[2+2] Cycloaddition of electron-poor acetylenes to (E)-3-dimethylamino-1-heteroaryl-prop-2-en-1-ones: synthesis of highly functionalized 1-heteroaroyl-1,3-butadienes

Article abstract of DOI:10.1016/j.tetlet.2010.04.106

Microwave-assisted [2+2] cycloaddition of (E)-3-dimethylamino-1-heteroaryl-prop-2-en-1-ones to dimethyl acetylenedicarboxylates gives (2E,3E)-dimethyl-2-[(dimethylamino)methylene]-3-(substituted)succinates in 8-91% yield. In the case of a 4,5-dihydrothiazoline derivative, cycloaddition also took place at the endocyclic C{double bond, long}N double bond.

Full text of DOI:10.1016/j.tetlet.2010.04.106

Tetrahedron Letters 51 (2010) 3392–3397
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
[
(
2+2] Cycloaddition of electron-poor acetylenes to
E)-3-dimethylamino-1-heteroaryl-prop-2-en-1-ones: synthesis of highly
functionalized 1-heteroaroyl-1,3-butadienes
Jure Bezenšek, Tanja Koleša, Uroš Grošelj, Jernej Wagger, Katarina Stare, Anton Meden, Jurij Svete,
Branko Stanovnik *
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ašker cˇ eva 5, PO Box 537, 1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
Microwave-assisted [2+2] cycloaddition of (E)-3-dimethylamino-1-heteroaryl-prop-2-en-1-ones to
dimethyl acetylenedicarboxylates gives (2E,3E)-dimethyl-2-[(dimethylamino)methylene]-3-(substi-
tuted)succinates in 8–91% yield. In the case of a 4,5-dihydrothiazoline derivative, cycloaddition also took
place at the endocyclic C@N double bond.
Received 4 March 2010
Revised 2 April 2010
Accepted 23 April 2010
Available online 28 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
[
2+2] Cycloaddition
(E)-3-Dimethylamino-1-heteroaryl-prop-2-
en-1-ones
-Heteroaroyl-1,3-butadienes
1
1
. Introduction
The [2+2] cycloaddition of an alkene to an alkyne represents an
amino)buta-1,3-dienes.11 In this Letter, we report the preparation
of highly functionalized 1-heteroaroylbuta-1,3-dienes via [2+2]
cycloaddition of dimethyl acetylenedicarboxylate (DMAD) to (E)-
important strategy for the synthesis of cyclobutene derivatives. It
can be achieved by photochemical methods, thermal reactions
3-dimethylamino-1-heteroaryl-prop-2-en-1-ones.
1
via biradical intermediates and by the use of Lewis acid cata-
2
. Results and discussion
2
a–c
lysts.
Recently, the formal [2+2] cycloaddition of strong elec-
tron-acceptors to electron-rich alkynes followed by retro-
electrocyclization has been studied in detail.3 On the other hand,
the wide applicability of 3-(dimethylamino)propenoates and re-
lated enaminones as versatile reagents in heterocyclic synthe-
When (E)-3-(dimethylamino)-1-heteroaryl-prop-2-en-1-ones
a–f and 12a prepared from acetyl heterocycles 1a–f and 12b
and N,N-dimethylformamide dimethylacetal (DMFDMA) or tert-
butoxy bis(dimethylamino)methane (TBDMAM) were reacted un-
der microwave irradiation at elevated temperatures with DMAD
3), as an electron-poor acetylene, the corresponding dimethyl
2E,3E)-2-[(dimethylamino)methylene]-3-(substituted)succinates
a–f and 14 were isolated as the only isomers in 8–91% yield
a–c
2
4
a–d
sis,
including natural products and their analogues, for
5a,b
6a,b
7a–c
example, the aplysinopsins,
meridianines,
dipodazines
(
(
4
(
8
and triprostatines, has been demonstrated. This encouraged us
to study their reactions with electron-poor acetylenes. Recently,
we reported regiospecific microwave-assisted [2+2] cycloadditions
of substituted 2-amino-3-(dimethylamino)propenoates with acet-
ylenedicarboxylates which gave highly functionalized 1-amino-4-
Schemes 1 and 3).
