A novel bis-thiourea organocatalyst for the asymmetric aza-Henry reaction

Article abstract of DOI:10.1002/adsc.200800214

A novel bis-thiourea/2,2′-diaminobinaphthalene (BINAM)-based catalyst for the asymmetric aza-Henry reaction has been developed. This catalyst promotes the reaction of N-Boc imines with nitroalkanes to afford β-nitroamines with good yields and high enantioselectivities. This catalyst has the advantage that it can be prepared in a single step from commercially available materials. A model is proposed for the catalyst action where both components of the reaction are activated simultaneously by hydrogen bonding. Regardless of the mechanism, the success of the present catalyst demonstrates the potential of bis-thioureas as an interesting class of relatively unexplored catalysts.

Full text of DOI:10.1002/adsc.200800214

COMMUNICATIONS
DOI: 10.1002/adsc.200800214
A Novel Bis-Thiourea Organocatalyst for the Asymmetric
Aza-HenryReaction
Constantinos Rampalakosa and William D. Wulffa,*
a
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA
Fax : (+1)-517-353-1793; e-mail: wulff@chemistry.msu.edu
Received: April 7, 2008; Published online: July 24, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A novel bis-thiourea/2,2’-diaminobinaph-
thalene (BINAM)-based catalyst for the asymmet-
ric aza-Henry reaction has been developed. This
catalyst promotes the reaction of N-Boc imines
with nitroalkanes to afford b-nitroamines with good
yields and high enantioselectivities. This catalyst
has the advantage that it can be prepared in a
single step from commercially available materials.
A model is proposed for the catalyst action where
both components of the reaction are activated si-
multaneously by hydrogen bonding. Regardless of
the mechanism, the success of the present catalyst
demonstrates the potential of bis-thioureas as an in-
teresting class of relatively unexplored catalysts.
Boc imines with nitroalkanes can be promoted by a
bifunctional asymmetric catalyst bearing both a thio-
urea function and an N,N-dimethylamino group lead-
ing to b-nitroamines with good enantioselectivities,[8]
while shortly thereafter, Yoon and Jacobsen were able
to promote the highly stereoselective addition of a
range of nitroalkanes to aromatic N-Boc imines using
a new thiourea based bifunctional catalyst.[9] A third
type of thiourea organocatalyst was recently reported
for this reaction by Ricci[10] and by Schaus[11] which in-
volved a thiourea-Cinchona alkaloid hybrid. Two
other thiourea catalysts have recently been reported
for the aza-Henry reaction.[12] Apart from the thiour-
eas, other organocatalysts have been reported for the
aza-Henry reaction which are either based on Cincho-
na alkaloids[13] or on the triflate salts of bis-
Keywords: asymmetric catalysis; aza-Henry reac-
tion; BINAM; nitroalkanes; organocatalyst; thiour-
eas
AHCTREUNG
(amidines).[14]
It is known that both imines and nitro compounds
will interact with thioureas[15] and yet a bis-thiourea
catalyst has not been reported for the aza-Henry reac-
tion. The first chiral bis-thiourea catalyst to be report-
ed was compound 1 by Nagasawa which was based on
The nucleophilic addition of nitroalkanes to imines, trans-cyclohexane-1,2-diamine.[16] This catalyst proved
known as the aza-Henry or nitro-Mannich reaction, is effective for the Morita–Baylis–Hillman reaction but
a fundamental C C bond forming reaction in organic not for a number of other reactions.[17] Other bis-thio-
À
chemistry.[1] The resulting 1,2-nitroamines are useful urea catalysts that have been reported include the in-
precursors to a variety of nitrogen-containing chiral teresting guanidine derivative 2 which has three
building blocks such as vicinal diamines via reduction double-hydrogen donors and was developed by Naga-
of the nitro group[2] and a-amino carbonyl compounds sawa for the Henry reaction.[18] Finally, Berkessel has
by means of the Nef reaction.[3] Given the importance developed the bis-thiourea catalyst 4 for the Morita–
of this transformation, considerable effort has been Baylis–Hillman reaction[19] and the catalyst 3 for the
directed towards the development of catalytic asym- kinetic resolution of azlactones (Figure 1).[20] We
metric aza-Henry reactions over the past several report here that the bis-thiourea catalyst 12 based on
years. The first pioneering examples were reported by the 2,2’-diaminobinaphthalene (BINAM) chiral scaf-
Shibasaki[4] and Jorgensen[5] and subsequently other fold is an effective catalyst for the aza-Henry reac-
metal-based catalysts have been reported.[6] Beyond tion.[21]
these metal-catalyzed variants, an increasing interest
The aza-Henry reactions of the Boc imine 14a and
in organocatalysts in the last few years[7] has led to nitromethane were screened with a number of thiour-
the appearance of the first examples of enantioselec- ea catalysts and the results are presented in Table 1.
