A chiral analog of the bicyclic guanidine TBD: Synthesis, structure and Br?nsted base catalysis

Article abstract of DOI:10.3762/bjoc.12.176

Starting from (S)-β-phenylalanine, easily accessible by lipase-catalyzed kinetic resolution, a chiral triamine was assembled by a reductive amination and finally cyclized to form the title compound 10. In the crystals of the guanidinium benzoate salt the six membered rings of 10 adopt conformations close to an envelope with the phenyl substituents in pseudo-axial positions. The unprotonated guanidine 10 catalyzes Diels-Alder reactions of anthrones and maleimides (25-30% ee). It also promotes as a strong Br?nsted base the retro-aldol reaction of some cycloadducts with kinetic resolution of the enantiomers. In three cases, the retroaldol products (48-83% ee) could be recrystallized to high enantiopurity (≥95% ee). The absolute configuration of several compounds is supported by anomalous X-ray diffraction and by chemical correlation.

Full text of DOI:10.3762/bjoc.12.176

A chiral analog of the bicyclic guanidine TBD: synthesis,
structure and Brønsted base catalysis
Mariano Goldberg1, Denis Sartakov1, Jan W. Bats1, Michael Bolte2
and Michael W. Göbel*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1870–1876.
1Institute for Organic Chemistry and Chemical Biology, Goethe
University Frankfurt, Max-von-Laue-Straße 7, D-60438 Frankfurt am
Main, Germany and 2Institute for Inorganic and Analytical Chemistry,
Goethe University Frankfurt, Max-von-Laue-Straße 7, D-60438
Frankfurt am Main, Germany
doi:10.3762/bjoc.12.176
Received: 01 June 2016
Accepted: 03 August 2016
Published: 19 August 2016
Email:
Associate Editor: M. Rueping
Michael W. Göbel* - m.goebel@chemie.uni-frankfurt.de
© 2016 Goldberg et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
absolute configuration; anthrone; cycloaddition; kinetic resolution;
lipase
Abstract
Starting from (S)-β-phenylalanine, easily accessible by lipase-catalyzed kinetic resolution, a chiral triamine was assembled by a re-
ductive amination and finally cyclized to form the title compound 10. In the crystals of the guanidinium benzoate salt the six mem-
bered rings of 10 adopt conformations close to an envelope with the phenyl substituents in pseudo-axial positions. The unproto-
nated guanidine 10 catalyzes Diels–Alder reactions of anthrones and maleimides (25–30% ee). It also promotes as a strong
Brønsted base the retro-aldol reaction of some cycloadducts with kinetic resolution of the enantiomers. In three cases, the retro-
aldol products (48–83% ee) could be recrystallized to high enantiopurity (≥95% ee). The absolute configuration of several com-
pounds is supported by anomalous X-ray diffraction and by chemical correlation.
Introduction
In guanidinium ions charge delocalization is an important factor The bicylic guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene
to stabilize the protonated form. As a result, guanidines are (TBD, 1, Figure 1) [4], another important Brønsted base in
exceptionally strong nitrogen bases. As part of the amino acid preparative chemistry, may also act as a powerful nucleophilic
arginine, they play an important role in biochemistry, mainly by catalyst [3]. Substituted analogs of TBD [5], such as the chiral
forming ion pairs. In addition, numerous guanidine derivatives compound 2, have become popular in the field of molecular
with complex cyclic structures can be found in natural products recognition, however, without being tested as catalysts [5-10].
[1]. Simple guanidines such as tetramethylguanidine have been A first example of enantioselective Michael addition has been
used as strong Brønsted bases in countless applications [2,3]. reported for guanidine 3, albeit with low selectivity [11]. Com-
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Figure 1: Structure of guanidines 1–10.
pounds 4 and 5, inspired by the structure of ptilomycalin A and hydrolysis with Amano lipase PS from Burkholderia cepacia
related natural products, have been used as chiral phase-transfer [22,23], a method already optimized for technical use [24]. The
catalysts [12,13].
