The fluoride ion-induced intramolecular conjugate addition of propargylsilanes to dihydropyridones. A novel method for the stereoselective construction of azabicyclic ring systems

Article abstract of DOI:10.1016/j.tet.2005.06.105

The fluoride ion-induced intramolecular conjugate addition of propargylsilanes to dihydropyridones is reported. Our results revealed that tetrabutylammonium triphenyldifluorosilicate (TBAT), an air-stable, non-hygroscopic fluoride ion source, catalyzes cyclocondensation to provide the corresponding 1-vinylidene indolizidines in a high yield as single isomers, while Lewis acid catalysts were ineffective. The scope of this method was further investigated in the reactions leading to compounds with larger ring size. In these cases dihydropyridones with the propargylsilane located in the side-chain underwent cyclization to give 9-vinylidene quinolizidines with significantly lower yields.

Full text of DOI:10.1016/j.tet.2005.06.105

Tetrahedron 61 (2005) 8641–8647
The fluoride ion-induced intramolecular conjugate addition of
propargylsilanes to dihydropyridones. A novel method for the
stereoselective construction of azabicyclic ring systems
a
Magdalena Dziedzic, Grzegorz Lipner, Jose M. Illangua and Bartłomiej Furman
a
b
a,
*
´
aInstitute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
´ ´ ´
Departamento de Quımica Organica, Facultad de Quımica, Universidad de Sevilla, Apartado 553, E-41071 Sevilla, Spain
b
Received 29 April 2005; revised 13 June 2005; accepted 30 June 2005
Available online 20 July 2005
Abstract—The fluoride ion-induced intramolecular conjugate addition of propargylsilanes to dihydropyridones is reported. Our results
revealed that tetrabutylammonium triphenyldifluorosilicate (TBAT), an air-stable, non-hygroscopic fluoride ion source, catalyzes
cyclocondensation to provide the corresponding 1-vinylidene indolizidines in a high yield as single isomers, while Lewis acid catalysts were
ineffective. The scope of this method was further investigated in the reactions leading to compounds with larger ring size. In these cases
dihydropyridones with the propargylsilane located in the side-chain underwent cyclization to give 9-vinylidene quinolizidines with
significantly lower yields.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
catalyst for the allylation of aromatic imines. Majetich
et al.,6 reported that the fluoride ion-catalyst is effective in
the conjugate allylation of a,b-unsaturated esters, nitriles,
and amides, a reaction, which cannot be achieved by the
TiCl4-promoted procedure. Occasionally, intramolecular
conjugate allylation of a,b-unsaturated ketones is also
catalyzed efficiently by TBAF.7 In contrast to the reaction of
allylsilanes, little is known about the reaction of propargyl-
silanes with electrophiles under fluoride ion catalysis. To
our knowledge, the fluoride ion-promoted 1,4-addition of
propargylsilanes to the a,b-unsaturated compounds has
never been reported.
The chemistry of propargylsilanes continues to be widely
exploited in organic synthesis. The main interest in
propargylsilanes relates to their ability to react with various
electrophiles activated by a Lewis acid to form a new C–C
bond.1 With propargylsilanes, the addition of an electro-
phile provides a quite general route to allenes, which are
emerging as versatile building blocks in organic synthesis.2
The intramolecular cyclizations, involving a terminal
propargylsilane function have been successfully employed
in the stereospecific synthesis of polycyclic compounds.3
The proper choice of Lewis acid can, in some cases, enable
control of the stereoselectivity and prevent protodesilyla-
tion, which is a common drawback of such a reaction.
Recently,8 we reported that the intramolecular fluoride-
promoted allylation of the appropriate dihydropyridones
proceeds in a conjugate fashion in good yield (Scheme 1).
The Lewis acid catalyzed allylation fails, in marked contrast
to the known efficiency of this method in the intermolecular
allylation of dihydropyridones.9
The fluoride ion is generally known to be a good activating
reagent for organosilicon reagents, which readily generates
reactive pentacoordinate silicates. In 1978, Hosomi and
Sakurai4 found that a fluoride ion source—tetrabutylammo-
nium fluoride (TBAF) catalyzed the allylation of carbonyl
compounds with allylsilanes to give the corresponding
homoallyl alcohols. Recently, Wang et al.,5 have found that
˚
TBAF (1 mol%) in the presence of 4 A MS is an effective
Keywords: Conjugate addition; Fluoride ions; Propargylsilanes;
Dihydropyridones.
*
Corresponding author. Tel.: C48 223 432 128; fax: C48 226 326 681;
e-mail: furbar@icho.edu.pl
Scheme 1.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.06.105

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M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
Scheme 2. Synthetic strategy.
We now report that the similar cyclocondensation of
dihydropyridones with propargylsilanes located in the
side-chain provides a concise protocol for the highly
stereoselective construction of the vinylidene azabicyclic
skeleton. The proposed reaction sequence consists of the
Lewis acid-mediated addition of an imine to the silyloxy-
diene followed by the fluoride ion-initiated intramolecular
1,4-addition of propargylsilanes to dihydropyridones
(Scheme 2).
to the Lewis acid-mediated reaction with Danishefsky’s
diene 4. We found that for aromatic and aliphatic imines of
the type 3 the highest yield of dihydropyridones 5 were
attained using Yb(OTf)3 (0.1 equiv) as a Lewis acid with
CH3CN as a solvent (Table 1).
The reactions of the imines 3 with dienes 4 shown in Table 1
take place smoothly to afford the corresponding adducts 5 in
good yield. The aliphatic imines worked well, however, the
dihydropyridones 5h and 5i were obtained in a slightly
decreased yield.
2. Results and discussion
Since imines, particularly those derived from aliphatic
aldehydes, are usually not stable, it is synthetically useful
when they are generated in situ and allowed to react under
one-pot reaction conditions. Recently, Kobayashi et al.12
reported that Yb(OTf)3 catalyzes the three-component
coupling reaction between aldehydes, amines, and
Danishefsky’s diene to afford dihydropyridone derivatives.
The two amines 110 and 211 were employed for the
construction of the desired dihydropyridones (Fig. 1).
Following this report, we studied the direct synthesis of
dihydropyridones as one-pot reaction (Table 2).
Figure 1.
