Heck reactions using aryldiazonium salts towards phosphonic derivatives

Article abstract of DOI:10.1055/s-2000-6482

A facile synthesis of aryl vinylphosphonates has been based on Heck reaction of aryldiazonium salts bearing electrowithdrawing or donating groups with vinylphosphonates. A one-pot procedure consisting of Heck reaction and hydrogenation permits the clean f

Full text of DOI:10.1055/s-2000-6482

LETTER
201
Heck Reactions Using Aryldiazonium Salts towards Phosphonic Derivatives
Heiko Brunner, Nathalie Le Cousturier de Courcy, Jean-Pierre Genêt*
Ecole Nationale Supérieure de Chimie de Paris, Laboratoire de Synthèse Sélective Organique et Produits Naturels, UMR 7523,
11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
Fax +33-1-44276743; E-mail: genêt@ext.jussieu.fr
Received 15 October 1999
cedure has the disadvantage that it involves two steps and
Abstract: A facile synthesis of aryl vinylphosphonates has been
the overall yields are quite low. We were motivated to
based on Heck reaction of aryldiazonium salts bearing electrowith-
look for a more convenient synthetic pathway for arylated
drawing or donating groups with vinylphosphonates. A one-pot pro-
vinylphosphonates using aryldiazonium salts. We now re-
port the first aerobic Heck reaction using aryldiazonium
salts for preparing aryl vinylphosphonates in the presence
of Pd/CaCO3 and Pd(OAc)2/CaCO3 as catalytic systems19
(Scheme 1). Compounds 3 have been prepared by using
these catalytic systems (Table 1).
cedure consisting of Heck reaction and hydrogenation permits the
clean formation of useful Wadsworth-Emmons reagents.
Key words: Heck reaction, palladium, arenediazonium salts, hy-
drogenation, vinylphosphonates, Wadsworth-Emmons reactives
The analogues of phosphonic acids are characterized by
their biological importance and have been frequently used
as suitable isosteric replacements for phosphates in nucle-
otides, phospholipides, and sugar phosphates.1 Further-
more, they are suitable isosters for carboxylic groups.
Phosphonic acids containing amino groups in the a-,b- or
g-position have also found considerable interest for natu-
ral amino acids and are described as an important class in
agricultural and medicinal chemistry.2 It is known that
these products can exert their biological activity as regu-
lators, mediators, or enzyme inhibitors.1,2 On the other
hand phosphonates have found a wide application in or-
ganic synthesis.3 In recent years substituted vinylphos-
phonates were beside their pharmacological activity4,
extensively used in polymer sciences5 as well in organic
synthesis, in particular, for preparing carbon- and hetero-
cycles compounds.3 Due to our recent works concerning
phosphonic acid analogues we were interested in the syn-
thesis of substituted vinylphosphonates.6
F4BN2
1) Pd/CaCO3 or
Pd(OAc)2 + 1.2 eq. CaCO3
OEt
OEt
P
O
+
P
OEt
OEt
O
MeOH
GF
GF
1
2
3
Scheme 1
Table 1 Palladium-catalysed reaction of functionalized aryldiazoni-
um with vinylphosphonate.
For their preparation non-catalytic methods, such as olefi-
nation of phosphonates bearing an active methylene group
and dehydration7,8 or dehalogenation of corresponding
substituted alkylphosphonates as well as nickel catalyzed
Arbusov reaction are known9. Currently palladium cata-
lyzed phosphonylation with vinyl halides10 and the phos-
phonylation of vinylzirconium(IV) complexes11 are re-
ported. Beside these methods, Heck reactions became of
greater importance.12 Next to the classical approach em-
ploying aryl halides13 and vinyl halides14 only palladium-
catalysed phosphonylation with aryl15 and vinyl triflates16
as pseudohalides is described for the synthesis of aryl vi-
nylphosphonates. Although the use of aryldiazonium salts
has received a major stimulus in the Heck reaction during
the last two decades17 the use of these electrophiles for the
preparation of arylated vinylphosphonates was not de-
scribed to date. Wada and Oga have merely reported the
reaction of an ethenephosphonate with aryldiazonium
salts followed by a dehydrohalogenation of the resulting
1-chloro-2-arylalkanephosphonates.18 However, this pro-
a Reaction conditions20: 50 °C in methanol; 1 mmol arenediazonium
salt; 0.85 mmol vinylphosphonate; 2 mol-% Pd-catalyst b Isolated
yields.