best results were obtained with methyl tert-butyl ether as a
cosolvent [24]. By simple precipitation, batches larger than 15 g
Bicyclic guanidines with five-membered rings are also known of the S-configurated acid 13 could be isolated in 90% yield
from the alkaloid isoalchornein [14,15]. In subsequent years, (45% based on rac-12). In a two-step procedure 13 was con-
synthetic compounds (6–9) of this structural type have been de- verted into amino alcohol 14 without recrystallization in order
veloped as chiral Brønsted bases [16-19] and used for highly to keep the enantiomeric excess unchanged. It was determined
enantioselective Strecker [17] and Diels–Alder reactions [19]. at this stage to be better than 99%. The S-configuration was
assigned to 13 in accord with published data [22-24]. Intermedi-
Compared to guanidine 7 ring expansion into structure 10 ate 14 was converted into amine 16 by mesylation (79%), reac-
would shift the stereogenic phenyl groups into closer proximity tion with NaN3 (96%) and hydrogenation (84%). Aldehyde 17,
to a hydrogen-bonded guest molecule and thereby might also accessible from 14 by oxidation (87%), then could be
improve the enantioselective control exerted by 10 when used coupled with amine 16 in a reductive amination to form 18
as a catalyst. In this article we describe the synthesis of guani- (58%). After removal of the Boc protecting group (quant.),
dine 10 together with some initial applications as a chiral triamine 19 was reacted with dimethyl trithiocarbonate in
Brønsted base.
refluxing nitromethane. The thiourea intermediate was acti-
vated in situ by S-alkylation with MeI. Upon further heating the
final cyclization occured forming the iodide salt 10a of S,S-
Results and Discussion
Previous syntheses of bicylic guanidines 2 and 6–9 started from configurated guanidine 10 (67%). The free base could be isolat-
enantiomerically pure α-amino acids [6,8,10,16,17] or from ed by extraction with CH2Cl2 from 20 M aqueous NaOH
their reduction products, chiral α-amino alcohols [9,18]. In (95%). For experimental details see Supporting Information
contrast, our approach used the racemic ester rac-12 of File 1.
β-phenylalanine, easily accessible in a Knoevenagel-type con-
densation of benzaldehyde 11, ammonium acetate and malonic A crystal structure could be obtained from the benzoate salt of
acid, followed by esterification (Scheme 1) [20,21]. A kinetic 10 (Figure 2). The asymmetric unit contains two cations, two
resolution of the enantiomers was achieved by enzymatic benzoate anions and ethyl acetate as a solvate molecule. Each
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Scheme 1: Synthesis of guanidine 10. Conditions: (a) 1 equiv HOOC-CH2-COOH, 2 equiv NH4OAc, EtOH, 78 °C, 5 h, 38%; (b) 10 equiv n-PrOH,
1.5 equiv H2SO4, 97 °C, 4 h, 82%; (c) Amano-Lipase PS (from Burkholderia cepacia), aqueous Na2HPO4 buffer, pH 7.00, 50 °C, 1 h, methyl tert-butyl
ether, 50 °C, 24 h, 90% (45% based on rac-12), >99% ee; (d) 2.5 equiv NaBH4, 1.2 equiv I2, THF, 66 °C, 18 h, 86%; (e) 1 equiv Boc2O, 1.2 equiv tri-
ethylamine, CH2Cl2, 0 °C, 3 h, 100%; (f) 1.1 equiv MsCl, 1.1 equiv triethylamine, CH2Cl2, 0 °C, 3 h, 79%; (g) 3 equiv NaN3, DMF, 24 °C, 120 h, 96%;
(h) 2 equiv SO3*Py, 2.3 equiv pyridine, 4.1 equiv triethylamine, DMSO, CH2Cl2, 0 °C, 10 min, 24 °C, 2 h, 87%; (i) H2, Pd/C, MeOH, overnight, 84%;
(k) 1 equiv 17, THF, 48 h, 2 equiv NaBH4; MeOH, 96 h, 58%; (l) 10 equiv TFA, CH2Cl2, 40 °C, 24 h, 100%; (m) 1.3 equiv dimethyl trithiocarbonate,
MeNO2, 101 °C, 2 h, 4 equiv AcOH, 2 equiv MeI, 101 °C, 3 h, 67% as iodide salt 10a.
cation is connected by two N–H···O hydrogen bonds to
benzoate ions. The rings of cation 10 adopt a conformation
close to an envelope with the phenyl substituents in pseudo-
axial positions. The ion pairs and the solvate molecule are also
connected by a number of very weak intermolecular C–H···π
(phenyl) and C–H···O contacts (see Supporting Information
File 2).
