Efficient carbon-Ferrier rearrangement on glycals mediated by ceric ammonium nitrate: Application to the synthesis of 2-deoxy-2-amino-C-glycoside

Article abstract of DOI:10.3762/bjoc.10.27

A carbon-Ferrier rearrangement on glycals has been performed by using ceric ammonium nitrate to obtain products in moderate to good yields with high selectivity. The versatility of this method has been demonstrated by applying it to differently protected

Full text of DOI:10.3762/bjoc.10.27

Efficient carbon-Ferrier rearrangement on glycals
mediated by ceric ammonium nitrate: Application to
the synthesis of 2-deoxy-2-amino-C-glycoside
Alafia A. Ansari, Y. Suman Reddy and Yashwant D. Vankar*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 300–306.
Department of Chemistry, Indian Institute of Technology Kanpur 208
doi:10.3762/bjoc.10.27
016, India
Received: 29 October 2013
Accepted: 09 January 2014
Published: 30 January 2014
Email:
Yashwant D. Vankar* - vankar@iitk.ac.in
*
Corresponding author
Associate Editor: N. Sewald
Keywords:
© 2014 Ansari et al; licensee Beilstein-Institut.
License and terms: see end of document.
C-glycosides; carbon-Ferrier rearrangement; ceric ammonium nitrate;
2-deoxy-2-aminoglycosides; Overman rearrangement
Abstract
A carbon-Ferrier rearrangement on glycals has been performed by using ceric ammonium nitrate to obtain products in moderate to
good yields with high selectivity. The versatility of this method has been demonstrated by applying it to differently protected
glycals and by employing several nucleophiles. The obtained C-allyl glycoside has been utilized for the synthesis of a orthogonally
protected 2-amino-2-deoxy-C-glycoside.
Introduction
The growing significance of C-glycosides can be attributed to glycosyl cyanides, and glycosyl azides have received consider-
their potential use as inhibitors of carbohydrate-processing able attention, as the allyl, cyanide and azide moieties can be
enzymes [1-3], their extraordinary stability compared to readily converted into a variety of other functional groups
O-glycosides, and their widespread applicability as intermedi- [6,28-30]. Furthermore, the resulting C-pseudoglycals are also
ates in the synthesis of biologically important molecules [4-8]. important as the double bond can also be easily functionalized
C-glycosides are also subunits of several biologically active in a variety of ways.
natural products [9-11]. Consequently, numerous reports are
available in literature on the synthesis of C-glycosides [12-14]. Although there is an abundance of methods reporting on the
Among these, the Ferrier rearrangement [15] of glycals with carbon-Ferrier rearrangement of glycals [9], efforts are ongoing
protic acids [5,16,17] or Lewis acids [18-22] and carbon for improvements in terms of efficiency, selectivity, time and
nucleophiles such as allylsilanes [23], silylacetylenes [24], silyl yields of reaction. In view of our continued interest in the devel-
enol ethers [25], olefins [26], and organozinc reagents [27] has opment of novel methods for the synthesis of O- and C-glyco-
emerged as a popular method. In particular, C-allyl glycosides, sides [31-36], we here report on the carbon-Ferrier rearrange-
300