The structures of the products were determined by spectro-
1
13
scopic methods (IR, H and C NMR, MS) and by elemental analy-
ses. Additionally, the structures of compounds 2f, 4d and 13a,b
were confirmed by X-ray diffraction analysis (Figs. 1, 2, 4 and 5).
Two pathways for this cycloaddition can be envisioned, con-
certed and stepwise. In the case of a concerted cycloaddition, the
(
dimethylamino)buta-1,3-dienes9 and their transformations into
highly substituted pyridines, pyrroles and pyrido[3,4-c]pyridazine
1
0
derivatives. These cycloadditions have been extended to (5Z)-5-
(dimethylamino)methylene]-3-methyl-imidazolidine-2,5-diones,
resulting in the formation of (1E,3E)-1-(acylamino)-4-(dimethyl-
[
s a
reaction should proceed as a [2 +2 ] cycloaddition resulting in cyc-
lobutene intermediate 9, which would undergo a conrotatory ret-
ro-electrocyclization (ring-opening) to afford (2E,3E)-10 and/or
(
2Z,3Z)-11. In the case of a two-step mechanism, the initially
*
Corresponding author. Tel.: +386 1 2419 100; fax: +386 1 2419 220.
E-mail address: branko.stanovnik@fkkt.uni-lj.si (B. Stanovnik).
0
formed intermediate 5 can be transformed into 5 by rotation
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.106

J. Bezenšek et al. / Tetrahedron Letters 51 (2010) 3392–3397
Me N
3393
2
CO Me
MeO C
2
2
O
MW
MeCN
+
CO Me
O
Het
NMe2
2
CO Me
2
Het
2
a-f
3
4a-f
Product
Het
Yield [%]
Time
Temp. [˚C]
100
O
4a
70
89
87
20 min
10 min
20 min
O
Me
4b
100
100
S
4c
S
4
d
70
91
20 min
24 h
100
r.t.
Me
N
4
e
Me
N
4f
16
15 min
100
Scheme 1. Microwave-assisted [2+2] cycloaddition.
around the single bond. Cyclization of 5 leads to trans-cyclobutene
intermediate 9, whereas cyclization of 5 would give sterically
integrating for 6 protons (NMe
4.55 ppm, each integrating for two protons, (J = 8.7 Hz, –CH
2
group), two triplets at 3.32 and
2
CH –
0
2
unfavoured cis-cyclobutene intermediate 6, which upon a conrota-
tory retro-electrocyclization would produce isomers (2Z,3E)-7 and/
or (2E,3Z)-8 (Scheme 2).
The single crystal X-ray structure of product 4d (Fig. 2) clearly
demonstrates the (2E,3E) configuration at the C@C double bonds.
On the other hand, the configurations at the C@C double bonds in
group), two singlets at 3.64 and 3.81 ppm, each integrating for
three protons (two ester Me groups) and two singlets, each inte-
0
0
grating for one proton at 7.70 and 7.89 ppm for C2 -H and C3 -H.
1
On the basis of these data and their close H NMR correlation with
analogous cycloadducts 4a–f (see Table 1), one can conclude that
the cycloaddition of DMAD took place at the exocyclic C@C double
bond of 12a, thus forming product 14.
products 4c and 4f were determined by NMR on the basis of long-
3
range coupling constants,
J
C–H
,
between the corresponding
The elemental analysis and HRMS for the major product 13a
methine protons and the carbonyl carbon atoms, measured from
the antiphase splitting of cross peaks in the HMBC spectrum.
gave a molecular formula of C14
H
18
N
2
O
5
S, also corresponding to
NMR spectrum exhibited singlet at
group), two multiplets at ꢀ3.41 ppm (3H)
CH – group, two singlets at
1
1:1 adduct. The
H
a
Generally, the magnitude of the coupling constants, 3
J
C–H, for nu-
3.00 ppm (6H, NMe
2
and ꢀ4.29 ppm (1H) for the –CH
clei with a cis-configuration at the C@C double bond is smaller
2
2
1
2
(
2–6 Hz) than that of trans-oriented nuclei (8–12 Hz). The mag-
3.72 (3H) and 3.80 ppm (3H) for the two ester Me groups and
two doublets at 5.98 (1H) and 8.49 ppm (1H) (J = 13.2 Hz) for the
–CH@CH– (trans) structural element. On the basis of these data,
we assumed that the cycloaddition of DMAD to 12a must have ta-
ken place at the C@N double bond of the thiazolidine ring. The sin-
gle crystal X-ray structure confirmed the structure of compound
13a (Fig. 4).