As shown in Table 2, the modified procedure afforded the
corresponding adducts in yields comparable to those
obtained by the reaction of the pre-formed and isolated
aldimines. The one-pot reaction can be carried out with
various aldehydes, including aliphatic, aromatic, and
heteroaromatic compounds.
These substrates were easily prepared in five steps using
literature methods from 3-butyn-1-ol and 4-butyn-1-ol in 52
and 35% overall yield, respectively.
The propargylamine 1 was converted into the required
Schiff bases 3 by treatment with series of different
aldehydes in CH2Cl2 in the presence of molecular sieves
˚
(4 A). These intermediates were not isolated. After
exchange of the solvent, they were immediately subjected
Very recently we reported8 that tetrabutylammonium
triphenyldifluorosilicate (TBAT)13 efficiently catalyzes the
Table 1. Lewis acid mediated synthesis of 2-substituted dihydropyridonesa
Entry
R
Product
Yield (%)b
1
2
3
4
5
6
7
8
9
Ph
p-MePh
p-MeOPh
p-ClPh
2-Furyl
2-Pyridyl
PhCH2OCH2
CH3(CH2)5
c-C6H11
5a
5b
5c
5d
5e
5f
5g
5h
5i
82
80
78
86
81
76
89
77
70
a All reactions were conducted in CH3CN at 20 8C in the presence of 0.1 equiv Yb(OTf)3.
b Isolated yield after chromatographic purification.

M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
Table 2. Results of direct synthesis of 2-substituted dihydropyridonesa
8643
with the aid of NOE experiments. Furthermore, the structure
and relative stereochemistry of the cyclized product was
unambiguously established by the single-crystal X-ray
analysis.14 As revealed in Figure 2, compound 6a possesses
the indolizidine skeleton and two hydrogens at C5 and C8a
are trans to each other.
Entry
R
Product
Time/h
Yieldb (%)
1
2
3
4
5
6
7
8
9
Ph
p-MePh
p-MeOPh
p-ClPh
2-Furyl
2-Pyridyl
PhCH2OCH2
CH3(CH2)5
c-C6H11
5a
5b
5c
5d
5e
5f
5g
5h
5i
5
6
4
5
8
9
2
3
3
80
83
81
79
74
72
88
80
77
a All reactions were conducted in CH3CN at 20 8C in the presence of
0.1 equiv Yb(OTf)3.
b Isolated yield after chromatographic purification.
intramolecular conjugate allylation of various 2-substituted
dihydropyridones with allylsilanes. To our satisfaction we
found that the use of 2.0 equiv of TBAT resulted in a nearly
quantitative formation of the corresponding vinylidene
indolizidines (Table 3). Other fluoride ion sources such as
CsF, KF or TBAF failed to promote the cyclization at all.
It is important to note that none of the desired products
could be obtained by treatment of respective dihydropyri-
dones with Lewis acids such as EtAlCl2, TiCl4, SnCl4,
TMSOTf or BF3$Et2O.
Figure 2. The X-ray structure of 6a.
Table 3. TBAT-initiated intramolecular conjugate addition of propargylsi-
lane to dihydropyridonesa
The stereochemical outcome of this cyclocondensation is
rationalized in Scheme 3.
We propose that the cyclization proceeds through the
transition state A, where the substituent R is located in the
equatorial position while the nitrogen electron pair is in the
axial orientation. The nucleophilic terminal of the double
bond approaches exclusively anti to the R substituent
leading to the compound 6 with trans-orientation between
C5 and C8a protons.
Entry
R
Product
Time/h
Yieldb (%)
1
2
3
4
5
6
7
8
9
Ph
p-MePh
p-MeOPh
p-ClPh
2-Furyl
2-Pyridyl
PhCH2OCH2
CH3(CH2)5
c-C6H11
6a
6b
6c
6d
6e
6f
6g
6h
6i
1
1
2
2
5
5
2
2
2
95
93
97
98
89
92
89
83
86
The generality of this transformation was further tested with
different ring size. The enaminones of the type 7 were
prepared by the similar strategy as described earlier, starting
from amine 2.
For the compounds with a longer side chain the
intramolecular 1,4-addition, under standard cyclization
conditions (2.0 equiv TBAT in THF at 30 8C), led to the
9-vinylidene quinolizidines 8a and 8b, obtained as single
diastereomers in 35 and 27% yield, respectively (Table 4).
However, significant amounts of protodesilylation products
were also observed. No cyclization occurred when
compound containing an aliphatic substituent 7c was used.
Numerous attempts to affect the reaction outcome, such as
applying alternative fluoride sources or selected Lewis
acids, were unsuccessful and only protodesilylation
products were observed.
a All reactions were conducted in THF at 30 8C in the presence of 2.0 equiv
of TBAT.
b Isolated yield after chromatographic purification.
For all entries in Table 3, treatment of the 2-substituted
dihydropyridones with 2.0 equiv of TBAT in THF at 30 8C
led to the desired indolizidines (single diastereomer), as
estimated from the H and 13C NMR data. It was observed
1
that, in the case of the reaction of dihydropyridones 6e and
6f (RZfuryl or pyridyl), longer reaction times (4–5 h) were
needed to afford products in a high yield. When we used
compounds containing aliphatic substituents 6h and 6i the
yield was slightly decreased, compared to other
dihydropyridones.
The relative stereochemistry of 8a was assigned by the NOE
experiments. Upon irradiation of the proton at C4 no
enhancement was observed for the proton at C9a. This
clearly reveals that the two hydrogens at C4 and C9a are
trans to each other. The 9-vinylidene quinolizidines have
similar relative stereochemistry at bridgehead carbon atoms
The relative stereochemistry at C5 and C8a was established

8644
M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
Scheme 3. Fluoride ion-initiated intramolecular 1,4-addition of propargylsilane to dihydropyridones.
Table 4. TBAT-initiated synthesis of 9-vinylidene quinolizidinesa
measured with a Perkin-Elmer FT-IR-1600 infrared spectro-
photometer. High-resolution mass spectra were taken on a
Mariner PerSeptive Biosystems mass spectrometer with
time-of-flight (TOF) detector. Unless stated otherwise, all
reagents and solvents were purchased from commercial
sources and used without additional purification.
