Catalytic asymmetric three-component synthesis of homoallylic amines

Article abstract of DOI:10.1002/anie.201209776

It takes three to make things go right: The first direct asymmetric three-component reaction of aldehydes, carbamates, and allyltrimethylsilane leading to enantioenriched homoallylic amines has been developed using a new chiral disulfonimide catalyst (see scheme). The method employs readily available, inexpensive, and nontoxic starting materials and is applicable to both aromatic and aliphatic aldehydes. Copyright

Full text of DOI:10.1002/anie.201209776

Angewandte
Chemie
DOI: 10.1002/anie.201209776
Asymmetric Catalysis
Catalytic Asymmetric Three-Component Synthesis of Homoallylic
Amines**
Shikha Gandhi and Benjamin List*
The direct catalytic three-component coupling of aldehydes
(1), carbamates (or amines; 2), and nontoxic allyltrimethyl-
silane (3) is a very effective approach to homoallylic amine
derivatives 4 [Eq. (1)]. However, despite its great potential
for the practical synthesis of chiral nitrogenous substances
metric version of the three-component coupling of aldehydes,
carbamates, and allyltrimethylsilane.
We started our investigation by studying the three-
component reaction of 2-naphthaldehyde (1a), benzyl carba-
mate (2a), and allyltrimethylsilane (3). An initial screening of
different chiral acid motifs showed the phosphoric acid 5a to
be completely inactive (Table 1, entry 1).[6] N-trifyl phosphor-
amide 6a was found to catalyze the reaction but in a low
conversion, as well as with low enantioselectivity (entry 2).
Chiral sulfonic acid 7a proved to be much more active but
provided no improvement in enantioselectivity (entry 3). As
anticipated, the most promising results were indeed obtained
and the enormous progress of asymmetric catalysis over the
last few decades, an enantioselective version of this reaction
has been entirely unknown.[1–3] Herein we report our finding
that the reaction of 9-fluorenylmethyl carbamate (Fmoc-
NH2) with a variety of aldehydes (1) and silane 3 in the
presence of a new chiral disulfonimide catalyst furnishes the
corresponding products 4 highly enantioselectively and in
good yield.
Table 1: Screening of different catalysts and reaction conditions.
We have recently introduced chiral enantiomerically pure
disulfonimides (DSI) as highly active and enantioselective
catalysts for the reaction of silylated nucleophiles with
aldehydes.[4] Preliminary mechanistic studies suggest that
these reactions proceed through an asymmetric counteranion-
directed Lewis acid catalysis mechanism operated by an
in situ silylated chiral disulfonimide.[5] While the ability of our
DSI catalysts to activate other electrophiles such as imines
has not yet been explored, we speculated on their potential
applicability in the above three-component reaction.
Although mechanistic details are currently unknown, we
were tempted to hypothesize that this and related processes
involve a silyl-transfer mechanism, and implies amenability to
our silyl asymmetric counteranion-directed catalysis (ACDC)
activation strategy. Our approach has proven particularly
suitable in reactions that have a strong silyl catalysis back-
ground and are hence not easily catalyzed by chiral Lewis
acids. Possibly, such a non-enantioselective background
reaction has hampered the development of a catalytic asym-
Entry[a]
R
Catalyst
t [d]
Yield [%]
e.r.[b]
1
2
3
4
5
6
7
8
Cbz
Cbz
Cbz
Cbz
5a
6a
7a
8a
8a
8a
8a
8b
8c
8d
8e
4
4
1
4
4
4
4
10
10
4
<1[c]
28[c]
95[c]
68
17
90
80
40
80
82
–
59:41
58:42
72:28
38:62
63:37
85:15
52:48
96:4
Boc
CO2Me
Fmoc
Fmoc
Fmoc
Fmoc
Fmoc
[*] Dr. S. Gandhi, Prof. Dr. B. List
9
10
11
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
E-mail: list@mpi-muelheim.mpg.de
93:7
92:8
1.5
88
[**] We gratefully acknowledge generous financial support from the
Max-Planck-Society, the European Research Council (Advanced
grant “High Performance Lewis Acid Organocatalysis, HIPOCAT”),
and the Alexander von Humboldt Foundation (fellowship for SG).