3
3
nitude of the coupling constants,
J
C(1)–H(2
0
)
= 4.5 Hz and
J
C(1)–
0
= 4.8 Hz, for compounds 4c and 4f, respectively, indicates a
H(2 )
(
2E)-configuration. Similarly, the (3E)-configuration was estab-
3
lished for both compounds 4c and 4f ( JC(3)–H(3
0
)
= 6.9 Hz)
1
(Fig. 3). Finally, selected H NMR data of cycloadducts 4a–f and
1
4 correlate well, implying their uniform (2E,3E) configuration.
This in turn indicates that the cycloaddition reactions could
proceed either by a concerted mechanism or by a two-step
mechanism in which rotation around the single bond is restricted
due to the formation of sterically less favored cis-cyclobutene
derivative 6.
The cycloaddition of (E)-1-(4,5-dihydrothiazol-2-yl)-3-(dimeth-
ylamino)prop-2-en-1-one (12a), prepared from 1-(4,5-dihy-
drothiazol-2-yl)ethanone (12b) and DMFDMA, with DMAD (3),
under microwave irradiation, afforded a mixture of the corre-
sponding [2+2] cycloadduct 14 in 8% yield and (E)-dimethyl 6-[2-
Similarly, the cycloaddition of DMAD to 1-(4,5-dihydrothiazol-
2-yl)ethanone (12b) produced a compound with the molecular for-
5
mula C11H13NO S, corresponding again to a 1:1 cycloadduct. The
1
H NMR spectrum showed a singlet at 2.19 ppm (3H) for the Me
group, three multiplets at ꢀ3.36 ppm (1H), ꢀ3.53 ppm (2H) and
ꢀ4.30 ppm (1H) for the –CH
2 2
CH – structural element and two
singlets at 3.77 ppm (3H) and 3.88 ppm (3H) for the two ester
Me groups. The structure of this cycloadduct is similar to the struc-
ture of compound 13a and was confirmed by single crystal X-ray
analysis (Fig. 5).
(
dimethylamino)vinyl]-5-oxo-2,3,5,7a-tetrahydropyrrolo[2,1-b]thi-
We rationalize the formation of compound 13b by initial [2+2]
cycloaddition of DMAD to the endocyclic C@N double bond of 12b
to form the intermediate cycloadduct 15, followed by a series of
1,3-sigmatropic shifts 15?16?17?18 to finally arrive at 13b
(Scheme 4).
azole-7,7a-dicarboxylate (13a) in 32% yield (Scheme 3).
The elemental analysis and HRMS spectrum for the minor prod-
uct 14 gave a molecular formula of C14
1
H
18
N
2
O
5
S, corresponding to
1
:1 adduct. The H NMR spectrum exhibited a singlet at 2.84 ppm

3
394
J. Bezenšek et al. / Tetrahedron Letters 51 (2010) 3392–3397
MeO C
CO Me
2
2
MeO C
CO Me
2
2
3
H
COHet
H
COHet
Me N
H
2
Me N
H
2
2
MeO C
2
MeO C
CO Me
2
2
CO Me
2
H
COHet
H
H
COHet
H
Me N
Me N
2
2
9
5
Me N
MeO C
2
2
MeO C
NMe2
MeO C
CO Me
2
2
2
3
and/or
Me N
COHet
H
2
2
HetOC
CO Me
CO Me
2
2
H
COHet
(2Z,3Z)-11
(2E,3E)-10
5'
Me N
MeO C
2
MeO C
CO Me
2
2
MeO C
NMe2
CO Me
2
2
and/or
Me N
COHet
H
2
HetOC
CO Me
2
2
H
COHet
(2E,3Z)-8
(2Z,3E)-7
6
Scheme 2. Proposed mechanisms for the cycloaddition–retro-electrocyclization.
