Unexpected insertion reaction of dimethoxycarbene with imidazole-2(3H)- thiones

Article abstract of DOI:10.1080/10426501003773761

The reaction of in situ generated dimethoxycarbene with 1,4,5-trisubstituted imidazole-2(3H)-thiones yields 3-(dimethoxymethyl) imidazole-2(3H)-thiones in good yields via insertion into the N-H bond. The structure of the products was established by spectr

Full text of DOI:10.1080/10426501003773761

This article was downloaded by: [University of Arizona]
On: 13 January 2013, At: 06:47
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Phosphorus, Sulfur, and Silicon and the
Related Elements
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gpss20
Unexpected Insertion Reaction of
Dimethoxycarbene with Imidazole-2(3H)-
thiones
Grzegorz Mlostoń a & Heinz Heimgartner b
a Department of Organic and Applied Chemistry, University of Łódź,
Łódź, Poland
b Institute of Organic Chemistry, University of Zürich, Zürich,
Switzerland
Version of record first published: 27 May 2010.
To cite this article: Grzegorz Mlostoń & Heinz Heimgartner (2010): Unexpected Insertion Reaction
of Dimethoxycarbene with Imidazole-2(3H)-thiones, Phosphorus, Sulfur, and Silicon and the Related
Elements, 185:5-6, 1235-1242
To link to this article: http://dx.doi.org/10.1080/10426501003773761
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae, and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

Phosphorus, Sulfur, and Silicon, 185:1235–1242, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003773761
UNEXPECTED INSERTION REACTION OF
DIMETHOXYCARBENE WITH IMIDAZOLE-2(3H)-THIONES
Grzegorz Mloston´1 and Heinz Heimgartner2
1Department of Organic and Applied Chemistry, University of Ło´dz´, Ło´dz´, Poland
2Institute of Organic Chemistry, University of Zu¨rich, Zu¨rich, Switzerland
The reaction of in situ generated dimethoxycarbene with 1,4,5-trisubstituted imidazole-
2(3H)-thiones yields 3-(dimethoxymethyl)imidazole-2(3H)-thiones in good yields via inser-
tion into the N H bond. The structure of the products was established by spectroscopic data
and, in the case of 7b, by X-ray crystallography.
Keywords Amide acetals; dimethoxycarbene; imidazole-2(3H)-thiones; insertion reactions
INTRODUCTION
Oxygen-containing carbenes with the general formula 1 (R1, R2 = alkyl, aryl, allyl,
acetyl), which belong to the class of the so-called “nucleophilic carbenes,” have been
studied extensively within the last two decades.1–3 The main attention has been focused on
dimethoxycarbene (1a, R1, R2 = Me), which was described for the first time simultaneously
by Hoffmann and Ha¨user 4 and Lemal et al.5 In both cases, thermal decomposition of 7,7-
dimethoxybicyclo[2.2.1]hepta-2,5-diene derivatives led to 1a via a cheletropic elimination.
Moss et al. described the thermolytic decomposition of 3,3-dimethoxy-3H-diazirine
as a method for the generation of 1a for spectroscopic studies.6 The third and most suitable
method based on the thermal cycloreversion of 2,2-dimethoxy-5,5-dimethyl-2,5-dihydro-
1,3,4-oxadiazole (2a, R = Me) was developed by El-Saidi et al.7 The same method was
successfully applied for the generation of nucleophilic carbenes of type 3 (Scheme 1), in
which X is an O- or S-atom.8,9
The easily available dimethoxycarbene (1a) found numerous applications for syn-
thetic purposes in two- or three-component reactions. The nucleophilic properties of 1a
are reflected in reactions with electron-deficient alkenes,10 carbonyl,11a and thiocarbonyl
compounds12 as well as some imines.13 Zwitterionic species generated in situ from 1a
Received 25 November 2008; accepted 26 November 2008.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
The authors thank the Rector of the University of Ło´dz´ for a grant (G.M.), F. Hoffmann-La Roche AG,
Basel, for financial support (H.H.), and PD Dr. Anthony Linden, Institute of Organic Chemistry, University of
Zu¨rich, for the X-ray crystal-structure determination.
