Catalytic asymmetric carbon-carbon bond forming reactions catalyzed by tetrahydroisoquinoline (TIQ) N,N′-dioxide ligands

Article abstract of DOI:10.1016/j.tetasy.2013.01.004

The use of TIQ-N,N′-dioxide ligands in asymmetric C-C bond forming reactions is described. In the Michael addition of cyclohexane-1,3-dione and malonates to β,γ-unsaturated α-ketoesters, excellent yields (up to 93%) and moderate to good enantioselectiviti

Full text of DOI:10.1016/j.tetasy.2013.01.004

Tetrahedron: Asymmetry 24 (2013) 191–195
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Tetrahedron: Asymmetry
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Catalytic asymmetric carbon–carbon bond forming reactions catalyzed
by tetrahydroisoquinoline (TIQ) N,N0-dioxide ligands
Zamani E. D. Cele a, Sphelele C. Sosibo a, Pher G. Andersson a,b, Hendrik G. Kruger a, Glenn E. M. Maguire a,
Thavendran Govender a,
⇑
a Catalysis and Peptide Research Unit, School of Health Sciences, University of KwaZulu-Natal, Durban, South Africa
b Department of Biochemistry and Organic Chemistry, Uppsala University, SE-751 23 Uppsala, Sweden
a r t i c l e i n f o
a b s t r a c t
The use of TIQ-N,N0-dioxide ligands in asymmetric C–C bond forming reactions is described. In the
Michael addition of cyclohexane-1,3-dione and malonates to b,c-unsaturated a-ketoesters, excellent
yields (up to 93%) and moderate to good enantioselectivities (70–89% ee) were obtained. The catalytic
hetero-ene reaction of 2-methoxypropene with phenylglyoxal gave the ene product in excellent yield
(95%) with moderate enantioselectivity (77% ee). The catalyst system performed well at temperatures
ranging from 0 to 30 °C and relatively low catalyst loading (0.2–5 mol %) with dichloromethane being
the preferred solvent for all reactions.
Article history:
Received 20 November 2012
Accepted 4 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
N-Oxides have emerged as promising ligands for chiral catalysis
because they offer excellent electron donating properties, which
Asymmetric catalysis has been a major subject of interest due to
the search for enantiomerically pure biologically active com-
pounds.1 The catalytic asymmetric Michael addition reaction is
very important in organic synthesis because it is a powerful method
for the construction of stereocenters.2,3 It is also the most widely
used reaction for C–C bond formations whereby nucleophiles (do-
nors) are added to alkenes or alkynes attached to an electron with-
drawing group (acceptors) to generate new stereogenic centers.4 In
addition, the broad range of donor atoms includes S, O, and N, which
has increased the scope of Michael addition reactions. The most
are required for complexation with metal ions.28 There have been
several reports on the use of N,N0-dioxides as tetradentate ligands
coordinated to metal ions for a range of reactions affording excel-
lent yields and selectivity. These reactions include asymmetric cyc-
loadditions29–33 and enantioselective nucleophilic additions to
C@O and C@N bonds.34–38,30 Stereoselective conjugate additions
between
also been achieved.
