Formal highly enantioselective organocatalytic addition of alkyl anions to α,β-unsaturated aldehydes: Application to the synthesis of isotope-enantiomers

Article abstract of DOI:10.1002/chem.200901806

A practical and powerful methodology that constitutes an organocatalytic alternative for organometallic 1,4-additions was developed. In a vial, bis(phenylsulfonyl)methane and catalyst V were added to toluene. Next, unsaturated aldehyde 1a was added and th

Full text of DOI:10.1002/chem.200901806

COMMUNICATION
DOI: 10.1002/chem.200901806
Formal Highly Enantioselective Organocatalytic Addition of Alkyl Anions to
a,b-Unsaturated Aldehydes: Application to the Synthesis of Isotope-
Enantiomers
Andrea-Nekane Alba,[a] Xavier Companyꢀ,[a] Albert Moyano,*[a] and Ramon Rios*[a, b, c]
Dedicated to Professor Andrew E. Greene on occasion of his retirement
During the last decade, chemists have devoted much
effort to the development of new asymmetric methodologies
to build complex scaffolds in a highly enantioselective fash-
ion.[1] Among the strategies employed, organocatalysis[2] has
received widespread attention due to its unique properties
such as metal-free environment, easy predictability of ste-
reoselectivity, use of inexpensive catalysts, and simplicity of
reaction conditions.
Since the work on proline catalysis by List, Lerner, and
Barbas III in 2000[3] and that on iminium catalysis by Mac-
Millan soon after,[4] organocatalysis has grown exponentially
both by improving catalysts and by the discovery of new re-
actions. However, there are still a number of challenges and
issues that lie ahead. One notable challenge is the use of
“naked” methyl or alkyl synthons as suitable nucleophiles.
The catalytic enantioselective conjugate addition of non-
functionalized hydrocarbon fragments to a,b-unsaturated al-
dehydes is an unsolved problem in organic synthesis. Where-
as in the past few years major breakthroughs have been re-
alized in the metal-catalyzed enantioselective conjugate ad-
dition of Grignard reagents[5] and of other organometallic
reagents[6] to a,b-unsaturated ketones, esters, or amides,
only recently Brꢀse and co-workers reported on the asym-
metric conjugate addition of diethylzinc to cinnamaldehyde,
by using N,O-planar chiral [2.2]paracyclophane-derived li-
gands.[7]
In the field of organocatalysis there are several carbon
À
nucleophiles that can be used to form C C bonds with unsa-
turated aldehydes. For example, Jorgensen and co-workers
developed the addition of malonate to unsaturated alde-
hydes in excellent yields and enantioselectivities.[8] Soon
after, several research groups developed similar routes to
furnish a wide range of interesting scaffolds.[9] Among them,
we can highlight useful asymmetric additions of ketoest-
ers,[10] nitroalkanes,[11] N-nucleophiles,[12] S-nucleophiles, and
O-nucleophiles[13] to unsaturated aldehydes. In spite of the
fact that the addition of new functionalities in a molecule
can be advantageous, the removal of these functional groups
can be difficult and sometimes demands a protection–depro-
tection approach to render the desired “naked” product.
To circumvent these disadvantages, we aimed to develop
an easy method to add “naked” methyl or alkyl chains to
unsaturated aldehydes. Bis(phenylsulfonyl)methane has
been extensively used in organic chemistry. For example, in
1997 Trost and co-workers used bis(phenylsulfonyl)methane
as a nucleophile in the palladium-catalyzed addition to al-
kenyl epoxides.[14] A recent application of this compound is
its use as a fluoromethyl anion equivalent: bis(phenylsulfo-
nyl)methane is easily fluorinated by treatment with NaH
and Selectfluor, which furnishes fluorobis(phenylsulfonyl)-
methane that can be used as a nucleophile in a broad range
of reactions.[15] However, there are few examples of the use
of bis(phenylsulfonyl)methane derivatives with unsaturated
aldehydes. Moreover, it is worth noting that for a,b-unsatu-
rated aldehydes, such as cinnamaldehyde, only 1,2-addition
products have been obtained. Only very recently, in our re-
search group we studied the addition of fluorobis(phenylsul-
fonyl)methane to a,b-unsaturated aldehydes, and obtained
the Michael adducts with excellent yields and enantioselec-
tivities.[16]
[a] A.-N. Alba, X. Companyꢁ, Prof. Dr. A. Moyano, Dr. R. Rios
Department of Organic Chemistry
Universitat de Barcelona
Martꢂ i Franquꢃs 1-11, 08028 Barcelona (Spain)
Fax : (+34)933397878
E-mail: riosramon@icrea.cat
amoyano@ub.edu
[b] Dr. R. Rios
ICREA Researcher at UB.
