Stannylated allylsulfones as versatile new building blocks

Article abstract of DOI:10.1055/s-2005-868478

Propargylic sulfones undergo regioselective hydrostannation in the presence of Mo(CO)₃(CNt-Bu)₃(MoBl₃), giving rise to stannylated allyl sulfones. These are interesting building blocks, because in the first step the vinyls

Full text of DOI:10.1055/s-2005-868478

LETTER
1271
Stannylated Allylsulfones as Versatile New Building Blocks
S
tannylated Allyls
l
ulfone
s
i
as Versatile
K
N
ew Building
B
lo
a
cks zmaier,* Alexander Wesquet
Institut für Organische Chemie, Universität des Saarlandes, Postfach 151150, 66041 Saarbrücken, Germany
Fax +49(681)3022409; E-mail: u.kazmaier@mx.uni-saarland.de
Received 1 March 2005
Abstract: Propargylic sulfones undergo regioselective hydrostan-
nation in the presence of Mo(CO)3(CNt-Bu)3(MoBl3), giving rise to
stannylated allyl sulfones. These are interesting building blocks, be-
cause in the first step the vinylstannane subunit can be modified via
Stille coupling, and in the second step the sulfone can be subjected
to a Julia–Lythgoe olefination. In principle, modifications at the
double bond should also be possible after the Stille coupling.
Figure 1
Stannane 1 was easily obtained from phenyl propargyl-
sulfone, which was prepared from thiophenol via propar-
gylation and subsequent oxidation. Under optimised
conditions (K2CO3, MeOH) phenyl propargylsulfide was
obtained nearly exclusively in excellent yields
(Scheme 2). Only traces of the corresponding allenyl-
sulfide could be detected. The next step was carried out
either with KMnO4 in acetic acid, or with H2O2 in an ace-
tic acid/diethyl ether mixture.9 Both protocols provided
the required propargylsulfone in 76% overall yield after
crystallisation. With this sulfone in hand, we investigated
the central step of our sequence, the hydrostannylation.
We found that it was not as trivial as expected. In contrast
to our previously investigated substrates we did not obtain
only the expected vinylstannane 1, but also the isomerised
product 1¢, and in a few cases also some distannylated
product,10 depending on the reaction conditions. But un-
der the optimised conditions shown in Scheme 2, these
side reactions could nearly be suppressed.11 Microwave
irradiation was definitely superior to conventional heat-
ing. In contrast to 20 hours refluxing in THF, the reaction
was finished in the focused microwave after 17 minutes at
150 W. Under these conditions the reaction temperature
raised to 135 °C building up a pressure of 6.4 bar. Even
under these relatively drastic conditions, radical hy-
drostannations could be suppressed completely by adding
2% hydroquinone to the reaction mixture.
Key words: hydrostannation, Julia–Lythgoe olefination, Stille
coupling, sulfones
Olefinations are important C–C coupling reactions in or-
ganic syntheses. Since the first report by Julia and Paris1
in 1973, the Julia–Lythgoe olefination2 became an impor-
tant alternative to the Wittig or Horner–Wadworth–
Emmons reaction.3 Starting from alkylsulfones A, their
deprotonation and reaction with an aldehyde gives rise to
b-hydroxysulfones B. Conversion of the hydroxy group
into a better leaving group and reduction with sodium
amalgam or samarium(III) iodide gives rise to the desired
alkene C, very often with good E-selectivity (Scheme 1).4
As O-leaving groups, several functionalities can be used.
In the original work by Julia, the hydroxysulfone was
converted into the corresponding acetate, tosylate or me-
sylate,1 but according to Barton et al. xanthates and thio-
carbonates also can be used.5 The mechanism of the
reductive elimination with sodium amalgam and samari-
um(III) iodide was investigated by Keck, who found dif-
ferent mechanisms depending on the reducing agent.6
Scheme 1 Julia–Lythgoe olefination.
To increase the flexibility of this synthetic protocol, e.g.
under a combinatorial point of view, we tried to combine
the olefination with cross coupling reactions such as Stille
couplings.7 For this purpose we were interested in the syn-
thesis of stannylated allylsulfone 1 (Figure 1). Very re-
cently we developed a new catalyst for hydrostannations
of alkynes based on molybdenum [Mo(CO)3(CNt-Bu)3,
MoBl3], which gives excellent regioselectivities with
electron-poor alkynes.8
Scheme 2 Reagents and conditions: (a) K2CO3, MeOH, 0 °C, 30
min; (b) 35% H2O2, HOAc/Et2O (2:1), reflux, 5 h; (c) 1.1 equiv
Bu3SnH, 3 mol% Mo(CO)3(CNt-Bu)3, 2% hydroquinone, THF,
microwave 150 W, 17 min.