Beilstein J. Org. Chem. 2014, 10, 300–306.
ment of glycals by using ceric ammonium nitrate. Ceric glycoside 2a in 88% yield (Table 1) [42]. We attempted the
ammonium nitrate (CAN) is a versatile and efficient reagent, same reaction with a catalytic amount (10–30 mol %) of CAN.
which has been well-explored for a variety of reactions in litera- However, under these conditions the reaction showed compli-
ture [37,38]. CAN was utilized successfully in carbohydrate cated TLC profiles and did not complete even after overnight
chemistry for important transformations such as the azidonitra- stirring or heating under reflux for several hours. Changing the
tion of glycals and the formation of 2-C-branched glycosides solvent to dichloromethane slowed down the reaction consider-
from glycals [39-41]. In this paper, we report on the addition of ably due to the poor solubility of CAN, whereas in acetone the
carbon nucleophiles onto differently protected glycals by using reaction gave lower yields. After several optimization experi-
CAN. The corresponding Ferrier rearrangement products were ments, the reaction was observed to be most efficient when
obtained in moderate to fairly good yields and with a high 2 equivalents of silane and one equivalent of CAN, with respect
selectivity.
to the glycal, was used in acetonitrile.
Results and Discussion
The reaction was then carried out with several glycals (Table 2).
We initially performed the carbon-Ferrier rearrangement on and yielded a single product in all cases studied. In the case of
,4,6-tri-O-acetyl-D-glucal (1a) by using 3 equivalents of glycals 1a–d, only the α-anomer was obtained as has been
3
allyltrimethylsilane and 2 equivalents of CAN in anhydrous reported by Danishefsky et al. in the first report on the
acetonitrile at room temperature. The reaction proceeded C-glycosidation of glycals with allyltrimethylsilane by using
smoothly over 1 hour and exclusively furnished the α-C-allyl TiCl4 [23]. The spectral data of the obtained C-allyl glycosides
Table 1: Optimization of reaction conditions.
Entry
Silane (equiv)
CAN (equiv)
Solvent
Temp (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
3.0
3.0
3.0
3.0
2.0
2.0
2.0
2.0
0.2
0.5
1.0
1.0
1.0
1.0
CH3CN
CH3CN
rt
reflux
reflux
rt
1 h
12 h
12 h
1.5 h
2 h
88
38
45
82
88
53
20
CH3CN
CH3CN
CH3CN
rt
CH3COCH3
CH2Cl2
rt
6 h
rt
12 h
Table 2: Reaction of glycals with allyltrimethylsilane by using CAN.
Entry
Glycal
Product
Time
Yield
86
Ref.
[42]
1
2 h
1
a
b
2
a
2
3 h
75
[42]
1
2b
301

Beilstein J. Org. Chem. 2014, 10, 300–306.
Table 2: Reaction of glycals with allyltrimethylsilane by using CAN. (continued)
3
4 h
69
52
[42]
[44]
1c
2c
4
8 h
1
d
e
2
d
e
5
6
7
8
2 h
3 h
4 h
8 h
80
84
71
61
[43,45]
[43,45]
[46]
2
1
1
f
2f
2g
1
g
h
[46]
1
2g
9
4 h
67
66
[46]
[43]
1
i
2
i
j
10.
4 h
1j
2
1
1.
2.
2 h
3 h
78
62
[43]
[46]
1k
2k
1
2l
1l
supported the known data [42-44]. In general, D-glucals were previous investigations [45]. Glycals 1i, with a TBDPS
found to react faster than D-galactals and provided the protecting group, and 1k, with a methyl protecting group, were
rearranged products in less time and better yields. Moreover, also found to undergo the reaction in good yields affording 2i
even the benzylated glycals 1c and 1d readily underwent the (see Supporting Information File 1) and 2k [42], respectively.
Ferrier rearrangement under these conditions. The pentose These findings indicate that the protecting groups were not
glycals 1e–h exclusively gave the anti-products, confirming affected by this reagent system. D-rhamnal 1j and D-ribose
302

Beilstein J. Org. Chem. 2014, 10, 300–306.
derived glycal 1l [46] yielded hitherto unknown C-allyl glyco- with C-3 substituted products. On the other hand, glucal 1a on
sides 2j and 2l, respectively (see Supporting Information reaction with triethylsilane afforded the product 7 in 43% yield,
File 1). However, the same reaction with benzylidene or while galactal 1b gave a complex mixture of products, which
isopropylidene glucals resulted in a complex mixture of prod- could not be analyzed.
ucts and the desired product was not isolated.
Proposed mechanism
Furthermore, the scope of this reaction was extended by The proposed mechanism of the formation of the C-allyl glyco-
subjecting glycals to reactions with other nucleophiles. Thus, sides is shown in Scheme 1. The ring oxygen could donate an
glycals 1a and 1b subjected to treatment with Me3SiCN electron to the Ce3+ leading to the formation of a radical cation
afforded a 5:1 and 4:1 mixture of glycosyl cyanides 3 and 4 in A. Subsequent migration of the double bond and loss of the
7
7% and 62% yields, respectively (Table 3). The reaction of acetyl radical could result in the formation of the delocalized
glycals 1a and 1b with Me3SiN3 furnished a mixture of C-1 and carbocation B. The acetate radical could accept an electron from
C-3 substituted glycosyl azides (5/5' and 6/6'). This observa- Ce2+ thereby forming an acetate anion, which could in turn
tion is in conformity with the report of Hayashi and co-workers attack the silyl moiety of allyltrimethylsilane leading to the for-
[
47], which described the obtainment of Ferrier products along mation of trimethylsilyl acetate and an allyl anion. The allyl
Table 3: Reaction of glycals with other nucleophiles.
Entry
Glycal
Nucleophile
Product(s)
Time
6 h
Yield
77
Ref.
[42]
1
1a
TMSCN
2
3
1b
1a
TMSCN
TMSN3
12 h
62
69
[42]
3 h
[42,47]
4
5
1b
1a
TMSN3
Et3SiH
4 h
2 h
59
43
[42,47]
[43]
Scheme 1: Proposed mechanism.
303