The chiral guanidine 10 was tested as a Brønsted base catalyst
to promote the reaction of anthrones 20 or 21 with maleimides
22–24 (Scheme 2) [19,25-28]. Depending on the substituents
and the strength of the Brønsted base either Diels–Alder
adducts or Michael products may be formed. Weaker bases such
as ion-free bisoxazolines [28] or triethylamine are known to in-
duce Diels–Alder adducts selectively whereas strongly basic
guanidines such as 8 may also form the Michael products [19]
which are the dominant products in polar solvents [25].
Figure 2: Crystal structure of guanidine 10 as a benzoate salt. Only
one of the ion pairs is shown for the sake of clarity.
Previous mechanistic studies by Koerner and Rickborn [25]
have collected strong arguments for a fast concerted
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Scheme 2: Reaction of anthrones and N-arylmaleimides catalyzed by guanidine 10. The guanidine deprotonates anthrones 20 or 21 and forms chiral
ion pairs. Primary products of the ion pair complexes are the Diels–Alder adducts that may be further transformed in a base catalyzed retro-aldol reac-
tion.
[4 + 2]-cycloaddition of the deprotonated anthrone. Michael uct 28 were found for reactions in solvents of increased
products were shown to be secondary products of a base-cata- polarity. Assuming that 28 is formed from 25 in a retro-aldol
lyzed retro-aldol reaction and they are not converted backwards step, one might expect similar ee values for both compounds.
into Diels–Alder adducts under such conditions [19,25,26]. Our This is clearly not the case. In THF the 83% ee for compound
results shown below give further support to this view. Riant, 28 strongly deviates from the low value for 25 (6% ee). Obvi-
Kagan and Ricard have demonstrated for the first time that ously, in the reaction of 20 and 22, a Diels–Alder step with low
deprotonated anthrones may coordinate to chiral counterions in to moderate stereoselectivity is superimposed with a kinetic
less polar solvents. Up to 61% ee could be obtained using resolution of enantiomers in the subsequent retro-aldol reaction.
cinchona alkaloids as catalysts [26]. The subsequent work of Both are catalyzed by the chiral guanidine 10. As a result, the
Tan and co-workers with guanidine catalyst 8 achieved enantio- numbers shown in Table 1 are not constant but depend on the
selectivities as high as 99% ee [19]. In recent years functionali- relative turnover of each reaction.
zed chiral amines have been successfully used as catalysts for
anthrone maleimide cycloadditions [29-35]. In the presence of
Table 1: Reaction of anthrone 20 and N-phenylmaleimide (22) in dif-
10, dichloroanthrone 21 reacted with maleimides 22 and 23 to
produce exclusively cycloadducts 26 and 27. In contrast, a mix-
ture of 25 and 28 resulted from the reaction of anthrone 20 and
N-phenylmaleimide (22). For the remaining combinations (20 +
23; 20 + 24; 21 + 24) only the Michael products 29–31 could be
observed (0.1 equiv of 10, CH2Cl2, −15 °C).
ferent solvents.a
solvent
Diels–Alder product 25
Michael product 28
yieldb
36%
eec
yieldb
eec
toluene
22%
6%
51%
8%
47%
35%
25%
41%
83%
74%
(CH2Cl)2 88%
CHCl3
CH2Cl2
THF
3%
37%
34%
6%
91%
70%
42%
53%
To determine the enantioselectivity of guanidine 10 we started
with the structurally simplest case, the reaction of anthrone 20
and N-phenylmaleimide (22) which finally turned out to be the
most complex one (Table 1). The best but still low ee values for
cycloadduct 25 were obtained in CHCl3 (37% ee) and CH2Cl2
(34% ee). Much better enantioselectivities of the Michael prod-
28%
51%
15%
CH3CN
1%
aAll reactions were carried out with 0.1 equiv of guanidine 10 for 64 h
at −15 °C. bIsolated yields after chromatographic separation.
cDetermined by HPLC on a chiral column.
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When rac-25 reacted with 0.1 equiv of 10 in THF at −15 °C, at 12.5 mg of catalyst 10 produced 51 mg of almost pure R-28.