tive organocatalytic aza-Henry reactions. Takemoto This screen includes Nagasawaꢀs bis-thiourea 1 and
et al. have reported that the aza-Henry reaction of N- the BINAM-based thiourea 5 developed by Wang for
Adv. Synth. Catal. 2008, 350, 1785 – 1790
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1785

Constantinos Rampalakos and William D. Wulff
COMMUNICATIONS
Figure 1. Chiral thioureas and bis-thiourea organocatalysts.
the Morita–Baylis–Hillman reaction and subsequently by the data in Table 1 this catalyst was capable of pro-
found to be effacious for a number of Michael addi- moting the reaction in the absence of triethylamine,
tion reactions.[22] The bis-thioureas 6–13 are all pre- but unfortunately, it was not able to provide any sig-
parable in one or two steps from the commercially nificant asymmetric induction (entry 4). The series of
available (R)-(+)-1,1’-binaphthyl-2,2’-diamine and the six bis-thiourea catalysts 7–12 were then prepared and
details can be found in the Supporting Information. evaluated and, while a few of them were active cata-
The bis-thiourea 1 was found to be very effective in lysts, only compound (R)-12 with the bis-3,5-trifluoro-
promoting the aza-Henry reaction of Boc imine 14a methylphenyl substituent on the thiourea units was
but the asymmetric induction for this reaction was capable of providing significant asymmetric induction
only 8% ee (entry 1). The bifunctional thiourea (R)-5 (74% ee, entry 10). The macrocyclic bis-thiourea
was not capable of promoting the reaction in the ab- (R,R)-13 was also an active catalyst, but as with most
sence of an added base and even in the presence of of the catalysts screened for this reaction very low
triethylamine the reaction only went to 50% comple- asymmetric induction was observed.
tion in 12 h and gave 16a in 4% ee. From these results
A brief survey of the reaction conditions for the re-
it can be inferred that dimethylamino group in (R)-5 action of imine 14a with nitromethane catalyzed by
is not sufficiently basic to deprotonate nitromethane. the chiral thiourea (R)-12 in the presence of 0.4 equiv.
For this reason, the bifunctional catalyst (R)-6 was of Et3N was conducted and the data in Table 2 reveal
prepared which has two thiourea functions and two the effects of changes in the solvent, temperature and
basic functions in the form of imidazoles. As shown catalyst loading. The least effective solvent was THF
1786
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1785 – 1790

A Novel Bis-Thiourea Organocatalyst for the Asymmetric Aza-Henry Reaction
COMMUNICATIONS
Table 1. Screen of thioureas catalysts with Boc imine 14a.[a]
and substrate. Solvents that are effective H-bond ac-
ceptors would be expected to disrupt such an associa-
tion in the catalyst-substrate complex. Toluene was
identifed as the best of the solvents that were exam-
ined and lowering the temperature of the reaction led
to an initial significant improvement in the asymmet-
ric induction when the temperature was dropped to
À58C (82% ee, entry 5) but then only to a small im-
provement as the temperature was further reduced to
À358C (86% ee, entry 6). Interestingly, the induction
was not increased when the temperature was further
lowered to À558C and the only effect observed was a
decrease in the rate of the reaction (entry 7). Lower-
ing the catalyst loading to 10 mol% at the optimal
temperature of À358C for the reaction in toluene
lead to a drop-off in rate as well as a slight lowering
of the asymmetric induction (entries 6 and 11). Thus,
the optimal conditions revealed by variations in the
conditions for the reaction of imine 14a with nitrome-
thane are those indicated in entry 6 of Table 2.