Beilstein J. Org. Chem. 2014, 10, 300–306.
anion could then attack at the anomeric position so that the benzyl ether by using Bu2SnO and benzyl bromide in the pres-
C-allyl pseudoglycoside is fomed. However, we were not able ence of triethylamine and tetrabutylammonium iodide to afford
to isolate Me3SiOAc. As the acetate ion and the acetyl radical alcohol 8 in 69% yield over 2 steps. The terminal olefin in com-
are better leaving groups the formation of the acyl radical seems pound 8 was then subjected to a Wacker oxidation [60] by using
to be more facile than the formation of the alkoxy radicals (e.g., a catalytic amount of PdCl2 and an excess of CuCl under O2
benzyloxy and methoxy). This explains faster reaction times in atmosphere to obtain the methyl ketone 9 in a good yield. The
the case of acetylated glycals compared to benzyl or methyl formation of the product was confirmed by the disappearance of
glycals. It is well-known in the literature that the participation the external olefinic protons and the appearance of a sharp
of a neighboring group is possible in the case of glucals thereby singlet at δ 2.11, corresponding to methyl protons in the
leading to a higher reactivity than the corresponding galactals 1H NMR spectrum, as well as a peak at δ 207 ppm in the
[
48,49].
13C NMR spectrum corresponding to the carbonyl group (see
Supporting Information File 1). The allylic alcohol 9 was
Having obtained the α-C-glycosides in an efficient manner, we converted to the corresponding trichloroacetimidate, which in
explored their synthetic utility to synthesize a 2-deoxy-2-amino- crude form was heated under reflux in xylene, to afford the
α-C-glycoside. 2-Deoxy-2-amino-α-C-glycosides have received single isomer 10 in 72% yield by an Overman rearrangement
considerable attention in recent years due to their use in the syn- [61-63].
thesis of glycopeptides [50,51], glycolipids [51] and glycosyl
amino acids [52] to name but a few. Amongst the various The trichloroacetamide group was hydrolyzed by heating under
reported methods to prepare these compounds [53-55], the most reflux in 6 N HCl, and the obtained free amine was protected as
common method is via C-glycosylation of 2-aminosugars [56- a benzyloxycarbamate group by using benzyl chloroformate and
5
8], which is challenging owing to the incompatibility of protic Na2CO3. The protected amide 11 was obtained in 74% yield
or Lewis acids with amino and amido functionality. In particu- over 2 steps. The regioselectivity of the Overman rearrange-
lar, very few reports on the synthesis of orthogonally protected ment step was determined from the 1H NMR and COSY experi-
C-glycosides appeared in literature [59]. Therefore, there is a ments of compound 11. These experiments showed that the
need for the development of efficient methods to allow easy anomeric proton (H-1) at δ 4.40 correlated with the H-2 proton
access to this important class of compounds.
at δ 4.00, which in turn was adjacent to the amide proton at
δ 4.85, thereby indicating that the amide moiety is present at the
For this purpose, Ferrier rearranged product 2a was subjected to C-2 position (see Supporting Information File 1). The internal
deacetylation by using K2CO3/MeOH (Scheme 2) followed by olefin was subjected to a dihydroxylation under the Upjohn
the selective protection of the primary hydroxy group as a conditions [64], followed by an acetonide protection of the
Scheme 2: Synthesis of 2-deoxy-2-amino-C-glycoside 12 from Ferrier product 2a.
304