[c] Dr. R. Rios
ICC Researcher at UB.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901806.
Chem. Eur. J. 2009, 15, 11095 – 11099
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11095

Spurred by these recent results, and taking into account
the facile removal of the phenylsulfonyl moiety that makes
this reactant a perfect choice to carry out the future conver-
sion of the resulting compounds into attractive and interest-
ing products for building blocks in medicinal chemistry, we
decided to examine the use of bis(phenylsulfonyl)methane
as a suitable nucleophile in the Michael addition to a,b-un-
saturated aldehydes under asymmetric iminium catalysis
(Scheme 1).
reaction led to the formation of the target Michael adduct
3a with 7% conversion in 14 h (Table 1, entry 1). Moreover,
when (+)-prolinol II was used as catalyst, the reaction ren-
dered the Michael adduct in 93% conversion after 14 h and,
more interestingly, with 54% ee (Table 1, entry 2). The best
enantioselectivities were obtained when diphenyl prolinol
derivatives were used. Catalyst IV afforded compound 3a
with 32% conversion and 70% ee after 14 h (Table 1,
entry 3). The best catalyst was the trimethylsilylACTHNUTRGENUG(N TMS)-pro-
tected (+)-diphenylprolinol V that showed full conversion
after 14 h and produced 3a with good enantioselectivity
(85% ee, Table 1, entry 5).
Further optimization of temperature, solvent, and addi-
tives was performed, allowing us to conclude that the best
conditions from the point of view of enantioselectivity were
toluene as a solvent at À208C as shown in Table 2.
Table 2. Conditions screening.[a]
Scheme 1. General output of the reaction.
Entry Solvent Temperature Additive
Conversion
(14 h) [%][b]
ee
[%][c]
1
2
3
4
5
6
toluene RT
CH2Cl2 RT
AcOEt RT
MeOH RT
toluene RT
toluene RT
–
–
–
–
96
97
91
traces
85
50
70
n.d.
64
To our delight, this unprecedented reaction was efficiently
catalyzed by chiral secondary amines, thus affording the cor-
responding Michael adducts in excellent conversions
(Table 1). When crotonaldehyde (1a) was treated with bis-
(phenylsulfonyl)methane using proline (I) as a catalyst, the
Et3N (1 equiv) 100
PhCOOH
(20 mol%)
100
77
7
8
toluene 48C
toluene À208C
–
–
100
40
88
93
Table 1. Catalyst screening.[a]
[a] In all cases, bis(phenylsulfonyl)methane (2) (1 equiv) was added to a
mixture of 1d (1.5 equiv) and catalyst (V) in the conditions specified in
Table 2; RT=room temperature. [b] Determined by NMR spectroscopy.
[c] Determined by chiral HPLC analysis.
Entry
1
Catalyst
Conversion (14 h) [%][b]
7
ee [%][c]
With these optimized conditions in hand, we decided to
examine the scope of this valuable methodology. The reac-
tion took place in good yields and enantioselectivities when
aliphatic unsaturated aldehydes were used (Table 3). For ex-
ample, when 2-pentenal (1b) or 2-hexenal (1c) were treated
with bis(phenylsulfonyl)methane (2) in toluene at 48C and
with 0.2 equivalents of catalyst V, the corresponding Michael
adducts 3b and 3c were isolated in 74% and 81% yields, re-
spectively, and more importantly, both in 99% ee (Table 3,
entries 2 and 3). The scope of the reaction was further ex-
panded by using different functionalities in the unsaturated
aldehydes, rendering the Michael adducts in good yields and
enantioselectivities (Table 3, entries 5, 6, 7, and 8). On the
other hand, when cinnamaldehyde derivatives were used the
reaction did not work, affording only, when more drastic
conditions were used, the undesired 1,2-addition product
(see the Supporting Information for details).
n.d.
2
3
93
À54
traces
n.d.
4
5
traces
100
n.d.