4.2. General procedure for the synthesis of imines
To a solution of amine 1 or 2 (0.5 mmol) in CH2Cl2 (5 mL)
was added the corresponding aldehyde (0.5 mmol) and
Entry
R
Product
Time/h
Yieldb (%)
˚
activated 4 A molecular sieves (200 mg). The mixture was
1
2
3
(7a) Ph
(7b) p-MeOPh
(7c) CH3(CH2)5
8a
8b
8c
2
2
2
35
27
0c
stirred for 12 h at ambient temperature, filtered through a
Celite pad and the solvent was evaporated in vacuo. The
resulting imines were used directly in the subsequent
reactions without further purification.
a All reactions were conducted in THF at 30 8C in the presence of 2.0 equiv
of TBAT.
b Isolated yield after chromatographic purification.
c Only protodesilylation products were observed.
4.3. General procedure for the aza-Diels–Alder reaction
of Danishefsky’s diene with imines
as indolizidines discussed earlier. Thus, the similar
rationalization for the stereochemical outcome observed in
these cyclizations should be applicable as well.
To a solution of the respective imine (0.5 mmol) in MeCN
(5 mL) was added Yb(OTf)3 (0.05 mmol) followed by diene
4 (0.6 mmol, 1.2 equiv). The reaction mixture was stirred
for 3–5 h at room temperature. After addition of saturated
NaHCO3 (10 mL), the aqueous phase was extracted with
CH2Cl2 (3!10 mL), the combined organic phases were
dried over MgSO4, and the solvent was evaporated in vacuo.
The crude material was purified by flash column chromato-
graphy on silica gel.
3. Conclusions
A novel method for the stereoselective construction of
vinylidene azabicyclic compounds has been described. It is
based on the fluoride ion-induced intramolecular 1,4-
addition of propargylsilanes to dihydropyridones. The
structure and relative stereochemistry of the cyclization
product, 1-vinylidene indolizidine, was confirmed by the
NMR and X-ray analysis. The scope of this method was
further investigated in the reactions leading to the com-
pounds with larger ring size. However, the one carbon
homologated compounds did not show the same cyclization
behavior, and the expected 9-vinylidene quinolizidines were
obtained in low yields.
4.3.1. 2-Phenyl-1-(5-trimethylsilanyl-pent-3-ynyl)-2,3-
dihydro-1H-pyridin-4-one (5a). Chromatography (80:20
Et2O/hexane) afforded 0.127 g (82%) of a colorless oil: IR
(neat) 2219, 1639, 1593, 1579, 849 cmK1 1H NMR
;
(CDCl3, 400 MHz) d 0.08 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H),
2.24–2.42 (m, 2H), 2.69 (dd, JZ16.4, 8.6 Hz, 1H), 2.86 (dd,
JZ16.4, 6.7 Hz, 1H), 3.21 (m, 2H), 4.69 (dd, JZ8.6,
6.7 Hz, 1H), 5.04 (d, JZ7.7 Hz, 1H), 7.23 (d, JZ7.7 Hz,
1H), 7.30–7.39 (m, 5H); 13C NMR (CDCl3, 100 MHz) d
K2.0, 6.9, 19.4, 43.9, 52.8, 60.9, 74.2, 81.3, 98.4, 126.9,
128.3, 129.1, 138.7, 154.5, 190.3; HRMS calcd for
C19H25NOSi (MC) 311.1705, found 311.1716.
4. Experimental
4.1. General
4.3.2. 2-p-Tolyl-1-(5-trimethylsilanyl-pent-3-ynyl)-2,3-
dihydro-1H-pyridin-4-one (5b). Chromatography (80:20
Et2O/hexane) afforded 0.130 g (80%) of a colorless oil: IR
Column chromatography was performed on silica gel,
Merck grade 60 (230–400 mesh). TLC plates were
visualized with UV and staining with phosphomolybdic
(neat) 2220, 1642, 1591, 849 cmK1 1H NMR (CDCl3,
;
1
acid. H and 13C NMR spectra were recorded on Bruker
500 MHz) d 0.08 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H), 2.24–2.33
(m, 2H), 2.34 (s, 3H), 2.68 (dd, JZ16.4, 9.2 Hz, 1H), 2.81
(dd, JZ16.4, 6.6 Hz, 1H), 3.18 (m, 2H), 4.69 (dd, JZ9.2,
6.6 Hz, 1H), 5.03 (d, JZ7.6 Hz, 1H), 7.15 (d, JZ7.6 Hz,
AM500 (500 MHz) and Varian (400 MHz) spectrometers
and the chemical shifts are reported in ppm from the solvent
resonans (CDCl3 7.26 ppm). Infrared (IR) spectra were

M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
8645
1H), 7.18–7.23 (m, 4H); 13C NMR (CDCl3, 125 MHz) d
K2.0, 6.9, 19.4, 21.1, 44.1, 52.7, 60.8, 74.3, 81.3, 98.3,
126.9, 129.7, 135.7, 138.1, 154.4, 190.5; HRMS calcd for
C20H27NOSi 325.1862, found 325.1870.
yellow oil: IR (neat) 2220, 1642, 1580, 849 cmK1; 1H NMR
(CDCl3, 500 MHz) d 0.07 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H),
2.36 (dd, JZ16.8, 2.3 Hz, 1H), 2.45 (m, 2H), 2.78 (dd, JZ
16.4, 6.8 Hz, 1H), 3.29 (m, 1H), 3.46 (m, 1H), 3.55 (dt, JZ
13.0, 6.0 Hz, 1H), 3.82 (m, 2H), 4.49 (s, 2H), 4.87 (d, JZ
7.4 Hz, 1H), 6.97 (d, JZ7.4 Hz, 1H), 7.27–7.36 (m, 5H);
13C NMR (CDCl3, 125 MHz) d K2.0, 6.9, 20.6, 37.7, 54.5,
56.2, 68.7, 73.6, 74.4, 80.9, 96.8, 127.6, 127.8, 128.4, 137.6,
152.4, 190.1; HRMS calcd for C21H29NO2Si (MC)
355.1968, found 355.1979.