SYNLETT 2005, No. 8, pp 1271–1274
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7.
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5.
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Advanced online publication: 21.04.2005
DOI: 10.1055/s-2005-868478; Art ID: G08005ST
© Georg Thieme Verlag Stuttgart · New York

1272
U. Kazmaier, A. Wesquet
LETTER
mixture 7s/8 can directly be used for reduction. Com-
pound 9 was formed with clean E-olefin configuration, in-
dicated by a J = 16.2 Hz coupling between the two vinylic
1
protons in the H NMR spectrum.16 As an alternative to
the xanthate route we also investigated the ‘classical con-
ditions’. Therefore, the b-hydroxysulfones 6a/6s were
converted into the corresponding acetates 10a/10s in ex-
cellent yield, which were also reduced under the condi-
tions described. But in this case, alkene 9 was obtained
only in 50% yield (Scheme 4).
Scheme 3 Reagents and conditions: 2 mol% [Pd(allyl)Cl]2, 4 mol%
PPh3; (a) 2 equiv PhCOCl, THF, 50 °C, 18h; (b) 2 equiv cinnamoyl
chloride, THF, 55 °C, 6 h; (c) 2 equiv allyl bromide, THF, 60 °C, 17
h; (d) 2.5 equiv BnBr, THF, 60 °C, 17 h.
With vinylstannane 1 in hand, we next investigated its
cross coupling reactions with several electrophiles using
allylpalladium chloride dimer (2 mol%)/PPh3 (4 mol%) as
a catalyst. Good results were obtained in the couplings of
acylhalides such as benzoyl- and cinnamoyl chloride (2,
3), with allyl- and benzylbromide the coupling products 4
and 5 were obtained in quantitative yield (Scheme 3).12
While 2 and 3 are interesting candidates for example for
Michael additions, we used 5 for our further investiga-
tions concerning the Julia–Lythgoe olefination. Aldol-
type addition of deprotonated 5 towards benzaldehyde
provided b-hydroxysulfone 6 as a 1:1 anti/syn diastereo-
meric mixture (6a/6s).13
Scheme 4 Reagents and conditions: (a) 1.05 equiv BuLi, THF,
–40 °C, 30 min, r.t., 3 min; (b) 1.1 equiv PhCHO, THF, –40 °C,
30 min; (c) 2.5 equiv CS2, THF, –40 °C, 2 h, –20 °C; (d) 3 equiv MeI,
THF, –40 °C, 2 h to r.t.; (e) 5 equiv Na/Hg (20% Na), MeOH–THF
(2:3), –20 °C, 3 h; (f) Ac2O, 5% pyridine, r.t., 2 h.
First of all we wanted to use the xanthate approach of
Barton for the radical elimination.5 Therefore we tried to
quench the ‘aldol’ reaction with CS2/MeI to convert the
mixture of 6 directly into the corresponding xanthates 7.
Surprisingly, only the xanthate 7s of the syn-isomer was
obtained, while the anti-isomer obviously underwent
elimination towards 8. Such an elimination was also re-
ported by Keck et al. as a crucial step in Julia–Lythgoe
olefinations using sodium amalgam as reducing agent.6
Therefore one might expect that 8 can also be reduced to
the alkene required. Obviously only the anti-isomer 7a
can take up a conformation suitable for elimination, while
7s cannot. 7s and 8 were formed as a 2:1 mixture, a ratio
different from that in the simple aldol step. This might be
caused by reversibility of the aldol step. To prove this, the
diastereomers 6a/6s were separated by flash chromatogra-
phy and pure 6a was subjected to the conditions of xan-
thate formation. And indeed, a ratio 7s/8 was obtained
very close to that of the direct approach.14
In conclusion we have shown, that stannylated allyl-
sulfone 1 is an interesting building block, because the two
different functionalities allow a wide range of modifica-
tions. Further investigations to improve the yield of the
hydrostannation step and to find other applications of the
allylsulfone chemistry are in progress.