Me N
MeO C
2
O
O
2
S
MeO C
CO Me
N
2
2
R
+
CO Me
2
R
MeCN, MW
S
N
O
MeO C
2
CO Me
2
N
1
2a,b
S
1
3a,b
2 %
14
8 %
NMe2
1
1
2a R =
2b R =
3
DMFDMA
MW, Δ
Me
Scheme 3. [2+2] Cycloaddition of 12a with DMAD.
3
3
. Experimental
nents were evaporated in vacuo and the residue was purified by
column chromatography (products 2a,e, 12a) or crystallization
(products 2b,c,d,f).
.1. Generalprocedureforthesynthesisof(E)-3-(dimethylamino)
-
1-substituted-prop-2-en-1-ones 2a–f and 12a13
3
.2. General procedure for the synthesis of (2E,3E)-dimethyl 2-
To compound 1a–f, 12b (1 equiv) was added DMFDMA or TBD-
[(dimethylamino)methylene]-3-(2-substituted)succinates 4a–f
MAM (1.2 equiv) and the resulting mixture was stirred in a closed
vessel under microwave irradiation (300 W) at an automatically
controlled constant temperature (CEM Corporation discover micro-
wave unit). After cooling to room temperature, the volatile compo-
To a solution of (E)-3-(dimethylamino)-1-substituted-prop-2-
en-1-one 2a–f (1 equiv) in MeCN (1–3 mL) was added DMAD (3)
(2.0–2.5 equiv) and the mixture was stirred in a closed vessel

J. Bezenšek et al. / Tetrahedron Letters 51 (2010) 3392–3397
3395
Figure 1. ORTEP view of compound 2f.
Figure 4. ORTEP view of compound 13a.
Figure 5. ORTEP view of compound 13b.
Figure 2. ORTEP view of compound 4d.
Table 1
Selected 1H NMR data of cycloadducts 4a–f and 14
0
0
Product
d NMe
2
(ppm)
d COOMe (ppm)
d H3 (ppm)
d H2 (ppm)
Me N
2
4
2'
MeO C
H
4a
2.89
3.62
3.83
3.62
7.53
7.47
7.52
7.48
7.53
7.39
7.70
7.77
2
3
2
4
4
b
c
2.89
2.89
2.89
2.90
2.90
2.84
7.72
7.67
7.56
7.56
7.55
7.89
H 3'
1
CO Me
2
3
.82
3.60
.83
O
R
3
4
c: R = 2-thienyl
4f: R = 1-methyl-1H-pyrrol-3-yl
4d
4e
3.60
3.83
3.82
3
3
JC(1)-H(2') = 4.5 Hz (cis)
JC(1)-H(2') = 4.8 Hz (cis)
3J
3J
= 6.9 Hz (cis)
= 6.9 Hz (cis)
C(4)-H(3')
C(4)-H(3')
3
.90
3.65
.81
3.64
.81
4
1
f
Figure 3. Determination of the configuration by HMBC spectroscopy.
3
4
3
under microwave irradiation (300 W) at automatically controlled
elevated constant temperature (CEM Corporation discover micro-
wave unit) or at room temperature. After cooling to room temper-
ature, volatile compounds were evaporated in vacuo and the
residue was purified by column chromatography (products
4a,b,c,e,f) or crystallization (product 4d).

3
396
J. Bezenšek et al. / Tetrahedron Letters 51 (2010) 3392–3397
CO Me
MeOC
S
2
CO Me
2
CO Me
2
S
S
[1,3]
COMe
+
N
N
CO Me
2
COMe
N
CO Me
CO Me
2
2
1
2b
3
15
16
CO Me
CO Me
MeO C
MeO C
2
MeO C
2
2
2
2
CO Me
2
S
S
S
[
1,3]
Me
O
N
N
N
Me
O
O
1
7
18
13b
Scheme 4. The proposed mechanism for the formation of 13b.