Beilstein J. Org. Chem. 2014, 10, 300–306.
resulting diol by using 2,2-dimethoxypropane and a catalytic
amount of para-toluenesulfonic acid (PTSA) to furnish the
desired orthogonally protected 2-deoxy-2-amino-C-glycoside
Supporting Information
Supporting Information File 1
1
2 as a single isomer in 82% yield (over 2 steps).
Analytical data and copies of the 1H NMR and 13C NMR
spectra of all new compounds.
The stereochemistry of the newly generated stereocenters in
compound 12 was determined with the help of 1H NMR,
COSY, nOe and decoupling experiments (Figure 1). Thus, the
irradiation of the signal at δ 3.93 corresponding to H-3 led to an
[
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-27-S1.pdf]
enhancement of the H-5 signal at δ 4.25 by 2.5%, while the Acknowledgements
peak at δ 4.63, corresponding to H-1, was not enhanced. This We thank the Department of Science and Technology, New
implies that the H-3 proton is cis to H-5 and trans to H-1 and Delhi, for a J. C. Bose National Fellowship (JCB/SR/S2/JCB-
thus axially oriented. Meanwhile, the irradiation of the H-1 26/2010) and the Council of Scientific and Industrial Research,
signal at δ 4.63 did not give any enhancement of the H-3 and New Delhi, for financial support [Grant No. 01(2298)/09/EMR-
H-5 peaks at δ 3.93 and δ 4.25, respectively. Moreover, the II] to Y.D.V. A.A.A. and Y.S.R. gratefully acknowledge a
homonuclear decoupling of the H-3 signal at δ 3.93 gave the Senior Research Fellowship from the Council of Scientific
coupling constant of H-4 and H-5 as J = 7.3 Hz, which indi- Industrial Research, New Delhi.
cates an axial-equatorial relationship. The decoupling of the
H-1' signals at δ 2.55 provided the coupling constant of H-1 and References
H-2 as J = 6.4 Hz, indicating an axial-equatorial relationship
1. Zou, W. Curr. Top. Med. Chem. 2005, 5, 1363–1391.
doi:10.2174/156802605774642999
(
see Supporting Information File 1). This suggests that the
2
.
.
Sutherlin, D. P.; Stark, T. M.; Hughes, R.; Armstrong, R. W.
J. Org. Chem. 1996, 61, 8350–8354. doi:10.1021/jo960119j
Weatherman, R. V.; Mortell, K. H.; Chervenak, M.; Kiessling, L. L.;
Toone, E. J. Biochemistry 1996, 35, 3619–3624.
rearrangement took place from the axial side, as expected, since
the hydroxy group at C-4 is axially oriented in compound 9.
Moreover, the dihydroxylation took place from the side oppo-
site to the amino group at C-2, which can be attributed to the
steric hindrance from the bulky amide group.
3
doi:10.1021/bi951916z
4.
5.
6.
7.
8.
Danishefsky, S. J.; DeNinno, S.; Lartey, P. J. Am. Chem. Soc. 1987,
109, 2082–2089. doi:10.1021/ja00241a028
Paterson, I.; Keown, L. E. Tetrahedron Lett. 1997, 38, 5727–5730.
doi:10.1016/S0040-4039(97)01257-4
Kira, K.; Isobe, M. Tetrahedron Lett. 2000, 41, 5951–5955.
doi:10.1016/S0040-4039(00)00988-6
Gallagher, B. M., Jr.; Zhao, H.; Pesant, M.; Fang, F. G.
Tetrahedron Lett. 2005, 46, 923–926. doi:10.1016/j.tetlet.2004.12.056
Sasaki, M.; Tsubone, K.; Shoji, M.; Oikawa, M.; Shimamoto, K.;
Sakai, R. Bioorg. Med. Chem. Lett. 2006, 16, 5784–5787.
doi:10.1016/j.bmcl.2006.08.082
9.
Fraser-Reid, B. Acc. Chem. Res. 1985, 18, 347–354.
doi:10.1021/ar00119a004
Figure 1: nOe and decoupling experiments of compound 12.
1
0.Kirshning, A.; Chen, G.-w.; Dräger, G.; Schuberth, I.; Tietze, L. F.
Bioorg. Med. Chem. 2000, 8, 2347–2354.
doi:10.1016/S0968-0896(00)00166-8
A D-galacto-configured C-glycoside 12 was obtained from
D-glucal 1a, which may serve as a versatile synthetic intermedi-
ate, since the carbonyl moiety and the double bond function-
ality in this compound can be synthetically manipulated in a
variety of ways.
1
1.Faul, M. M.; Huff, B. E. Chem. Rev. 2000, 100, 2407–2474.
doi:10.1021/cr940210s
12.Dondoni, A.; Marra, A. Chem. Rev. 2000, 100, 4395–4422.
doi:10.1021/cr9903003
1
1
1
1
3.Du, Y.; Linhardt, R. J.; Vlahov, I. R. Tetrahedron 1998, 54, 9913–9959.
doi:10.1016/S0040-4020(98)00405-0
4.Ansari, A. A.; Lahiri, R.; Vankar, Y. D. ARKIVOC 2013, No. ii, 316–362.
doi:10.3998/ark.5550190.0014.223
Conclusion
We have developed an efficient method for the Ferrier
rearrangement of glycals by using ceric ammonium nitrate and
several carbon nucleophiles. We have successfully employed
the obtained C-allyl glycoside 2a for the stereoselective syn-
thesis of a orthogonally protected 2-deoxy-2-amino-C-glyco-
side 12 via an Overman rearrangement as a key step.
5.Ferrier, R. J. J. Chem. Soc. 1964, 5443–5449.
doi:10.1039/JR9640005443
6.Yadav, J. S.; Satyanarayana, M.; Balanarsaiah, E.; Raghavendra, S.
Tetrahedron Lett. 2006, 47, 6095–6098.
doi:10.1016/j.tetlet.2006.06.084
305

