A palladium-catalyzed aminoalkynylation strategy towards bicyclic heterocycles: Synthesis of (±)-trachelanthamidine

Article abstract of DOI:10.1002/anie.201100718

Sweet cyclizations: The synthesis of pyrrolizidines and indolizidines has been achieved. Olefins were subjected to an intramolecular palladium-catalyzed aminoalkynylation with the hypervalent iodine reagent TIPS-EBX. After removal of the protecting group, a two-step cyclization sequence and subsequent reduction led to the natural product (±)-trachelanthamidine (see scheme; TIPS-EBX=triisopropylsilyl ethynylbenziodoxolone).

Full text of DOI:10.1002/anie.201100718

Communications
DOI: 10.1002/anie.201100718
Heterocyclic Synthesis
A Palladium-Catalyzed Aminoalkynylation Strategy towards Bicyclic
Heterocycles: Synthesis of (ꢀ )-Trachelanthamidine**
Stefano Nicolai, Cyril Piemontesi, and Jꢀrꢁme Waser*
Dedicated to Professor Barry M. Trost on the occasion of his 70th birthday
Nitrogen-containing heterocycles are omnipresent in natural
products and biologically active compounds. Among them,
pyrrolizidine and indolizidine alkaloids have attracted broad
interest for their potential pharmacological applications.
Furthermore, their polycyclic structures, which frequently
incorporate multiple chiral centers, have been the testing
tionalization. In 2010, our research group reported the
intramolecular oxyalkynylation of alkenes.[5] However, we
were unable to use our protocol efficiently in the case of
amination. The second required cyclization could be envis-
aged through hydroamination of the triple bond,[2,6] or a two-
step alternative route through vinyl halides.[7] Both
approaches are well established for the intermolecular syn-
thesis of enamides, but they have never been reported to
access pyrrolizidinones or indolizidinones.
Herein, we report the exceptional efficiency of lithium
palladate catalysts for the intramolecular aminoalkynylation
of olefins using hypervalent iodine reagents. The reaction is
broadly applicable, working not only for g- and d-lactams but
also for oxazolidinones, imidazolidinones, and indole or
pyrrole piperazinones. The importance of the aminoalkynyl-
ation reaction is demonstrated in the elaboration of the
lactam acetylenes into pyrrolizidines and indolizidines using a
two-steps procedure (indium-mediated iodination[8] and
copper-catalyzed vinylation),[7c,d] which culminated in the
total synthesis of the natural product (ꢀ )-trachelanthami-
dine.
ꢁ
ꢁ
ground for new C C and C N bond-forming methods for
several decades.[1]
In this context, we envisaged a novel strategy to access
both pyrrolizidine and indolizidine heterocycles involving the
initial aminoalkynylation of olefins to afford 5-propargyl
ꢁ
lactams (Scheme 1). This step allows both C N bond
A preliminary screening was performed by subjecting
protected/activated amides derived from 4-pentenoic acid to
the protocol developed for oxyalkynylation (triisopropylsilyl
ethynylbenziodoxolone (TIPS-EBX (2a)), 10 mol% of [Pd-
(hfacac)2], dichloromethane).[5,9] N-tosyl amide 1a was the
most promising substrate, but the yield was low (33%)
because of decomposition (Table 1, entry 1).
Scheme 1. Strategy for the synthesis of pyrrolizidines and indolizidines.
PG=protecting group.
formation and introduction of the two missing carbon atoms
in a single transformation. The amination of olefins is still a
major challenge in organic synthesis and only recently
progress has been realized in intramolecular hydroamina-
tion,[2] the aza-Wacker reaction,[3] and more challenging
multiple aminofunctionalization of alkenes.[4] In particular,
Optimization of the reaction conditions using the pre-
viously discovered catalyst was not successful.[10] An interest-
ing lead result was obtained with PdCl2 in EtOH (Table 1,
entry 2). Better yields were observed with LiCl as the additive
(Table 1, entry 3), while other Li or Cl salts were less efficient
(Table 1, entries 4 and 5). Three equivalents of LiCl with
respect to 2a was optimal (Table 1, entry 6). Under these
reaction conditions, the active catalyst could be Li2[PdCl4]
formed in situ. Indeed, when Li2[PdCl4] was used as the
catalyst with 1a, product 3a was isolated in the same yield as
when PdCl2/LiCl was used (Table 1, entry 7).[11] One possible
explanation for this effect would be that halide ions can slow
down side reactions, in particular b-hydride elimination.[12] To
the best of our knowledge, this is the first example of the use
of a palladate complex for the carboamination of olefins. The
reaction was performed at room temperature using analytical
grade EtOH in an ambient atmosphere. Contributing factors
that make this method highly practical are: LiCl is inex-
pensive, PdCl2 is the most commonly used Pd salt, and TIPS-
EBX (2a) is commercially available. The use of EBX reagents
ꢁ
ꢁ
the simultaneous formation of C N and C C bonds has been
focused mostly on aminoarylation and aminocarbonylation.