Synlett 2000, No. 2, 201–204 ISSN 0936-5214 © Thieme Stuttgart · New York

202
H. Brunner et al.
LETTER
We have found that the reaction is stereoselective and that products isolated from plants and fungi23 this strategy al-
the E-isomers are exclusively formed.21According to our lows an elegant application in organic synthesis in partic-
previous results19 of the Heck reaction using aryldiazoni- ular for natural products24 (Scheme 3).
um salts we found an optimum of the reaction using meth-
anol or ethanol at a temperature between 30 °C for
electron deficient and 50 °C for electron rich arenediazo-
P
nium salts within 5 min and 2 h. Thus, the use of aryldia-
Li(TMS)2
zonium salts as electrophiles is itself characterized by
more gentle reaction conditions compared with previous-
ly published palladium catalysed vinylations.
MeO
MeO
OEt
OEt
MeO
MeO
MeO
O
DME, acetone
57%
MeO
4b
5
O
P
OMe
OMe
Beside these advantages this approach offers a second
synthetic opportunity. The use of palladium as catalyst
and of alcoholic solvents permits an one-pot procedure for
the preparation of new Wadsworth-Emmons reagents
combining Heck reaction and hydrogenation22 (Scheme
2).
N2BF4
COOMe
Pd2(µ-OAc)2(P(o-tolyl)2)2
+
COOMe
(0.5%)
OEt
OEt
P
I
I
O
6a,b
7
8a,b
8a : 2-I : 74%
8b : 4-I : 92%
Scheme 3
OEt
OEt
1) Pd(OAc)2 + 1.2 eq. CaCO3
/
N2BF4
P
OEt
OEt
However not only unsubstituted vinylphosphonates, but
also α−substituted vinylphosphonates like trimethyl 2-
phosphonoacrylate 7 were reacted with arenediazonium
salts.25 Instead of Pd(OAc)2/CaCO3 or Pd/CaCO3 as cata-
lytic system, the use of the palladacycle26,27 Pd2(µ-
OAc)2(P(o-tolyl)2)2 was also feasible in methanol under
aerobic conditions and gave good yields (Scheme 3).
GF
P
O
MeOH
2) H2
O
GF
2
4
Scheme 2
By using this protocol it is possible to gain in easily one-
pot manner and in good yields several Wadsworth-Em-
mons reagents bearing different functional groups includ-
ing chlorides and ester-groups (Table 2). In the case of
halogenated arylvinylphosphonates bearing iodine or bro-
mine atoms only the dehalogenated products could be ob-
tained.
3
1
The coupling constants JPH = 24.5 Hz in the H NMR-
spectra indicate that the stereochemistry corresponds to an
E-configuration on the basis of the phosphorous-cis vinyl
proton NMR.28,29 Furthermore it is very remarkable that
not only iodine-substitution in para-position but also in
ortho-position is completely compatible with this reac-
tion. In the light of practicability we emphasise that this
approach using Pd2(µ-OAc)2(P(o-tolyl)2)2 as catalytic sys-
Table 2 Synthesis of phosphonates : one-pot Heck reaction-Hydro- tem can also be carried out under aerobic conditions using
not degassed solvents.28
genation procedure.
In conclusion, this protocol is characterized by the mild
conditions and an excellent variety of functional groups
tolerated and promises to be an useful method for the in-
troduction of vinyl- and ethylene phosphonate functional-
ities into organic substrates. This method permits also the
synthesis of arylated vinyl phosphonates bearing iodide
and bromide functionalities via Heck reactions. Further
investigations concerning phosphonic analogues are cur-
rently underway in our laboratory.
Acknowledgement
H.B. thanks the exchange program between CNRS and Max-
Planck-Gesellschaft for a post-doctoral scholarship. N. de Courcy.
a Reaction conditions: 50 °C in methanol; 1 mmol arenediazonium
thanks the CROUS for a scholarship (Allocation Spéciale 3ème cy-
salt; 0.85 mmol vinylphosphonate; 2 mol-% Pd-catalyst; 1atm. H2
cle). Financial support by the CNRS is gratefully acknowledged.
b Isolated yields.