Acknowledgment
Financial support by the Deutsche Forschungsgemeinschaft as well
as the Fonds der Chemischen Industrie is gratefully acknowledged.
We also thank Prof. J. Jauch for providing the microwave facilities.
References
Compound 7s was than subjected to the reductive elimi-
nation. According to Bernard and Piras et al.15 sodium
amalgam (5 equiv) was used in MeOH–THF and the reac-
tion was kept at –20 °C. If the reaction was quenched after
45 minutes, the expected alkene 9 was obtained in 54%
yield, accompanied by 18% of 8, the postulated interme-
diate.6 But after 3.5 hours at this temperature, quantitative
conversion of 7s into 9 was observed. Therefore the
(1) Julia, M.; Paris, J.-M. Tetrahedron Lett. 1973, 49, 4833.
(2) (a) Kocienski, P. J.; Lythgoe, B.; Ruston, S. J. Chem. Soc.,
Perkin Trans. 1 1978, 829. (b) Kocienski, P. J.; Lythgoe, B.;
Waterhouse, U. J. Chem. Soc., Perkin Trans. 1 1980, 1045.
(3) Review: Bestmann, H. J.; Vostrowsky, O. Top. Curr. Chem.
1983, 109, 85; and references cited.
(4) Marko, I. E.; Murphy, F.; Dolan, S. Tetrahedron Lett. 1996,
37, 2089.
Synlett 2005, No. 8, 1271–1274 © Thieme Stuttgart · New York

LETTER
Stannylated Allylsulfones as Versatile New Building Blocks
1273
(5) Barton, D. H. R.; Jaszberenyi, J. C.; Taschdjian, C.
Tetrahedron Lett. 1981, 24, 2703.
(6) Keck, G. E.; Savin, K. A.; Weglarz, M. A. J. Org. Chem.
1995, 60, 3194.
(7) Review: Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25,
508; Angew. Chem. 1986, 98, 504.
(8) (a) Pourcelot, G.; Cadiot, P. Bull. Soc. Chim. Fr. 1966,
3024. (b) Denmark, S. E.; Harmata, M. A.; White, K. S. J.
Org. Chem. 1987, 52, 4031.
(9) (a) Kazmaier, U.; Schauß, D.; Pohlman, M. Org. Lett. 1999,
1, 1017. (b) Kazmaier, U.; Schauß, D.; Pohlman, M.;
Raddatz, S. Synthesis 2000, 914. (c) Kazmaier, U.;
Pohlman, M.; Schauß, D. Eur. J. Org. Chem. 2000, 2761.
(d) Kazmaier, U.; Schauß, D.; Raddatz, S.; Pohlman, M.
Chem. Eur. J. 2001, 7, 456. (e) Kazmaier, U.; Braune, S. J.
Organomet. Chem. 2002, 641, 26. (f) Braune, S.; Pohlman,
M.; Kazmaier, U. J. Org. Chem. 2004, 69, 468.
(10) Braune, S.; Kazmaier, U. Angew. Chem. Int. Ed. 2003, 42,
306; Angew. Chem. 2003, 115, 318.
(11) Synthesis of (1-Benzenesulfonylmethylvinyl)tributyl-
stannane (1).
Hb), 6.48 (d, J15,16 = 7.0 Hz, 2 H, 15-H), 7.03–7.10 (m, 3 H,
C-16, C-17), 7.24–7.31 (m, 5 H, C-10, C-11, C-12), 7.48
(ddd, J3,2 = 7.9 Hz, J3,4 = 7.9 Hz, J3,3¢ = 1.9 Hz, 2 H, 3-H),
7.63 (tt, J4,3 = 8.0 Hz, J4,2 = 1.2 Hz, 1 H, 4-H), 7.73 (dd,
J2,3 = 8.4 Hz, J2,4 = 1.1 Hz, 2 H, 2-H). 13C NMR (125 MHz,
CDCl3): d = 45.1 (t, C-8), 70.1 (d, C-13), 72.9 (d, C-5),
121.6 (t, C-7), 126.0, 128.3, 128.3, 129.0, 129.0, 129.3
(6 × 2 d, C-2, C-3, C-10, C-11, C-12, C-15, C-16, C-17),
126.2 (d, C-17), 127.9 (d, C-12), 133.8 (C-4), 135.9, 136.8,
137.8 (3s, C-1, C-9, C-14), 139.3 (s, C-6).