In this context, the aminoalkynylation process is still
unknown, despite its tremendous potential for further func-
[*] S. Nicolai, C. Piemontesi, Prof. Dr. J. Waser
Laboratory of Catalysis and Organic Synthesis
Ecole Polytechnique Fꢀdꢀrale de Lausanne
EPFL SB ISIC LCSO, BCH 4306, 1015 Lausanne (CH)
Fax: (+41)21-693-97-00
E-mail: jerome.waser@epfl.ch
Homepage: http://lcso.epfl.ch/
[**] The EPFL and the SNF (grant number 200021_119810) are
acknowledged for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100718.
4680
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 4680 –4683

Table 1: Optimization of the aminoalkynylation reaction.[a]
Table 2: Aminoalkynylation of activated amides.[a]
Entry
Substrate
Product
Yield [%][b]
1
2
3
1a Ar=4-MeC6H4
1b Ar=4-MeOC6H4
1c Ar=4-NO2C6H4
3a Ar=4-MeC6H4
3b Ar=4-MeOC6H4
3c Ar=4-NO2C6H4
88
80
50
Entry
R
Catalyst
Additive
Solvent
Yield [%][b]
1
2
3
4
TIPS
TIPS
TIPS
TIPS
TIPS
TIPS
TIPS
TMS
Ph
[Pd(hfacac)2]
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Li2[PdCl4]
PdCl2
–
–
CH2Cl2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
33
57
LiCl
Bu4NCl
LiBF4
LiCl
–
76 (75)[c,d]
4
5
6
7
8
1d
1e
1 f
3d
3e
3 f
84
88
77
48
20
5
6[e]
7[c]
8[e]
9[e]
88
84[d]
23
LiCl
LiCl
PdCl2
58
[a] Reaction conditions: 0.069 mmol of 1a, 0.014 mmol of catalyst,
0.084 mmol of R-EBX (2), 0.084 mmol of additive in 1.75 mL of solvent,
15 h, RT. [b] Yield was determined by H NMR spectroscopy. [c] Using
0.40 mmol of 1a, 0.040 mmol of catalyst, 0.48 mmol of R-EBX (2),
0.48 mmol of additive in 10 mL of solvent, 3.5 h, RT. [d] Yield of isolated
product. [e] As for [c], but using 1.44 mmol of LiCl. Yield of isolated
product. hfacac=hexafluoroacetylacetonate, TIPS=triisopropylsilyl,
TMS=trimethylsilyl, Ts=4-toluenesulfonyl.
1
71
1g
1h
3g
3h
(d.r. 90:10)
67
(d.r. 70:30)
was required, as no product was obtained using alkynyl
halides or alkynyliodonium salts. The smaller TMS group
gave a lower yield (Table 1, entry 8), but the introduction of a
phenyl group led to 58% yield (Table 1, entry 9).[13]
9
1i
1j
1k
3i
3j
3k
57
84
88
10
11
The scope of the reaction was then examined (Table 2).
Modification of the benzenesulfonyl group showed that tosyl
amide 1a was optimal (Table 2, entry 1). 4-Methoxybenzene-
sulfonyl amide 1b gave a similar yield (Table 2, entry 2). The
lactam was obtained in 50% yield with the easily removable
nosyl group (Table 2, entry 3). Good results were obtained for
the synthesis of nitrogen-protected 5-propargyl pyrrolidi-
nones (Table 2, entries 4–8). The reaction was tolerant to
substitution a to the carbonyl group (Table 2, entries 4–6),
thus allowing facile access to azaspiro compounds (Table 2,
entries 5 and 6). Substitution at the allylic position with a
phenyl group gave the trans product in 71% yield and
90:10 d.r. (Table 2, entry 7). The reaction was also successful
for the synthesis of bicyclic heterocycle 3h (Table 2, entry 8).