With the preparation and screen of the eleven cata-
lysts indicated in Table 1 and the survey of the reac-
tion conditions summarized in Table 2, it was time to
review the substrate compatiblity of the reaction with
the set of ten different imines and three nitroalkanes
presented in Table 3 which serves to bring the reac-
tion scope into focus. The asymmetric induction for
these reactions does not appear to be particularly sen-
sitive to the presence of either electron-donating or
electron-withdrawing substituents on the phenyl
group of the imine. It was also of note to observe that
the reactions of the methoxy-substituted imines did
not appear to be slower than that of the imine from
benzaldehyde, however, the imines with electron-
withdrawing groups were more reactive and were
complete in shorter reactions times. It was also of in-
terest to find that the reaction will tolerate the pres-
ence of a pyridine ring in the imine substrate
(entry 10). A decrease in induction was noted for the
ortho-substituted arylimines 14d and 14g (entries 4
and 7) and an explanation for this is not exactly clear
especially upon consideration that the 1-naphthyl-
[a]
Reactions conducted with 10 equiv CH3NO2 and 20
mol% catalyst in toluene at 258C for 12 h, except
entry 10 which was 2 h.
Determined from the H NMR spectrum of the crude re-
action mixture.
[b]
[c]
1
Determined by HPLC on a Chiralpak OJ-H column.
Table 2. Optimization of the reaction of imine 14a with thio-
ureas (R)-12.[a]
AHCTREiUNG mine 14i does not give an induction that is sup-
pressed relative to that of imine 14a (entries 1 vs. 9).
Finally, the asymmetric induction drops from 86% ee
for nitromethane to 70% ee for nitroethane with the
same imine (entries 1 vs. 11). Interestingly, the drop is
much smaller for nitropropane (entry 12). In the last
two entries of the Table, the major diastereomer is
the syn-adduct 16 with a syn:anti ratio of approxi-
mately 4:1.
[a]
Reactions conducted with 10 equiv CH3NO2 in 0.2m solu-
tion in imine 14a with x mol% (R)-12 and 0.4 equiv
Et3N. Reaction time at 258C was 24 h and for all other
temperatures it was 36 h.
[b]
[c]
1
Determined from the H NMR spectrum of the crude re-
action mixture with Ph3CH as internal standard.
% ee of (R)-16a determined by HPLC on a Chiralpak
OJ-H column.
The absolute stereochemistry of the adducts 16 was
determined by comparing the HPLC data and optical
rotations of the products with the data reported for
which was not unanticipated since the catalyst design those derivatives of 16 that have been previously re-
was predicated on the assumption that there would be ported and then the remaining derivatives of 16 were
H-bonds involved in the association of the catalyst assigned by correlation and the details can be found
Adv. Synth. Catal. 2008, 350, 1785 – 1790
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1787

Constantinos Rampalakos and William D. Wulff
COMMUNICATIONS
Table 3. Scope of the aza-Henry reaction of the N-Boc
imine 14 catalyzed by the bis-thiourea (R)-12.[a]
[a]
Reactions conducted with 10 equiv CH3NO2 with 0.2m 14
in toluene at À358C with 0.4 equiv Et3N and 20 mol%
Figure 2. Catalyst-substrate complex 17 as a proposed model
for the reaction with catalyst (S)-12 as it is hydrogen bonded
to imine 14a and to the anion of CH3NO2.
(R)-12.
[b]
Isolated yield after purification byꢀchromatography on
silica gel.
[c]
Determined by HPLC on a Chiralpak OJ-H column.
Combined yield of syn- and anti-adducts.
% ee of syn-isomer; syn:anti=77:23.
% ee of syn-isomer; syn:anti=80:20.
[d]
[e]
imine shown in Figure 2 is based on the following rea-
soning. The conformation of the 3,5-bistrifluorometh-
yl-substituted arene ring is known to be co-planar
with the thiourea as a result of hydrogen bonding of
[f]
in the Supporting Information. To account for the ab- one of the ortho-hydrogens of the arene with the
solute sense of the enantioselectivity of the reactions sulfur of the thiourea.[23]} The consequence of this is
with the bis-thiourea catalyst 12 we propose structure that there would not be any obvious non-covalent
17 as a model for the catalyst-substrate complex. The positive interactions between the phenyl of the imine
proposed structure 17 shown in Figure 2 is generated and the 3,5-bistrifluoromethylphenyl group of the cat-
from (S)-12 and would give the (S)-enantiomer of the alyst in the alternative binding mode. In contrast, the
adduct 16a. On the right side of structure 17 one of binding mode of the imine that is shown in Figure 2
the thiourea groups is interacting with the N-Boc could benefit from CH–p interactions between the
imine via two hydrogen bonds with the nitrogen and edge of the phenyl group of the imine and the face of
oxygen atoms of the imine. On the left side of struc- one of the naphthalene rings of the catalyst. In addi-
ture 17 the other thiourea group is interacting with tion it is likely that the nitromethane is deprotonated
the anion of nitromethane via hydrogen bonds to after it is hydrogen bonded to the catalyst. Triethyl-
each of the oxygens of the nitronate anion. This
ACHTREaUNG mine is not sufficiently basic to completely deproto-
would lead to re-face addition to the imine and the nate nitromethane since its pKa in DMSO is 17 and in
formation of the (S)-enantiomer of 16a which is in water it is 10. The pKa of nitromethane should be
accord with the experimental observation that the lowered upon binding to the catalyst via two hydro-
(R)-enantiomer of 12 gives rise to the (R)-enantiomer gen bonds. This is consistent with the observation that
of 16a.
