number of diamondoid
amines in good yields from their respective acids and al-
cohols. Testing of the neurophysiological activities of
these structural analogues of Memantine“ is currently
underway.
HP5890 GC instrument with an H5971A mass-selective detector;
HP GC–MS capillary column 50 m × 0.2 mm, Ultra 1, silicone, 80–
205 °C. All compounds showed adequate IR and DEPT 13C NMR
spectra.
Method A; General Procedure
NMR: Bruker Avance II spectrometers operating at 400.130 and
200.13 MHz (1H NMR) and 100.613 and 50.32 MHz (13C NMR)
and an Avance II spectrometer at 600.130 MHz (1H NMR) and
150.903 MHz (13C NMR); internal standard = Me4Si. GC–MS:
A mixture of the chosen diamondoid carboxylic acid (1 mmol),
Et3N (0.14 mL, 1 mmol) and diphenylphosphoryl azide (275 mg, 1
mmol; Acros) and t-BuOH (2 mL) was stirred for 12 h, quenched
with aq NaHCO3, extracted with CHCl3, and dried over Na2SO4.
Synthesis 2009, No. 6, 909–912 © Thieme Stuttgart · New York

PAPER
Synthetic Routes to Aminotriamantanes
911
The residue after evaporation was dissolved in MeOH and stored in
a flow of anhydrous HCl. Evaporation of MeOH, washing of the re-
sulting hydrochloride with Et2O, neutralization with 10% NaOH,
extraction with Et2O, and drying over Na2SO4, gave the pure amines
6 and 7, whose spectral data were identical to those of standard sam-
ples.22,24
MS (EI): m/z (%) = 255 (100), 240, 239, 197, 183, 144, 106, 91.
HRMS: m/z calcd for C18H25N: 255.1987; found: 255.1989.
N-Chloroacetyltriamantane-2-amine (10)
Colorless solid; mp 164–166 °C.
1H NMR (400 MHz, CDCl3): d = 6.67 (br s, 1 H), 4.00 (s, 2 H),
2.67–2.61 (m, 2 H), 1.91–1.54 (m, 17 H), 1.49–1.42 (m, 2 H), 1.15–
1.08 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 163.9 (C=O), 60.9 (C–N), 49.2
(CH), 43.4 (CH2), 39.5 (CH2), 39.7 (CH2), 38.3 (CH2), 38.1 (CH),
37.6 (CH), 36.7 (CH), 35.6 (C), 32.7 (CH2), 27.0 (CH).
Method B; General Procedure
A solution of the respective diamondoid alcohol (1 mmol) in a mix-
ture of AcOH (1.7 mL, 30 mmol), ClCH2CN (0.4 mL, 6.3 mmol)
and concd H2SO4 (0.5 mL, 9.4 mmol) was stirred at r.t. for 20 h. The
reaction mixture was poured onto ice and extracted with CHCl3.
Evaporation of the extracts gave the chloroacetamide, which was
dissolved in a mixture of EtOH (0.8 mL, 13.7 mmol), thiourea (20
mg, 0.26 mmol) and AcOH (0.3 mL, 5.2 mmol) and heated at 95 °C
for 20 h. The mixture was neutralized with 10% NaOH, extracted
with CHCl3, and dried over Na2SO4. Evaporation gave amines 2–5,
which were purified through their hydrochlorides as described in
Method A.
MS (EI): m/z (%) = 331, 239, 238 (100), 197, 167, 129, 128, 91.
HRMS: m/z calcd for C20H26ClNO: 331.1703; found: 331.1668.
N-Chloroacetyltriamantane-3-amine (11)
Colorless solid; mp 146–147 °C.
1H NMR (400 MHz, CDCl3): d = 6.31 (br s, 1 H), 3.94 (s, 2 H),
2.16–2.11 (m, 1 H), 2.08–2.04 (m, 2 H), 2.03–1.97 (m, 1 H), 1.94–
1.80 (m, 5 H), 1.77–1.56 (m, 8 H), 1.47–1.40 (m, 2 H), 1.38–1.28
(m, 3 H), 1.25–1.19 (m, 1 H).
13C NMR (100 MHz, CDCl3): d = 164.3 (C=O), 57. 2 (C–N), 47.9
(CH), 46.1 (CH), 44.6 (CH2), 44.4 (CH2), 43.1 (CH2), 41.1 (CH2),
38.8 (CH), 37.6 (CH2), 37.5 (CH2), 37.4 (CH), 37.0 (CH2), 36.8
(CH), 34.9 (C), 34.2 (CH), 33.2 (CH2), 32.4 (CH), 29.1 (CH), 27.4
(CH).