Cyclization reactions to give six-membered rings are usually
more challenging. Nevertheless, simple N-tosyl 5-hexenamide
gave propargyl piperidone 3i in 57% yield (Table 2, entry 9).
The yield increased for more rigid substrates (Table 2,
entries 10–12). Arylpiperazines derived from pyrroles or
indoles are key structural elements in bioactive compounds.[14]
We then examined substitution on the double bond. Although
no reactivity was observed with vicinally disubstituted olefins,
the cyclized product could be isolated in 62% yield with
geminally disubstituted alkene 1m under modified reaction
conditions (Table 2, entry 13). This result is promising,
because it indicates that the reaction could be extended to
substituted olefins after adequate optimization.
12
1l
3l
89
13
1m Ar=4-NO2C6H4
3m Ar=4-NO2C6H4
62[c]
[a] Reaction conditions: 0.40 mmol of 1, 0.040 mmol of PdCl2, 0.48 mmol of
TIPS-EBX (2a), 1.44 mmol of LiCl in 10 mL of EtOH, 2–5 h, RT. [b] Yield after
purification by column chromatography. [c] 0.40 mmol of 1, 0.040 mmol of
[Pd(hfacac)2], 0.48 mmol of TIPS-EBX (2a), 0.48 mmol of 4-chlorosalicylic
acid in 10 mL of CH2Cl2, 15 h, RT.
tolerated. We concentrated first on O-allyl and homoallyl
carbamates (Table 3, entries 1–4). Allyl tosylcarbamate 6a
was converted into the corresponding 4-propargyl oxazolidi-
none 7a in 83% yield (Table 3, entry 1). Substitution at the
allylic position was also tolerated (Table 3, entries 2 and 3),
although the diastereoselectivity was lower than for N-tosyl
pentenamides. Homoallyl tosylcarbamate 6d afforded the six-
membered ring product in 59% yield (Table 3, entry 4).
Finally, we studied the behavior of allyl ureas under our
As our protocol was successful for the synthesis of
lactams, we wondered if a second heteroatom would be
Angew. Chem. Int. Ed. 2011, 50, 4680 –4683
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4681

Communications
Table 3: Aminoalkynylation of activated carbamates and ureas.[a]
Entry
Substrate
Product
Yield [%][b]
1
2
6a R=H, R1 =H
6b R=Me, R1 =H
7a R=H, R1 =H
7b R=Me, R1’=H
83
66
(d.r. 61:39)
58
3
6c R=Cy, R1 =H
7c R=Cy, R1 =H
(d.r. 66:34)
Scheme 2. Cyclization to form bicyclic heterocycles. The scheme shows
individual reactions under the same conditions (i.e. 8a!9a, 8b!9b,
8c!9c). DIBALH=diisobutylaluminum hydride, THF=tetrahydro-
furan.
4
6d
7d
59
(Z)-vinyl iodide in 99% yield (Z/E 15:1).[8] This is the first
example of the use of Oshimaꢀs method in the presence of an
amide group and demonstrates the strength of this under-
utilized protocol. The conditions described by Buchwald for
the intermolecular vinylation of amides gave pyrrolizidine 9a
in 75% yield.[7c,d] The same procedure could be successfully
applied to the cyclization of oxazolidinone 8b and indolizi-
dinone 8c in 72% and 65% overall yields, respectively.
Our strategy was then applied to the synthesis of the
5
6
7
6e R=H
6 f R=allyl
6g R=Bn
7e R=H
7 f R=allyl
7g R=Bn
79
67
65
[a] Reaction conditions: 0.40 mmol of 6, 0.040 mmol of PdCl2,
0.48 mmol of TIPS-EBX (2a), 1.44 mmol of LiCl in 10 mL of EtOH, 2–
5 h, RT. [b] Yield after purification by column chromatography. Bn=
benzyl, Cy=cyclohexyl.
pyrrolizidine
alkaloid
(ꢀ )-trachelanthamidine
(16;
Scheme 3).[16] Amide 11 was obtained through Johnson–
Claisen rearrangement of protected butene diol 10,[17] sub-
sequent hydrolysis, and treatment with p-tosylisocyanate. The
aminoalkynylation of tosyl amide 11 proceeded in 72% yield
and 83:17 dr. The tosyl and silyl groups were removed and
cyclization on the triple bond proceeded in 69% overall yield.