at À358C the reaction only goes to 9% completion
The imine could have been hydrogen-bonded to the for the formation of 16a without catalyst under the
catalyst in two different ways. The alternative to the conditions in entry 1 in Table 3. Therefore, the cata-
binding mode for the imine shown in Figure 2 is to lyst does have a significant effect on the rate of the
have the imine rotated 1808 such that the oxygen and reaction which may be the result of many factors. The
nitrogen atoms of the imine switch hydrogens on the coordination of the imine by the catalyst is supported
1
thiourea. Our preference for the binding mode of the by H NMR experiments. The signal for the HC=N
1788
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1785 – 1790

A Novel Bis-Thiourea Organocatalyst for the Asymmetric Aza-Henry Reaction
COMMUNICATIONS
[2] a) D. H. Lloyd, D. E. Nichols, J. Org. Chem. 1986, 51,
4294–4295; b) A. G. M. Barrett, C. D. Spilling, Tetrahe-
dron Lett. 1988, 29, 5733–5734; c) A. S. Kende, J. S.
Mendoza, Tetrahedron Lett. 1991, 32, 1699–1702;
d) M. S. Sturgess, D. J. Yarberry, Tetrahedron Lett.
1993, 34, 4743–4746; e) H. Adams, J. C. Anderson, S.
Peace, A. M. K. Pennell, J. Org. Chem. 1998, 63, 9932–
9934.
[3] a) H. W. Pinnick, Org. React. 1990, 38, 655–792; b) C.
Matt, A. Wagner, C. Moiskowski, J. Org. Chem. 1997,
62, 234–235; c) R. Ballini, M. Petrini, Tetrahedron
2004, 60, 1017–1047.
proton of the amine is shifted from 8.88 ppm in the
uncomplexed imine to 8.65 ppm in the presence of
two equivalents of the catalyst. The interpretation of
this shift is not clear at this point and this will be one
of many subjects of future efforts to gain additional
experimental evidence towards the understanding of
the mechanism of this catalyst in the aza-Henry reac-
tion.
Experimental Section
[4] a) K. Yamada, S. J. Harwood, H. Groger, M. Shibasaki,
Angew. Chem. 1999, 111, 3713–3715; Angew. Chem.
Int. Ed. 1999, 38, 3504–3506; b) K. Yamada, G. Moll,
M. Shibasaki, Synlett 2001, 980–982; c) N. Tsuritani, K.
Yamada, N. Yoshikawa, M. Shibasaki, Chem. Lett.
2002, 276–277.
[5] a) K. R. Knudsen, T. Risgaard, N. Nishiwaki, K. V.
Gothelf, K. A. Jørgensen, J. Am. Chem. Soc. 2001, 123,
5843–5844; b) N. Nishiwaki, K. R. Knudsen, K. V.
Gothelf, K. A. Jørgensen, Angew. Chem. 2001, 113,
3080–3083; Angew. Chem. Int. Ed. 2001, 40, 2992–
2995; c) K. R. Knudsen, K. A. Jorgensen, Org. Biomol.
Chem. 2005, 3, 1362–1364.
[6] a) J. C. Anderson, G. P. Howell, R. M. Lawrence, C. S.
Wilson, J. Org. Chem. 2005, 70, 5665–5670; b) F. Gao,
J. Zhu, Y. Tang, M. Deng, C. Qian, Chirality, 2006, 18,
741–745; c) C. Palomo, M. Liarbide, R. Halder, A.
Laso, R. Lopez, Angew. Chem. 2006, 118, 123–126;
Angew. Chem. Int. Ed. 2006, 45, 117–120; d) S. Handa,
V. Gnanadesikan, S. Matsunaga, S. Masakatsu, J. Am.
Chem. Soc. 2007, 129, 4900–4901; e) B. M. Trost, D. W.
Lupton, Org. Lett. 2007, 9, 2023–2026.