2-Aminotriamantane (2)
Colorless solid; mp 236–239 °C.
1H NMR (400 MHz, CD3OD): d = 2.15–2.10 (m, 2 H), 1.90–1.87
(m, 2 H), 1.86–1.81 (m, 2 H), 1.81–1.71 (m, 7 H), 1.70–1.64 (m,
4 H), 1.56–1.51 (m, 4 H), 1.08–1.04 (m, 2 H).
13C NMR (100 MHz, CD3OD): d = 56.0 (C), 50.5 (CH), 44.2 (CH),
40.1 (CH2), 39.9 (CH), 39.8 (CH), 39.6 (CH2), 39.3 (CH2), 38.2 (C),
33.5 (CH2), 28.9 (CH).
MS (EI): m/z (%) = 331, 295, 282, 239, 238 (100), 167, 129, 128,
91.
MS (EI): m/z (%) = 255, 239, 238 (100), 197, 167, 129, 128.
HRMS: m/z calcd for C18H25N: 255.1987; found: 255.1968.
HRMS: m/z calcd for C20H26ClNO: 331.1703; found: 331.1697.
N-Chloroacetyltriamantane-4-amine (12)
Colorless solid; mp 147–149 °C.
3-Aminotriamantane (3)
Colorless solid; mp 206–210 °C.
1H NMR (400 MHz, CDCl3): d = 6.36 (br s, 1 H), 3.96 (s, 2 H),
2.19–2.13 (m, 2 H), 2.03–1.98 (m, 2 H), 1.96–1.88 (m, 2 H), 1.86–
1.79 (m, 3 H), 1.78–1.74 (m, 2 H), 1.74–1.60 (m, 6 H), 1.54–1.45
(m, 2 H), 1.33–1.25 (m, 4 H).
13C NMR (100 MHz, CDCl3): d = 164.4 (C=O), 47.7 (CH), 45.2
(CH2), 44.6 (CH2), 43.1 (CH2), 41.4 (CH2), 39.4 (CH), 37.5 (CH),
37.3 (C), 37.2 (CH), 37.0 (CH2), 33.2 (C), 32.8 (CH2), 27.4 (CH),
26.8 (CH).
1H NMR (400 MHz, CDCl3): d = 2.06–1.93 (m, 3 H), 1.85–1.78 (m,
1 H), 1.77–1.70 (m, 2 H), 1.68–1.50 (m, 9 H), 1.47–1.30 (m, 6 H),
1.28–1.15 (m, 4 H).
13C NMR (100 MHz, CDCl3): d = 52.8 (CH), 51.1 (C), 46.7 (CH2),
46.1 (CH), 45.1 (CH2), 44.7 (CH2), 41.5 (CH), 39.1 (CH), 37.9
(CH2), 37.7 (CH2), 37.6 (CH2), 37.5 (CH), 34.9 (C), 34.8 (CH), 33.2
(CH2), 32.1 (CH), 29.7 (CH), 27.6 (CH).
MS (EI): m/z (%) = 255, 239, 238 (100), 167, 128.
MS (EI): m/z (%) = 331, 282, 239, 238 (100), 167, 142, 129, 85, 84,
83.
HRMS: m/z calcd for C18H25N: 255.1987; found: 255.1992.
HRMS: m/z calcd for C20H26ClNO: 331.1703; found: 331.1720.
4-Aminotriamantane (4)
Colorless solid; mp 207–209 °C.
N-Chloroacetyltriamantane-9-amine (13)
Colorless solid; mp 128–131 °C.
1H NMR (400 MHz, CDCl3): d = 2.09–2.01 (m, 2 H), 1.85–1.75 (m,
3 H), 1.72–1.65 (m, 6 H), 1.60–1.55 (m, 2 H), 1.51–1.37 (m, 8 H),
1.31–1.26 (m, 2 H), 1.25–1.21 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 49.2 (C), 48.4 (CH), 47.1 (CH2),
45.5 (CH2), 44.8 (CH2), 44.1 (CH), 39.3 (CH), 37.9 (CH), 37.5
(CH2), 32.9 (C), 32.7 (CH2), 27.6 (CH), 27.3 (CH).