[a] Reactions were run with aldehyde (0.1 mmol), carbamate
(0.15 mmol), allyltrimethylsilane (0.3 mmol), and catalyst (10 mol%) in
CHCl3 (1.0m) at 188C. [b] Determined by HPLC on a chiral stationary
phase. [c] Determined by 1H NMR spectroscopy using triphenylmethane
as an internal standard. Boc=tert-butyloxycarbonyl, Cbz=carbo-
benzyloxy, Fmoc=9-fluorenylmethyloxycarbonyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209776.
Angew. Chem. Int. Ed. 2013, 52, 2573 –2576
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2573

.
Angewandte
Communications
Table 2: Substrate scope.
with the chiral disulfonimide 8a (entry 4), which formed the
desired product with an enantiomeric ratio of 72:28.
We next carried out screening of a series of carbamates.
Both rate and enantioselectivity substantially varied with
different carbamates. The reaction with tert-butyl carbamate
(2b) furnished the desired product in only low yield and an
enantioselectivity which was inferior to benzyl carbamate
(Table 1, entry 5). The less bulky methyl carbamate (2c)
reacted well and gave the corresponding product in a high
yield but the enantioselectivity remained poor (entry 6).
Continuing with our screening of carbamates, we then tested
9-fluorenylmethyl carbamate (Fmoc-NH2; 2d). Remarkably,
this reagent has never been investigated, even in non-
enantioselective three-component couplings with aldehydes
and allyltrimethylsilane, despite the fact that it introduces
a common protecting group. Encouragingly, the reaction with
Fmoc-NH2 showed an improvement in the enantioselectivity
and gave the desired product with an enantiomeric ratio of
85:15 and in 80% yield (entry 7).
Having optimized the carbamate component, we focussed
our attention towards modifying the aryl substituents at the
3,3’-position of the disulfonimide catalyst. Catalyst 8b, which
has recently been reported by our group as an efficient
catalyst for hetero-Diels–Alder reactions of aldehydes gave
inferior results compared to catalyst 8a (compare entries 7
and 8 in Table 1). This suggested to us that bulky substituents
at the 3,5-positions of the aryl groups are detrimental to both
reactivity and enantioselectivity of the reaction. Learning
from this result, we next tested catalyst 8c, bearing only
a single CF3 substituent at the 4-position of the 3,3’-aryl
groups. To our delight, this catalyst proved to be highly
enantioselective and furnished the desired product with an
enantiomeric ratio of 96:4 in a yield of 80% upon isolation
(entry 9). We also synthesized the corresponding nitro-
substituted catalyst 8d, which, however, formed the product
with slightly lower enantioselectivity (entry 10). Also,
a decreased enantioselectivity was observed with the more
active catalyst 8e bearing additional fluorine substituents at
the 3,5-positions (entry 11). Considering all these results,
disulfonimide 8c was chosen as the optimal catalyst.
Entry[a]
1
R
Product
Yield [%]
80
e.r.[b,c]
96:4
4d
2
4e
4 f
70
84
98.5:1.5
95.5:4.5
3[d,e]
4
5
4g
4h
66
84
94:6
92:8
6[d,e]
7[f]
4i
4j
4k
80
71
70
95:5
96:4
94:6
8[f]
9[e,g]
4l
83
78
65
92.5:7.5
92:8
10[e,g]
11[g]
4m
4n
91:9
[a] Unless noted otherwise, all reactions were run with aldehyde
(0.1 mmol), Fmoc-NH2 (0.15 mmol), allyltrimethylsilane (0.3 mmol),
and catalyst (10 mol%) in CHCl3 (1.0m) at 188C for 10 d. [b] Determined
by HPLC on a chiral stationary phase. [c] For determination of absolute
configuration, see the Supporting Information. [d] Reaction at 158C.
[e] Reaction was run for 12 d. [f] Used 0.3 mmol of Fmoc-NH2 and
0.6 mmol of allyltrimethylsilane. [g] Used 0.6 mmol of allyltrimethyl-
silane.