a
,b-unsaturated compounds and nucleophiles39–41 have
Over the past few years, we have developed the tetrahydroiso-
quinoline (TIQ) scaffold either as ligands or catalysts for various
applications. These reactions include catalytic asymmetric transfer
hydrogenations (ATH),42,43 Henry type C–C bond formations,44,45
high pressure hydrogenations of unsymmetrical olefins,46 conju-
gate addition reactions,47 and as organocatalysts for Diels–Alder
reactions.48 We recently demonstrated the use of TIQ based N-oxi-
des as organocatalysts for the asymmetric allylation of aldehydes49
and for enantioselective conjugate addition of thioglycolate to a
range of chalcones using metal complexes.50 Herein, we report
the use of C2-symmetric TIQ N,N0-dioxide ligands complexed with
metal ions for enantioselective Michael addition of cyclic diketones
common acceptors reported are based on
a
,b-unsaturated carbonyl
a
compounds as compared to b, -unsaturated
c
-ketoesters.5–19 The
use of this latter group in the synthesis of biological active com-
pounds is steadily gaining interest.20
The catalytic asymmetric hetero-ene reaction is also one of the
simplest ways for C–C bond formation and for the derivatization of
allylic C–H bonds.21 There are only a few cases where enol ethers
have been reported for this type of reaction due to their instability
in the presence of a Lewis acid and the competitive Mukaiyama al-
dol reaction (Scheme 1).22–26 Jacobsen et al. reported the only suc-
cessful enantioselective reaction of aryl aldehydes with an alkyl
enol ether via Lewis acid activation, which afforded the ene prod-
uct with up to 97% yield and 96% ee.26 The first catalytic enantiose-
lective hetero-ene reaction of alkyl enol ethers with 1,2-dicarbonyl
compounds in excellent yields (up to 98%) with >99% ee was re-
ported by Feng et al. using N-oxide ligands.27
and malonates to b,c-unsaturated a-ketoesters. We also report the
catalytic hetero ene reaction of glyoxal with enol ether.
2. Results and discussion
Since Dong et al. reported the use of N,N0-dioxide(II) complexes
as effective chiral Lewis acid catalysts for enantioselective Michael
addition reactions,51 we first chose the chiral N0N-dioxide ligand L1
(Fig. 1). Ligand L1 was previously synthesized for the asymmetric
⇑
Corresponding author.
E-mail address: govenderthav@ukzn.ac.za (T. Govender).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.01.004

192
Z. E. D. Cele et al. / Tetrahedron: Asymmetry 24 (2013) 191–195
OH
OR3
OH
O
O
OR3
Lewis acid catalyst
+
R1
R1
R1
R2
R2
R2
enol ethers
A
B
Mukaiyama
ene product
aldol product
silyl enol ethers: R3 = SiR3 unstable
alkyl enol ethers: R3 = alkyl more stable but lower activity
Scheme 1. Competing formation of ene product A and the Mukaiyama aldol product B in the Lewis acid catalyzed addition of enol ethers to electrophiles.
we showed that ligands with bulky groups at the ortho positions of
aniline, such as isopropyl in L2, could give the Michael adduct with
higher enantioselectivities52 (Table 1, entry 1 vs 2). With L2 being
the best ligand, we next carried out the reaction of 1a with 2a in
different solvents. The results indicated that the choice of solvent
had a significant effect on the rate and enantioselectivity of the
reaction (Table 1, entries 2–6). Solvents such as chloroform, tetra-
hydrofuran, acetonitrile, and 1,2-dichloroethane provided lower
enantioselectivities of 3a compared to dichloromethane. Several
other Lewis acids were also investigated however there was no fur-
ther improvement in the results (Table 1, entries 7–10). It should
also be noted that neither the metal nor the ligand alone promoted
the reaction (Table 1, entries 11 and 12), therefore the preparation
of a metal complex beforehand was unnecessary.
R
HN
O
R
O
NH
O
N
N
O
L1
R = phenyl
L2 R = 2,6-diisopropyl phenyl
Figure 1. Chiral N-oxide ligands used herein.
Under the optimized conditions (Table 1, entry 2), the scope of
substrates for the asymmetric conjugate addition of cyclic dike-
tones to various b,c-unsaturated a-ketoesters was extended and
conjugate addition of thioglycolate to a range of chalcones.52 This
ligand was complexed with Cu(OTf)2, and evaluated in the reaction
of cyclic diketone 2a with b,c-unsaturated a-ketoester 1a in CH2Cl2
at room temperature to afford the addition product 3a in 72% yield
and with 30% ee (Table 1, entry 1). The 1H NMR and 13C NMR spec-
tra showed that the product was obtained as an equilibrating mix-
ture of anomers and that the cyclic compound was the major
product.53 Only one pair of enantiomers was detected by HPLC be-
cause the equilibrium was very rapid, although it was slow enough
for both compounds to show up in NMR.53,51 In our previous report
the results are shown in Table 2. Ketoesters with both electron-
donating and electron-withdrawing groups at the para position
were well tolerated in terms of enantioselectivity and yields, with
up to 91% yield and 89% ee being obtained (Table 2, entries 1–7). A
fused ring b,c-unsaturated a-ketoester 1h was also a suitable sub-
strate for the reaction, giving the corresponding product in moder-
ate yield and ee (Table 2, entry 8).