Separation of the two diastereoisomers was achieved after
cyclization. Reduction of the enamide and debenzylation of
the alcohol were achieved in quantitative yield. Finally,
reduction of the amide group using LiAlH4 gave racemic
trachelanthamidine (16) in nine steps and 22% overall yield
from 10.
optimal reaction conditions (Table 3, entries 5–7). Control of
O versus N cyclization is more difficult for urea, owing to the
enhanced nucleophilicity of the oxygen atom. Nevertheless,
only propargyl imidazolidinones were isolated in 65–79%
yields when using either free or allyl- and benzyl-protected
ureas.
As several new bonds are formed during the reaction,
understanding the mechanism is challenging. Complete con-
version of the hypervalent iodine reagent (TIPS-EBX) into 2-
iodobenzoic acid and the dimerized acetylene was observed
by 1H NMR spectroscopy when it was mixed with a stoichio-
metric amount of Li2[PdCl4]. A shift of the olefin and
aliphatic signals was observed in the 1H NMR spectrum of the
N-tosyl amide 1a when added to a stoichiometric amount of
the catalyst.[15] The addition of TIPS-EBX (2a) to this mixture
then resulted in rapid formation of the product. These
preliminary results seem to indicate that an initial amino-
palladation and subsequent oxidative alkynylation could be
envisaged. Further investigation is obviously required to
elucidate the mechanism.
At this point, we turned our attention to the second
cyclization step required to access pyrrolizidine and indolizi-
dine heterocycles. Removal of the tosyl group from 3a could
be performed in 97% yield by treatment with [Li(naphtha-
lenide)]. The desilylation of the triple bond was then
accomplished with TBAF to give 87% yield. Cyclization of
pyrrolizidinone 8a was then examined (Scheme 2). As no
product was obtained using ruthenium-catalyzed methods
reported for the intermolecular hydroamination of alkynes
with amides,[6b,c] we investigated a two-step approach via a
vinyl iodide intermediate. Hydroindiation of the acetylene
with HInCl2 and subsequent quenching with iodine gave the
Scheme 3. Total synthesis of (ꢀ)-trachelanthamidine (16). Reaction
conditions: a) CH3C(OEt)3, EtCO2H, 1008C to 1608C; then KOH,
MeOH, reflux, 80%; b) p-TsNCO, Et3N, THF, RT, 80%; c) 5 mol% of
PdCl2, LiCl, TIPS-EBX (2a), EtOH, RT, 72%; 83:17 d.r.; d) Li/naphtha-
lene, THF, ꢁ788C, 77%; e) TBAF, THF, 08C to RT, 98%; f) 1) InCl3,
DIBALH, 2) Et3B, 3) I2, THF, ꢁ508C, 95%; g) 40 mol% CuI, Cs2CO3,
N,N’-dimethylethylenediamine, toluene, 858C, 73%; h) H2, Pd/C,
MeOH, RT, quantitative; i) LiAlH4, THF, reflux, 94%. TBAF=tetra-n-
butylammonium fluoride.
4682 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 4680 –4683

L. Cui, Y. Z. Wang, L. M. Zhang, J. Am. Chem. Soc. 2010, 132,
1474; others: s) A. W. Lei, X. Y. Lu, G. S. Liu, Tetrahedron Lett.
2004, 45, 1785; t) M. R. Manzoni, T. P. Zabawa, D. Kasi, S. R.
Chemler, Organometallics 2004, 23, 5618; u) F. E. Michael, P. A.
Sibbald, B. M. Cochran, Org. Lett. 2008, 10, 793; reviews: v) A.
Minatti, K. Muꢂiz, Chem. Soc. Rev. 2007, 36, 1142; w) E. M.
Beccalli, G. Broggini, M. Martinelli, S. Sottocornola, Chem. Rev.
2007, 107, 5318.