85
[a] In all cases, bis(phenylsulfonyl)methane (2) (1 equiv) was added to a
solution of 1a (1.5 equiv) and catalyst I–V (0.2 equiv) in toluene at room
temperature; n.d.= not determined. [b] Determined by NMR spectrosco-
py. [c] Determined by chiral HPLC analysis.
Once we had defined the scope of the reaction, and with
a range of enantiomerically enriched b-(bisphenylsulfonyl-
methyl)aldehydes in our hands, we were in position to inves-
11096
www.chemeurj.org
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 11095 – 11099

Organocatalytic Addition of Alkyl Anions to a,b-Unsaturated Aldehydes
COMMUNICATION
Table 3. Aldehyde scope.[a]
This methodology can be also regarded as an organocata-
lytic version of previous metal-based conjugate addition
methodologies. As shown in Scheme 4, compound 3a can be
easily protected as the cyclic acetal 9a; this compound can
be deprotonated and subsequently reacted with an electro-
phile to afford a wide range of products (Scheme 4). This
makes this methodology an efficient short-cut to highly
enantiopure conjugate addition products that avoids the use
of transition metals and chiral auxiliaries or ligands.
The absolute configuration of the obtained adducts was
ascertained by chemical correlation (Scheme 4). Thus, com-
pound 9a was fluorinated by treatment with NaH and then
with Selectfluor. Comparison of the optical rotation data of
11a ([a]2D5 =+16.3 (c=1.2, CHCl3) with those obtained for
material derived from the known compound 12a ([a]2D5 =+
15.6 (c=1.0, CHCl3)[16] revealed that the absolute configura-
tion of this aldehyde 3a was (3R). The absolute configura-
tion of the remaining adducts 3b–i was assigned by analogy.
The stereochemical outcome of the reaction can be easily
rationalized by the mechanistic proposal outlined in
Scheme 5. Thus, efficient shielding of the Re-face of the
chiral iminium intermediate 15 by the bulky aryl groups of
V leads to stereoselective Si-facial nucleophilic conjugate
attack on the b-carbon of 15. This mechanism is in accord-
ance with those proposed for other amine-catalyzed reac-
tions between nucleophiles and enals.[17]
Entry
1[d]
2
3
4
R
Compound
Yield [%][b]
ee [%][c] (Conf)
Me
Et
nPr
nBu
CO2Et
3a
3b
3c
3d
3e
90
74
77
79
74
93 (R)
99 (R)
99 (R)
90 (R)
94 (R)
5
6
7
3 f
3g
91
72
94 (R)
97 (R)
8[e]
3i
61
91 (R)
[a] In all cases, bis(phenylsulfonyl)methane (2) (1 equiv) was added to a
mixture of 1a–i (1.5 equiv) and catalyst V (0.2 equiv) in toluene at room
temperature. [b] Yield of isolated product. [c] Determined by chiral
HPLC analysis. [d] Reaction performed at À208C. [e] determined by
Mosher ester analysis.
tigate the reductive desulfonylation of compounds 3 to com-
plete the sequence of enantioselective monomethylation. To
show the viability of this sequence, we chose compound 3a;
the aldehyde was oxidized to the corresponding acid 7a,
which was subsequently treated with activated magnesium
in methanol for the reductive removal of the sulfonyl
groups. We obtained isoamylic acid 13a in almost quantita-
tive yield from the starting aldehyde 3a (Scheme 2).
Both deuterium- and 13C-labeled amino acids have found
wide use in metabolite studies. For example, in 1980, Tanaka
et al. described a metabolism study of valine in vitamin B12-
folate deficiency in rats using
valine chirally labeled with 13C
and deuterium.[18] In 1997,
Meinwald
synthesized
l-
in
(2S,3S)-4,4,4-[2H3]valine
eight steps in low overall yield,
employing the Sharpless epoxi-
dation of allyl alcohols as the
source of chirality and using
highly valuable deuterium start-
ing materials.[19] Furthermore,
Scheme 2. Deprotection of addition products.
Furthermore, we synthesized different derivatives from 3a
to showcase the versatility of our method. We obtained
amine 6a, acid 7a, and ester 8a in excellent yields and in
highly enantiopure form (Scheme 3).
the synthetic route only affords this diastereomer.