With the results from our previous work on asymmetric conju-
gate addition reactions52 and from these optimized reaction condi-
tions (Table 1), we concluded that the TIQ-N0N-dioxide ligands are
Table 1
Asymmetric conjugate addition of cyclohexane-1,3-dione 1a to ketoester 2a catalyzed by ligands L1 and L2
O
O
Ph
O
∗
L
/ metal (2 mol%)
+
OMe
OH
Ph
solvent, RT, 12 h
O
O
COOMe
O
1a
2a
3a
Entry
Ligand
Metal
Solvent
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
L1
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
—
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
La(OTf)3
Sc(OTf)3
Yb(OTf)3
In(OTf)3
—
CH2Cl2
CH2Cl2
CHCl3
ClCH2CH2Cl
CH3CN
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
72
90
89
81
76
65
92
40
88
30
82
20
43
10
10
rac
36
41
rac
—
9
10
11c
12d
72
No reaction
No reaction
Cu(OTf)2
—
Unless otherwise noted, the reactions were performed with 1a (0.12 mmol), 2a (0.10 mmol), N,N0-dioxide (0.01 mmol), and
metal (0.01 mmol) in solvent (1.0 mL) at room temperature for 12 h.
a
Yield of the isolated product.
Determined by HPLC analysis (Chiralpak IA).
With the ligand only.
With the metal only.
b
c
d

Z. E. D. Cele et al. / Tetrahedron: Asymmetry 24 (2013) 191–195
193
Table 2
Substrate scope for the asymmetric conjugate addition of hexane-1,3-dione to b,
c
-unsaturated
a
-ketoesters
O
O
Ph
∗
O
L2 (2 mol%)
Cu(OTf)2 (2 mol %)
+
OMe
OH
COOMe
R
CH2Cl2, RT, 12 h
O
O
O
3
1a
2
Entry
R
Yielda (%)
eeb (%)
Product
1
2
3
4
5
6
Ph
90
88
91
87
79
68
82
74
89
79
78
70
3a
3b
3c
3d
3e
3f
4-MeC6H4
4-MeOC6H4
4-FC6H4
4-ClC6H4
4-NO2C6H4
O
7
8
70
74
77
71
3g
3h
O
2-Naphthyl
Unless otherwise noted, the reactions were performed with 1a (0.12 mmol), 2 (0.10 mmol), N,N0-dioxide L2 (2 mol %), and
Cu(OTf)2 (2 mol %) in CH2Cl2 (1.0 mL) at room temperature for 12 h.
a
Yield of the isolated product.
Determined by HPLC analysis (Chiralpak IA).
b
similar to the N-oxide ligands reported by Feng et al.1 We decided
to investigate different substrates for the asymmetric Michael
addition of malonates to b,c-unsaturated a-ketoester using L2-
Y(OTf)3.
(Table 3, entries 4 and 5). It should be noted that the condensed
ring performed well, giving the corresponding product in relatively
good yield and enantioselectivity (Table 3, entry 6). When the ester
group on the malonate was changed from methyl to ethyl, the ee
did not change (Table 3, entries 1–9) and even when it was altered
to a larger group such as i-Pr (Table 2, entries 10–13), the ee was
unaffected.