22% conversion ee values of 18% and 70% were found for Analogous treatment of 20 and 23 yielded 21% of 29 (61% ee).
compounds 25 and 28. The dominant isomer of Michael prod- After recrystallization 15% of 29 with 98% ee were obtained.
uct 28 is shown below to be R configurated which corresponds Finally, Michael product 30, obtained from 20 and 24 (42%,
to the slower running peak in Figure 3B. The R isomer of 28 is 48% ee) yielded 26% of material with 95% ee.
formed from the S,S isomer of 25 with retention of configura-
tion (Scheme 2). The change from S to R is caused by a change Assignment of absolute configurations (Scheme 3): Well
in the CIP priorities of the substituents. Thus the faster running grown crystals of enantiopure Michael product 30 suitable for
smaller peak in Figure 3A must correspond to the S,S enantio- X-ray structural analysis could be obtained by a second recrys-
mer of 25. Assuming independent first order rate laws for the tallization from CH2Cl2/cyclohexane. The 4-bromophenyl
opening of the S,S and R,R enantiomers of 25, the best numeri- residue allowed us to assign the R configuration by anomalous
cal fit is obtained for kS,S/kR,R ≈ 6.5. When both steps, dispersion (Supporting Information File 4). This isomer corre-
Diels–Alder and Michael reaction, are catalyzed by guanidine sponds to the slower running isomer on a Chiralpak IA column.
10 in THF (see Table 1), at 45% conversion the R isomer of 28 By catalytic hydrogenation with Pd on charcoal the bromo
and the R,R isomer of 25 are the dominant species. However, residue of enantiopure 30 was replaced with hydrogen thus
due to the faster reaction of S,S-25 the numerical simulation converting R configurated 30 into R configurated Michael prod-
shows that not the R,R but the S,S isomer is the preferred prod- uct 28 (slower running isomer on Chiralpak IA and preferred
uct of the Diels–Alder step with approximately 33% ee. This product under catalysis with 10). Both compounds must result
value comes close to the numbers obtained for the production of from the S,S configurated Diels–Alder adducts. In the case of 25
26 (25% ee; 30% ee in THF) and 27 (28% ee) in CH2Cl2 when this is the faster running isomer on Chiralpak IA. In an earlier
no secondary conversion occurs. In contrast, when compound
21 reacts with 24 ring opening is so fast that no Diels–Alder
product accumulates. Here the ee value of the Michael product
31 (21%) must correspond to the stereoselectivity of the
Diels–Alder step. Due to low stereoselectivities and fast retro-
aldol reactions guanidine 10 is not optimal for the preparation
of Diels–Alder adducts.
Figure 3: A) Chromatogram of rac-25 after incubation with 0.1 equiv of
10 in THF at −15 °C for 64 h. The faster running isomer shows in-
creased conversion into 28. B) The slower running isomer (R) of 28 is
formed preferentially. The faster running isomer of 25, therefore, must
be S,S configurated.
We therefore focussed our efforts on the Michael products:
Anthrone 20 and maleimide 22 reacted with 0.1 equiv of 10 in
THF at −15 °C for 64 h. Chromatographic separation then
Scheme 3: Assignment of the absolute configurations by chemical
correlation. The R configuration of compound 30 is derived from the
crystal structure.
yielded 51% of 25 and 42% of Michael product 28 (83% ee).
Recrystallization afforded 32% of 28 with 99% ee. Thus,
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Beilstein J. Org. Chem. 2016, 12, 1870–1876.
study [28] with bisoxazoline catalyst 33, we have assigned the
S,S configuration to the major isomer of product 32. By hydro-
genolysis, we could now convert such a sample (40% ee) into
25. The product S,S-25 was formed as the dominant product
(35% ee) thus confirming our previous assignment. Under mild
basic conditions, S,S-32 finally could be opened into R configu-
rated Michael product 30 [25,26] (31% ee) to generate a stereo-
chemically consistent view.
Supporting Information File 4
X-ray data of compound 30 (CCDC-1482613).