Very recently, Soai et al. reported on the use of cryptoe-
nantiomers and/or of isotope-enantiomers as chirality induc-
ers in the autocatalytic diisopropylzinc addition to pyrimid-
yl-5-carbaldehydes.[20] In this context, we examined the pos-
sibility of applying the present methodology to the synthesis
of isotope-enantiomers. We envisaged an easy entry to deu-
terium-labeled compounds by simply using CD3OD (a
common NMR solvent) with magnesium in the desulfonyla-
tion step. To test this idea, we treated carboxylic acid 7a in
deuterated methanol and Mg, and obtained the chiral deu-
terolabelled compound 14a in moderate yield (Scheme 6).
This methodology allows us to synthesize both enantiomers
of 14a simply by changing the enantiomer of the catalyst. In
addition, the use of 13C-labeled bis(phenylsulfonyl)methane
would result in the formation of the chirally 13C-labelled
compound 14a. As described in the literature, 14a could be
Scheme 3. Synthetic transformations of product 3a.
Chem. Eur. J. 2009, 15, 11095 – 11099
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11097

R. Rios, A. Moyano et al.
valuable for the synthesis of in-
teresting compounds for biolog-
ical studies or for the investiga-
tion of autocatalytic asymmetric
processes.
Experimental Section
In a vial, bis(phenylsulfonyl)methane
(2) (74 mg, 0.25 mmol) and catalyst V
(16 mg, 0.05 mmol) were added to tol-
uene (2 mL). Next, unsaturated alde-
hyde 1a (0.375 mmol, 1.5 equiv) was
added and the reaction mixture was
stirred at 48C overnight. The crude
product was purified by column chro-
matography to afford the final com-
pound 4a.
Scheme 4. Determination of absolute configuration and scope of the reaction.
Yellow oil; 1H NMR (300 MHz, CDCl3, TMSint): d=9.65 (s, 1H), 7.90–
7.83 (m, 4H), 7.70–7.61 (m, 2H), 7.57–7.47 (m, 4H), 4.77 (m, 1H), 3.28–
3.12 (m, 2H), 2.92 (dd, J1 =7.0 Hz, J2 =18.7 Hz, 1H), 1.39 ppm (d, J=
6.7 Hz, 3H); 13C NMR (100 MHz. CDCl3): d=200.1, 139.4, 138.9, 134.5,
134.4, 129.4, 129.1, 129.0, 85.2, 47.3, 28.0, 18.2 ppm; HRMS [M+H]+: cal-
culated for [C17H19O5S2]+: 347.1254; found: 347.1255; [a]2D5 = +6.7 (c=
1.2, CHCl3, 93% ee).
Acknowledgements
We thank the Spanish Ministry of Science and Education for financial
support (Project AYA2006-15648-C02-01). A.N.A. thanks the Universitat
de Barcelona for a predoctoral fellowship. We are grateful to Irene Fer-
nandez for her valuable help in HRMS spectrometry and we are indebt-
ed to Prof. Dr. Mitsuhiro Yoshimatsu for his advice in the synthesis of 2-
chloro-bisphenylsulfonylmethane.
Keywords: enantioselective addition
organocatalysis · sulfones · unsaturated aldehydes
· isotope effects ·
Scheme 5. Proposed mechanism.
[1] a) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New
York, 1994; b) Comprehensive Asymmetric Catalysis (Eds.: E. N. Ja-
cobsen, A. Pfaltz, H. Yamamoto), Springer, Berlin, 1999; c) Catalytic
Asymmetric Synthesis, 2nd ed. (Ed.: I. Ojima), Wiley-VCH, Wein-
heim, 2000; d) Transition Metals for Organic Synthesis, 2nd ed.
(Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim, 2004.
[2] For authoritative reviews on the topic of organocatalysis: a) P. I.
Dalko, L. Moisan, Angew. Chem. 2001, 113, 3840; Angew. Chem.
Int. Ed. 2001, 40, 3726; b) B. List, Synlett 2001, 1675; c) B. List, Tet-
rahedron 2002, 58, 2481; d) P. I. Dalko, L. Moisan, Angew. Chem.
2004, 116, 5248; Angew. Chem. Int. Ed. 2004, 43, 5138; e) A. Berkes-
sel, H. Groger, Asymmetric Organocatalysis, Wiley-VCH, Weinheim,
2005; f) J. Seayad, B. List, Org. Biomol. Chem. 2005, 3, 719; g) G.
Lelais, D. W. C. MacMillan, Aldrichimica Acta 2006, 39, 79; h) D.