Spectroscopic data of 6s: 1H NMR (300 MHz, CDCl3):
d = 2.45 (d, J8a,8b = 16.1 Hz, 1 H, 8-Ha), 2.65 (d, J8b,8a = 16.1
Hz, 1 H, 8-Hb), 3.93 (d, J5,13 = 9.5 Hz, 1 H, 5-H), 4.27 (s, 1
H, OH), 4.67 (s, 1 H, 7-Ha), 5.22 (s, 1 H, 7-Hb), 5.37 (d,
J13,5 = 9.5 Hz, 1 H, 13-H), 6.35 (dd, J15,16 = 6.6 Hz,
J15,17 = 1.6 Hz, 2 H, 15-H), 7.02–7.09 (m, 3 H, 16-H, 17-H),
7.24–7.28 (m, 5 H, C-10, C-11, C-12), 7.53 (ddd, J3,2 = 7.9
Hz, J3,4 = 7.9 Hz, J3,3¢ = 1.6 Hz, 2 H, 3-H), 7.66 (tt, J4,3 = 6.0
Hz, J4,2 = 1.1 Hz, 1 H, 4-H), 7.85 (dd, J2,3 = 7.3 Hz, J2,4 = 1.1
Hz, 2 H, 2-H). 13C NMR (125 MHz, CDCl3): d = 43.8 (t, C-
8), 74.4 (d, C-13), 75.0 (d, C-5), 120.5 (t, C-7), 126.3 (2 d,
C-17), 127.9, 128.2, 128.3 (3 × 2 d, C-10, C-11, C-16),
128.4 (d, C-12), 128.9 (2 d, C-3), 129.4 (2 d, C-15), 129.5 (2
d, C-2), 134.0 (C-4), 136.4, 137.8, 139.2 (3 s, C-1, C-9, C-
14), 139.5 (s, C-6). Anal. Calcd for C23H22O3S (378.49): C,
72.99; H, 5.86. Found: C, 72.89; H, 5.96.
A mixture of phenylpropargylsulfone (270 mg, 1.50 mmol),
Mo(CO)3 (CNt-Bu)3 (32.0 mg, 74.5 mmol) and Bu3SnH (495
mg, 1.70 mmol) were solved in dry THF (5 mL) in a flame-
dried flask and closed under dry argon atmosphere. The
solution was then heated in the microwave oven (CEM
Discover) for 17 min at 150 W. After cooling to r.t. the
solvent was evaporated and the residue purified by
chromatography (hexanes–EtOAc, 9:1) to give 1 (422 mg,
0.90 mmol) as a colorless oil.
(14) Preparation of Compounds 7s/8.
A solution of sulfone 5 (135 mg, 0.50 mmol) in THF (3 mL)
was cooled to –40 °C before 1.6 M BuLi (0.32 mL, 0.51
mmol) was added. The mixture was stirred for 30 min, the
cooling bath was removed for 3 min, and after cooling again
to –40 °C fresh distilled benzaldehyde (60 mg, 0.56 mmol)
was added. After stirring for further 30 min at this
temperature, CS2 (95 mg, 0.125 mmol) was added, and the
mixture was allowed to warm to –20 °C during 2 h. After
cooling to –40 °C MeI (210 mg, 1.5 mmol) was added and
during 2 h the mixture was warmed to r.t. The solvent was
evaporated in vacuo and the residue purified by
Spectroscopic data of 1: 1H NMR (300 MHz, CDCl3):
d = 0.87–1.06 (m, 15 H, 9-H, 11-H), 1.28–1.35 (m, 6 H, 10-
H), 1.46–1.54 (m, 6 H, 8-H), 3.91 (s, J5,Sn = 19.9 Hz, 2 H, 5-
H), 5.41 (d, J7cis,5 = 1.9 Hz, J7cis,Sn = 25.7 Hz, 1 H, 7-Hcis),
5.60 (d, J7trans,5 = 1.25 Hz, J7trans,Sn = 56.7 Hz, 1 H, 7-Htrans),
7.51 (dd, J3,2 = 7.7 Hz, J3,4 = 7.7 Hz, 2 H, 3-H), 7.60 (t,
J4,3 = 7.6 Hz, 1 H, 4-H), 7.81 (d, J2,3 = 7.25 Hz, 2 H, 2-H).
