The first phosphine oxide ligand precursors for transition metal catalyzed cross-coupling reactions: C-C, C-N, and C-S bond formation on unactivated aryl chlorides

Article abstract of DOI:10.1002/1521-3773(20010417)40:8<1513::AID-ANIE1513>3.0.CO;2-C

Rapid and regioselective oxidative addition of unactivated aryl chlorides in the presence of bases can be achieved by using metal complexes of phosphinous acid (RR′POH), for which air-stable phosphine oxides (RR′P(O)H) serve as ideal ligand precursors. Such processes can be incorporated into efficient catalytic cycles for a variety of C-C, C-N, and C-S bond-forming processes (see scheme, Ar = aryl).

Full text of DOI:10.1002/1521-3773(20010417)40:8<1513::AID-ANIE1513>3.0.CO;2-C

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The First Phosphine Oxide Ligand Precursors
for Transition Metal Catalyzed Cross-Coupling
Reactions: C C, C N, and C S Bond
Formation on Unactivated Aryl Chlorides
George Y. Li*
Dedicated to Professor Fred Basolo
on the occasion of his 81st birthday
In recent years, transition metal catalyzed cross-coupling
reactions of aryl chlorides have developed into a versatile and
efficient method for a variety of synthetic transformations.[1, 2]
Furthermore, promising results in the Pd- (or Ni)-catalyzed
C C (Suzuki,[3±6] Kumada,[7] and Stille[8] cross-coupling) and
C N[9] bond-forming processes prompted us to search for
new, inexpensive, and efficient air-stable ligands for potential
industrial applications of these less expensive but readily
available aryl chlorides. One potentially valuable and cost-
saving approach is to apply combinatorial synthesis and
screening methods for the discovery and development of
efficient, air-stable novel catalysts.[10, 11] Air-stable phosphine
oxides (RR'P(O)H) in the presence of transition metals
undergo tautomerization to the less stable phosphinous acids
(RR'POH), which subsequently and expectedly coordinate to
the metal centers through phosphorus atoms to form metal ±
phosphinous acid compounds.[12] Deprotonolysis of the result-
ing phosphinous acid complex with base (e.g. NaOR, Na2CO3,
CsF) can be anticipated to yield an electron-rich anionic
compound. These results raise the interesting question of
whether these phosphane-containing anionic complexes could
be utilized to facilitate oxidative addition of aryl chlorides
(rate-limiting step) in homogeneous catalysis[13] to generate a
variety of C C, C N, and C S[14] bonds in different environ-
ments by using cross-coupling reactions of aryl chlorides.
Herein, we report the first examples of simple, readily
accessible, air-stable phosphine oxides used as ligand pre-
cursors for the efficient transition metal catalyzed C C, C N,
and C S bond-forming reactions of aryl chlorides to access a
variety of such architectures, as well as initial observations
regarding the scope and mechanism. We also describe a
general synthesis of binuclear chloride-bridged PdII mono-
phosphinous acid complexes used as catalyst precursors for a
variety of cross-coupling reactions and the X-ray crystallo-
graphic characterization of one of them.
Scheme 1. Synthesis of air-stable phosphine oxide ligand precursors and
their corresponding phosphinous acid ligand complexes. cod ˆ 1,5-cyclo-
octadiene.
elevated temperature. The formation of these catalyst pre-
cursors was conveniently monitored by solution 31P NMR
spectroscopy.[16] The catalytic cross-coupling reactions were
carried out in organic solvents (e.g. toluene, DMSO, dioxane)
under anhydrous/anaerobic or open-to-air conditions (cata-
lyst loading: ꢀ1 ± 5 mol%).[17] The yields in Table 1 refer
to products isolated by column chromatography. Known
products were identified by comparison with literature
data[4±9, 14] and/or with those of authentic samples. New
1
compounds were characterized by 1D and 2D H/13C NMR
spectroscopy, high-resolution mass spectrometry, and elemen-
tal analysis.[17]
The [Pd2(dba)3]/RR'POH precatalysts are capable of cata-
lyzing the cross-coupling reactions of aryl chlorides and
arylboronic acids to yield the desired biaryls in high yields
(Table 1, entries 1 ± 6). Entries 4 ± 6 illustrate that the elec-
tron-rich 4-chloroanisole could be coupled with arylboronic
acids quantitatively. Entries 2 and 3 demonstrate that the
more sterically demanding and electron-rich substrate
2-chloroanisole was coupled with 4-methylphenylboronic acid
(Table 1, entry 2) and phenylboronic acid (Table 1, entry 3).