Almasi, D. A. Alonso, C. Nꢅjera, Tetrahedron: Asymmetry 2007, 18,
299–365.
Scheme 6. Synthesis of chiral deutero-labeled isoamylic acid.
a convenient starting compound for synthesizing valine in
an organocatalytic fashion by the a-amination protocol re-
ported by List[21] and Jorgensen and co-workers[22] or by the
method described by Aberhardt and Weiller[23] starting from
isoamyl alcohol.
In summary, we have described a practical, inexpensive,
and powerful methodology that constitutes an organocata-
lytic alternative for organometallic 1,4-additions. We have
developed an asymmetric methyl or alkyl addition to a,b-
unsaturated aldehydes with excellent yields and enantiose-
lectivities. This versatile reaction can also be used in the syn-
thesis of isotope-enantiomers, making this procedure highly
[3] B. List, R. A Lerner, C. F. Barbas III, J. Am. Chem. Soc. 2000, 122,
2395–2396.
[4] K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan, J. Am. Chem. Soc.
2000, 122, 4243–4244.
[5] S. R. Harutyunyan, T. den Hartog, K. Geurts, A. J. Minnaard, B. L.
Feringa, Chem. Rev. 2008, 108, 2824–2852.
11098
www.chemeurj.org
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 11095 – 11099

Organocatalytic Addition of Alkyl Anions to a,b-Unsaturated Aldehydes
COMMUNICATION
[6] A. Alexakis, J. E. Bꢀckvall, N. Krause, O. Pamies, M. Dieguez,
Chem. Rev. 2008, 108, 2796–2823.
[7] S. Ay, M. Nieger, S. Brꢀse, Chem. Eur. J. 2008, 14, 11539–11556.
[8] S. Brandau, A. Landa, J. Franzꢆn, M. Marigo, K. A. Jorgensen,
Angew. Chem. 2006, 118, 4411–4415; Angew. Chem. Int. Ed. 2006,
45, 4305–4309.
[9] For recent application of malonate addition for the synthesis of pyr-
rolidines: a) R. Rios, J. Vesely, I. Ibrahem, H. Sunden, A. Cordova,
Tetrahedron Lett. 2007, 48, 8695–8699; for the synthesis of piperi-
dines: b) G. Valero, J. Schimer, I. Cisarova, J. Vesely, A. Moyano, R.
Rios, Tetrahedron Lett. 2009, 50, 1943–1946; for the synthesis of qui-
nolizilidine derivatives: c) J. Franzꢆn, A. Fisher, Angew. Chem. 2009,
4936; c) T. Fukuzumi, N. Shibata, M. Sugiura, H. Yasui, S. Naka-
mura, T. Toru, Angew. Chem. 2006, 118, 5095–5099; Angew. Chem.
Int. Ed. 2006, 45, 4973–4977; d) S. Mizuta, N. Shibata, Y. Goto, T.
Furukawa, S. Nakamura, T. Toru, J. Am. Chem. Soc. 2007, 129,
6394–6395; e) T. Furukawa, N. Shibata, S. Mizuta, S. Nakamura, T.
Toru, M. Shiro, Angew. Chem. 2008, 120, 8171–8174; Angew. Chem.
Int. Ed. 2008, 47, 8051–8054; f) during the preparation of this manu-
script, Prakash et al. described a stereoselective 1,4-addition of a-
fluoro-a-nitro(phenylsulfonyl)methane to chalcones, using cinchona-
based bifunctional catalysts: G. K. S. Prakash, F. Wang, T. Stewart,
T. Matthew, G. A. Olah, Proc. Natl. Acad. Sci. USA 2009, 106,
4090–4094.
121, 1377–1377; Angew. Chem. Int. Ed. 2009, 48, 1351–1354.
[10] a) S. Cabrera, J. Aleman, P. Bolze, S. Bertelsen, K. A. Jorgensen,
Angew. Chem. 2008, 120, 127–131; Angew. Chem. Int. Ed. 2008, 47,
121–125; b) A. Carlone, M. Marigo, C. North, A. Landa, K. A. Jor-
gensen, Chem. Commun. 2006, 4928–4930.
[11] a) Y. Wang, P. Li, X. Liang, T. Zhang, J. Ye, Chem. Commun. 2008,
1232–1234; b) L. Zu, H. Xie, H. Li, J. Wang, W. Wang, Adv. Synth.
Catal. 2007, 349, 2660–2664.