3
.3. (E)-Dimethyl 6-[2-(dimethylamino)vinyl]-5-oxo-2,3,5,7a-
tetrahydropyrrolo[2,1-b]thiazole-7,7a-dicarboxylate (13a) and
2E,3E)-dimethyl 2-[2-(4,5-dihydrothiazol-2-yl)-2-
CH
3.88 (3H, s, COOMe); 4.29–4.40 (1H, m, 1H of thiazolidine CH
NMR (CDCl , 75.5 MHz): d 11.0, 36.9, 44.6, 52.5, 53.8, 78.3, 141.5,
143.0, 162.9, 168.5, 172.6. (C11 S requires: C, 48.70; H,
.83; N, 5.16; found C, 48.75; H, 4.66; N, 5.07); EI-HRMS: m/
2 2
); 3.48–3.57 (2H, m, CH of thiazolidine); 3.77 (3H, s, COOMe);
13
2
).
C
(
3
oxoethylidene]-3-[(dimethylamino)methylene]succinate (14)
H13NO
5
4
+
+
Prepared from (E)-1-(4,5-dihydrothiazol-2-yl)-3-(dimethyl-
amino)prop-2-en-1-one (12a) (276 mg, 1.5 mmol) and DMAD (3)
z = 272.0588 (MH ); C11
5
H14NO S requires: m/z = 272.0593 (MH );
m
max (KBr) 3008, 2959, 1734, 1706, 1464, 1438, 1347, 1326, 1278,
ꢁ
1
(
366
l
L, 3.0 mmol) in MeCN (1.5 mL), 60 °C, 10 min. The two prod-
1217, 1161, 1071, 990, 955, 925 cm .
ucts were separated by column chromatography (EtOAc/petroleum
ether = 1:2). Fractions containing the products were combined and
evaporated in vacuo.
3.7. X-ray structure analysis for compounds 2f, 4d, 13a and 13b
Single crystal X-ray diffraction data of compounds 2f, 4d, 13a
and 13b were collected at room temperature on a Nonius Kappa
3
.4. Major product 13a
14
CCD diffractometer using the Nonius Collect Software.
DENZO
1
5
Elutes first, recrystallized from EtOAc/petroleum ether. Yield:
56 mg (32%); yellow solid; mp 103–115 °C. H NMR (CDCl
and SCALEPACK were used for indexing and scaling of the data and
1
16
1
3
3
,
the structures were solved by means of SIR97. Refinement was
1
7
00 MHz): d 3.00 (6H, s, NMe
2
); 3.31–3.53 (3H, m, 3H of thiazoli-
performed using the XTAL3.4 program package and the crystallo-
1
8
dine); 3.72 (3H, s, COOMe); 3.80 (3H, s, COOMe); 4.29–4.36 (1H,
m, 1H of thiazolidine); 5.98 (1H, d, J = 13.2 Hz, CH); 8.49 (1H, d,
graphic plots were prepared by ORTEP III. Crystal structures were
refined on F values using the full-matrix least-squares procedure.
The non-hydrogen atoms were refined anisotropically in all cases,
while the positions of the hydrogen atoms were geometrically cal-
culated and their positional and isotropic atomic displacement
parameters were not refined. Absorption correction was not neces-
1
3
J = 13.2 Hz, CH). C NMR (CDCl
3.5, 79.0, 90.1, 120.4, 140.4, 150.9, 164.4, 169.6, 172.3.
S requires: C, 51.52; H, 5.56; N, 8.58; found C, 51.30;
3
, 75.5 MHz): d 36.0, 43.9, 51.4,
5
14 18 2 5
(C H N O
+
H, 5.56; N, 8.42); EI-HRMS: m/z = 327.1012 (MH ); C14
H
19
N
2
O
5
S re-
max (KBr) 2948, 1755, 1735, 1697,
611, 1565, 1431, 1406, 1341, 1251, 1191, 1162, 1109, 1040,
+
19
quires: m/z = 327.1015 (MH );
1
1
m
sary. Regina weighting scheme was used in all cases.
ꢁ1
Acknowledgements
011, 974, 861 cm
.
Financial support from the Slovenian Research Agency through
grants P0-0502-0103, P1-0179 and J1-6689-0103-04 is gratefully
acknowledged. We also thank the Krka d.d. (Novo mesto, Slovenia)
and Lek d.d., a Sandoz Company (Ljubljana, Slovenia) for financial
support.