We then set out to evaluate the substrate scope for our
new direct asymmetric three-component reaction. Aromatic
aldehydes formed the corresponding homoallylic amines in
high yields and high enantiomeric ratios (Table 2, entries 1–
6). Substituted naphthaldehydes containing electron-donat-
ing as well as electron-withdrawing substituents in the 6-
position served as suitable substrates and furnished the
products in high yields and enantioselectivities (entries 3
and 6). Also, 9-phenanthrene carboxaldehyde could be
employed as the substrate and the corresponding homoallylic
amine was isolated with an excellent enantiomeric ratio of
98.5:1.5 and a yield of 70% (entry 2). A meta-substituted
benzaldehyde as well as benzaldehyde bearing substituents at
both the meta and para-positions also delivered the corre-
sponding products in high yields and enantioselectivities
(entries 4 and 5). Gratifyingly, even aliphatic aldehydes could
be converted into the corresponding homoallylic amines
under these reaction conditions (entries 7–11). For example,
hydrocinnamaldehyde gave the corresponding product in an
excellent enantiomeric ratio of 96:4 and a yield of 71%
(entry 7). p-Methoxy hydrocinnamaldehyde reacted similarly
(entry 8). b-branched aliphatic aldehydes were also well
tolerated and formed the corresponding products in high
yields and good enantioselectivities (entries 9 and 10). Even
an a-branched aliphatic aldehyde (isobutyraldehyde) could
be used as substrate and gave the corresponding product in
good yield and enantioselectivity (entry 11).
To further demonstrate the synthetic utility of our
methodology, the product 4d was oxidatively converted into
the enantioenriched b3-amino acid 9 (Scheme 1). b-Amino
acids[7] such as derivative 9 are important motifs which serve
as precursors for b-lactams, are constituents of several
medicinally important compounds,[8] and most importantly,
serve as monomers in the synthesis of peptidomimetic b-
2574
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 2573 –2576

Angewandte
Chemie
can be used in our chiral disulfonimide-based Lewis acid
catalysis, and towards the design of even more enantioselec-
tive and active catalysts is currently ongoing in our labora-
tories.
Received: December 6, 2012
Published online: January 25, 2013
Scheme 1. Synthesis of the enantioenriched b-amino acid 9.
peptides.[9] Especially, the N-Fmoc-protected b-amino acids
are commonly applied in solid-phase peptide synthesis.
Compound 4d was oxidized with NaIO4 in the presence of
a catalytic amount of OsO4 to form the corresponding
aldehyde, which without purification was further oxidized to
furnish the desired Fmoc-protected b-amino acid 9 with an
overall yield of 67% and without any loss in optical purity
(Scheme 1). The enantiomer of 9 has recently been used in the
solid-phase synthesis of a peptide hydroxamate, which is
highly potent for the inhibition of an insulin-degrading
enzyme (IDE) and is thus effective for the treatment of
diabetes.[10]
Keywords: asymmetric catalysis · disulfonimides ·
homoallylic compounds · multicomponent reactions ·
synthetic methods
.
[1] For recent reviews on allyl addition to imines, see: a) M. Yus,
J. C. Gonzꢀlez-Gꢁmez, F. Foubelo, Chem. Rev. 2011, 111, 7774 –
7854; b) T. R. Ramadhar, R. A. Batey, Synthesis 2011, 1321 –
1346; c) G. K. Friestad, A. K. Mathies, Tetrahedron 2007, 63,
2541 – 2569; d) P. E. Ramachandran, T. E. Burghardt, Pure Appl.
Chem. 2006, 78, 1397 – 1406; e) H. Ding, G. K. Friestad, Syn-
thesis 2005, 2815 – 2829; f) C. O. Puentes, V. Kouznetsov, J.
Heterocycl. Chem. 2002, 39, 595 – 614.
[2] For catalytic asymmetric addition of allyl nucleophiles to imines,
see: allylboron reagents: a) Y. Luo, H. B. Hepburn, N. Chot-
saeng, H. W. Lam, Angew. Chem. 2012, 124, 8434 – 8438; Angew.
Chem. Int. Ed. 2012, 51, 8309 – 8313; b) E. M. Vieira, M. L.