4.3.3. 2-(4-Methoxyphenyl)-1-(5-trimethylsilanyl-pent-
3-ynyl)-2,3-dihydro-1H-pyridin-4-one (5c). Chromato-
graphy (80:20 Et2O/hexane) afforded 0.133 g (78%) of a
yellow oil: IR (neat) 2220, 1640, 1591, 848 cmK1; 1H NMR
(CDCl3, 400 MHz) d 0.08 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H),
2.22–2.40 (m, 2H), 2.69 (dd, JZ16.4, 9.3 Hz, 1H), 2.80 (dd,
JZ16.4, 6.6 Hz, 1H), 3.18 (m, 2H), 3.81s, 3H), 4.62 (dd,
JZ9.3, 6.6 Hz, 1H), 5.02 (d, JZ7.6 Hz, 1H), 6.88 (m, 2H),
7.21 (d, JZ7.6 Hz, 1H), 7.24 (m, 2H); 13C NMR (CDCl3,
100 MHz) d K2.0, 6.9, 19.4, 30.3, 44.1, 52.6, 55.3, 81.3,
98.2, 114.4, 120.2, 128.2, 130.6, 154.5, 159.5, 190.7; HRMS
calcd for C20H27NNaO2Si (MCNa) 364.1703, found
364.1698.
4.3.8. 2-Hexyl-1-(5-trimethylsilanyl-pent-3-ynyl)-2,3-
dihydro-1H-pyridin-4-one (5h). Chromatography (60:40
Et2O/hexane) afforded 0.112 g (77%) of a colorless oil: IR
(neat) 2220, 1640, 1588, 851 cmK1 1H NMR (CDCl3,
;
500 MHz) d 0.08 (s, 9H), 0.87 (t, JZ7.1 Hz, 3H), 1.20–1.35
(m, 6H), 1.43 (t, JZ2.6 Hz, 2H), 1.57–1.75 (m, 4H), 2.32
(ddd, JZ16.3, 3.2, 1.0 Hz, 1H), 2.45 (m, 2H), 2.76 (dd, JZ
16.3, 6.8 Hz, 1H), 3.30 (m, 2H), 3.51 (m, 1H), 4.89 (dd, JZ
7.4, 1.1 Hz, 1H), 6.95 (dd, JZ7.4, 1.0 Hz, 1H); 13C NMR
(CDCl3, 125 MHz) d K2.0, 6.9, 14.0, 20.7, 22.5, 25.6, 28.9,
29.2, 31.7, 39.5, 53.6, 56.6, 74.3, 81.0, 96.9, 152.2, 190.7;
HRMS calcd for C19H34NOSi (MCH) 320.2404, found
320.2400.
4.3.4. 2-(4-Chlorophenyl)-1-(5-trimethylsilanyl-pent-3-
ynyl)-2,3-dihydro-1H-pyridin-4-one (5d). Chromatography
(90:10 Et2O/hexane) afforded 0.127 g (86%) of a colorless oil:
IR (neat) 2220, 1639, 1589, 1574, 849 cmK1 1H NMR
;
(CDCl3, 500 MHz) d 0.08 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H),
2.25–2.41 (m, 2H), 2.62 (dd, JZ16.4, 8.0 Hz, 1H), 2.88 (dd,
JZ16.4, 6.9 Hz, 1H), 3.15–3.25 (m, 2H), 4.67 (dd, JZ8.0,
6.9 Hz, 1H), 5.06 (d, JZ7.7 Hz, 1H), 7.18 (d, JZ7.7 Hz, 1H),
7.25 (m, 2H), 7.32 (m, 2H); 13C NMR (CDCl3, 125 MHz) d
K2.0,7.0,19.5, 43.7, 52.9, 60.4, 74.1,81.5,98.6, 128.3,129.3,
134.2, 137.3, 154.1, 189.8; HRMS calcd for C19H24-
ClNNaOSi (MCNa) 368.1208, found 368.1207.
4.3.9. 2-Cyclohexyl-1-(5-trimethylsilanyl-pent-3-ynyl)-2,
3-dihydro-1H-pyridin-4-one (5i). Chromatography (60:40
Et2O/hexane) afforded 0.100 g (70%) of a colorless oil: IR
(neat) 2220, 1638, 1586, 850 cmK1 1H NMR (CDCl3,
;
400 MHz) d 0.08 (s, 9H), 0.86–1.31 (m, 6H), 1.42 (t, JZ
2.7 Hz, 2H), 1.69–1.93 (m, 5H), 2.41–2.49 (m, 3H), 2.76
(dd, JZ16.5, 7.4 Hz, 1H), 3.26–3.39 (m, 3H), 4.87 (dd, JZ
7.3, 1.1 Hz, 1H), 7.02 (dd, JZ7.3, 1.0 Hz, 1H); 13C NMR
(CDCl3, 100 MHz) d K2.0, 6.9, 21.0, 26.1, 28.8, 30.1, 37.4,
38.7, 54.8, 61.4, 74.3, 80.7, 97.2, 152.8, 192.2; HRMS calcd
for C19H32NOSi (MCH) 318.2248, found 318.2239.
4.3.5. 2-Furan-2-yl-1-(5-trimethylsilanyl-pent-3-ynyl)-2,
3-dihydro-1H-pyridin-4-one (5e). Chromatography (70:30
EtOAc/hexane) afforded 0.120 g (81%) of a yellow oil: IR
(neat) 2217, 1644, 1589, 848 cmK1 1H NMR (CDCl3,
;
500 MHz) d 0.08 (s, 9H), 1.42 (t, JZ2.6 Hz, 2H), 2.35–2.51
(m, 2H), 2.78 (dd, JZ16.4, 6.5 Hz, 1H), 2.85 (dd, JZ16.4,
6.6 Hz, 1H), 3.27 (m, 1H), 3.38 (m, 1H), 4.76 (m, 1H), 4.99
(d, JZ7.6 Hz, 1H), 6.30 (d, JZ3.3 Hz, 1H), 6.33 (dd, JZ
3.3, 1.8 Hz, 1H), 7.03 (d, JZ7.6 Hz, 1H), 7.39 (d, JZ
1.8 Hz, 1H); 13C NMR (CDCl3, 125 MHz) d K2.0, 6.9,
20.0, 39.9, 53.5, 54.1, 74.4, 81.1, 98.0, 108.5, 110.4, 142.6,
151.2, 152.8, 190.3; HRMS calcd for C17H23NO2Si (MC)
301.1498, found 301.1494.