General Procedure for the Asymmetric Aza-Henry
Reaction (Table 3)
A
flame-dried round-bottom flask was loaded with
0.2 equiv. of catalyst 12 (0.2 mmol, 160 mg) and 1 equiv. of
imine 14a–j (1 mmol). The solid mixture was dissolved in
4 mLof toluene and then RCH 2NO2 (10 equiv., 0.52 mL)
was added at À358C. After 5 min, Et3N (0.4 equiv., 56 mL)
was added and then the mixture was stirred at À358C for
17–36 h. The volatiles were evaporated and the crude prod-
uct was purified by column chromatography on silica gel
(20% acetone in hexanes) to afford products 16a–l.
For all the optimization studies of the aza-Henry reaction
detailed in Table 1 and Table 2, the experimental procedure
is the same with slight modifications that are indicated in
each Table. Whenever a % conversion is mentioned, this
means the product was not purified, and the % conversion
was determined from the 1H NMR spectrum of the crude re-
action mixture by integration of product peaks versus the
C(=N)H proton of the imine. The only species observed in
1
the crude H NMR spectrum of the crude reaction mixture
was the catalyst, the desired product and the starting imine.
[7] For general reviews on asymmetric organocatalysis,
see: a) P. I. Dalko, L. Moisan, Angew. Chem. 2001, 113,
3840–3864; Angew. Chem. Int. Ed. 2001, 40, 3726–
3748; b) P. I. Dalko, L. Moisan, Angew. Chem. 2004,
116, 5248–5286; Angew. Chem. Int. Ed. 2004, 43, 5138–
5175; c) J. Seayad, B. List, Org. Biomol. Chem. 2005, 3,
719–724.
Supporting Information
Experimental protocols, characterization procedures, spec-
tral data for all compounds and X-ray data for 13 are avail-
able as Supporting Information.
[8] a) T. Okino, S. Nakamura, T. Furukawa, Y. Takemoto,
Org. Lett. 2004, 6, 625–627; b) X. Xu, T. Furukawa, T.
Okino, H. Miyabe, Y. Takemoto, Chem. Eur. J. 2006,
12, 466–476.
[9] T. P. Yoon, E. N. Jacobsen, Angew. Chem. 2005, 117,
470–472; Angew. Chem. Int. Ed. 2005, 44, 466–468.
[10] L. Bernardi, F. Fini, R. P. Herrera, A. Ricci, V. Sgarza-
ni, Tetrahedron 2006, 62, 375–380.
Acknowledgements
This work was supported by a grant from the NIH (GM
63019).
[11] C. M. Bode, A. Ting, S. E. Schaus, Tetrahedron 2006,
62, 11499–11505.
References
[12] a) Y.-W. Chang, J.-J. Yang, J.-N. Dang, Y.-X. Xue, Syn-
lett 2007, 2283–2285; b) M. T. Robak, M. Trincado, J.
A. Elman, J. Am. Chem. Soc. 2007, 129, 15110–15111.
[13] a) F. Fini, V. Sgarzani, D. Pettersen, R. P. Herrera, L.
Bernardi, A. Ricci, Angew. Chem. 2005, 117, 8189–
8192; Angew. Chem. Int. Ed. 2005, 44, 7975–7978;
b) C. Polomo, M. Oiarbide, A. Laso, R. Lopez, J. Am.
Chem. Soc. 2005, 127, 17622–17623.
[1] a) H. H. Baer, L. Urbas, in: The Chemistry of the Nitro
and Nitroso Groups, (Ed.: S. Patai), Interscience, New
York, 1970, Part 2, p 117; b) N. Ono, The Nitro Group
in Organic Synthesis, 1st edn., Wiley, New York, 2001;
c) B. Westermann, Angew. Chem. 2003, 115, 161–163;
Angew. Chem. Int. Ed. 2003, 42, 151–153; d) C.
Palomo, M. Oiarbide, A. Mielgo, Angew. Chem. 2004,
116, 5558–5560; Angew. Chem. Int. Ed. 2004, 43, 5442–
5444; e) J. C. Anderson, A. J. Blake, G. P. Howell, C.
Wilson, J. Org. Chem. 2005, 70, 549–555.
[14] a) B. M. Nugent, R. A. Yoder, J. N. Johnston, J. Am.
Chem. Soc. 2004, 126, 3418–3419; b) A. S. Singh, R. A.