Snapper, A. H. Hoveyda, J. Am. Chem. Soc. 2011, 133, 3332 –
3335; c) A. Chakrabarti, H. Konishi, M. Yamaguchi, U.
Schneider, S. Kobayashi, Angew. Chem. 2010, 122, 1882 – 1885;
Angew. Chem. Int. Ed. 2010, 49, 1838 – 1841; d) M. Fujita, T.
Nagano, U. Schneider, T. Hamada, C. Ogawa, S. Kobayashi, J.
Am. Chem. Soc. 2008, 130, 2914 – 2915; e) S. Lou, P. N. Moquist,
S. E. Schaus, J. Am. Chem. Soc. 2007, 129, 15398 – 15404;
allylstannane reagents: f) R. A. Fernandes, D. A. Chaudhari,
Eur. J. Org. Chem. 2012, 1945 – 1952; g) F. Colombo, R.
Annunziata, M. Benaglia, Tetrahedron Lett. 2007, 48, 2687 –
2690; h) J. Aydin, K. S. Kumar, M. J. Sayah, O. A. Wallner,
K. J. Szabꢁ, J. Org. Chem. 2007, 72, 4689 – 4697; i) R. A.
Fernandes, A. Stimac, Y. Yamamoto, J. Am. Chem. Soc. 2003,
125, 14133 – 14139; j) T. Gastner, H. Ishitani, R. Akiyama, S.
Kobayashi, Angew. Chem. 2001, 113, 1949 – 1951; Angew. Chem.
Int. Ed. 2001, 40, 1896 – 1898; k) X. Fang, M. Johannsen, S. Yao,
N. Gathergood, R. G. Hazell, K. A. Jørgensen, J. Org. Chem.
1999, 64, 4844 – 4849; l) H. Nakamura, K. Nakamura, Y.
Yamamoto, J. Am. Chem. Soc. 1998, 120, 4242 – 4243; allylsilane
reagents: m) N. Momiyama, H. Nishimoto, M. Terada, Org. Lett.
2011, 13, 2126 – 2129; n) M. Miyazaki, N. Ando, K. Sugai, Y.
Seito, H. Fukuyoka, T. Kanemitsu, K. Nagata, Y. Odanaka, K. T.
Nakamura, T. Itoh, J. Org. Chem. 2011, 76, 534 – 542; o) M.
Naodovic, M. Wadamoto, H. Yamamoto, Eur. J. Org. Chem.
2009, 5129 – 5131; p) H. Kiyohara, Y. Nakamura, R. Matsubara,
S. Kobayashi, Angew. Chem. 2006, 118, 1645 – 1647; Angew.
Chem. Int. Ed. 2006, 45, 1615 – 1617; q) I. Fernꢀndez, V. Valdivia,
B. Gori, F. Alcudia, E. ꢂlvarez, N. Khiar, Org. Lett. 2005, 7,
1307 – 1310; r) C. Ogawa, M. Sugiura, S. Kobayashi, Angew.
Chem. 2004, 116, 6653 – 6655; Angew. Chem. Int. Ed. 2004, 43,
6491 – 6493; s) R. A. Fernandes, Y. Yamamoto, J. Org. Chem.
2004, 69, 735 – 738; t) T. Hamada, K. Manabe, S. Kobayashi,
Angew. Chem. 2003, 115, 4057 – 4060; Angew. Chem. Int. Ed.
2003, 42, 3927 – 3930; u) D. Ferraris, B. Young, C. Cox, T.
Dudding, W. J. Drury III, L. Ryzhkov, A. E. Taggi, T. Lectka, J.
Am. Chem. Soc. 2002, 124, 67 – 77; v) K. Nakamura, H.
Nakamura, Y. Yamamoto, J. Org. Chem. 1999, 64, 2614 – 2615;
allylindium reagents: w) S. J. Kim, D. O. Jang, J. Am. Chem. Soc.
2010, 132, 12168 – 12169; x) K. L. Tan, E. N. Jacobsen, Angew.