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-176-S4.cif]
References
1. Ma, Y.; De, S.; Chen, C. Tetrahedron 2015, 71, 1145–1173.
doi:10.1016/j.tet.2014.11.056
Conclusion
2. Ishikawa, T.; Kumamoto, T. Synthesis 2006, 737–752.
doi:10.1055/s-2006-926325
(S)-β-Phenylalanine (13), readily accessible by enantioselective
hydrolysis of ester rac-12, is the starting material for an effi-
cient synthesis of the chiral TBD analog 10. The cyclization
steps in particular, which convert the triamine 19 into the final
guanidine 10, give considerably higher yields compared to
guanidines with five-membered rings such as compounds 6–9.
Apart from the better synthetic accessibility the formal ring
expansion to the six-membered structure of 10 does not increase
the stereoselectivity when it is used as a catalyst. We could not
observe the near-perfect selectivities of the Tan catalyst 8 in the
base-induced reaction of anthrones and maleimides. Neverthe-
less ee values of up to 83% for the formal Michael products
allowed us to isolate 3 compounds in almost enantiopure form
after a single recrystallization. The Tan guanidine 8 [19], the
bisoxazoline 33 [28], and a stereochemically related bisamidine
[27] consistently favor the S,S isomer of Diels–Alder adduct 25.
Although guanidine 10 has the opposite configuration, it also
forms S,S-25 preferentially. Accordingly, transition-state struc-
tures in the reactions of guanidines 8 and 10 must differ from
each other.
3. Selig, P. Synthesis 2013, 45, 703–718. doi:10.1055/s-0032-1318154
4. McKay, A. F.; Kreling, M.-E. Can. J. Chem. 1957, 35, 1438–1445.
doi:10.1139/v57-190
5. Schmidtchen, F. P. Chem. Ber. 1980, 113, 2175–2182.
doi:10.1002/cber.19801130612
6. Echavarren, A.; Galán, A.; de Mendoza, J.; Salmerón, A.; Lehn, J.-M.
Helv. Chim. Acta 1988, 71, 685–693. doi:10.1002/hlca.19880710402
7. Echavarren, A.; Galán, A.; Lehn, J.-M.; de Mendoza, J.
J. Am. Chem. Soc. 1989, 111, 4994–4995. doi:10.1021/ja00195a071
8. Kurzmeier, H.; Schmidtchen, F. P. J. Org. Chem. 1990, 55,
3749–3755. doi:10.1021/jo00299a013
9. Jadhav, V. D.; Schmidtchen, F. P. J. Org. Chem. 2008, 73, 1077–1087.
doi:10.1021/jo7021026
10.Turočkin, A.; Honeker, R.; Raven, W.; Selig, P. J. Org. Chem. 2016,
81, 4516–4529. doi:10.1021/acs.joc.6b00283
11.Davis, A. P.; Dempsey, K. J. Tetrahedron: Asymmetry 1995, 6,
2829–2840. doi:10.1016/0957-4166(95)00374-X
12.Kita, T.; Georgieva, A.; Hashimoto, Y.; Nakata, T.; Nagasawa, K.
Angew. Chem., Int. Ed. 2002, 41, 2832–2834.
doi:10.1002/1521-3773(20020802)41:15<2832::AID-ANIE2832>3.0.CO
;2-Q
13.Kita, T.; Shin, B.; Hashimoto, Y.; Nagasawa, K. Heterocycles 2007, 73,
241–247. doi:10.3987/COM-07-S(U)49
14.Khuong-Huu, F.; Le Forestier, J.-P.; Goutarel, R. Tetrahedron 1972,
28, 5207–5220. doi:10.1016/S0040-4020(01)88940-7
15.Büchi, G.; Rodriguez, A. D.; Yakushijin, K. J. Org. Chem. 1989, 54,
4494–4496. doi:10.1021/jo00280a009
Supporting Information
Supporting Information File 1
Synthetic procedures, characterization data, copies of
chromatograms on chiral columns and of 1H and 13C NMR
spectra.
16.Corey, E. J.; Ohtani, M. Tetrahedron Lett. 1989, 30, 5227–5230.
doi:10.1016/S0040-4039(01)93748-7
17.Corey, E. J.; Grogan, M. J. Org. Lett. 1999, 1, 157–160.
doi:10.1021/ol990623l
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-176-S1.pdf]
18.Ye, W.; Leow, D.; Goh, S. L. M.; Tan, C.-T.; Chian, C.-H.; Tan, C.-H.