The [Ni(cod)2]/RR'POH precatalyst can be employed to
mediate Kumada cross-coupling reactions of aryl chlorides
and RMgX at room temperature (Table 1, entries 7 and 8).
The electron-rich aryl chloride is tolerated by the present
system as demonstrated by 4-chloroanisole (Table 1, entry 7).
Entries 9 ± 12 reveal that the present catalysts are also
effective in the C N bond cross-coupling reactions of aryl
chlorides with a variety of amines. Entries 9, 10, and 12
illustrate aminations of aryl chlorides and alkylamines.
Entry 11 shows that an aryl chloride/arylamine cross-coupling
reaction produces a diarylamine. Also noteworthy are the
successful catalytic activities on C S bond-forming processes
of unactivated aryl chlorides. The present air-stable phosphine
oxides are capable of forming precatalysts with Pd(OAc)2 or
[Pd2(dba)3] to efficiently catalyze cross-coupling reactions of
aryl chlorides and alkylthiols to yield novel thioethers
(Table 1, entry 13). In the case of aryl bromides as substrates,
the present phosphinous acids can be employed as ligands for
the efficient C S bond formations at room temperature
(Table 1, entry 14).
The ligand precursors used for cross-coupling reactions
were prepared either in situ or by standard sublimations
(Scheme 1).[10, 11, 15] The precatalysts were synthesized by
treating transition metal precursors with monophosphine
oxides in a variety of solvents at either room temperature or
[*] Dr. G. Y. Li
The DuPont Company
Central Research and Development
Experimental Station
P.O. Box 80328, Wilmington, DE 19880-0328 (USA)
Fax : (‡1)302-695-3786
E-mail: george.y.li@usa.dupont.com
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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Table 1. Transition metal/phosphinous acid catalyzed cross-couplings of aryl chlorides with Ar B(OH)2, RMgX, RNH2, and RSH compounds.[a]
Entry
Halide
Coupling
partner
Base
Temp
[8C]
Time
[h]
Product
Yield
[%] (isolated)
1
2
CsCO3
CsF
100
100
12
12
88[b]
83[b]
3
CsF
100
12
91[c]
4
5
6
7
CsF
CsF
CsF
none
100
100
100
RT
12
12
12
12
97[b]
99[c]
99[b,d]
93[e]
8
9
none
RT
100
100
12
12
12
96[e]
51[b]
61[b]
NaOtBu
NaOtBu
10
11
12
13
14
NaOtBu
NaOtBu
NaOtBu
NaOtBu
100
100
100
RT
12
12
24
24
44[b]
67[b]
26[f]
49[f]
[a] General reaction conditions (not optimized): aryl halide (1.0 equiv), boronic acid (or RMgX, RNH2, RSH) (1 ± 1.5 equiv), CsF (or NaOtBu) (1.0 ±
3.0 equiv), [Pd2(dba)3] (0.5 ± 2.5 mol%; dba ˆ trans,trans-dibenzylideneacetone), 1.0 ± 5.0 mol% ligand. [b] [Pd2(dba)3]/(tBu)2P(O)H as precatalyst.
[c] [Pd2(dba)3]/(tBu)2PCl/H2O as precatalyst. [d] [Pd2(dba)3]/(tBu)2P(O)H or [Pd2(dba)3]/(2,4-(OMe)2C6H3)2P(O)H as precatalyst. [e] [Ni(cod)2]/
(tBu)2P(O)H as precatalyst. [f] [Pd2(dba)3]/(tBu)2PCl/H2O as precatalyst.