Address correspondence to Grzegorz Mloston´, Department of Organic and Applied Chemistry, University
of Ło´dz´, Tamka 12, PL-91-403, Ło´dz´, Poland. E-mail: gmloston@uni.lodz.pl
1235

´
1236
G. MLOSTON AND H. HEIMGARTNER
N N
OMe
OMe
Me
R
C
C
R1O
OR2
R1S
XR2
O
1
2
3
(X = O, S)
(R = Me, 4-MeOC6H4)
Scheme 1
and dimethyl acetylenedicarboxylate can be trapped with various dipolarophiles yielding
five-membered hetero- or carbocyclic products.14
Very little is known about insertion reactions of carbenes of type 1. In the case of 1a,
insertion reactions into C C bonds led to ring enlargement of some cyclopropanone and
butanone derivatives.11,12b The reaction of 1a with 2,4-dinitrofluorobenzene occurred via the
insertion into the C F bond, leading, after hydrolysis, to 2,4-dinitrobenzoic acid.15 Insertion
of nucleophilic carbenes 1 into O-H bonds were reported as inter-10a and intramolecular
processes.16 Thermal decomposition of 2b (R = 4-MeOC6H4) in benzene in the presence
of t-BuOH gave the orthoester 4 in 73% yield (Scheme 2).10a
H
O
OMe
N
H
OMe
H
C
Me3COH
O
C
Me3C O C
MeO
OMe
OMe
N
OMe
4
1a
5
Scheme 2
An earlier described example of an N H insertion with 1a is the formation of product
5 (76%) from the parent azetidinone.11a In a very recent paper analogous products were
reported to be formed from 1a and both, lactones and thiolactones, with a variable ring
size.11b It is worth mentioning that in all reported cases, relating to azetidine derivatives, no
ring-enlargement, analogous to that one obtained with cyclobutanone, was observed. Other
insertion reactions of nucleophilic carbenes 1a and 3 (R1 = Pr, R2X = PrS) were described
in reactions with 2-pyrrolidones incorporated in fused aromatic systems.17 To the best of
our knowledge, there are no reports published on the insertion of nucleophilic carbenes 1
or 3 into S H bonds.∗
In some recent articles, we described a new method for the preparation of imidazole-
2(3H)-thiones20 as well as their reactions with some electrophilic reagents.21 The results
showed that, depending on the reagent, the reactions occurred selectively either at the S-
(with dimethyl acetylenedicarboxylate) or N-atom (with phenylisocyanate). For the present
study, selected imidazole-2(3H)-thiones of type 6 were used in reactions with 1a.
RESULTS AND DISCUSSION
For preparation purposes, dimethoxycarbene has been generated, in general, by heat-
ing the precursor 2a in hydrocarbon solutions, e.g., in toluene. Other solvents are less
∗The reaction of the electrophilic dichlorocarbene with tautomerizable pyrimidine-2-thione derivatives
yielded products of the insertion into the S H bond.18 The unsaturated carbene Me2C C: undergoes the insertion
into the S H bond of enethiols to give divinylsulfides.19

DIMETHOXYCARBENE AND IMIDAZOLE-2(3H)-THIONES
1237
useful, as some undesired side reactions can occur. It is also known that imidazole-2(3H)-
thiones 6 are crystalline compounds with limited solubility in nonpolar organic solvents.
For this reason, four imidazole-2(3H)-thiones 6a–d bearing aliphatic substituents were
selected, which could be dissolved in toluene in reasonable concentration.∗∗ In all cases,
a 20% excess of 2a was used, and the reaction was complete after 8 h in boiling toluene.
After evaporation of the solvent, the residue was analyzed by 1H NMR spectroscopy. Thus,
in the case of 6b, the crude product was obtained in almost quantitative yield as a solid
1
material. In the H NMR spectrum, along with three signals of Me groups at 3.55, 2.20,
and 2.08 ppm, two signals appeared at 6.87 and 3.42 ppm. The intensities of these two
signals corresponded to one and six H-atoms, respectively, indicating that a 1:1-adduct of
6b and dimethoxycarbene was formed. The presence of the signal at low field suggested the
formation of a product containing a (MeO)2CH group as the result of an insertion reaction.
However, the location of the signal is not decisive for the insertion into the N H (in 6) or
S H (in 6ꢁ) bond (Scheme 3).