References and Notes
(1) Engel, R. Chem. Rev. 1977, 77, 349-367. Klachburn, G.M.;
In regard of prenylated phenolic moieties (like 4b), which
Perree, T.D. Chem. Scr. 1986, 26, 21.
represent a widespread motif in the structure of natural
Synlett 2000, No. 2, 201–204 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Heck Reactions Using Aryldiazonium Salts towards Phosphonic Derivatives
203
(2) Kafarski, P.; Lejczak, B. Phosphorus, Sulfur Silicon Relat.
Elem. 1991, 63, 193-215. O’Donnell, M.J.; Lawley, L.K.;
Pushpavanam, P.B.; Burger, A.; Bordwell, F.G.; Zhang, X.-
M. Tetrahedron Lett. 1994, 35, 6421-6424 and literature cited
therein; for a recent review on racemic and optically active
synthesis of α-aminophosphonic acids see Uziel, J.; Genêt, J.-
P. Russ. J. Org. Chem. 1997, 33, 1521-1542.
(3) Minami, T.; Motoyoshiya, J. Synthesis 1992, 333-349.
(4) several examples of pharmaceutical active arylated vinyl-
phosphonates are published in the literature of patents e.g.
Graeve, R.; Thorwart, W.; Raiss, R.; Weithmann, K.U.;
Mullner, S., US 5,627,173 (Hoechst AG 1997). Lennon, P.J.,
US 5,434,288 (Monsanto Company 1995).
(5) Römpp Lexikon Chemie 10; Falbe, J.; Regitz, M., Eds.;
Georg Thieme Verlag: Stuttgart; Vol. 6 1999. Jin, S.;
Gonsalves, K.E., Macromolecules 1998, 31, 1010-1015 and
literature cited therein. Davies, K.P.; Walker, D.R.E.;
Woodward, G.; Smith, A.C., EP 861846 A2 (Albright &
Wilson Ltd 1998). For applications in the field of flame
retardancy in styrenic and acrylic polymers with covalently
bound phosphorous-containing groups see Ebdon, J.R, Recent
Adv. Flame Retard. Polym. Materials 1997, 8, 161.
(6) Henry, J.C.; Lavergne, D.; Ratovelomanana-Vidal, V.;
Beletskaya, I. P.; Dolgina, T.M. Tetrahedron Lett. 1998, 39,
3473-3476.
(7) Ahlbrecht, H.; Farnung, W. Synthesis 1977, 336-338.
Venugopalan, B.; Bevin Hamlet, A.; Durst, T. Tetrahedron
Lett. 1981, 22, 191-194.
(8) Minami, T.; Nishimura, K.; Hirao, I.; Suganuma, H.; Agawa,
T. J. Org. Chem. 1982, 47, 2360-2363 and literature cited
therein.
(9) Tavs, P.; Weitkamp, H. Tetrahedron 1970, 26, 5529-5534.
(10) Hirao, T.; Masunaga, Y.; Ohshiro, T.; Agawa, T. Tetrahedron
Lett. 1980, 21, 3595-3598. Hirao, T.; Masunaga, T.; Agawa,
T. Bull. Chem. Soc. Jpn. 1982, 55, 909-913.
(11) Zhong, P.; Huang, X.; Xiong, Z.-X. Synlett 1999, 721-722.
(12) Milton, L.; Donald, M. Phosphorus 1974, 4, 63.
(13) Xu, Y.; Jin, X.; Huang, G.; Huang, Y. Synthesis 1983, 556-
558. Demik, N.N.; Kabachnik, M.M.; Novikova, Z.S.;
Beletskaya, I.P. Russ. J. Org. Chem. 1995, 31, 57-60. and Zh.
Org. Khim. 1995, 31, 64-68. Ortwine, D.F.; Malone, T.C.;
Bigge, C.F.; Drummond, J.T.; Humblet, C.; Johnson, G.;
Pinter, G.W. J.Med.Chem. 1992, 35, 1345-1370. Bigge, C.F;
Johnson, G.; Ortwine, D.F.; Drummond, J.T.; Retz, D.M.;
Brahce, L.J.; Coughenour, L.L.; Marcoux, F.W.; Probert,
A.W., Jr. J. Med. Chem. 1992, 35, 1371-1384.
the Pd-catalyst and 1 mmol of CaCO3 were suspended in 5 mL
of solvent at room temperature. Subsequently 0.85 mmol of
the vinylphosphonate was added at room temperature and the
resulting mixture was stirred at 50 °C. The progress of the
reaction was monitored by measuring the volume of given off
gas (5 min − 2 h). After the reaction was completed the solvent
was evaporated and the residue was purified by flash
chromatography on silica.