Chem. 2007, 119, 1337 – 1339; Angew. Chem. Int. Ed. 2007, 46,
1315 – 1317; y) R. Kargbo, Y. Takahashi, S. Bhor, G. R. Cook,
Concerning the mechanism of our new asymmetric three-
component reaction, we envision two distinct pathways
(Scheme 2). Accordingly, the disulfonimide itself may act as
a Brønsted acid which activates the in situ generated imine by
Scheme 2. Plausible catalytic pathways.
protonation. Alternatively, the catalyst may be silylated
in situ from the allylsilane as shown before.[4a] This catalyst
species in turn may activate the imine by silicon-based Lewis
acid catalysis. Preliminary experiments utilizing a preformed
imine and the preformed silylated catalyst gave nearly
identical enantioselectivity as that obtained in the corre-
sponding three-component reaction (see the Supporting
Information). We therefore currently tend towards the
Lewis acid catalysis pathway. However, further studies are
clearly needed to confirm this notion.
In summary, we have developed the first catalytic
asymmetric three-component synthesis of chiral homoallylic
amines starting directly from aldehydes, carbamates, and
allyltrimethylsilane. The use of readily available, inexpensive,
and nontoxic starting materials is an attractive feature of our
method. Moreover, the products bear the Fmoc substituent as
a synthetically useful protecting group and can be readily
converted into the corresponding Fmoc-protected b3-amino
acids. Importantly, both aromatic and aliphatic aldehydes can
be converted into the corresponding homoallylic amines in
high yields and enantioselectivities. With the successful
development of this protocol, we present the first application
of chiral disulfonimide in the activation of imines. Further
work towards expanding the scope of the electrophiles which
Angew. Chem. Int. Ed. 2013, 52, 2573 –2576
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2575

.
Angewandte
Communications
G. C. Lloyd-Jones, I. R. Shepperson, J. Am. Chem. Soc. 2007,
129, 3846 – 3847; z) G. R. Cook, R. Kargbo, B. Maity, Org. Lett.
2005, 7, 2767 – 2770; allyl alcohol: aa) X.-C. Qiao, S.-F. Zhu, W.-
Q. Chen, Q.-L. Zhou, Tetrahedron: Asymmetry 2010, 21, 1216 –
1220. For a catalytic asymmetric three-component synthesis
using an allylstannane, see: ab) X. Li, X. Liu, Y. Fu, L. Wang, L.
Zhou, X. Feng, Chem. Eur. J. 2008, 14, 4796 – 4798. For catalytic
asymmetric allylboron addition to N,O-aminals, see: ac) Y.-Y.
Huang, A. Chakrabarti, N. Morita, U. Schneider, S. Kobayashi,
Angew. Chem. 2011, 123, 11317 – 11320; Angew. Chem. Int. Ed.
2011, 50, 11121 – 11124.
603 – 614; c) M. Mahlau, B. List, Isr. J. Chem. 2012, 52, 630 – 638;
d) L. Ratjen, S. Mꢅller, B. List, Nachr. Chem. 2010, 58, 640 – 646;
e) J. Lacour, D. Moraleda, Chem. Commun. 2009, 7073 – 7089;
f) J. Lacour, D. Linder, Science 2007, 317, 462 – 463; g) J. Lacour,
V. Hebbe-Viton, Chem. Soc. Rev. 2003, 32, 373 – 382.
[6] For reviews on chiral Brønsted acid catalysis, see: a) M.
Rueping, B. J. Nachtsheim, W. Ieawsuwan, I. Atodiresei,
Angew. Chem. 2011, 123, 6838 – 6853; Angew. Chem. Int. Ed.
2011, 50, 6706 – 6720; b) M. Terada, Synthesis 2010, 1929 – 1982;
c) D. Kampen, C. M. Reisinger, B. List, Top. Curr. Chem. 2010,
291, 395 – 456; d) T. Akiyama, Chem. Rev. 2007, 107, 5744 – 5758;
e) T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal. 2006, 348,
999 – 1010; f) M. Terada, Chem. Commun. 2008, 4097 – 4112.
[7] a) Enantioselective Synthesis of b-amino acids, 2nd ed. (Eds.: E.
Juaristi, V. A. Soloshonok), Wiley-VCH, New York, 2005; b) M.
Liu, M. P. Sibi, Tetrahedron 2002, 58, 7991 – 8035.