OMe
R2
R2
R2
R2
H
NH
N
Toluene
reflux
OMe
S
S
N
N
R1
R1
a R1 = Me, R2 = H
b R1 = R2 = Me
c R1 = Bu, R2 = Me
d R1 = cHex, R2 = Me
6a-d
7a-d
N
N
OMe
OMe
Me
Me
+
O
R2
R2
R2
R2
2a
N
N
OMe
H
OMe
SH
S
N
N
R1
R1
6a'-d'
8a-d
Scheme 3
The 13C NMR data of the crystalline product solved the problem unambiguously as
the absorption at 161.3 ppm corresponds well with the value reported for C(2) of imidazole-
2(3H)-thiones20a but differs significantly from the value for C(2) found for imidazole-2-
thiol derivatives.21 In addition, the signals for (MeO)2CH appear at 105.3 and 54.2 ppm.
Therefore, the structure of the product is 7b, i.e., the product of the insertion into the N H
bond. Finally, the structure 7b was established by X-ray crystallography (Figure 1).
The products obtained in the reactions of 2a with 6a, 6c, and 6d displayed similar
spectroscopic properties as 7b, which confirmed the structures of NH-insertion products of
type 7. In the case of 7b and 7d, pure compounds were obtained by crystallization. In the
two other cases, the products were oily materials, and all attempts to crystallize them were
unsuccessful. On the other hand, preparative TLC resulted in the conversion of the product
7 to the corresponding imidazole-2(3H)-thione 6.
Taking into account the nucleophilic properties of dimethoxycarbene, the typical
mechanism for insertion reactions of electrophilic carbenes into X H (X = N, O, S) bonds,
in which the carbene attacks the lone electron pair,23 cannot be applied. It seems likely that
∗∗In the case of very low concentrations of 6, the dimerization of dimethoxycarbene leading to tetra-
methoxyethene was observed (for the formation of tetramethoxyethene, see ref. 7,11).

´
G. MLOSTON AND H. HEIMGARTNER
1238
Figure 1 ORTEP plot22 of the molecular structure of 7b (50% probability ellipsoids, arbitrary numbering of the
atoms).
the final product 7 is formed via an ion pair of type A (Figure 2), which results from the
proton transfer between 6 and dimethoxycarbene.1,24 The mechanism of the proton transfer
is not clear, and transition states of type B and/or C, which could explain the selective
formation of the N C bond, may be proposed.
In an extension of the presented study, the exploration of the new “amide acetals” 7
for the synthesis of polycyclic sulfur compounds was examined. The strategy was based on
the described reaction of thiourea with dimethylformamide dimethyl acetal, which yields
bis(N,N-dimethylmethylidene)thiourea.25 The latter is the key building block for a series
of fused S,N-heterocycles. Unfortunately, the reaction of 7a with thiourea carried out in
boiling dichloromethane did not afford the product analogous to the already mentioned
thiourea derivative. After 4 h heating, the unchanged 7a was recovered. Apparently, the
incorporation of the N-atom into the imidazole ring significantly reduced the reactivity of
the “acetal” function of 7a.
H
C(OMe)2
OMe
C(OMe)2
N
N
N
HC
H
OMe
S
S
S
N
N
N
C
A
B
Figure 2 Possible intermediate ion pair A and transition states B and C.

DIMETHOXYCARBENE AND IMIDAZOLE-2(3H)-THIONES
1239
CONCLUSIONS
A series of experiments with imidazole-2(3H)-thiones 6 and in situ generated nucleo-
philic dimethoxycarbene evidenced that products of the carbene insertion into the N H
bond of type 7 are formed exclusively and in good yields. This chemoselectivity corresponds
with that observed in the reactions of 6 with phenylisocyanate.21 Neither products of
the nucleophilic attack onto the C S group nor products of the insertion into the S H
bond could be detected. To the best of our knowledge, these are the first reactions of
dimethoxycarbene with thiourea-type reagents, and only the third case of an insertion into
an N H bond (see Schemes 2 and 3).