(21) The stereochemistry was determinated by 1H NMR
spectroscopy. In all cases the coupling constants 3JHH indicate
an E-configuration. Characteristic NMR-data:
3a: 1H NMR δ (CDCl3) 7.46-7.27 (m, 3H), 6.80 (dd, 3JHH = 8.8
Hz, 4JHH = 2.4 Hz, 2H), 6.00 (t, 3JHH = 17.6 Hz, 1H), 4.03
(quint. 3JHH = 7.1 Hz, 4H), 3.74 (s, 3H), 1.26 (t, 3JHH = 7.1 Hz,
6H); 13C NMR δ (CDCl3) 161.1 (s), 148.2 (d, 2JCP = 6.1 Hz,
CH), 129.1 (d), 124.0 (s), 114.0 (d), 110.7 (d, 1JCP = 191.6 Hz,
CH), 61.6 (t, 2JCP = 5.9 Hz), 55.1 (q), 16.20 (q).
3b: 1H NMR: δ (CDCl3) 7.64-7.25 (m, 3H), 7.07 (d, 3JHH = 8.5
Hz, 1H), 6.31 (t, 3JHH = 17.8 Hz, 1H), 4.11 (quint. 3JHH = 7.5
Hz, 4H), 3.90 (s, 3H), 1.31 (t, 3JHH = 7.0 Hz, 6H); 13C NMR δ
(CDCl3) 158.1 (s), 148.7 (d, 2JCP = 6.7 Hz, CH), 136.9 (s),
129.4 (d), 124.0 (s), 113.4 (d), 112.8 (d, 1JCP = 191.3 Hz, CH),
63.4 (t, 2JCP = 5.5 Hz), 56.9 (q), 16.8 (q).
3c: 1H NMR: δ (CD3OD) 8.85-8.74 (m, 1H), 8.53-7.76 (m,
3H), 7.10 (td, 3JHH = 17.5 Hz, 4JHH = 2.9 Hz, 1H), 4.75 (quint.,
3JHH = 6.2 Hz, 4H), 2.01 (t, 3JHH = 6.2 Hz, 6H); 13C NMR δ
(CD3OD) 154.5 (s, 1JCF = 258.9 Hz), 148.3 (d, 2JCP = 8.4 Hz),
136.1 (d, 3JCF = 8.6 Hz), 134.0 (s, 3JCF = 8.4 Hz), 127.8 (d,
2JCF = 22.6 Hz), 121.5 (s, 2JCF = 22.0 Hz), 116.9 (d,
1JCP = 189.6 Hz), 64.1 (t, 2JCP = 5.5 Hz), 17.9 (q).
3d: 1H NMR δ (CDCl3) 8.25 (t, 3JHH = 17.6 Hz, 1H), 6.57 (s,
2H), 6.25 (t, 3JHH = 17.6 Hz, 1H), 4.14 (quint., 3JHH = 7.5 Hz,
4H), 3.82 (s, 6H), 3.74 (s, 3H), 1.35 (t, 3JHH = 7.1 Hz, 6H); 13
C
NMR δ (CDCl3) 149.2 (d, 2JCP = 6.9 Hz), 144.3 (s), 132.8 (s),
118.4 (d, 1JCP = 194.4 Hz), 107.3 (d), 62.4 (t, 2JCP = 5.0 Hz),
56.4 (q), 52.3 (q), 13.8 (q).
3e: 1H NMR δ (CDCl3) 7.62 (t, 3JHH = 17.5 Hz, 1H), 7.50 (dd,
3JHH = 6.0 Hz, 4JHH = 1.7 Hz, 2H), 7.27-7.07 (m, 2H), 6.16 (t,
3JHH = 17.5 Hz, 1H), 4.07 (quint., 3JHH = 7.0 Hz, 4H), 1.28 (t,
3JHH = 7.0 Hz, 6H); 13C NMR δ (CDCl3) 146.3 (d, 2JCP = 7.6
Hz), 133.2 (d), 131.0 (d), 127.6 (d), 127.4 (d), 124.6 (s), 117.5
(d, 1JCP = 189.3 Hz), 61.9 (t, 2JCP = 5.5 Hz), 16.3 (q).