13C NMR (125 MHz, CDCl3): d = 10.4 (3 t, J8,Sn = 172.1 Hz,
C-8), 13.7 (3 q, C-11), 27.3 (3 t, J9,Sn = 30.0 Hz, C-9), 28.9
(3 t, J10,Sn = 9.9 Hz, C-10), 66.3 (t, C-5), 128.5 (2 d, C-2),
128.8 (2 d, C-3), 133.4 (d, C-4), 135.9 (s, C-1), 138.6 (t, C-
7), 141.4 (s, C-6).
chromatography (hexanes–EtOAc, 9:1) giving rise to 7s
(140 mg, 0.30 mmol, 60%) as a colourless solid, mp 148–
149 °C. Compound 8 was obtained as minor product (50 mg,
0.13 mmol, 26%) as a colourless oil.
(12) Synthesis of (2-Benzylprop-2-enyl)phenylsulfone (5).
A mixture of vinylstannane 1 (235 mg, 0.50 mmol) and
benzylbromide (231 mg, 1.35 mmol) was solved in dry THF
(3 mL) in a Schlenk flask under argon. A solution of allyl
palladium chloride dimer (3.7 mg, 10.1 mmol) and PPh3 (5.3
mg, 20.2 mmol) was added in THF (2 mL) and the mixture
was warmed to 60 °C overnight. After cooling to r.t. the
solvent was evaporated and the residue purified by
chromatography (hexanes–EtOAc, 9:1) to give 5 (135 mg,
0.50 mmol) as a white solid, mp 63–65 °C.
Spectroscopic data of 7s: 1H NMR (300 MHz, CDCl3):