EXPERIMENTAL
Melting points were determined in capillary using a Meltemp 2 apparatus and are
uncorrected. 1H and 13C NMR spectra were registered with a Tesla BS 687 (80 and 20 MHz,
respectively) or a Bruker 300 (300 and 75 MHz, respectively) spectrometer using TMS
(δTMS = 0) as an internal standard. The multiplicity of the signals was elucidated based
on DEPT experiments. IR spectra were registered with a Nexus spectrophotometer. MS
(EI, 70 eV) were recorded using a Finnigan-Mat-95 spectrometer. Elemental analyses were
performed by the Analytical Laboratory of the Institute of Organic Chemistry, University
of Zu¨rich.
Starting Materials
Imidazole-2(3H)-thiones 6a–d used in the present study were prepared from the
corresponding imidazole N-oxides using the sulfur transfer method described in earlier ar-
ticles of our group.20 2,2-Dimethoxy-5,5-dimethyl-2,5-dihydro-1,3,4-oxadiazole (2a) used
as the precursor of dimethoxycarbene (DMC) (1) was synthesized following Warkentin’s
protocol.7
Reactions of Imidazole-2(3H)-thiones 6a-d with Dimethoxycarbene (1):
General Procedure
A solution of the corresponding imidazole-2(3H)-thione 6 (1 mmol) and the DMC
precursor 2a (192 mg, 1.2 mmol) in 2 mL abs. toluene was heated under reflux for 8 h
with no special precautions. After this time, 2a was completely decomposed. Toluene was
evaporated, and the crude products, after preliminary inspection in the 1H NMR spectrum,
were purified either by microdistillation in vacuum (oil bath 130–135◦C/0.08 hPa) or by
crystallization.
1-Dimethoxymethyl-3-methylimidazole-2(3H)-thione (7a). Yield: 128 mg
(68%). Colorless oil, purified by microdistillation. IR (neat): 3170m, 3138m, 3103m, 2941s,
2839m, 1573m, 1455s, 1403s, 1370s, 1321m, 1287s, 1212s, 1142m, 1112s, 1073s, 979m,
1
912m, 824s, 794m, 722s, 674s. H-NMR (CDCl3): 3.44 (s, 2 MeO), 3.60 (s, MeN), 6.48
(s, (MeO)2CH), 6.68, 6.89 (2d, JH,H = 2.5 Hz, 2 CH, imidazole). 13C-NMR (CDCl3): 33.9
(MeN), 54.2 (2 MeO), 103.8 ((MeO)2CH), 112.6, 118.2 (2 CH, imidazole), 162.3 (C S).
EI-MS: 188 (19, M+.), 157 (22), 129 (10), 128 (100), 127 (21), 114 (57), 113 (23), 95
(53), 83 (18), 82 (20), 81 (20), 75 (57). HR-EI-MS: Calcd. C7H12N2O2S: 188.0619; found:

´
G. MLOSTON AND H. HEIMGARTNER
1240
188.0617. Anal. Calcd. for C7H12N2O2S (188.25): C 44.66, H 6.43. Found: C 44.55, H
6.49.
1-Dimethoxymethyl-3,4,5-trimethylimidazole-2(3H)-thione (7b). Yield:
188 mg (87%). Colorless plates, mp 70–71◦C (petroleum ether/CH2Cl2). IR (KBr): 3009w,
2943m, 2928m, 2837m, 1666w, 1459m, 1437m, 1395vs, 1376s, 1367s, 1330vs, 1224s,
1200m, 1112vs, 1094vs, 1075vs, 1021m, 996s, 978m, 871m. 1H-NMR (CDCl3): 2.08, 2.20
(2s, 2 Me), 3.42 (s, 2 MeO), 3.55 (s, MeN), 6.87 (s, (MeO)2CH). 13C-NMR (CDCl3):
8.0, 8.2 (2 Me), 30.8 (MeN), 54.2 (2 MeO), 105.3 ((MeO)2CH), 119.5, 121.5 (2 Cq,
imidazole), 161.3 (C S). EI-MS: 216 42, M+.), 185 (33), 157 (12), 156 (93), 155 (10), 143
(7), 142 (62), 141 (32), 123 (44), 109 (11), 75 (100). HR-EI-MS: Calcd. for C9H16N2O2S:
216.0932; found: 216.0933. Anal. Calcd. for C9H16N2O2S (216.30): C 49.97, H 7.46.
Found: C 50.10, H 7.31.