3f: 1H NMR δ (CDCl3) 8.22 (t, 3JHH = 18.0 Hz, 1H), 7.73 (d,
3JHH = 8.4 Hz, 2H), 7.23 (d, 3JHH = 8.4 Hz, 2H), 6.23 (t,
3JHH = 18.0 Hz, 1H), 4.25 (quint., 3JHH = 7.0 Hz, 4H), 1.39 (t,
3JHH = 7.1 Hz, 1H); 13C NMR δ (CDCl3) 148.0 (d, 2JCP = 7.0
Hz), 138.0 (d), 133.5 (s), 129.6 (d), 117.2 (d, 1JCP = 192.4 Hz),
97.4 (s), 62.1 (t, 2JCP = 5.0 Hz), 14.2 (q).
(14) Axelrad, G.; Laosooksathit, S.; Engel, R. J. Org. Chem. 1981,
46, 5200-5204.
(15) Petrakis, K.S.; Nagabhushan, T.L. J.Am.Chem.Soc. 1987,
109, 2831-2833.
(16) Burini, A.; Cacchi, S.; Pace, P.; Pietroni, B.R. Synlett 1995,
677-679. Holt, D.A.; Erb, J.M. Tetrahedron Lett. 1989, 30,
5393-5396.
(17) For some recent works in this area : see Beller, M.,
Kühlein,K., Synlett 1995, 441-442. Ikenaga, K.; Matsumoto,
S.; Kikukawa, K.; Matsuda, T., Chem.Lett. 1988, 873-876.
Sengupta, S., Sadhukhan, S.K., Tetrahedron Lett. 1998; 39,
715-718. Sengupta, S., Sadhukan, S.K., Bhattacharyya, S.,
Guha, J., J. Chem. Soc., Perkin Trans 1 1998, 407-410.
(18) Wada, Y.; Oda, R. Kogyo Kagaku Zasshi 1964, 67, 2093. C.A.
1965, 62, 13177.
(19) Brunner, H., Le Cousturier de Courcy, N.; Genêt, J-P.
Tetrahedron Lett. 1999, 40, 4815-4818.
(20) Preparation of aryldiazonium salts : Roe, A. Org. React.
1949, 5, 193-228. Doyle, M.P.; Bryker, W.J. J. Org. Chem.
1979, 44, 1572-1574. Typical procedure: In a reaction flask 1
mmol of the corresponding aryldiazonium salt, 0.02 mmol of
3g: 1H NMR δ (CDCl3) 8.21 (t, 3JHH = 19.9 Hz, 1H), 7.97 (dd,
3JHH = 7.6 Hz, 4JHH = 1.4 Hz, 1H), 7.61-7.40 (m, 3H), 6.16 (t,
3JHH = 19.8 Hz, 1H), 4.40 (q, 3JHH = 7.1 Hz, 2H), 4.19 (quint.
3JHH = 7.0 Hz, 4H), 1.39 (t, 3JHH = 7.1 Hz, 9H); 13C NMR δ
(CDCl3) 166.3 (s), 147.8 (d, 2JCP = 7.0 Hz), 136.5 (s), 132.1
(d), 130.3 (d), 129.2 (d), 127.4 (d), 119.8 (s), 116.1 (d,
1JCP = 189.6 Hz), 62.1 (t, 2JCP = 4.5 Hz), 60.39 (t), 16.1 (q),
13.9 (q).
(22) To our knowledge some examples using one-pot sequences
Heck reaction − hydrogenation are known: Meyer, W.;
Reinehr, D.; Oertle, K.; Schurter, R., EP 0 102 925 A2 and EP
0 248 245 A2 (Ciba-Geigy 1983). Bräse, S.; Schroen, M.
Angew. Chem. Int. Ed. 1999, 38, 1071-1073. Typical
procedure: In a reaction flask 1 mmol of the corresponding
aryldiazonium salt, 0.02 mmol of the Pd-catalyst and 1 mmol
of CaCO3 were suspended in 5 mL of solvent at room
temperature. Subsequently 0.85 mmol of the
vinylphosphonate was added at room temperature and the
resulting mixture was stirred at 50 °C. The progress of the
Synlett 2000, No. 2, 201–204 ISSN 0936-5214 © Thieme Stuttgart · New York

204
H. Brunner et al.
LETTER
reaction was monitored by measuring the volume of given off
gas (15 min − 2 h). After the Heck reaction was completed the
mixture was cooled to room temperature and stirred at room
temperature under hydrogen (1 atm). The progress of the
reaction was monitored TLC (1 − 2 days). After the
(27) A new review article concerning the catalytic applications of
the palladacycle trans-di(µ-acetato)-bis[o-(di-o-tolylphos-
phino)benzyl]dipalladium(II) in cross couplings was recently
published by Speicher, A; Schulz, T.; Eicher, T. J. Prakt.