[8] a) K. R. Guertin, Y.-M. Choi, Curr. Med. Chem. 2007, 14, 2471 –
2481; b) K. B. Hansen, J. Balsells, S. Dreher, Y. Hsiao, M.
Kubryk, M. Paluki, N. Rivera, D. Steinhuebel, J. D. Armstron-
g III, D. Askin, E. J. J. Grabowski, Org. Process Res. Dev. 2005, 9,
634 – 639; c) N. Yasuda, Y. Hsiao, M. S. Jensen, N. R. Rivera, C.
Yang, K. M. Wells, J. Yau, M. Paluki, L. Tan, P. G. Dormer, R. P.
Volante, D. L. Hughes, P. J. Reider, J. Org. Chem. 2004, 69,
1959 – 1966; d) E. K. Rowinsky, Annu. Rev. Med. 1997, 48, 353 –
374.
[9] For reviews on b-peptides, see: a) W. S. Horne, S. M. Gellman,
Acc. Chem. Res. 2008, 41, 1399 – 1408; b) D. Seebach, J.
Gardiner, Acc. Chem. Res. 2008, 41, 1366 – 1375; c) D. Seebach,
A. K. Beck, D. J. Bierbaum, Chem. Biodiversity 2004, 1, 1111 –
1239; d) D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes, J. S.
Moore, Chem. Rev. 2001, 101, 3893 – 4011; e) R. P. Cheng, S. H.
Gellman, W. F. DeGrado, Chem. Rev. 2001, 101, 3219 – 3232.
[10] M. A. Leissring, D. Selkoe, G. D. Cuny, S. Choi, R. L. Stein, B.
Turk, L. C. Cantley, PCT Int. Appl. WO 2008156701, 2008.
[3] For catalytic asymmetric synthesis of homoallylic amines by
transfer aminoallylation, see: a) H. Ren, W. D. Wulff, J. Am.
Chem. Soc. 2011, 133, 5656 – 5659; b) M. Rueping, A. P. Anton-
chick, Angew. Chem. 2008, 120, 10244 – 10247; Angew. Chem.
Int. Ed. 2008, 47, 10090 – 10093; c) M. Sugiura, C. Mori, S.
Kobayashi, J. Am. Chem. Soc. 2006, 128, 11038 – 11039.
[4] a) M. Mahlau, P. Garcꢃa-Garcꢃa, B. List, Chem. Eur. J. 2012, 18,
16283 – 16287; b) J. Guin, C. Rabalakos, B. List, Angew. Chem.
2012, 124, 8989 – 8993; Angew. Chem. Int. Ed. 2012, 51, 8859 –
8863; c) L. Ratjen, P. Garcꢃa-Garcꢃa, F. Lay, M. E. Beck, B. List,
Angew. Chem. 2011, 123, 780 – 784; Angew. Chem. Int. Ed. 2011,
50, 754 – 758; d) P. Garcꢃa-Garcꢃa, F. Lay, P. Garcꢃa-Garcꢃa, C.
Rabalakos, B. List, Angew. Chem. 2009, 121, 4427 – 4430; Angew.
Chem. Int. Ed. 2009, 48, 4363 – 4366. For other publications on
chiral DSIs, see: e) M. Barbero, S. Cadamuro, S. Dughers, G.
Ghero, Org. Biomol. Chem. 2012, 10, 4058 – 4068; f) L.-Y. Chen,
H. He, W.-H. Chan, A. W. M. Lee, J. Org. Chem. 2011, 76, 7141 –
7147; g) A. Berkessel, P. Christ, N. Leconte, J. M. Neudçrfl, M.
Schꢄfer, Eur. J. Org. Chem. 2012, 5165 – 5170; h) M. Treskow, J.
Neudçrfl, R. Giernoth, Eur. J. Org. Chem. 2009, 3693 – 3697.
[5] For reviews on ACDC, see: a) M. Mahlau, B. List, Angew. Chem.
2013, 125, 540 – 556; Angew. Chem. Int. Ed. 2013, 52, 518 – 533;
b) R. J. Phipps, G. L. Hamilton, F. D. Toste, Nat. Chem. 2012, 4,
2576
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 2573 –2576