3
.5. Minor product 14
Elutes second. Yield: 38 mg (8%); red oil. 1H NMR (CDCl
00 MHz): d 2.84 (6H, s, NMe ); 3.32 (2H, t, J = 8.7 Hz, CH of thia-
3
,
3
2
2
zolidine); 3.64 (3H, s, COOMe); 3.81 (3H, s, COOMe); 4.55 (2H, t,
J = 8.7 Hz, CH
C(2 )-H). C NMR (CDCl
0
2
of thiazolidine); 7.70 (1H, s, C(3 )-H); 7.89 (1H, s,
0
13
Supplementary data
3
, 75.5 MHz): d 32.9, 43.9, 51.6, 53.1,
6
6.4, 93.3, 120.7, 142.4, 155.5, 168.7, 169.1, 172.2, 181.0. EI-HRMS:
+
Crystallographic data (excluding structure factors) for the struc-
tures in this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication numbers
CCDC 768092 (2f), 768093 (4d), 768094 (13a) and 768095(13b).
Copies of these data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK [fax: +44(0)-
m/z = 327.1006 (MH );
C
14
19
H N
2
O
5
S
requires: m/z = 327.1015
+
(
MH );ax (KBr) 2949, 1723, 1692, 1604, 1538, 1433, 1400, 1328,
ꢁ1
1
255, 1217, 1135, 1089, 1046, 997, 942, 923 cm
.
3
.6. Dimethyl 6-methyl-5-oxo-2,3,5,7a-tetrahydropyrrolo[2,1-
b]thiazole-7,7a-dicarboxylate (13b)
1
223-336033 or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2010.04.106.
Prepared from 1-(4,5-dihydrothiazol-2-yl)ethanone (12b)
(
440 mg, 3.4 mmol) and DMAD (3) (830
lL, 6.8 mmol) in MeCN
(
3 mL), 60 °C, 15 min, column chromatography (EtOAc/petroleum
References and notes
ether = 1:3), recrystallized from EtOAc/petroleum ether. Yield:
1
3
1
71 mg (19%); white solid; mp 106–109 °C. H NMR (CDCl
3
,
1.
Trost, B. M., Fleming, I., Paquette, L. A., Eds.Comprehensive Organic Synthesis;
Oxford: Pergamon, 1991; Vol. 5,. Chapters 2.1–2.6.
00 MHz): d 2.19 (3H, s, CH ); 3.31–3.42 (1H, m, 1H of thiazolidine
3

J. Bezenšek et al. / Tetrahedron Letters 51 (2010) 3392–3397
3397
2
.
.
(a) Cockburn, N.; Karimi, E.; Tam, W. J. Org. Chem. 2009, 74, 5762–5765. and
reference cited therein; (b) Fan, B.-M.; Li, X.-J.; Peng, F.-Z.; Zhang, H.-B.; Chan,
A. S. C.; Shao, Z. H. Org. Lett. 2010, 12, 304–306; (c) Shibata, T.; Tsuchikama, K.
Org. Biomol. Chem. 2008, 6, 1317–1323.
(a) Kivala, M.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.; Diederich, F.
Chem. Commun. 2007, 4731–4733; (b) Jarowski, P. D.; Wu, Y. L.; Boudon, C.;
Gisselbrecht, J.-P.; Gross, M.; Schweizer, W. B.; Diederich, F. Org. Biomol. Chem.
11. Urši cˇ , U.; Grošelj, U.; Meden, A.; Svete, J.; Stanovnik, B. Helv. Chim. Acta 2009,
92, 481–490.
12. (a) Bax, A.; Freeman, R. J. Am. Chem. Soc. 1982, 104, 1099–1100; (b) Titman, J. J.;
Foote, J.; Jarvis, J.; Keeler, J.; Neuhaus, D. J. Chem. Soc., Chem. Commun. 1991,
419–421; (c) Ando, T.; Koseki, N.; Toia, R. F.; Casida, J. E. Magn. Reson. Chem.
1993, 31, 90–93; (d) Fischer, P.; Schweizer, E.; Langner, J.; Schmidt, U. Magn.
Reson. Chem. 1994, 32, 567–568; (e) Willker, W.; Leibfritz, D. Magn. Reson.