1H NMR (400 MHz, CDCl3): d = 6.20 (br s, 1 H), 3.90 (s, 2 H),
2.00–1.91 (m, 4 H), 1.86–1.79 (m, 3 H), 1.74–1.56 (m, 12 H), 1.48–
1.43 (m, 2 H), 1.33–1.29 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 164.7 (C), 52.4 (C), 48.4 (CH2),
45.6 (CH), 44.7 (CH2), 42.9 (CH2), 41.6 (CH2), 39.1 (CH), 37.9
(CH), 37.6 (CH2), 37.4 (CH2), 34.8 (CH), 34.7 (C), 34.0 (CH), 37.1
(CH).
MS (EI): m/z (%) = 255, 238 (100), 181, 167, 142, 130, 129, 91.
HRMS: m/z calcd for C18H25N: 255.1987; found: 55.1933.
MS (EI): m/z (%) = 331, 296, 282, 256, 239, 238 (100), 167, 142,
91.
9-Aminotriamantane (5)
Colorless solid; mp 193–194 °C.
HRMS: m/z calcd for C20H26ClNO: 331.1703; found: 331.1684.
1H NMR (400 MHz, CDCl3): d = 1.83–1.79 (m, 1 H), 1.78–1.74 (m,
2 H), 1.73–1.59 (m, 9 H), 1.58–1.43 (m, 7 H), 1.36–1.31 (m, 2 H),
1.29–1.25 (m, 2 H), 1.14–1.09 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 53.7 (CH2), 47.3 (C), 46.3 (CH2),
44.7 (CH), 44.0 (CH2), 38.6 (CH), 37.8 (CH2), 37.7 (CH2), 37.5
(CH), 34.9 (CH), 34.8 (C), 34.0 (CH), 27.3 (CH).
N-Chloroacetyldiamantane-4-amine (14)
Colorless solid; mp 166 °C.
1H NMR (400 MHz, CDCl3): d = 6.24 (br s, 1 H), 3.95 (s, 2 H),
2.04–1.99 (m, 6 H), 1.93 (br s, 3 H), 1.85–1.74 (m, 10 H).
Synthesis 2009, No. 6, 909–912 © Thieme Stuttgart · New York

912
A. A. Fokin et al.
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13C NMR (100 MHz, CDCl3): d = 164.8 (C=O), 51.3 (C), 43.0
(CH2), 41.8 (CH2), 38.5 (CH), 37.3 (CH2), 36.4 (CH), 25.5 (CH).
(5) Sonkusare, S. K.; Kaul, C. L.; Romarao, P. Pharmacol. Res.
2005, 51, 1.
(6) Dahl, J. E.; Moldowan, J. M.; Peters, E. M.; Claypool, G. E.;
Rooney, M. A.; Michael, G. E.; Mello, M. R.; Kohen, M. L.
Nature 1999, 399, 54.
HRMS: m/z calcd for C16H22ClNO: 279.1390; found: 279.1391.
Anal. Calcd for C16H22ClNO: C, 68.68; H, 7.93; N, 5.01. Found: C,
68.48; H, 7.85; N, 4.79.
(7) Dahl, J. E.; Liu, S. G.; Carlson, R. M. K. Science 2003, 299,
96.
Bis(N-chloroacetyl)diamantane-4,9-diamine (15)
(8) Dahl, J. E. P.; Moldowan, J. M.; Peakman, T. M.; Clardy, J.
C.; Lobkovsky, E.; Olmstead, M. M.; May, P. W.; Davis, T.
J.; Steeds, J. W.; Peters, K. E.; Pepper, A.; Ekuan, A.;
Carlson, R. M. K. Angew. Chem., Int. Ed. Engl. 2003, 42,
2040.
(9) Fokin, A. A.; Butova, E. D.; Chernish, L. V.; Fokina, N. A.;
Dahl, J. E. P.; Carlson, R. M. K.; Schreiner, P. R. Org. Lett.
2007, 9, 2541.
Colorless solid; mp 292–293 °C (dec.).
1H NMR (400 MHz, DMSO-d6): d = 7.69 (s, 2 H), 3.95 (s, 4 H),
1.91 (s, 12 H), 1.82 (s, 6 H).
13C NMR (100 MHz, DMSO-d6): d = 165.0 (C=O), 49.7 (C), 43.4
(CH2), 40.3 (CH2), 36.9 (CH).
HRMS: m/z calcd for C18H24Cl2N2O2: 370.1215; found: 370.1190.
(10) Fokin, A. A.; Schreiner, P. R.; Fokina, N. A.; Tkachenko, B.
A.; Hausmann, H.; Serafin, M.; Dahl, J. E. P.; Liu, S.;
Carlson, R. M. K. J. Org. Chem. 2006, 71, 8532.