[12] N-Nucleophiles: a) I. Ibrahem, R. Rios, J. Vesely, G.-L. Zhao, A.
Cordova, Chem. Commun. 2007, 849–852; b) Y. K. Chen, Y. Masa-
nori, D. W. C. MacMillan, J. Am. Chem. Soc. 2006, 128, 9328–9329.
[13] O-Nucleophiles: a) H. Sundꢆn, I. Ibrahem, G.-L. Zhao, L. Erikson,
A. Cordova, Chem. Eur. J. 2007, 13, 574–581; b) H. Li, J. Wang, T.
E-Nunu, L. Zu, W. Jiang, W. Wang, Chem. Commun. 2007, 507–509.
S-Nucleophiles: c) M. Marigo, T. Schulte, J. Franzen, K. A. Jorgen-
sen, J. Am. Chem. Soc. 2005, 127, 15710–15711; d) R. Rios, H.
Sunden, I. Ibrahem, G.-L. Zhao, L. Erikson, A. Cordova, Tetrahe-
dron Lett. 2006, 47, 8679–8682; e) W. Wang, H. Hao, J. Wang, L.
Zu, J. Am. Chem. Soc. 2006, 128, 10354–10355.
[16] A.-N. Alba, X. Companyꢁ, A. Moyano, R. Rios, Chem. Eur. J. 2009,
15, 7035–7038.
[17] a) A.-N. Alba, N. Bravo, A. Moyano, R. Rios, Tetrahedron Lett.
2009, 50, 3067–3069; b) J. Vesely, R. Rios, I. Ibrahem, G.-L. Zhao,
L. Eriksson, A. Cꢁrdova, Chem. Eur. J. 2008, 14, 2693–2698; c) R.
Rios, H. Sundꢆn, I. Ibrahem, G.-L Zhao, A. Cꢁrdova, Tetrahedron
Lett. 2006, 47, 8679–8682; d) R. Rios, H. Sundꢆn, I. Ibrahem, A.
Cꢁrdova, Tetrahedron Lett. 2007, 48, 2181–2184; e) R. Rios, I. Ibra-
hem, J. Vesely, G.-L. Zhao, A. Cꢁrdova, Tetrahedron Lett. 2007, 48,
5701–5705; f) R. Rios, J. Vesely, H. Sundꢆn, I. Ibrahem, A. Cꢁrdo-
va, Tetrahedron Lett. 2007, 48, 5835–5839.
[18] K. Tanaka, D. J. Aberhardt, J. L. Amster, E. E. Jenkins, C.-T. Hau, J.
Biol. Chem. 1980, 255, 7763–7768.
[19] J. C. Shattuck, J. Meinwald, Tetrahedron Lett. 1997, 38, 8461–8464.
[20] a) T. Kawasaki, Y. Matsumura, T. Tsutsumi, K. Suzuki, M. Ito, K.
Soai, Science 2009, 324, 492–495; b) T. Shibata, K. Iwahashi, T. Ka-
wasaki, K. Soai, Tetrahedron: Asymmetry 2007, 18, 1759–1762.
[21] B. List, J. Am. Chem. Soc. 2002, 124, 5656–5657.
[22] A. Bogevig, K. Juhl, N. Kumaragurubaran, W. Zhuang, K. A. Jor-
gensen, Angew. Chem. 2002, 114, 1868–1871; Angew. Chem. Int. Ed.
2002, 41, 1790–1793.
[14] B. M. Trost, M. A. Ceschi, B. Kçnig, Angew. Chem. 1997, 109, 1562–
1564; Angew. Chem. Int. Ed. Engl. 1997, 36, 1486–1489.
[15] a) G. K. S. Prakash, X. Zhao, S. Chacko, F. Wang, H. Vaghoo, G. A.
Olah, Beilstein J. Org. Chem. 2008, 4, 17; b) G. K. S. Prakash, S.
Chacko, S. Alconcel, T. Stewart, T. Mathew, G. A. Olah, Angew.
Chem. 2007, 119, 5021–5024; Angew. Chem. Int. Ed. 2007, 46, 4933–
[23] D. J. Aberhart, B. H. Weiller, J. Labelled Comd. Radiopharm. 1982,
20, 663–668.
Received: June 30, 2009
Published online: September 24, 2009
Chem. Eur. J. 2009, 15, 11095 – 11099
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11099