Chem. 1995, 33, 632–638; (f) Goli cˇ Grdadolnik, S.; Stanovnik, B. Magn. Reson.
3
2
009, 7, 1312–1322; (c) Wu, Y. L.; Jarowski, P. D.; Schweizer, W. B.; Diederich,
}
F. Chem. Eur. J. 2010, 16, 202–211.
Chem. 1997, 35, 482–486; (g) Osz, E.; Szilágyi, L.; Marton, J. J. Mol. Struct. 1998,
4
.
.
(a) Stanovnik, B. J. Heterocycl. Chem. 1999, 36, 1581–1593; (b) Stanovnik, B.;
Svete, J. Synlett 2000, 1077–1091; (c) Stanovnik, B.; Svete, J.. In Attanasi, O. A.,
Spinelli, D., Eds.; Targets in Heterocyclic Systems. Synthesis, Reactions and
Properties; Italian Society of Chemistry: Rome, 2000; Vol. 3, pp 105–137; (d)
Stanovnik, B.; Svete, J. Chem. Rev. 2004, 104, 2433–2480.
(a) Seli cˇ , L.; Jakše, R.; Lampi cˇ , K.; Goli cˇ , L.; Goli cˇ Grdadolnik, S.; Stanovnik, B.
Helv. Chim. Acta 2000, 83, 2802–2811; (b) Seli cˇ , L.; Stanovnik, B. Tetrahedron
442, 267–274; (h) Furihata, K.; Seto, H. Tetrahedron Lett. 1999, 40, 6271–6275;
(i) Seki, H.; Tokunaga, T.; Utsumi, H.; Yamaguchi, K. Tetrahedron 2000, 56,
2935–2939; (j) Tokunaga, T.; Seki, H.; Yasuike, S.; Ikoma, M.; Kurita, J.;
Yamaguchi, K. Tetrahedron Lett. 2000, 41, 1031–1034; (k) Ding, K. Magn. Reson.
Chem. 2000, 38, 321–323.
5
13. Pleier, A.-K.; Glas, H.; Grosche, M.; Sirsch, P.; Thiel, W. R. Synthesis 2001, 55–62.
14. Collect Software, Nonius, BV, Delft, The Netherlands, 1998.
15. Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307–326.
16. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.;
Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr.
1999, 32, 115–119.
2
001, 57, 3159–3164.
6
.
.
(a) Jakše, R.; Svete, J.; Stanovnik, B. Tetrahedron 2004, 60, 4601–4608; (b) Cˇ asar,
Z.; Bevk, D.; Svete, J.; Stanovnik, B. Tetrahedron 2005, 61, 7508–7519.
(a) Wagger, J.; Bevk, D.; Meden, A.; Svete, J.; Stanovnik, B. Helv. Chim. Acta 2006,
7
8
9, 240–248; (b) Wagger, J.; Goli cˇ Grdadolnik, S.; Grošelj, U.; Meden, A.;
17. Hall, S. R.; King, G. S. D.; Stewart, J. M. The Xtal3.4 User’s Manual, University of
Western Australia, Lamb, Perth, 1995.
18. Burnett, M. N.; Johnson, C. K. In ORTEP-III: Oak Ridge Thermal Ellipsoid Plot
Program for Crystal Structure Illustrations, Oak Ridge National Laboratory
Report ORNL-6895, 1996.
Stanovnik, B.; Svete, J. Tetrahedron: Asymmetry 2007, 18, 464–475; (c) Wagger, J.;
Grošelj, U.; Meden, A.; Svete, J.; Stanovnik, B. Tetrahedron 2008, 64, 2801–2815.
Wagger, J.; Svete, J.; Stanovnik, B. Synthesis 2008, 1436–1442.
8
9
.
.
Urši cˇ , U.; Grošelj, U.; Meden, A.; Svete, J.; Stanovnik, B. Tetrahedron Lett. 2008,
4
9, 3775–3778.
19. Wang, H.; Robertson, B. E. In Structure and Statistics in Crystallography; Wilson,
A. J. C., Ed.; Adenine Press: New York, 1985.
1
0. Urši cˇ , U.; Svete, J.; Stanovnik, B. Tetrahedron 2008, 64, 9937–9946.