1-Dimethoxymethyl-3-butyl-4,5-dimethylimidazole-2(3H)-thione (7c).
Yield: 190 mg (74%). Colorless oil, purified by microdistillation. IR (neat): 2957s, 2935s,
2872m, 1657m, 1443s, 1410s, 1389s, 1329s, 1220s, 1194s, 1105s, 1070s, 992m, 869m.
1H-NMR (CDCl3): 0.95 (t, JH,H = 7.3 Hz, MeCH2), 1.15–1.95 (m, -CH2-CH2-), 2.09,
2.30 (2s, 2 Me), 3.43 (s, 2 MeO), 4.02 (t, JH,H = 7.2 Hz, CH2N), 6.89 (s, (MeO)2CH).
13C-NMR (CDCl3): 8.3, 8.7, 13.4 (3 Me), 19.7, 30.3 (2 CH2), 44.4 (CH2N), 54.7 (2 MeO),
105.7 ((MeO)2CH), 120.3, 121.5 (2 Cq, imidazole), 161.5 (C S). EI-MS: 258 (8, M+.),
227 (8), 198 (36), 184 (10), 183 (11), 169 (15), 151 (23), 142 (11), 141 (8), 128 (9), 127
(10), 123 (8), 116 (17), 109 (17), 101 (8), 75 (100). HR-EI-MS: Calcd. for C12H22N2O2S:
258.1402; found: 258.1398. Anal. Calcd. for C12H22N2O2S (258.38): C 55.78, H 8.58.
Found: C 55.90, H 8.32.
1-Dimethoxymethyl-3-cyclohexyl-4,5-dimethylimidazole-2(3H)-thione
(7d). Yield: 187 mg (66%). Colorless plates, mp 84–86◦C (petroleum ether/CH2Cl2).
IR (KBr): 2954s, 2851w, 1649w, 1447w, 1405s, 1377vs, 1362s, 1335s, 1219m, 1189m,
1200m, 1108vs, 1067vs, 993s, 859m. 1H-NMR (CDCl3): 2.18 (br. s, 2 Me), 1.00–2.30 (m,
10H, 5 CH2), 3.43 (s, 2 MeO), 5.15 (m, CH), 6.98 (s, (MeO)2CH). 13C-NMR (CDCl3):
8.5, 10.3 (2 Me), 25.1, 25.8, 30.3 (5 CH2), 54.7 (2 MeO), 57.0 (CHN), 106.0 ((MeO)2CH),
120.9, 121.5 (2 Cq, imidazole), 161.5 (C S). EI-MS: 284 (26, M+.), 253 (17), 225 (9),
224 (57), 210 (13), 209 (10), 171 (7), 143 (10), 142 (91), 141 (9), 128 (36), 127 (16), 109
(61), 75 (100). HR-EI-MS: Calcd. for C14H24N2O2S: 284.1558; found: 284.1553. Anal.
Calcd. for C14H24N2O2S (284.42): C 59.12, H 8.51. Found: C 59.27, H 8.77.
X-Ray Crystal-Structure Determination of 7b
All measurements were performed on a Nonius KappaCCD area-diffractometer26 us-
ing graphite-monochromated MoKα radiation (λ 0.71073 Å) and an Oxford Cryosystems
Cryostream 700 cooler. The data collection and refinement parameters are given below,27
and a view of the molecule is shown in Figure 1. Data reduction was performed with HKL
Denzo and Scalepack.28 The intensities were corrected for Lorentz and polarization effects,
and an absorption correction based on the multi-scan method29 was applied. Equivalent
reflections were merged. The structure was solved by direct methods using SIR92,30 which
revealed the positions of all non-H-atoms. The non-H-atoms were refined anisotropically.
All of the H-atoms were placed in geometrically calculated positions and refined using a
riding model where each H-atom was assigned a fixed isotropic displacement parameter
with a value equal to 1.2Ueq of its parent C-atom (1.5Ueq for the methyl groups). The

DIMETHOXYCARBENE AND IMIDAZOLE-2(3H)-THIONES
1241
refinement of the structure was carried out on F2 by using full-matrix least-squares proce-
2
2 2
dures, which minimized the function ꢀw(Fo −Fc ) . A correction for secondary extinction
was not applied. Neutral atom scattering factors for non-H-atoms were taken from ref.31,
and the scattering factors for H-atoms were taken from ref.32 Anomalous dispersion effects
were included in Fc;33 the values for fꢁ and fꢁꢁ were those of ref.34 The values of the
mass attenuation coefficients are those of ref.35 All calculations were performed using the
SHELXL9736 program.