Several features of these phosphinous acid ligands are
noteworthy and provide both informative parallels and
contrasts to the corresponding phosphane ligands[5, 8, 9, 14] in
the cross-coupling reactions of aryl chlorides. Air-stable
phosphine oxides (RR'P(O)H) can be employed in the
present process as ligand precursors to produce electron-rich
phosphane-containing anionic complexes in the presence of
bases. We found that [Pd(cod)Cl2] reacts smoothly with
phosphine oxides (RR'P(O)H), in dioxane solution, affording
a new binuclear derivative shown in Scheme 1. Well-charac-
terized examples of this type of PdII complex are extremely
rare.[12] Complex I was isolated and characterized by X-ray
crystallography (see Supporting Information) and the struc-
ture is shown in Figure 1. As expected, the core structure of
complex I is based on P ± Pd coordination with phosphinous
acid ligands (RR'POH). Important features of the structure of
I include the O1 P1 and P1 Pd1 bond lengths of 1.5922 and
2.2434(6) Š, respectively. The O1 P1 distance is about
ˆ
0.105 Š longer than the P O double bond length in
RRP(O)H and to a first approximation can be interpreted
as a O P single bond. Also noteworthy is the facile
preparation of these air-stable phosphine oxide ligand pre-
cursors which can be generated as libraries by using polymer-
supported synthesis [Eqs. (1) and (2)].[10, 11]
Figure 1. Structure of the dichloro(di-tert-butylphosphinous acid)palladi-
um(ii) dimer (ORTEP view; 50% probability). The 12 methyl groups are
Ê
omitted for clarity. Selected interatomic distances [A ] and angles [8]: Pd1-
P1 2.2434(6), Pd1-Cl2 2.2917(6), Pd1-Cl1 2.3188(6), Pd1-Cl1* 2.4500(6),
Cl1-Pd1* 2.4500(6), P1-O1 1.5922(17); P1-Pd1-Cl2 90.36(2), P1-Pd1-Cl1
95.39(2), Cl2-Pd1-Cl1 174.23(2), P1-Pd1-Cl1* 179.109(19), Cl2-Pd1-Cl1*
88.76(2), Cl1-Pd1-Cl1* 85.48(2), Pd1-Cl1-Pd1 94.52(2), O1-P1-C5
100.47(10), O1-P1-C1 102.65(10), C5-P1-C1 116.54(10), O1-P1-Pd1
111.10(7).
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[6] For Suzuki cross-coupling reactions of aryl chlorides mediated by
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Resouly, Tetrahedron 1999, 55, 11889 ± 11894; c) S. Saito, S. Oh-tani,
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[7] For Kumada cross-coupling reactions of aryl chlorides mediated by
palladium or nickel complexes, see: a) V. P. W. Bohm, T. Weskamp,
C. W. K. Gstottmayr, W. A. Herrmann, Angew. Chem. 2000, 112,
1672 ± 1674; Angew. Chem. Int. Ed. 2000, 39, 1602 ± 1604; b) J. Huang,
S. P. Nolan, J. Am. Chem. Soc. 1999, 121, 9889 ± 9890.
In conclusion, these results demonstrate that air-stable
phosphine oxides (RR'P(O)H) are ideal ligand precursors for
the formation of phosphinous acid (RR'POH) ± metal com-
plexes for the rapid and regioselective oxidative addition of
unactivated aryl chlorides in the presence of bases, and that
such processes can be incorporated into efficient catalytic
cycles for a variety of C C, C N, and C S bond-forming
processes. Noteworthy are the efficiency for unactivated aryl
chlorides, as well as the simplicity, low cost, air-stability, and
ready accessibility by using polymer-supported synthesis of
these ligand precursors.
[8] a) A. F. Littke, G. C. Fu, Angew. Chem. 1999, 111, 2568 ± 2570; Angew.
Chem. Int. Ed. 1999, 38, 2411 ± 2412; b) E. Shirakawa, K. Yamamoto,
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[9] Leading references for the amination of aryl chlorides mediated by
transition metal complexes, see: a) J. P. Wolfe, H. Tomori, J. P. Sadighi,
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references therein; b) C. Desmarets, R. Schneider, Y. Fort, Tetrahe-
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Tetrahedron Lett. 2000, 41, 2881 ± 2884; d) J. Huang, G. Grasa, S. P.
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dron 1999, 55, 12829 ± 12842; g) X. Bei, A. S. Guram, H. W. Turner,
W. H. Weinberg, Tetrahedron Lett. 1999, 40, 1237 ± 1240; h) B. C.
Hamann, J. F. Hartwig, J. Am. Chem. Soc. 1998, 120, 7369 ± 7370.
[10] For combinatorial approaches to the synthesis of phosphane ligands,
see: a) G. Y. Li, P. J. Fagan, P. L. Watson, Angew. Chem. 2001, 113,
1140 ± 1143; Angew. Chem. Int. Ed. 2001, 40, 1106 ± 1109; b) G. Y. Li
(DuPont), US patent application, No. 60/197031, case number:
BC 1048 US PRV, 2000; c) P. J. Fagan, G. Y. Li, PCT Int. Appl.