4.3.10. 2-Phenyl-1-(6-trimethylsilanyl-hex-4-ynyl)-2,3-
dihydro-1H-pyridin-4-one (7a). Chromatography (70:30
Et2O/hexane) afforded 0.123 g (78%) of a colorless oil: IR
(neat) 2219, 1641, 1593, 1579, 850 cmK1 1H NMR
;
(CDCl3, 500 MHz) d 0.06 (s, 9H), 1.39 (t, JZ2.6 Hz, 2H),
1.60–1.72 (m, 2H), 2.12–2.23 (m, 2H), 2.67 (dd, JZ16.4,
7.7 Hz, 1H), 2.88 (dd, JZ16.4, 7.1 Hz, 1H), 3.18 (m, 1H),
3.27 (m, 1H), 4.61 (t, JZ7.4 Hz, 1H), 5.01 (d, JZ7.6 Hz,
1H), 7.20 (d, JZ7.6 Hz, 1H), 7.28–7.37 (m, 5H); 13C NMR
(CDCl3, 125 MHz) d K2.0, 6.9, 16.0, 28.1, 43.7, 52.3, 61.0,
76.5, 79.3, 98.3, 126.9, 128.2, 129.0, 138.7, 154.1, 190.0;
HRMS calcd for C20H28NOSi (MCH) 326.1935, found
326.1940.
4.3.6. 1-(5-Trimethylsilanyl-pent-3-ynyl)-2,3-dihydro-
1H-[2,20]bipyridinyl-4-one (5f). Chromatography (30:70
i-PrOH/hexane) afforded 0.118 g (76%) of a brown oil: IR
(neat) 2219, 1640, 1594, 1584, 850 cmK1 1H NMR
;
(CDCl3, 500 MHz) d 0.08 (s, 9H), 1.43 (t, JZ2.6 Hz, 2H),
2.33–2.50 (m, 2H), 2.85 (dd, JZ16.5, 5.6 Hz, 1H), 2.99 (dd,
JZ16.4, 7.5 Hz, 1H), 3.33 (m, 2H), 4.83 (dd, JZ7.5,
5.5 Hz, 1H), 5.01 (d, JZ7.6 Hz, 1H), 7.21 (d, JZ7.6 Hz,
1H), 7.22 (ddd, JZ7.7, 4.8, 1.1 Hz, 1H), 7.33 (d, JZ7.7 Hz,
1H), 7.67 (m, 1H), 8.6 (m, 1H); 13C NMR (CDCl3,
125 MHz) d K2.0, 6.9, 19.8, 41.5, 53.8, 61.8, 74.2, 81.2,
98.3, 121.0, 122.9, 136.9, 150.0, 153.7, 157.9, 189.9; HRMS
calcd for C18H24N2OSi (MC) 312.1658, found 312.1652.
4.3.11. 2-(4-Methoxyphenyl)-1-(6-trimethylsilanyl-hex-
4-ynyl)-2,3-dihydro-1H-pyridin-4-one (7b). Chromato-
graphy (70:30 Et2O/hexane) afforded 0.125 g (71%) of a
colorless oil: IR (neat) 2219, 1641, 1593, 1579, 850 cmK1
;
1H NMR (CDCl3, 500 MHz) d 0.06 (s, 9H), 1.39 (t, JZ
2.6 Hz, 2H), 1.58–1.68 (m, 2H), 2.10–2.23 (m, 2H), 2.66
(dd, JZ16.4, 8.3 Hz, 1H), 2.82 (dd, JZ16.4, 6.8 Hz, 1H),
3.17 (m, 1H), 3.23 (m, 1H), 3.80 (s, 3H), 4.55 (dd, JZ8.3,
6.8 Hz, 1H), 5.01 (d, JZ7.6 Hz, 1H), 6.88 (m, 2H), 7.17 (d,
JZ7.6 Hz, 1H), 7.22 (m, 2H); 13C NMR (CDCl3, 125 MHz)
d K2.0, 6.9, 16.0, 28.1, 43.9, 52.0, 55.3, 60.5, 76.5, 79.2,
4.3.7. 2-Benzyloxymethyl-1-(5-trimethylsilanyl-pent-3-
ynyl)-2,3-dihydro-1H-pyridin-4-one (5g). Chromato-
graphy (70:30 Et2O/hexane) afforded 0.150 g (89%) of a

8646
M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
98.1, 114.4, 128.2, 130.8, 154.1, 159.5, 190.4; HRMS calcd
for C21H29NNaO2Si (MCNa) 378.1860, found 378.1852.
103.1, 127.9, 129.1, 136.4, 137.4, 201.4, 208.9; HRMS
calcd for C17H19NO (MC) 253.1467, found 253.1468.
4.5.3. (5R*,8aR*)-5-(4-Methoxyphenyl)-1-vinylidene-
hexahydro-indolizin-7-one (6c). Chromatography (40:60
Et2O/hexane) afforded 0.025 g (97%) of a yellow oil: IR
4.3.12. 2-Hexyl-1-(6-trimethylsilanyl-hex-4-ynyl)-2,3-
dihydro-1H-pyridin-4-one (7c). Chromatography (60:40
Et2O/hexane) afforded 0.107 g (65%) of a colorless oil: IR
1
(neat) 1965, 1718, 1512, 1251 cmK1; H NMR (CDCl3,
(neat) 2220, 1641, 1587, 850 cmK1 1H NMR (CDCl3,
;
500 MHz) d 2.47–2.76 (m, 7H), 2.88 (ddd, JZ10.9, 7.7,
3.3 Hz, 1H), 3.80 (s, 3H), 3.81 (m, 1H), 4.10 (t, JZ6.1 Hz,
1H), 4.81 (dq, JZ10.0, 4.5 Hz, 1H), 4.89 (dq, JZ10.0,
4.4 Hz, 1H), 6.89 (m, 2H), 7.14 (m, 2H); 13C NMR (CDCl3,
125 MHz) d 28.3, 43.3, 47.1, 49.8, 55.3, 58.1, 59.9, 79.1,
103.2, 113.8, 129.1, 131.7, 159.1, 201.4, 208.8; HRMS
calcd for C17H20NO2 (MCH) 270.1489, found 270.1501.