In summary, a novel strategy for the synthesis of nitrogen-
bridged bicyclic heterocycles has been reported. The palla-
dium-catalyzed intramolecular aminoalkynylation of terminal
olefins was operationally simple and could be successfully
applied to tosyl amides, carbamates, and ureas. Both five- and
six-membered rings were obtained in good to excellent yields.
After facile removal of protecting groups, iodination of the
triple bond, and subsequent copper-catalyzed vinylation of
the amide gave access to the core of pyrrolizidine and
indolizidine heterocycles and led to a new total synthesis of
(ꢀ )-trachelanthamidine (16). Further work towards the
elucidation of the reaction mechanism and the development
of an asymmetric version of the transformation are ongoing.
[5] S. Nicolai, S. Erard, D. Fernꢃndez Gonzꢃlez, J. Waser, Org. Lett.
2010, 12, 384.
[6] a) J. S. Prasad, L. S. Liebeskind, Tetrahedron Lett. 1988, 29, 4253;
b) L. J. Goossen, J. E. Rauhaus, G. J. Deng, Angew. Chem. 2005,
117, 4110; Angew. Chem. Int. Ed. 2005, 44, 4042; c) L. J.
Goossen, K. S. M. Salih, M. Blanchot, Angew. Chem. 2008,
120, 8620; Angew. Chem. Int. Ed. 2008, 47, 8492.
[7] a) Y. Kozawa, M. Mori, Tetrahedron Lett. 2002, 43, 111; b) Y.
Kozawa, M. Mori, J. Org. Chem. 2003, 68, 3064; c) L. Jiang, G. E.
Job, A. Klapars, S. L. Buchwald, Org. Lett. 2003, 5, 3667;
d) L. L. W. Cheung, A. Yudin, Org. Lett. 2009, 11, 1281.
[8] K. Takami, H. Yorimitsu, K. Oshima, Org. Lett. 2002, 4, 2993.
[9] The exceptional properties of ethynyl benziodoxolone reagents
were key for success; a) M. Ochiai, Y. Masaki, M. Shiro, J. Org.
Chem. 1991, 56, 5511; b) V. V. Zhdankin, C. J. Kuehl, A. P.
Krasutsky, J. T. Bolz, A. J. Simonsen, J. Org. Chem. 1996, 61, 654;
c) J. P. Brand, J. Charpentier, J. Waser, Angew. Chem. 2009, 121,
9510; Angew. Chem. Int. Ed. 2009, 48, 9346; d) D. Fernꢃndez
Gonzꢃlez, J. P. Brand, J. Waser, Chem. Eur. J. 2010, 16, 9457;
e) J. P. Brand, J. Waser, Angew. Chem. 2010, 122, 7462; Angew.
Chem. Int. Ed. 2010, 49, 7304; For reviews, see: f) T. Wirth,
Hypervalent Iodine Chemistry: Modern Developments in
Organic Synthesis, Vol. 224, Springer, New York, 2003; g) V. V.
Zhdankin, P. J. Stang, Chem. Rev. 2008, 108, 5299; h) J. P. Brand,
D. Fernꢃndez Gonzꢃlez, S. Nicolai, J. Waser, Chem. Commun.
2011, 47, 102.
Received: January 28, 2011
Published online: April 15, 2011
Keywords: alkaloids · alkynylation · amination ·
.
heterocyclic compounds · hypervalent iodine
[1] a) J. R. Liddell, Nat. Prod. Rep. 2002, 19, 773; b) J. P. Michael,
Nat. Prod. Rep. 2007, 24, 191.
[2] T. E. Mꢁller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem.
Rev. 2008, 108, 3795.
[3] For a few selected references, see: a) L. S. Hegedus, G. F. Allen,
J. J. Bozell, E. L. Waterman, J. Am. Chem. Soc. 1978, 100, 5800;
b) S. R. Fix, J. L. Brice, S. S. Stahl, Angew. Chem. 2002, 114, 172;
Angew. Chem. Int. Ed. 2002, 41, 164; c) R. I. McDonald, S. S.
Stahl, Angew. Chem. 2010, 122, 5661; Angew. Chem. Int. Ed.
2010, 49, 5529; d) R. M. Trend, Y. K. Ramtohul, B. M. Stoltz, J.