Crystal data for 7b: C9H16N2O2S, M = 216.30, colorless, plate, crystal dimensions
0.08 × 0.15 × 0.30 mm, monoclinic, space group P21/c, Z = 4, reflections for cell deter-
mination 64835, a = 9.9728(2) Å, b = 15.8607(4) Å, c = 7.5623(2) Å, β = 108.752(2◦),
V = 1132.68(5) Å3, T = 160(1) K, Dx = 1.268 g·cm−3, µ(MoKα) = 0.265 mm−1, scan
type φ and ω, 2θ(max) = 60◦, transmission factors (min; max) 0.915; 0.981, total reflections
measured 33116, symmetry independent reflections 3303, reflections with I > 2σ(I) 2224,
reflections used in refinement 3303, parameters refined 132, R(F) [I > 2σ(I) reflections] =
0.0488, wR(F2) [all data] = 0.1238 (w = [σ2(Fo )+ (0.0507P)2 + 0.6043P]−1, where P =
2
2
2
(Fo + 2Fc )/3), goodness of fit 1.019, final ꢁmax/σ 0.001, ꢁρ (max; min) = 0.27; −0.29
e Å−3
.
REFERENCES
1. J. Warkentin, Adv. Carbene Chem., 2, 245 (1998).
2. J. Warkentin, J. Chem. Soc., Perkin Trans. 1, 2161 (2000).
3. J. Warkentin, Acc. Chem. Res., 42, 205 (2009).
4. R. W. Hoffmann and H. Ha¨user, Tetrahedron Lett., 197 (1964).
5. D. M. Lemal, E. P. Gosselink, and A. Ault, Tetrahedron Lett., 579 (1964).
6. (a) R. A. Moss, M. Wlostowski, J. Terpinski, G. Kmiecik-Lawrynowicz, and K. Krogh-Jespersen,
J. Am. Chem. Soc., 109, 3811 (1987); (b) R. A. Moss, Acc. Chem. Res., 39, 267 (2006).
7. M. El-Saidi, K. Kassam, D. L. Pole, T. Tadey, and J. Warkentin, J. Am. Chem. Soc., 114, 8751
(1992).
8. J. H. Rigby, S. Laurent, W. T. Dong, and M. D. Danca, Tetrahedron, 56, 10101 (2000).
9. P. R. Schreiner, H. P. Reisenauer, J. Romanski, and G. Mloston´, Angew. Chem. Int. Ed., 45, 3989
(2006).
10. (a) H. Zhou, G. Mloston´, and J. Warkentin, Org. Lett., 7, 487 (2005); (b) A. Sliwinska, W.
Czardybon, and J. Warkentin, Org. Lett., 9, 695 (2007).
11. (a) P. C. Venneri and J. Warkentin, Can. J. Chem., 78, 1194 (2000); (b) G. Mloston, G. Kania,
and H. Heimgartner, J. Sulfur Chem., 30, 278 (2009).
12. (a) M. Dawid, G. Mloston´, and J. Warkentin, Org. Lett., 3, 2455 (2001); (b) M. Dawid, G.
Mloston´, and J. Warkentin, Chem. Eur. J., 8, 2184 (2002).
13. (a) G. Mloston´ and H. Heimgartner, Helv. Chim. Acta, 90, 1758 (2007); (b) G. Mloston´, J.
Warkentin, A. Linden, and H. Heimgartner, Helv. Chim. Acta, 90, 2024 (2007); (c) R. W.
Hoffmann, K. Steinbach, and W. Lilienblum, Chem. Ber., 109, 1759 (1976).
14. (a) V. Nair, C. Rajesh, A. U. Vinod, S. Bindu, A. R. Sreekanth, J. S. Mathen, and L. Balagopal,
Acc. Chem. Res., 36, 899 (2003); (b) V. Nair, P. B. Beneesh, V. Sreekumar, S. Bindu, R. S.