500 MHz) d 0.1 (s, 9H), 0.88 (t, JZ7.0 Hz, 3H) 1.15–1.37
(m, 7H), 1.44 (t, JZ2.6 Hz, 2H), 1.53–1.62 (m, 2H), 1.70–
1.80 (m, 3H), 2.20–2.28 (m, 2H), 2.33 (dd, JZ16.4, 3.0 Hz,
1H), 2.73 (dd, JZ16.4, 6.8 Hz, 1H), 3.32 (m, 2H), 3.44 (m,
1H), 4.88 (d, JZ7.4 Hz, 1H), 6.94 (d, JZ7.4 Hz, 1H); 13C
NMR (CDCl3, 125 MHz) d K2.0, 6.9, 14.0, 16.0, 22.5,
25.6, 28.6, 29.1, 29.2, 31.7, 39.4, 52.6, 56.7, 76.5, 79.3,
96.7, 152.3, 190.4; HRMS calcd for C20H36NOSi (MCH)
334.2561, found 334.2575.
4.5.4. (5R*,8aR*)-5-(4-Chlorophenyl)-1-vinylidene-hexa-
hydro-indolizin-7-one (6d). Chromatography (60:40
Et2O/hexane) afforded 0.026 g (98%) of a yellow oil: IR
4.4. General procedure for the direct aza-Diels–Alder
reaction
(neat) 1965, 1719 cmK1 1H NMR (C6D6, 500 MHz) d
;
2.20–2.35 (m, 6H), 2.43 (dd, JZ15.4, 8.5 Hz, 1H), 2.54 (dd,
JZ15.4, 4.8 Hz, 1H), 3.53 (t, JZ6.5 Hz, 1H), 3.64 (m, 1H),
4.70 (m, 1H), 4.78 (m, 1H), 6.82 (m, 2H), 7.09 (m, 2H); 13C
NMR (CDCl3, 125 MHz) d 28.3, 42.8, 46.9, 50.1, 58.4,
60.3, 79.2, 102.9, 128.6, 129.1, 133.4, 138.6, 201.4, 208.2;
HRMS calcd for C16H17ClNO (MCH) 274.0993, found
274.1006.
The aldehyde (0.1 mmol), amine 1 or 2 (0.1 mmol) and
Danishefsky’s diene (0.11 mmol) were dissolved in MeCN
(5 mL) at room temperature and Yb(OTf)3 (0.01 mmol) was
added in one portion. The reaction mixture was stirred for
2–9 h. Then water was added and the mixture was extracted
with CH2Cl2 (3!5 mL) and the combined organic layers
were washed with brine and dried (MgSO4). The residue
was purified by flash column chromatography.
4.5.5. (5R*,8aR*)-5-Furan-2-yl-1-vinylidene-hexahydro-
indolizin-7-one (6e). Chromatography (40:60 t-BuOMe/
hexane) afforded 0.020 g (89%) of a yellow oil: IR (neat)
4.5. General procedure for the TBAT-mediated
cyclocondensation
1
1972, 1722 cmK1; H NMR (CDCl3, 500 MHz) d 2.42 (m,
1H), 2.50–2.77 (m, 5H), 2.88 (dd, JZ15.3, 6.9 Hz, 1H),
3.10 (m, 1H), 3.34 (m, 1H), 4.47 (dd, JZ6.9, 2.3 Hz, 1H),
4.79 (dq, JZ9.8, 4.8 Hz, 1H), 4.89 (dq, JZ9.9, 4.6 Hz, 1H),
6.13 (d, JZ3.2 Hz, 1H), 6.31 (dd, JZ3.2, 1.8 Hz, 1H), 7.36
(d, JZ1.8 Hz, 1H); 13C NMR (CDCl3, 125 MHz) d 28.4,
44.0, 45.3, 49.1, 53.2, 57.3, 79.2, 103.5, 109.1, 109.9, 142.4,
152.1, 201.1, 207.5; HRMS calcd for C14H15NO2 (MC)
229.1103, found 229.1106.
The dihydropyridones (0.1 mmol) and TBAT (0.2 mmol)
were dissolved in THF (5 mL) at 30 8C. After 20 min, the
reaction turned orange, and after 1–5 h the reaction was
complete (TLC monitoring). The THF was removed under
reduced pressure, the residue was redissolved in Et2O
(10 mL), and filtered. The filtrate was concentrated, and the
crude material obtained was purified by flash column
chromatography.
4.5.6. (5R*,8aR*)-5-Pyridin-2-yl-1-vinylidene-hexahydro-
indolizin-7-one (6f). Chromatography (50:50 t-BuOMe/
hexane) afforded 0.022 g (92%) of a yellow oil: IR (neat)
1967, 1712 cmK1; 1H NMR (CDCl3, 500 MHz) d 2.44–2.54
(m, 3H), 2.65–2.80 (m, 4H), 3.01 (m, 1H), 3.75 (m, 1H),
4.37 (t, JZ4.8 Hz, 1H), 4.79 (dq, JZ9.9, 4.7 Hz, 1H), 4.87
(dq, JZ9.8, 4.6 Hz, 1H), 7.17 (ddd, JZ1.1, 4.8, 7.7 Hz,
1H), 7.21 (d, JZ7.7 Hz, 1H), 7.66 (m, 1H), 8.57 (m, 1H);
13C NMR (CDCl3, 125 MHz) d 28.3, 44.0, 44.6, 49.3, 57.0,
61.1, 79.0, 103.3, 122.3, 122.8, 136.1, 149.2, 157.9, 201.1,
207.6; HRMS calcd for C15H16N2NaO (MCNa) 263.1155,
found 263.1152.
4.5.1. (5R*,8aR*)-5-Phenyl-1-vinylidene-hexahydro-
indolizin-7-one (6a). Chromatography (50:50 Et2O/hexane)
afforded 0.023 g (95%) of white needles. Recrystallisation
from TBME/pentane afforded 6a as white needles: mp 118–
119 8C; IR (neat) 1966, 1718 cmK1 1H NMR (CDCl3,
;
500 MHz) d 2.52 (m, 1H), 2.57–2.75 (m, 6H), 2.88 (m, 1H),
3.85 (m, 1H), 4.12 (dd, JZ7.1, 5.5 Hz, 1H), 4.81 (dq, JZ
9.9, 4.4 Hz, 1H), 4.89 (dq, JZ10.0, 4.4 Hz, 1H), 7.23–7.36
(m, 5H); 13C NMR (CDCl3, 125 MHz) d 28.3, 43.1, 47.0,
49.9, 58.3, 60.7, 79.1, 103.1, 127.7, 127.9, 128.4, 139.8,
201.4, 208.6; HRMS calcd for C16H17NO (MC) 239.1310,
found 239.1320.