We next decided to examine the benchmark reaction for the
addition of alkyl enol ether 7 to phenylglyoxal 6 as another C–C
bond formating reaction. Zheng et al. showed that the Mukaiyama
aldol product, which arises due to the high reactivity of glyoxals
and the hydrolysis of 9 under acidic conditions can be avoided
by low catalytic loading and by using the less reactive Cu(OTf)2
In general, the scope of this transformation was found to be rel-
atively wide as different substituents on the b, -unsaturated a-
c
ketoesters led to the corresponding addition product in good yields
and enantioselectivities (Table 3, entries 1–6). The electronic nat-
ure of the substituents on the ketoesters had an influence on the
reaction yield, but did not have an effect on the selectivity of the
products. For example, the ketoesters with an electron-donating
group showed higher reactivity (Table 3, entry 2), than electron-
withdrawing substituents, while the selectivity remained similar
Table 3
Substrate scope for the asymmetric Michael addition of malonates to b,
c
-unsaturated
a
-ketoesters
OR2 R1
O
O
O
O
L2 (5 mol%)
∗
OMe
Y(OTf)3 (5 mol %)
O
COOMe
R1
R2O
OR2
+
CH2Cl2, 0 °C
O
OR2
O
5
2
4
Entry
R2, R1
Time (h)
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
Me, Phe
Me, 4-MeC6H4
5
12
8
8
8
12
10
10
10
8
9
10
10
89
76
93
63
61
84
85
76
72
68
70
69
67
85
81
87
71
83
73
81
84
80
82
84
79
82
Me, 4-MeOC6H4
Me, 4-FC6H4
Me, 4-ClC6H4
Me, 2-napthyl
Et, Phe
Et, 4-MeOC6H4
Et, 4-ClC6H4
i-Pr, Phe
9
10
11
12
13
i-Pr, 4-MeOC6H4
i-Pr, 4-FC6H4
i-Pr, 4-ClC6H4
Unless otherwise noted, the reactions were performed with 2 (0.10 mmol), N,N0-dioxide L2 (5 mol %), and Y(OTf)3 (5 mol %) in
a CH2Cl2, then malonates 4 were added at 0 °C. The reaction mixture was stirred at 0 °C for the indicated time.
a
Isolated yield.
Determined by HPLC analysis (Chiralpak IA).
b

194
Z. E. D. Cele et al. / Tetrahedron: Asymmetry 24 (2013) 191–195
complex compared to Mg(OTf)2.27 The reaction shown in Scheme 2,
proceeded smoothly in excellent yield and with moderate ee using
L2-Cu(OTf)2 complex at 0.2 mol % catalyst loading to hinder the
Mukaiyama aldol reaction.
at room temperature for 30 min. The reaction mixture was cooled
to 0 °C and dimethyl malonate (0.12 mmol, 14 L) was added, after
l
which the reaction mixture was stirred at 0 °C for 8 h while being
monitored by TLC. The solvents were evaporated under reduced
pressure and the residue was purified through column chromatog-
raphy (hexane/ethyl acetate = 4:1) to afford the pure conjugated
product as a yellow oil in 95% yield. The enantioselectivity was
determined by chiral HPLC (Chiralpak IA, hexane/i-PrOH = 80/10,
flow rate 1.0 mL/min, k = 254 nm).
OH
OMe
O
L2
(0.2 mol%)
Cu(OTf)2 (0.2 mol%)
OMe
Ph
∗
H
+
Ph
CH2Cl2, 30 °C
4Å M.S., 2 h
O
O
4.4. Typical procedure for the catalytic asymmetric hetero-ene
reaction of glyoxal with 2-methoxypropene
6
8
7
yield = 95%, ee = 77%
A mixture of 100 lL (0.002 mmol) of catalyst solution (0.002 M
Scheme 2. Catalytic asymmetric hetero ene reaction of glyoxal with 2-
Cu(OTf)2-L2 in CH2Cl2), phenylglyoxal (0.1 mmol), and 3 Å MS
(50 mg) was stirred at 30 °C for 30 min. Then, 2-methoxypropene
(1.25 equiv) was added at room temperature under nitrogen. The
reaction mixture was then allowed to reflux for 2 h while being
monitored by TLC and was directly purified by column chromatog-
raphy on silica gel (ethyl acetate/hexane = 1:10). The pure ene
product was obtained in 90% yield as a colorless liquid. The enanti-
oselectivity was determined by chiral HPLC using a DAICEL CHI-
RALCEL AS-H column, 2-propanol/n-hexane = 10/80, flow
rate = 0.8 mL/min, k = 254 nm.
methoxypropene.