Menon, and A. Deepthi, Tetrahedron Lett., 46, 201 (2005); (c) V. Nair, A. Deepthi, M. Poonoth,
B. Santhamma, S. Vellalath, B. P, Babu, R. Mohan, and E. Suresh, J. Org. Chem., 71, 2313
(2006).
15. J. P. Ross, P. Couture, and J. Warkentin, Can. J. Chem., 75, 1331 (1997).
16. P. Couture and J. Warkentin, Can. J. Chem., 75, 1281 (1997).
17. J. H. Rigby and P. J. Burke, Heterocycles, 67, 643 (2006).
18. H. Singh and P. Singh, Tetrahedron, 37, 1215 (1981).

´
G. MLOSTON AND H. HEIMGARTNER
1242
19. P. J. Stang and S. B. Christensen, J. Org. Chem., 46, 823 (1981).
20. (a) G. Mloston´, T. Gendek, and H. Heimgartner, Helv. Chim. Acta, 81, 1585 (1998); (b) M.
Jasin´ski, G. Mloston´, P. Mucha, A. Linden, and H. Heimgartner, Helv. Chim. Acta, 90, 1765
(2007); (c) P. Mucha, G. Mloston´, M. Jasin´ski, A. Linden, and H. Heimgartner, Tetrahedron:
Asymmetry, 19, 1600 (2008).
21. G. Mloston´, T. Gendek, A. Linden, and H. Heimgartner, Pol. J. Chem., 82, 1561 (2008).
22. C. K. Johnson, ORTEP II, Report ORNL-5138 (Oak Ridge National Laboratory, Oak Ridge,
TN, 1976).
23. (a) Y. Chiang, A. J. Kresge, P. Pruszynski, N. P. Schlepp, and J. Wirz, Angew. Chem. Int. Ed., 30,
1356 (1991); (b) D. J. Miller and C. J. Moody, Tetrahedron, 51, 10811 (1995); (c) C. J. Moody,
Angew. Chem. Int. Ed., 46, 9148 (2007).
24. W. Kirmse, In Advances of Carbene Chemistry, U. H. Brinker, Ed. (JAI Press, Stanford, CT,
1994), pp. 1–57.
25. C. Landreau, D. Deniaud, and J. C. Meslin, J. Org. Chem., 68, 4912 (2003).
26. R. Hooft, KappaCCD Collect Software (Nonius BV, Delft, The Netherlands, 1999).
27. CCDC-707800 contains the supplementary crystallographic data for this article. These data can
be obtained free of charge from the Cambridge Crystallographic Data Centre, via www.ccdc.
cam.ac.uk/data request/cif.
28. Z. Otwinowski and W. Minor, In Methods in Enzymology, Vol. 276, Macromolecular Crystal-
lography, Part A, C. W. Carter, Jr. and R. M. Sweet, Eds. (Academic Press, New York, 1997),
pp. 307–326.
29. R. H. Blessing, Acta Crystallogr., Sect. A, 51, 33 (1995).
30. A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori, and M.
Camalli, J. Appl. Crystallogr., 27, 435 (1994).
31. E. N. Maslen, A. G. Fox, and M. A. O’Keefe, In International Tables for Crystallography, A. J.
C. Wilson, Ed. (Kluwer Academic, Dordrecht, 1992), vol. C, pp. 477–486; Table 6.1.1.1.
32. R. F. Stewart, E. R. Davidson, and W. T. Simpson, J. Chem. Phys., 42, 3175 (1965).
33. J. A. Ibers and W. C. Hamilton, Acta Crystallogr., 17, 781 (1964).
34. D. C. Creagh and W. J. McAuley, In International Tables for Crystallography, A. J. C. Wilson,
Ed. (Kluwer Academic, Dordrecht, 1992), vol. C, pp. 219–222; Table 4.2.6.8.
35. D. C. Creagh and J. H. Hubbell, In International Tables for Crystallography, A. J. C. Wilson,
Ed. (Kluwer Academic, Dordrecht, 1992), vol. C, pp. 200–206; Table 4.2.4.3.
36. G. M. Sheldrick, SHELXL97, Program for the Refinement of Crystal Structures (University of
Go¨ttingen, Go¨ttingen, Germany, 1997).