4.5.7. (5R*,8aR*)-5-Benzyloxymethyl-1-vinylidene-hexa-
hydro-indolizin-7-one (6g). Chromatography (40:60 t-
BuOMe/hexane) afforded 0.025 g (89%) of a colorless oil:
IR (neat) 1965, 1713 cmK1; 1H NMR (CDCl3, 500 MHz) d
2.36 (m, 1H), 2.41–2.51 (m, 2H), 2.59–2.71 (m, 3H), 3.03
(m, 2H), 3.44 (m, 1H), 3.51 (dd, JZ9.7, 4.6 Hz, 1H), 3.58
(dd, JZ9.7, 4.6 Hz, 1H), 3.95 (m, 1H), 4.49 (d, JZ2.2 Hz,
2H), 4.80 (m, 1H), 4.84 (m, 1H), 7.27–7.36 (m, 5H); 13C
NMR (CDCl3, 125 MHz) d 28.7, 42.2, 44.0, 49.6, 56.2,
58.4, 70.3, 73.4, 78.9, 103.9, 127.5, 127.6, 128.4, 137.9,
4.5.2. (5R*,8aR*)-5-p-Tolyl-1-vinylidene-hexahydro-
indolizin-7-one (6b). Chromatography (50:50 Et2O/hexane)
afforded 0.023 g (93%) of a colorless oil: IR (neat) 1966,
1
1715 cmK1; H NMR (CDCl3, 400 MHz) d 2.35 (s, 3H),
2.47–2.77 (m, 7H), 2.89 (m, 1H), 3.85 (m, 1H), 4.06 (t, JZ
6.1 Hz, 1H), 4.81 (dq, JZ9.9, 4.4 Hz, 1H), 4.89 (dq, JZ
10.0, 4.4 Hz, 1H), 7.10–7.18 (m, 4H); 13C NMR (CDCl3,
100 MHz) d 21.0, 28.2, 43.2, 46.9, 49.7, 58.1, 60.2, 79.1,

M. Dziedzic et al. / Tetrahedron 61 (2005) 8641–8647
8647
201.0, 208.3; HRMS calcd for C18H21NO2 (MC) 283.1572,
found 283.1578.
References and notes
1. (a) Marshal, J. A.; Adams, N. D. J. Org. Chem. 1997, 62,
8976–8977. (b) Kobayashi, S.; Nishio, K. J. Am. Chem. Soc.
1995, 117, 6392–6393. (c) Santelli, M.; Abed, D. E.; Jellal, A.
J. Org. Chem. 1986, 51, 1199–1206. (d) Pornet, J.; Miginiac,
L. Tetrahedron 1986, 42, 2017–2024. (e) Pornet, J.; Miginiac,
L.; Jaworski, K.; Randrianoelina, B. Organometallics 1985, 4,
333–338. (f) Pornet, J.; Mesnard, D.; Miginiac, L. Tetrahedron
Lett. 1982, 23, 4083–4086. (g) Pornet, J.; Kolani, N.; Mesnard,
D.; Miginiac, L.; Jaworski, K. J. Organomet. Chem. 1982, 236,
177–183. (h) Pornet, J. Tetrahedron Lett. 1981, 22, 453–454.
(i) Pornet, J.; Randrianoelina, B. Tetrahedron Lett. 1981, 22,
1327–1328. (j) Pornet, J.; Kolani, N. Tetrahedron Lett. 1981,
22, 3609–3610. (k) Pornet, J. Tetrahedron Lett. 1980, 21,
2049–2050.
4.5.8. (5S*,8aR*)-5-Hexyl-1-vinylidene-hexahydro-indo-
lizin-7-one (6h). Chromatography (40:60 Et2O/hexane)
afforded 0.020 g (83%) of a colorless oil: IR (neat) 1967,
1
1715 cmK1; H NMR (CDCl3, 500 MHz) d 0.87 (t, JZ
7.1 Hz, 3H), 1.20–1.56 (m, 10H), 2.28 (ddd, JZ14.2, 4.0,
1.5 Hz, 1H), 2.34–2.44 (m, 2H), 2.62 (dd, JZ14.2, 5.6 Hz,
1H), 2.68 (m, 2H), 2.92–3.03 (m, 2H), 3.21 (m, 1H), 3.79
(m, 1H), 4.80 (m, 1H), 4.84 (m, 1H); 13C NMR (CDCl3,
125 MHz) d 14.0, 22.6, 26.5, 28.8, 29.3, 29.8, 31.7, 43.6,
44.3, 48.9, 57.1, 57.9, 78.8, 103.9, 201.1, 209.3; HRMS
calcd for C16H26NO (MCH) 248.2009, found 248.2009.
4.5.9. (5R*,8aR*)-5-Cyclohexyl-1-vinylidene-hexahydro-
indolizin-7-one (6i). Chromatography (40:60 Et2O/hexane)
afforded 0.021 g (86%) of a colorless oil: IR (neat) 1966,
2. For reviews, see: (a) Shuster, H. F.; Coppola, G. M. Allenes in
Organic Synthesis; Wiley: New York, 1984. (b) Landor, S. R.
The Chemistry of the Allenes; Academic: New York, 1982. (c)
Patai, S. The Chemistry of Ketenes, Allenes and Related
Compounds; Wiley: Chichester, 1980. (d) Hashmi, A. S. K.
Angew. Chem., Int. Ed. 2000, 39, 3590–3593.