3. Conclusion
We have demonstrated the use of C2-symmetric TIQ-N0N-diox-
ides as efficient ligands for C–C bond forming reactions. High yields
(up to 95%), moderate enantioselectivities (up to 89% ee), a broad
range of substrates, and mild reaction conditions demonstrate
the potential of this catalytic system for other asymmetric trans-
formations. The choice of solvent also played an important role
in the selectivity of these reactions with dichloromethane being
the most efficient for all three of the C–C bond forming reactions
reported herein. The Michael addition reaction of ketoesters with
cyclohexane1,3-dione (Tables 1 and 2) and malonate (Table 3) re-
quired 2 and 5 mol % catalytic loading, respectively, to achieve the
best results. The benchmark hetero-ene reaction proceeded
smoothly with relatively low catalyst loading (0.2 mol %) allowing
us to avoid the Mukaiyama product. A comparative study of our
TIQ-N0N-dioxide with Fengfs bis N-oxide ligands demonstrated
complementarity in their behavior, however our system did not
achieve equivalent yields and selectivities. Further studies of the
application of this catalyst to other reactions are currently ongoing
in our laboratory.
References
1. Liu, X.; Lin, L.; Feng, X. Acc. Chem. Res. 2011, 44, 574–587.
2. Sibi, M. P.; Manyem, S. Tetrahedron 2000, 56, 8033–8061.
3. Berner, O. M.; Tedeschi, L.; Enders, D. Eur. J. Org. Chem. 2002, 1877–1894.
4. Perlmutter, P. In Conjugate Addition Reactions in Organic Synthesis; Pergamon:
Oxford; New York, 1992; Vol. 9,
5. Rueping, M.; Sugiono, E.; Merino, E. Angew. Chem., Int. Ed. 2008, 47, 3046–3049.
6. Rueping, M.; Merino, E.; Sugiono, E. Adv. Synth. Catal. 2008, 350, 2127–2131.
7. Rueping, M.; Sugiono, E.; Merino, E. Chemistry – A European Journal 2008, 14,
6329–6332.
8. Halland, N.; Hansen, T.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2003, 42, 4955–
4957.
9. Itoh, K.; Hasegawa, M.; Tanaka, J.; Kanemasa, S. Org. Lett. 2005, 7, 979–981.
10. Xie, J.-W.; Yue, L.; Chen, W.; Du, W.; Zhu, J.; Deng, J.-G.; Chen, Y.-C. Org. Lett.
2007, 9, 413–415.
11. Kim, H.; Yen, C.; Preston, P.; Chin, J. Org. Lett. 2006, 8, 5239–5242.
12. Kristensen, T. E.; Vestli, K.; Hansen, F. K.; Hansen, T. Eur. J. Org. Chem. 2009,
2009, 5185–5191.
13. Dong, Z.; Wang, L.; Chen, X.; Liu, X.; Lin, L.; Feng, X. Eur. J. Org. Chem. 2009,
5192–5197.
4. Experimental
4.1. Ligands L1 and L2
14. Herrera, R. P.; Monge, D.; Martin-Zamora, E.; Fernandez, R.; Lassaletta, J. M. Org.
Lett. 2007, 9, 3303–3306.
15. Zhao, S.-L.; Zheng, C.-W.; Zhao, G. Tetrahedron: Asymmetry 2009, 20, 1046–
1051.
16. Wang, H.-F.; Zheng, C.-W.; Yang, Y.-Q.; Chai, Z.; Zhao, G. Tetrahedron:
Asymmetry 2008, 19, 2608–2615.
The TIQ-N0N-dioxide ligands L1 and L2 were synthesized using
the same procedure reported in the literature.52
17. Wang, X.-S.; Zheng, C.-W.; Zhao, S.-L.; Chai, Z.; Zhao, G.; Yang, G.-S.
Tetrahedron: Asymmetry 2008, 19, 2699–2704.