Chem. 1999, 341, 605.
hydrogenation was completed the solvent was evaporated and
the residue was purified by flash chromatography on silica.
(23) Donnelly, D.M.X.; Boland, G.M. Nat. Prod. Rep. 1995, 12,
321-338.
(24) Huneck, S.; Connolly, J.D.; Harrison, L.J.; Joseph, R.S.I.;
Pocs, T. Phytochemistry 1984, 23, 2396-2397.
(25) For the reaction conditions see Yue, X.; Li, Y. Synthesis 1996,
736-740. The spectroscopical data were identical to
previously reported data.24
(26) Herrmann, W.A.; Broßmer, C.; Öfele, K.; Reisinger, C.-P.;
Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem. Int. Ed.
Engl. 1995, 34, 1844-1848. Beller, M.; Fischer, H.; Herrmann,
W.A.; Öfele, K.; Broßmer, C. Angew. Chem. Int. Ed. Engl.
1995, 37, 1848-1849. Herrmann, W.A; Broßmer, C.;
Reisinger, C.-P.; Riermeier, T.H.; Öfele, K.; Beller, M. Chem.
Eur. J. 1997, 3, 1357-1364. Herrmann, W.A.; Reissinger, C.-
P.; Öfele, K.; Broßmer, C.; Beller, M.; Fischer, H. J. Mol.
Catal. 1996, 103, 133-146. Beller, M.; Riermeier, H.
Tetrahedron Lett. 1996, 37, 6535-6538. Beller, M.; Riermeier,
H.; Haber, S.; Kleiner, H.J.; Herrmann, W.A. Chem. Ber.
1996, 129, 1259-1264.
(28) Typical procedure for the synthesis of 8b: In a reaction flask
318 mg (1mmol) 4-iodobenzenediazoniumtetrafluoroborate,
9.4 mg (0.01mmol 0.5mol-% Pd) trans-di(µ-acetato)-bis[o-
(di-o-tolylphosphino)benzyl]dipalladium(II) were suspended
in 5mL methanol. Subsequently 0.14 mL (0.9mmol) methyl-
2-(diethylphosphono)acrylate was added and the reaction
mixture was stirred at 50 °C. After the reaction was completed
the solvent was evaporated and the residue was
chromatographed on silica gel eluting with cyclohexane/
EtOAc mixture (30/70 v/v) to give 327 mg (92%) of 8b as
white solid: mp:142 °C; 1H NMR δ (CD3OD) 8.30 (dd,
3JHH = 8.8 Hz, 4JHH = 2.2 Hz, 2H), 7.73 (dd, 3JHH = 8.8 Hz,
4JHH = 2.2 Hz, 2H), 7.37 (t, 3JPH = 24.5Hz), 3.80 (s, 3H) 3.72
(d, 3JPH = 10.1 Hz, 6H); 13C NMR δ (CD3OD) 166.5 (d,
J
CP = 13.2 Hz), 149.2 (d, JCP = 6.2 Hz), 142.2 (s), 139.4 (d),
137.2 (d), 115.3 (d, JCP = 190.2 Hz), 97.6 (s), 53.7 (q, JCP = 5.6
Hz), 52.9 (q); IR (KBr) 1718, 1254; MS m/s (relative
intensity) 384 (M+, 6).
(29) Aboujaoude, E.E.; Lietjé, S.; Collignon, N.; Teulade, M.P.;
Savignac, P. Tetrahedron Lett. 1985, 26, 4435-4438. Minami,
T.; Utsonomiya, T.; Nakamura, S.; Okubo, M.; Kitamura, N;
Okada, Y.; Ichikawa, J. J. Org. Chem. 1994, 59, 6717-6727.
Article Identifier:
1437-2096,E;2000,0,02,0201,0204,ftx,en;G24999ST.pdf
Synlett 2000, No. 2, 201–204 ISSN 0936-5214 © Thieme Stuttgart · New York