1
1714 cmK1; H NMR (CDCl3, 500 MHz) d 0.83–0.96 (m,
2H), 1.09–1.25 (m, 4H), 1.37 (m, 1H), 1.63–1.86 (m, 4H),
2.30 (ddd, JZ15.0, 4.5, 1.5 Hz, 1H), 2.36–2.42 (m, 2H),
2.49 (dd, JZ14.4, 5.1 Hz, 1H), 2.64 (m, 1H), 2.74 (m, 1H),
2.90 (m, 1H), 3.00 (m, 1H), 3.07 (m, 1H), 4.05 (m, 1H),
4.79–4.83 (m, 2H); 13C NMR (CDCl3, 125 MHz) d 26.2,
26.5, 29.2, 30.4, 39.3, 40.2, 43.2, 50.5, 58.8, 63.2, 78.4,
104.3, 201.1, 209.9; HRMS calcd for C16H24NO (MCH)
246.1852, found 246.1853.
3. (a) Karstens, W. F. J.; Moolenaar, M. J.; Rutjes, F. P. J. T.;
Grabowska, U.; Speckamp, W. N.; Hiemstra, H. Tetrahedron
Lett. 1999, 40, 8629–8632. (b)Luker, T.;Koot, W-J. ;Speckamp,
W. N.; Hiemstra, H. J. Org. Chem. 1998, 63, 220–221. (c)
Schinzer, D.; Ruppelt, M. Chem. Ber. 1991, 124, 247–248. (d)
Guay, D.; Johnson, W. S.; Schubert, U. J. Org. Chem. 1989, 54,
4731–4732. (e) Johnson, W. S.; Elliott, J. D.; Hanson, G. J. J. Am.
Chem. Soc. 1984, 106, 1138–1139. (f) Schinzer, D. Synthesis
1988, 263–273. (g) Schinzer, D.; Dettmer, G.; Ruppelt, M.;
Solyom, S.; Steffen, J. J. Org. Chem. 1988, 53, 3823–3828. (h)
Schinzer, D.; Solyom, S.; Steffen, J. J. Chem. Soc., Chem.
Commun. 1986, 829–830. (i) Schinzer, D.; Solyom, S.; Becker,
M. Tetrahedron Lett. 1985, 26, 1831–1834.
4.5.10. (4R*,9aR*)-4-Phenyl-9-vinylidene-octahydro-
quinolizin-2-one (8a). Chromatography (40:60 Et2O/hexane)
afforded 0.008 g (35%) of a colorless oil: IR (neat) 1968,
1
1720 cmK1; H NMR (C6D6, 500 MHz) d 0.88–0.95 (m,
1H), 1.58–1.70 (m, 1H), 1.91 (m, 1H), 2.30–2.41 (m, 2H),
2.47 (dd, JZ14.6, 9.5 Hz, 1H), 2.53–2.62 (m, 2H), 2.74
(ddd, JZ14.8, 3.6, 2.1 Hz, 1H), 2.82 (m, 1H), 3.61 (m, 1H),
4.12 (dd, JZ9.5, 4.2 Hz, 1H), 4.76 (dt, JZ10.1, 4.1 Hz,
1H), 4.84 (dq, JZ13.7, 3.5 Hz, 1H), 7.13–7.25 (m, 5H); 13C
NMR (CDCl3, 125 MHz) d 20.4, 29.9, 44.0, 49.0, 49.8,
57.7, 59.4, 77.5, 98.8, 127.8, 127.7, 140.8, 203.7, 207.4;
HRMS calcd for C17H19NO (MC) 253.1467, found
253.1462.
4. Hosomi, A.; Shirahata, A.; Sakurai, H. Tetrahedron Lett. 1978,
19, 3043–3046.
5. Wang, D.-K.; Zhou, Y.-G.; Tang, Y.; Hou, X.-L.; Dai, L.-X.
J. Org. Chem. 1999, 64, 4233–4237.
6. Majetich, G.; Casares, A.; Chapman, D.; Behnke, J. J. Org.
Chem. 1986, 51, 1745–1753.
7. (a) Majetich, G.; Desmond, R. W., Jr.; Soria, J. J. J. Org. Chem.
1986, 51, 1753–1769. (b) Majetich, G.; Hull, K.; Casares, A. M.;
Khateni, V. J. Org. Chem. 1991, 56, 3958–3973.
4.5.11. (4R*,9aR*)-4-(4-Methoxyphenyl)-9-vinylidene-
octahydro-quinolizin-2-one (8b). Chromatography (40:60
Et2O/hexane) afforded 0.006 g (27%) of a brown oil: IR
8. Furman, B.; Dziedzic, M. Tetrahedron Lett. 2003, 44,
6629–6631.
1
(neat) 1972, 1718 cmK1; H NMR (CDCl3, 500 MHz) d
9. Sato, M.; Aoyagi, S.; Yago, S.; Kibayashi, Ch. Tetrahedron
Lett. 1996, 37, 9063–9066.
0.83–0.94 (m, 1H), 1.70–1.80 (m, 1H), 2.10–2.21 (m, 1H),
2.38–2.43 (m, 1H), 2.49–2.54 (m, 2H), 2.57–2.64 (m, 1H),
2.68 (dd, JZ15.2, 3.8 Hz, 1H), 2.73 (dd, JZ15.2, 6.0 Hz,
1H), 2.86–2.92 (m, 1H), 3.81 (s, 3H), 3.84–3.89 (m, 1H),
4.13–4.17 (m, 1H), 4.74–4.81 (m, 2H), 6.88 (d, JZ8.7 Hz,
2H) 7.23 (d, JZ8.7 Hz, 2H); HRMS calcd for C18H21NO2
(MC) 283.1573, found 283.1582.
10. Hiemstra, H.; Fortgens, H. P.; Stegenga, S.; Speckamp, W. N.
Tetrahedron Lett. 1985, 26, 3151–3154.
11. Wasserman, H. H.; Vu, Ch. B.; Cook, J. D. Tetrahedron 1992,
48, 2101–2112.
12. Kobayashi, S.; Ishitani, H.; Nagayama, S. Synthesis 1995,
1195–1202.
13. Pilcher, A. S.; Ammon, H.; DeShong, P. J. Am. Chem. Soc.
1995, 117, 5166–5167.
Acknowledgements
14. Crystallographic data for the structure 6a in this paper have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication number CCDC 270291.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK [fax: C44 1223 336033 or e-mail: deposit@ccdc.cam.ac.
uk].
The authors would like to express our sincere gratitude to
Prof. Z. Lipkowska (Institute of Organic Chemistry PAS)
for X-ray crystallographic analysis. We also thank the State
Committee for Scientific Research (Grant 4 T09A 064 25)
for financial support.