18. Sasai, H.; Arai, T.; Shibasaki, M. J. Am. Chem. Soc. 1994, 116, 1571–1572.
19. Kim, Y. S.; Matsunaga, S.; Das, J.; Sekine, A.; Ohshima, T.; Shibasaki, M. J. Am.
Chem. Soc. 2000, 122, 6506–6507.
20. Kumar, A.; Sharma, S.; Tripathi, V. D.; Maurya, R. A.; Srivastava, S. P.; Bhatia, G.;
Tamrakar, A. K.; Srivastava, A. K. Bioorg. Med. Chem. 2010, 18, 4138–4148.
21. Mikami, K.; Shimizu, M. Chem. Rev. 1992, 92, 1021–1050.
22. Mikami, K.; Matsukawa, S. J. Am. Chem. Soc. 1993, 115, 7039–7040.
23. Ruck, R. T.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2003, 42, 4771–4774.
24. Zeng, W.; Chemler, S. R. J. Am. Chem. Soc. 2007, 129. 12948-+.
25. Carreira, E. M.; Lee, W.; Singer, R. A. J. Am. Chem. Soc. 1995, 117, 3649–3650.
26. Ruck, R. T.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124, 2882–2883.
27. Zheng, K.; Yin, C.; Liu, X.; Lin, L.; Feng, X. Angew. Chem., Int. Ed. 2011, 50, 2573–
2577.
4.2. General procedure for the enantioselective conjugate
addition of cyclic diketones to b,c-unsaturated a-ketoesters
Dichloromethane (1.0 mL) was added to a mixture of ligand L2
(2.98 mg, 0.004 mmol), Cu(OTf)2 (1.45 mg, 0.004 mmol), and b,
unsaturated -ketoesters (38.0 mg, 0.2 mmol) and diketone
c
-
a
(22.4 mg, 0.2 mmol), then stirred at room temperature for 12 h.
After the reaction was complete (monitored by TLC), the mixture
was purified by column chromatography on silica gel (ethyl ace-
tate/hexane = 1:2) to afford the desired product as a white solid
in 90% yield. The enantioselectivity was determined by chiral HPLC
(Chiralpak IA, hexane/i-PrOH = 80/10, flow rate 1.0 mL/min,
k = 254 nm).
28. Su, Z. S.; Hu, C. W.; Qin, S.; Feng, X. M. Tetrahedron 2006, 62, 4071–4080.
29. Yu, Z.; Liu, X.; Dong, Z.; Xie, M.; Feng, X. Angew. Chem., Int. Ed. 2008, 47, 1308–
1311.
30. Shang, D.; Xin, J.; Liu, Y.; Zhou, X.; Liu, X.; Feng, X. J. Org. Chem. 2008, 73, 630–
637.
31. Guillaneux, D.; Zhao, S. H.; Samuel, O.; Rainford, D.; Kagan, H. B. J. Am. Chem.
Soc. 1994, 116, 9430–9439.
4.3. General procedure for the asymmetric Michael addition of
malonates to b,c-unsaturated a-ketoesters
32. Xie, M.; Chen, X.; Zhu, Y.; Gao, B.; Lin, L.; Liu, X.; Feng, X. Angew. Chem., Int. Ed.
2010, 49, 3799–3802.
33. Zhu, Y.; Chen, X.; Xie, M.; Dong, S.; Qiao, Z.; Lin, L.; Liu, X.; Feng, X. Chem.-Eur. J.
2010, 16, 11963–11968.
A solution of ligand L2 (3.8 mg, 0.005 mmol), Y(OTf)3 (2.68 mg,
0.005 mmol), and b, -unsaturated -ketoesters (19.1 mg,
0.10 mmol) in anhydrous dichloromethane (0.2 mL) was stirred
c
a

Z. E. D. Cele et al. / Tetrahedron: Asymmetry 24 (2013) 191–195
195
34. Wang, F.; Liu, X.; Zhang, Y.; Lin, L.; Feng, X. Chem. Commun. 2009, 7297–7299.
35. Zheng, K.; Liu, X.; Zhao, J.; Yang, Y.; Lin, L.; Feng, X. Chem. Commun. 2010, 3771–
3773.