Adv. Synth. Catal. 2008, 350, 1785 – 1790
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1789

Constantinos Rampalakos and William D. Wulff
COMMUNICATIONS
Yoder, B. Shen, J. N. Johnston, J. Am. Chem. Soc. 2007,
129, 3466–3467.
[15] a) S. J. Connon, Chem. Eur. J. 2006, 12, 5418–5427;
b) M. S. Taylor, E. N. Jacobsen, Angew. Chem. 2006,
118, 1550–1573; Angew. Chem. Int. Ed. 2006, 45, 1520–
1543.
[21] While this work was in progress the bis-thiourea 12 was
reported as a catalyst for the Friedel–Crafts reaction of
nitroalkenes which gave 12–50% ee: E. M. Fleming, T.
McCabe, S. J. Connon, Tetrahedron Lett. 2006, 47,
7037–7042.
[22] a) J. Wang, H. Li, X. Yu, L. Zu, W. Wang, Org. Lett.
2005, 7, 4293–4296; b) J. Wang, H. Li, W. Duan, L. Zu,
W. Wang, Org. Lett. 2005, 7, 4713–4716; c) J. Wang, H.
Li, L. Zu, W. Wang, Org. Lett. 2006, 8, 1391–1394;
d) H. Li, L. Zu, J. Wang, W. Wang, Tetrahedron Lett.
2006, 47, 3145–3148; e) J. Wang, H. Li, L. Zu, W.
Jiang, W. Wang, Adv. Synth. Catal. 2006, 348, 2047–
2050; f) A. Berkessel, S. Mukherjee, T. N. Mꢂller, F.
Cleemann, K. Roland, M. Brandenburg, J. M. Neudçrfl,
J. Lex, Org. Biomol. Chem. 2006, 4, 4319–4330; g) J.
Wang, H. Li, L. Zu, W. Jiang, H. Xie, W. Duan, W.
Wang, J. Am. Chem. Soc. 2006, 128, 12652–12653;
h) G. Bartoli, M. Bosco, A. Carlone, M. Locatelli, A.
Mazzanti, L. Sambri, P. Melchiorre, Chem. Commun.
2007, 722–724; i) L. Zu, J. Wang, H. Li, H. Xie, W.
Jiang, W. Wang, J. Am. Chem. Soc. 2007, 129, 1036;
j) L. Zu, H. Xie, H. Li, J. Wang, W. Jiang, W. Wang,
Adv. Synth. Catal. 2007, 349, 1882–1886; k) J. Wang, L.
Zu, H. Li, H. Xie, W. Wang, Synthesis 2007, 2576–
2580.
[16] Y. Sohtome, A. Tanatani, Y. Hashimoto, K. Nagasawa,
Tetrahedron Lett. 2004, 45, 5589–5592.
[17] a) R. P. Herrera, V. Sgarzani, L, Bernardi, A. Ricci,
Angew. Chem. 2005, 117, 6734–6737; Angew. Chem.
Int. Ed. 2005, 44, 6576–6579; b) B. Procuranti, S. J.
Connon, Chem. Commun. 2007, 1421–1423; c) R. P.
Herrera, D. Monge, E. Martin-Zamora, R. Ferandez,
J. M. Lassaletta, Org. Lett. 2007, 9, 3303; d) N. J. A.
Martin, L. Ozores, B. List, J. Am. Chem. Soc. 2007, 129,
8976–8977.
[18] a) Y. Sohtome, Y. Hashimoto, K. Nagasawa, Adv.
Synth. Catal. 2005, 347, 1643–1648; b) Y. Sohtome, N.
Takemura, T. Iguchi, Y. Hashimoto, K. Nagasawa, Syn-
lett 2006, 144–146; c) Y. Sohtome, Y. Hashimoto, K.
Nagasawa, Eur. J. Org. Chem. 2006, 2894–2897; d) Y.
Sohtome, N. Takemura, K. Takada, R. Takagi, T.
Iguchi, K. Nagasawa, Chem. Asian J. 2007, 2, 1150–
1160.
[19] A. Berkessel, K. Roland, J. M. Neudçrfl, Org. Lett.
2006, 8, 4195–4198.
[20] A. Berkessel, S. Mukherjee, T. N. Mꢂller, F. Cleemann,
K. Roland, M. Brandenburg, J. M. Neufçrfl, J. Lex,
Org. Biomol. Chem. 2006, 4, 4319–4330.
[23] A. Wittkopp, P. R. Schreiner, Chem. Eur. J. 2003, 9,
407.
1790
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1785 – 1790