(11) Fokin, A. A.; Tkachenko, B. A.; Gunchenko, P. A.; Gusev,
D. V.; Schreiner, P. R. Chem. Eur. J. 2005, 11, 7091.
(12) Fokina, N. A.; Tkachenko, B. A.; Merz, A.; Serafin, M.;
Dahl, J. E. P.; Carlson, R. M. K.; Fokin, A. A.; Schreiner, P.
R. Eur. J. Org. Chem. 2007, 4738.
(13) Schreiner, P. R.; Fokina, N. A.; Tkachenko, B. A.;
Hausmann, H.; Serafin, M.; Dahl, J. E. P.; Liu, S.; Carlson,
R. M. K.; Fokin, A. A. J. Org. Chem. 2006, 71, 6709.
(14) Schwertfeger, H.; Fokin, A. A.; Schreiner, P. R. Angew.
Chem. Int. Ed. 2008, 47, 1022.
(15) Tkachenko, B. A.; Fokina, N. A.; Chernish, L. V.; Dahl, J.
E.; Liu, S.; Carlson, R. M. K.; Fokin, A. A.; Schreiner, P. R.
Org. Lett. 2006, 8, 1767.
(16) Willey, T. M.; Fabbri, J. D.; Lee, J. R. I.; Schreiner, P. R.;
Fokin, A. A.; Tkachenko, B. A.; Fokina, N. A.; Dahl, J. E.
P.; Carlson, R. M. K.; Vance, A. L.; Yang, W.; Terminello,
L. J.; van Buuren, T.; Melosh, N. A. J. Am. Chem. Soc. 2008,
130, 10536.
N-Chloroacetyldiamantane-1-amine (16)
Colorless solid; mp 125 °C.
1H NMR (400 MHz, CDCl3): d = 6.40 (br s, 1 H), 3.99 (s, 2 H), 2.25
(s, 2 H), 2.10–1.91 (m, 7 H), 1.83–1.62 (m, 8 H), 1.59–1.50 (m,
2 H).
13C NMR (100 MHz, CDCl3): d = 164.3 (C=O), 56.6 (C), 43.1
(CH2), 41.2 (CH2), 39.0 (CH), 38.7 (CH), 38.0 (CH2), 37.2 (CH2),
36.9 (CH), 32.7 (CH2), 28.5 (CH), 24.9 (CH).
HRMS: m/z calcd for C16H22ClNO: 279.1390; found: 279.1391.
Anal. Calcd for C16H22ClNO: C, 68.68; H, 7.93; N, 5.01. Found: C,
68.62; H, 7.96; N, 4.97.
Bis(N-chloroacetyl)diamantane-1,6-diamine (17)
Colorless solid; mp 313 °C (dec.).
1H NMR (400 MHz, DMSO-d6): d = 7.60 (s, 2 H), 4.01 (s, 4 H),
2.36 (br s, 4 H), 1.99–1.81 (m, 10 H), 1.40–1.31 (m, 4 H).
13C NMR (100 MHz, DMSO-d6): d = 164.9 (C=O), 54.8 (C), 43.5
(CH2), 41.0 (CH2), 39.0 (CH), 31.4 (CH2), 27.1 (CH).
(17) Yang, W. L.; Fabbri, J. D.; Willey, T. M.; Lee, J. R. I.; Dahl,
J. E.; Carlson, R. M. K.; Schreiner, P. R.; Fokin, A. A.;
Tkachenko, B. A.; Fokina, N. A.; Meevasana, W.; Mannella,
N.; Tanaka, K.; Zhou, X. J.; van Buuren, T.; Kelly, M. A.;
Hussain, Z.; Melosh, N. A.; Shen, Z.-X. Science 2007, 316,
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(21) Jirgensons, A.; Kauss, V.; Kalvinsh, I.; Gold, M. R.
Synthesis 2000, 1709.
HRMS: m/z calcd for C18H24Cl2N2O2: 370.1215; found: 370.1185.
Acknowledgment
This work was supported by the Fonds der Chemischen Industrie,
the Ukrainian Basic Research Foundation, and MolecularDiamond
Technologies, Chevron Technology Ventures. We are thankful to
M. Serafin and C. Würtele for X-ray structure analyses.
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(26) Davis, M. C.; Nissan, D. A. Synth. Commun. 2006, 36, 2113.
(27) The structures have been deposited in the Cambridge
Crystallographic Data Centre and can be retrieved using the
following numbers: CCDC 701883 (14) and 701884 (16).
Synthesis 2009, No. 6, 909–912 © Thieme Stuttgart · New York