Am. Chem. Soc. 2005, 127, 17778.
[10] See the Supporting Information for a complete list of tested
reaction conditions.
[4] Aminocarbonylation: a) Y. Tamaru, M. Hojo, Z. Yoshida, J. Org.
Chem. 1988, 53, 5731; b) T. A. Cernak, T. H. Lambert, J. Am.
Chem. Soc. 2009, 131, 3124; aminooxygenation: c) E. J. Alexa-
nian, C. Lee, E. J. Sorensen, J. Am. Chem. Soc. 2005, 127, 7690;
d) L. V. Desai, M. S. Sanford, Angew. Chem. 2007, 119, 5839;
Angew. Chem. Int. Ed. 2007, 46, 5737; e) P. H. Fuller, J. W. Kim,
S. R. Chemler, J. Am. Chem. Soc. 2008, 130, 17638; f) H. M.
Lovick, F. E. Michael, J. Am. Chem. Soc. 2010, 132, 1249; g) T.
de Haro, C. Nevado, Angew. Chem. 2011, 123, 936; Angew.
Chem. Int. Ed. 2011, 50, 906; diamination: h) T. P. Zabawa, D.
Kasi, S. R. Chemler, J. Am. Chem. Soc. 2005, 127, 11250; i) J.
Streuff, C. H. Hovelmann, M. Nieger, K. Muꢂiz, J. Am. Chem.
Soc. 2005, 127, 14586; j) K. Muꢂiz, J. Am. Chem. Soc. 2007, 129,
14542; k) K. Muꢂiz, C. H. Hovelmann, J. Streuff, J. Am. Chem.
Soc. 2008, 130, 763; l) P. A. Sibbald, C. F. Rosewall, R. D. Swartz,
F. E. Michael, J. Am. Chem. Soc. 2009, 131, 15945; arylamina-
tion: m) J. E. Ney, J. P. Wolfe, Angew. Chem. 2004, 116, 3689;
Angew. Chem. Int. Ed. 2004, 43, 3605; n) J. P. Wolfe, Synlett 2008,
2913; o) J. D. Neukom, N. S. Perch, J. P. Wolfe, J. Am. Chem. Soc.
2010, 132, 6276; p) C. F. Rosewall, P. A. Sibbald, D. V. Liskin,
F. E. Michael, J. Am. Chem. Soc. 2009, 131, 9488; q) W. E.
Brenzovich, D. Benitez, A. D. Lackner, H. P. Shunatona, E.
Tkatchouk, W. A. Goddard, F. D. Toste, Angew. Chem. 2010,
122, 5651; Angew. Chem. Int. Ed. 2010, 49, 5519; r) G. Z. Zhang,
[11] The in situ generation of Li2[PdCl4] from cheaper PdCl2 and LiCl
was still preferred for preparative reactions. Other metal
additives, such as CuI, were not successful, either with hyper-
valent iodine reagents or alkynyl halides (See Table S3 and S4 in
the Supporting Information).
[12] a) K. Fagnou, M. Lautens, Angew. Chem. 2002, 114, 26; Angew.
Chem. Int. Ed. 2002, 41, 26; b) X. Y. Lu, Top. Catal. 2005, 35, 73.
[13] This result is interesting in terms of further extension of the
scope of the reaction. Nevertheless, we decided to focus first on
the use of TIPS-EBX (2a), as the protecting groups of the silyl
acetylenes obtained were easily removed.
[14] a) J. L. Mokrosz, B. Duszynska, M. H. Paluchowska, Arch.
Pharm. 1994, 327, 529; b) S. Butini et al., J. Med. Chem. 2009,
52, 151. See the Supporting Information for full citation.
[15] See Figures S1 and S2. Investigations are ongoing to identify the
intermediate that is formed.
[16] a) G. P. Menschikov, Zh. Obshch. Khim. 1946, 16, 1311; b) S.
Danishefsky, R. McKee, R. K. Singh, J. Am. Chem. Soc. 1977, 99,
4783; c) H. Ishibashi, M. Sasaki, T. Taniguchi, Tetrahedron 2008,
64, 7771.
[17] S. Couty, C. Meyer, J. Cossy, Tetrahedron 2009, 65, 1809.
Angew. Chem. Int. Ed. 2011, 50, 4680 –4683
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4683