Tetrahedron Lett. 2006, 47, 1007–1010.
doi:10.1016/j.tetlet.2005.11.133
Supporting Information File 2
X-ray data of guanidine 10 as benzoate salt
(CCDC-1482611).
19.Shen, J.; Nguyen, T. T.; Goh, Y.-P.; Ye, W.; Fu, X.; Xu, J.; Tan, C.-H.
J. Am. Chem. Soc. 2006, 128, 13692–13693. doi:10.1021/ja064636n
20.Rodionow, W. M.; Postovskaja, E. A. J. Am. Chem. Soc. 1929, 51,
841–847. doi:10.1021/ja01378a027
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-176-S2.cif]
21.Rodionow, W. M. J. Am. Chem. Soc. 1929, 51, 847–852.
doi:10.1021/ja01378a028
22.Faulconbridge, S. J.; Holt, K. E.; Garcia Sevillano, L.; Lock, C. J.;
Tiffin, P. D.; Tremayne, N.; Winter, S. Tetrahedron Lett. 2000, 41,
2679–2681. doi:10.1016/S0040-4039(00)00222-7
23.Tasnádi, G.; Forró, E.; Fülöp, F. Tetrahedron: Asymmetry 2008, 19,
2072–2077. doi:10.1016/j.tetasy.2008.08.009
Supporting Information File 3
X-ray data of compound 29 (CCDC-1482612).
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-176-S3.cif]
1875

Beilstein J. Org. Chem. 2016, 12, 1870–1876.
24.Grayson, J. I.; Roos, J.; Osswald, S. Org. Process Res. Dev. 2011, 15,
1201–1206. doi:10.1021/op200084g
25.Koerner, M.; Rickborn, B. J. Org. Chem. 1990, 55, 2662–2672.
doi:10.1021/jo00296a024
26.Riant, O.; Kagan, H. B.; Ricard, L. Tetrahedron 1994, 50, 4543–4554.
doi:10.1016/S0040-4020(01)89385-6
27.Akalay, D.; Dürner, G.; Bats, J. W.; Göbel, M. W.
Beilstein J. Org. Chem. 2008, 4, No. 28. doi:10.3762/bjoc.4.28
28.Akalay, D.; Dürner, G.; Göbel, M. W. Eur. J. Org. Chem. 2008,
2365–2368. doi:10.1002/ejoc.200800179
29.Uemae, K.; Masuda, S.; Yamamoto, Y. J. Chem. Soc., Perkin Trans. 1
2001, 1002–1006. doi:10.1039/b100961n
30.Hau, C.-K.; He, H.; Lee, A. W. M.; Chik, D. T. W.; Cai, Z.;
Wong, H. N. C. Tetrahedron 2010, 66, 9860–9874.
doi:10.1016/j.tet.2010.10.064
31.Zea, A.; Valerao, G.; Alba, A.-N. R.; Moyano, A.; Rios, R.
Adv. Synth. Catal. 2010, 352, 1102–1106.
doi:10.1002/adsc.201000031
32.Zea, A.; Alba, A.-N. R.; Bravo, N.; Moyano, A.; Rios, R. Tetrahedron
2011, 67, 2513–2529. doi:10.1016/j.tet.2011.02.032
33.Bai, J.-F.; Guo, Y.-L.; Peng, L.; Jia, L.-N.; Xu, X.-Y.; Wang, L.-X.
Tetrahedron 2013, 69, 1229–1233. doi:10.1016/j.tet.2012.11.011
34.Kumagai, J.; Otsuki, T.; Reddy, U. V. S.; Kohari, Y.; Seki, C.; Uwai, K.;
Okuyama, Y.; Kwon, E.; Tokiwa, M.; Takeshita, M.; Nakano, H.
Tetrahedron: Asymmetry 2015, 26, 1423–1429.
doi:10.1016/j.tetasy.2015.10.022
35.Tawde, T. S.; Wagh, S. J.; Sapre, J. V.; Khose, V. N.; Badani, P. M.;
Karnik, A. V. Tetrahedron: Asymmetry 2016, 27, 130–135.
doi:10.1016/j.tetasy.2016.01.002
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