WO 2000021663 A2 20000420, 169 pp. Appl. WO 1999-US23509
19991013.
[11] Use of phosphine oxides as ligands for homogeneous catalysis,
see: a) G. Y. Li, US patent 6124462, 2000; b) G. Y. Li (DuPont), US
patent application, No. 09/602714, Case Number: BC 1029 US NA
CIP 2000.
[12] For metal ± phosphinous acid complexes, for example, see: a) C. J.
Cobley, M. van den Heuvel, A. Abbadi, J. G. de Vries, Tetrahedron
Lett. 2000, 41, 2467 ± 2470; b) T. Ghaffar, A. W. Parkins, Tetrahedron
Lett. 1995, 36, 8657 ± 8660, UK patent Application 9506389.7; c) J. R.
Goerlich, A. Fischer, P. G. Jones, R. Z. Schmutzler, Z. Naturforsch. B
1994, 49, 801 ± 811; d) A. M. Arif, T. A. Bright, R. A. Jones, J. Coord.
Chem. 1987, 16, 45 ± 50.
[13] The low catalytic reactivity of aryl chlorides in cross-coupling
reactions is usually attributed to their reluctance towards oxidative
addition to Pd0. For a discussion, see: a) V. V. Grushin, H. Alper,
Chem. Rev. 1994, 94, 1047 ± 1062, and reference therein; b) ref. [1a].
[14] For cross-coupling reactions of C S bond-forming processes mediated
by transition metal complexes, see: a) M. S. Harr, A. L. Presley, A.
Thorarensen, Synlett, 1999, 10, 1579 ± 1581; b) N. Zheng, J. C. McWil-
liams, F. J. Fleitz, J. D. Armstrong III, R. P. Volante, J. Org. Chem.
1998, 63, 9606 ± 9607; c) D. Baranano, J. F. Hartwig, J. Am. Chem. Soc.
1995, 117, 2937 ± 2938; d) K. Takagi, Chem. Lett. 1987, 2221 ± 2224;
e) M. Kosugi, T. Ogata, M. Terada, H. Sano, T. Migita, Bull. Chem.
Soc. Jpn. 1985, 58, 3657 ± 3658; f) T. Migita, T. Shimiza, Y. Asami, J.
Shiobara, Y. Kato, M. Kosugi, Bull. Chem. Soc. Jpn. 1980, 53,
1385 ± 1389; g) M. Kosugi, T. Shimizu, T. Migita, Chem. Lett. 1978,
13 ± 14.
Received: December 4, 2000 [Z16219]
[1] For general references, see: a) V. V. Grushin, H. Alper in Topics in
Organometallic Chemisty, Vol. 3 (Ed.: S. Murai), 1999, Springer, New
York, pp. 193 ± 226, and references therein; b) A. Suzuki in Metal-
Catalyzed Cross-Coupling Reactions (Eds.: F. Diederich, P. J. Stang),
Wiley-VCH, Weinheim, 1998, Chap. 2; c) M. Beller, T. H. Riermeier
in Transition Metals for Organic Synthesis, Vol. 1 (Eds.: M. Beller, C.
Bolm), Wiley-VCH, Weinheim, 1998, pp. 184 ± 193; d) E.-i. Negishi, F.
Liu in Metal-Catalyzed Cross-Coupling Reactions (Eds.: F. Diederich,
P. J. Stang), Wiley-VCH, Weinheim, 1998, pp. 1 ± 48; e) W. A. Herr-
mann in Applied Homogeneous Catalysis with Organometallic Com-
pounds (Eds.: B. Cornils, W. A. Herrmann), Wiley-VCH, Weinheim,
1996, pp. 764 ± 765.
[2] For leading reviews, see: a) B. H. Yang, S. L. Buchwald, J. Organomet.
Chem. 1999, 576, 125 ± 146; b) A. Suzuki, J. Organomet. Chem. 1999,
576, 147 ± 168, and references therein; c) R. Sturmer, Angew. Chem.