36. Qin, B.; Xiao, X.; Liu, X.; Huang, J.; Wen, Y.; Feng, X. J. Org. Chem. 2007, 72,
9323–9328.
37. Zhang, X.; Chen, D.; Liu, X.; Feng, X. J. Org. Chem. 2007, 72, 5227–5233.
38. Chen, W.; Hui, Y.; Zhou, X.; Jiang, J.; Cai, Y.; Liu, X.; Lin, L.; Feng, X. Tetrahedron
Lett. 2010, 51, 4175–4178.
39. Chang, L.; Shang, D.; Xin, J.; Liu, X.; Feng, X. Tetrahedron Lett. 2008, 49, 6663–
6666.
45. Pawar, S. S.; Chakka, S. K.; Andersson, P. G.; Kruger, H. G.; Maguire, G. E. M.;
Govender, T. S. Afr. J. Chem. 2012, 65, 23–29.
46. Chakka, S. K.; Peters, B. K.; Andersson, P. G.; Maguire, G. E. M.; Kruger, H. G.;
Govender, T. Tetrahedron: Asymmetry 2010, 21, 2295–2301.
47. Naicker, T.; Arvidsson, P. I.; Kruger, H. G.; Maguire, G. E. M.; Govender, T. Eur. J.
Org. Chem. 2012, 3331–3337.
48. Naicker, T.; Petzold, K.; Singh, T.; Arvidsson, P. I.; Kruger, H. G.; Maguire, G. E.
M.; Govender, T. Tetrahedron: Asymmetry 2010, 21, 2859–2867.
49. Naicker, T.; Arvidsson, P. I.; Kruger, H. G.; Maguire, G. E. M.; Govender, T. Eur. J.
Org. Chem. 2011, 6923–6932.
40. Liu, Y.; Zhou, X.; Shang, D.; Liu, X.; Feng, X. Tetrahedron 2010, 66, 1447–1457.
41. Hui, Y.; Jiang, J.; Wang, W.; Chen, W.; Cai, Y.; Lin, L.; Liu, X.; Feng, X. Angew.
Chem., Int. Ed. 2010, 49, 4290–4293.
42. Chakka, S. K.; Andersson, P. G.; Maguire, G. E. M.; Kruger, H. G.; Govender, T.
Eur. J. Org. Chem. 2010, 972–980.
43. Peters, B. K.; Chakka, S. K.; Naicker, T.; Maguire, G. E. M.; Kruger, H. G.;
Andersson, P. G.; Govender, T. Tetrahedron: Asymmetry 2010, 21, 679–687.
44. Kawthekar, R. B.; Chakka, S. K.; Francis, V.; Andersson, P. G.; Kruger, H. G.;
Maguire, G. E. M.; Govender, T. Tetrahedron: Asymmetry 2010, 21, 846–852.
50. Chakka, S. K.; Cele, Z. E. D.; Sosibo, S. C.; Francis, V.; Arvidsson, P. I.; Kruger, H.
G.; Maguire, G. E. M.; Govender, T. Tetrahedron: Asymmetry 2012, 23, 616–622.
51. Dong, Z.; Feng, J.; Fu, X.; Liu, X.; Lin, L.; Feng, X. Chem.-Eur. J. 2011, 17, 1118–
1121.
52. Chakka, S. K.; Cele, Z. E. D.; Sosibo, S. C.; Francis, V.; Arvidsson, P. I.; Kruger, H.
G.; Maguire, G. E. M.; Govender, T. Tetrahedron: Asymmetry 2012, 23, 616–622.
53. Zhao, S.-L.; Zheng, C.-W.; Wang, H.-F.; Zhao, G. Adv. Synth. Catal. 2009, 351,
2811–2816.