d = 2.38 (s, 3 H, 19-H), 2.92 (d, J8a,8b = 16.1 Hz, 1 H, 8-Ha),
2.98 (d, J8b,8a = 16.1 Hz, 1 H, 8-Hb), 4.37 (d, J5,13 = 10.1 Hz,
1 H, 5-H), 4.95 (s, 1 H, 7-Ha), 5.47 (s, 1 H, 7-Hb), 6.60 (ddd,
J15,16 = 7.6 Hz, J15,17 = 1.9 Hz, J15,15¢ = 1.9 Hz, 2 H, 15-H),
7.04 (d, J13,5 = 10.4 Hz, 1 H, 13-H), 7.12–7.17, 7.20–7.26 (2
m, 8 H, H-10, H-11, H-12, H-16, H-17), 7.52 (ddd, J3,2 = 7.7
Hz, J3,4 = 7.7 Hz, J3,3¢ = 1.6 Hz, 2 H, 3-H), 7.62 (tt, J4,3 = 7.6
Hz, J4,2 = 1.4 Hz, 1 H, 4-H), 7.82 (ddd, J2,3 = 7.9 Hz,
J2,4 = 1.3 Hz, J2,2¢ = 1.3 Hz, 2 H, 2-H). 13C NMR (125 MHz,
CDCl3): d = 19.0 (q, C-19), 44.4 (t, C-8), 72.0 (d, C-5), 82.6
(d, C-13), 121.5 (t, C-7), 126.5 (d, C-17), 128.2, 128.4,
128.8, 128.9, 129.0 (5 × 2 d, C-2, C-3, C-10, C-11, C-16),
128.7 (d, C-12), 129.7 (2 d, C-15), 133.4 (d, C-4), 135.4,
136.5, 138.5 (3 s, C-1, C-9, C-14), 140.2 (d, C-6), 213.0 (s,
C-18). Anal. Calcd for C25H24O3S3 (468.66): C, 64.07; H,
5.16. Found: C, 63.91; H, 5.35.
Spectroscopic data of 5: 1H NMR (300 MHz, CDCl3):
d = 3.49 (s, 2 H, 8-H), 3.67 (s, 2 H, 5-H), 4.82 (s, 1 H, 7-Ha),
5.05 (d, J7b,5 = 1.0 Hz, 1 H, 7-Hb), 7.12 (d, J10,11 = 7.3 Hz, 2
H, 10-H), 7.20 (t, J12,11 = 7.4 Hz, 1 H, 12-H), 7.27 (dd,
J11,10 = 7.4 Hz, J11,12 = 7.4 Hz, 2 H, 11-H), 7.54 (dd,
J3,2 = 7.9 Hz, J3,4 = 7.9 Hz, 2 H, 3-H), 7.64 (tt, J4,3 = 7.4 Hz,
J4.2 = 0.9 Hz, 1 H, 4-H), 7.88 (dd, J2,3 = 7.3 Hz, J2,4 = 1.3 Hz,
2 H, 2-H). 13C NMR (125 MHz, CDCl3): d = 42.2 (t, C-8),
61.6 (t, C-5), 121.5 (t, C-7), 126.6 (d, C-12), 128.5, 128.6,
129.0, 129.2 (4 × 2 d, C-2, C-3, C-10, C-11), 133.7 (d, C-4),
136.8 (d, C-9), 137.9 (d, C-6), 138.4 (s, C-1).
Spectroscopic data of 8: 1H NMR (300 MHz, CDCl3):
d = 3.20 (s, 1 H, 8-H), 4.85 (d, J7a,8 = 1.3 Hz, 1 H, 7-Ha), 4.94
(d, J7b,8 = 1.0 Hz, 1 H, 7-Hb), 6.96 (dd, J15,16 = 7.9 Hz,
J15,17 = 1.3 Hz, 2 H, 15-H), 7.16 (tt, J17,16 = 7.3 Hz,
(13) Spectroscopic data of 6a: 1H NMR (300 MHz, CDCl3):
d = 2.45 (d, J8a,8b = 15.8 Hz, 1 H, 8-Ha), 2.73 (d, J8b,8a = 15.8
Hz, 1 H, 8-Hb), 3.70 (d, J5,13 = 1.9 Hz, 1 H, 5-H), 5.06 (s, 1
H, 7-Ha), 5.73 (d, J13,5 = 2.2 Hz, 1 H, 13-H), 5.79 (s, 1 H, 7-
J17,15 = 2.2 Hz, 1 H, 17-H), 7.22 (dd, J16,15 = 7.3 Hz,
J16,17 = 7.3 Hz, 2 H, 16-H), 7.33–7.38 (m, 3 H, 10-H, 12-H),
7.51 (dd, J3,2 = 7.7 Hz, J3,4 = 7.7 Hz, 2 H, 3-H), 7.58–7.62
(m, 3 H, 4-H, 11-H), 7.82 (s, 1 H, 13-H), 7.90 (dd, J2,3 = 8.4
Synlett 2005, No. 8, 1271–1274 © Thieme Stuttgart · New York

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U. Kazmaier, A. Wesquet
LETTER
(16) Spectroscopic data of 9: 1H NMR (300 MHz, CDCl3):
Hz, J2,4 = 1.1 Hz, 2 H, 2-H). 13C NMR (125 MHz, CDCl3):
d = 41.4 (t, C-8), 122.1 (t, C-7), 126.5 (d, C-11), 128.4,
128.7, 128.8, 129.0, 129.8, 130.0 (6 × 2 d, C-2, C-3, C-10,
C-11, C-15, C-16), 130.3 (d, C-17), 133.0 (s, C-14), 133.3
(d, C-4), 136.8 (d, C-13), 137.0 (s, C-9), 138.6 (s, C-1),
140.8 (s, C-5), 141.9 (c, C-6).
d = 3.69 (s, 2 H, 9-H), 4.99 (s, 1 H, 8-Ha), 5.31 (s, 1 H, 8-Hb),
6.51 (d, J5,6 = 16.1 Hz, 1 H, 5-H), 6.88 (d, J6,5 = 15.2 Hz, 1
H, 6-H), 7.21–7.39 (m, 10 H, 2-H, 3-H, 4-H, 11-H, 12-H, 13-
H). 13C NMR (125 MHz, CDCl3): d = 38.7 (t, C-9), 118,7 (t,
C-7), 126.1 (d, C-13), 126.4 (2 d, C-2), 127.5 (d, C-4), 128.4,
128.6, 128.8 (3 × 2 d, C-3, C-11, C-12), 129.1 (d, C-5),
130.6 (d, C-6), 137.2 (s, C-10), 139.4 (s, C-1), 145.0 (s, C-7).
(15) Bernard, A. M.; Frongia, A.; Piras, P. P.; Secci, F. Synlett
2004, 1064.
Synlett 2005, No. 8, 1271–1274 © Thieme Stuttgart · New York