1999, 111, 3509 ± 3510; Angew. Chem. Int. Ed. 1999, 38, 3307 ± 3308;
d) A. J. Belfield, G. R. Brown, A. J. Foubister, Tetrahedron 1999, 55,
11399 ± 11428, and references therein; e) J. P. Wolfe, S. Wagaw, J.-F.
Marcoux, S. L. Buchwald, Acc. Chem. Res. 1998, 31, 805 ± 818; f) J. F.
Hartwig, Angew. Chem. 1998, 110, 2154 ± 2177; Angew. Chem. Int.
Ed. 1998, 37, 2046 ± 2067; g) N. Miyaura, A. Suzuki, Chem. Rev.
1995, 95, 2457 ± 2483; h) M. Kumada, Pure Appl. Chem. 1980, 52,
669 ± 679.
[3] For Suzuki cross-coupling reactions of aryl chlorides mediated by
palladacycle complexes, see: M. Beller, H. Fischer, W. A. Herrmann,
K. Ofele, C. Brossmer, Angew. Chem. 1995, 107, 1992 ± 1993; Angew.
Chem. Int. Ed. Engl. 1995, 34, 1848 ± 1849.
[4] For Suzuki cross-coupling reactions of aryl chlorides mediated by
palladium/N-heterocyclic carbene complexes, see: a) C. Zhang, M. L.
Trudell, Tetrahedron Lett. 2000, 41, 595 ± 598; b) C. Zhang, J. Huang,
M. L. Trudell, S. P. Nolan, J. Org. Chem. 1999, 64, 3804 ± 3805; c) T.
Weskamp, V. P. W. Bohm, W. A. Herrmann, J. Organomet. Chem.
1999, 585, 348 ± 352; d) W. A. Herrmann, C.-P. Reisinger, M. J.
Spiegler, J. Organomet. Chem. 1998, 557, 93 ± 96.
[5] For Suzuki cross-coupling reactions of aryl chlorides mediated by
palladium/phosphane complexes, see: a) ref. [2b]; b) J. P. Wolfe, S. L.
Buchwald, Angew. Chem. 1999, 111, 2570 ± 2573; Angew. Chem. Int.
Ed. 1999, 38, 2413 ± 2416; c) J. P. Wolfe, R. A. Singer, B. H. Yang, S. L.
Buchwald, J. Am. Chem. Soc. 1999, 121, 9550 ± 9561; d) X. Bei, A. S.
Guram, H. W. Turner, W. H. Weinberg, Tetrahedron Lett. 1999, 40,
3855 ± 3858; e) X. Bei, H. W. Turner, W. H. Weinberg, A. S. Guram, J.
Org. Chem. 1999, 64, 6797 ± 6803; f) F. Firooznia, C. Gude, K. Chan, Y.
Satoh, Tetrahedron Lett. 1998, 39, 3985 ± 3988; g) A. F. Littke, G. C.
[15] For a typical synthesis of (tBu)2PH(O), see: a) N. D. Gomelya, N. G.
Feshchenko, J. Gen. Chem. USSR 1987, 57, 1518 ± 1523; b) A. P.
Stewart, S. J. Trippett, J. Chem. Soc. C 1970, 9, 1263 ± 1266; c) H. P.
Angstadt, J. Am. Chem. Soc. 1964, 86, 5040 ± 5041.
[16] A reaction of [Pd2(dba)3] (100 mg, 0.109 mm) and (tBu)2PH(O)
((36.0 mg, 0.218 mmol), 31P{1H} NMR: d ˆ 69.8 (d, 1J(P,H) ˆ
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434.2 Hz)) in 1,4-dioxane (6.0 mL) at room temperature for 6 h
adjacent to the heteroatom. This cycloaddition protocol can
be classified as a sequential anionic ± pericyclic strategy for
which the significance in natural product synthesis has been
elegantly summarized by Tietze and Beifuss.[8] Our recent
work on the use of vinylogous amides in this formal cyclo-
addition[1] has allowed us to envision an intramolecular
version of this reaction. Since the vinylogous amide 1, which
contains a functionalized tether, provided the formal cyclo-
adduct 3 in good yields,[1] it is conceivable that the vinylogous
amides 4, which are tethered with an a,b-unsaturated iminium
ion, may lead to piperidinyl heterocycles 5, attractive as
intermediates in the syntheses of natural alkaloids
(Scheme 1). We report herein the first stereoselective intra-
molecular formal [3‡3] cycloaddition reaction by using
vinylogous amides that are tethered with iminium ions, and
its application in a formal total synthesis of (‡)-gephyro-
toxin.[9±11]
Based on our previous studies, the formation of the a,b-
unsaturated iminium salt prior to the addition of a b-diketo
nucleophile is crucial to the regio- and chemoselectivity of the
intermolecular formal cycloaddition reaction.[1, 2a] Conditions
and protocols that generate a,b-unsaturated iminium salts in
the presence of the b-diketo nucleophile lead to low yields of
the desired products, and/or to synthetically less useful by-
products resulting from various competing reaction path-
ways.[7b±e] However, the enal precursor to a,b-unsaturated
iminium ions in the intramolecular variant also contains a
vinylogous amide. Thus, controlling the extent of iminium ion
formation before competing reactions take place is a major
challenge.
To demonstrate the feasibility of an intramolecular formal
[3‡3] cycloaddition, the vinylogous amides 6 and 7 were
prepared[12, 13] and treated with piperidine (2.0 equiv) and
Ac2O (2.0 equiv) in EtOAc/toluene (1:2; concentration
of vinylogous amides: 0.03 ± 0.30m) at 858C for 1 h
(Scheme 2). These are the standard reaction conditions that
have been consistently employed for the formation of
iminium salts.[1, 2] Subsequent heating at 1508C in a sealed
tube led to the isolation of the desired heterocycles 8 and 9 in
yields of 80 and 68%, respectively. Furthermore, the chiral
vinylogous amide 10[12] led to the desired tricycle 11 in 55%
yield as a single diastereomer under the same reaction
conditions. The stereochemistry of 11 was assigned by
NOESY experiments.
generated
resonance at d ˆ 123.4 (singlet).
a brown mixture exhibiting a
1H-coupled 31P NMR
[17] See Supporting Information for full synthetic details and character-
ization of new compounds. Crystallographic data (excluding structure
factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-157421. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
A Novel and Highly Stereoselective
Intramolecular Formal [3‡3] Cycloaddition
Reaction of Vinylogous Amides Tethered with
a,b-Unsaturated Aldehydes: A Formal Total
Synthesis of (‡)-Gephyrotoxin**
Lin-Li Wei, Richard P. Hsung,* Heather M. Sklenicka,
and Aleksey I. Gerasyuto
We have been exploring reactions of b-diketo equivalents 1
with a,b-unsaturated iminium salts 2 for the construction of
heterocycles such as 3 (Scheme 1).[1±3] These reactions involve
a sequence consisting of a Knoevenagel condensation fol-
lowed by a six p-electron electrocyclic ring-closure,[4] thereby
constituting a stepwise formal [3‡3] cycloaddition[5±7] in which
two s bonds are formed, along with a new stereocenter
These examples suggest that our initial concern regarding
the extent of iminium salt formation was not necessary; under
conditions that do not promote the formation of iminium ions,
these reactions were essentially arrested. This control study
suggests that even a low concentration of the iminium species
is sufficient to promote this intramolecular formal [3‡3]
cycloaddition reaction. On the other hand, under the same
standard reaction conditions, the vinylogous amide 12 did not
provide the desired cycloadduct 13 in useful yields
(Scheme 2). We were, however, able to observe the formation
of 13 by 1H NMR spectroscopy; thus our inability to isolate 13
from 12 may be a result of the instability of 13 under the
reaction conditions and/or under the work-up conditions. This
prompted us to explore other protocols suitable for generat-
ing iminium salts.
Scheme 1. [3‡3] Cycloaddition reactions of vinylogous amides and a,b-
unsaturated iminium salts. Reagents and conditions: a) EtOAc/toluene 1:2,
1508C, 15 ± 30 h, 76 ± 80%; TBS ˆ tert-butyldimethylsilyl.
[*] Prof. R. P. Hsung, L.-L. Wei, H. M. Sklenicka, A. I. Gerasyuto[‡]
Department of Chemistry, University of Minnesota
Minneapolis, MN, 55455 (USA)
Fax : (‡1)612-626-7541
E-mail: hsung@chem.umn.edu
‡
[ ] An Undergraduate Research Participant from the Higher Chemical
College of the Russian Academy of Sciences.
[**] This work was supported by R. W. Johnson Pharmaceutical Research
Institutes.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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Angew. Chem. Int. Ed. 2001, 40, No. 8