Beilstein J. Org. Chem. 2014, 10, 300–306.
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
7.Tiwari, P.; Agnihotri, G.; Misra, A. K. Carbohydr. Res. 2005, 340,
49–752. doi:10.1016/j.carres.2004.12.022
8.De Las Heras, F. G.; Felix, A. S.; Fernández-Resa, P. Tetrahedron
983, 39, 1617–1620. doi:10.1016/S0040-4020(01)88571-9
9.Ghosh, R.; De, D.; Shown, B.; Maiti, S. B. Carbohydr. Res. 1999, 321,
–3. doi:10.1016/S0008-6215(99)00219-0
0.Yadav, J. S.; Reddy, B. V. S.; Chand, P. K. Tetrahedron Lett. 2001, 42,
057–4059. doi:10.1016/S0040-4039(01)00629-3
47.Kawabata, H.; Kubo, S.; Hayashi, M. Carbohydr. Res. 2001, 333,
153–158. doi:10.1016/S0008-6215(01)00128-8
48.Ferrier, R. J.; Zubkov, O. A. Org. React. 2004, 569–736.
49.Gómez, A. M.; Lobo, F.; Uriel, C.; López, J. C. Eur. J. Org. Chem.
2013, 7221–7262. doi:10.1002/ejoc.201300798
See for a recent review.
7
1
1
50.Wang, L.-X.; Fan, J.-Q.; Lee, Y. C. Tetrahedron Lett. 1996, 37,
1975–1978. doi:10.1016/0040-4039(96)00261-4
51.McGarvey, G. J.; Benedum, T. E.; Schmidtmann, F. W. Org. Lett. 2002,
4, 3591–3594. doi:10.1021/ol0264861
4
1.Balamurugan, R.; Koppolu, S. R. Tetrahedron 2009, 65, 8139–8142.
doi:10.1016/j.tet.2009.07.087
2.Takhi, M.; Rahman, A. A.-H. A.; Schmidt, R. R. Tetrahedron Lett. 2001,
52.Westermann, B.; Walter, A.; Diedrichs, N. Angew. Chem., Int. Ed.
1999, 38, 3384–3386.
4
2, 4053–4056. doi:10.1016/S0040-4039(01)00628-1
3.Danishefsky, S. J.; Kerwin, J. F., Jr. J. Org. Chem. 1982, 47,
803–3805. doi:10.1021/jo00140a053
4.Ichikawa, Y.; Isobe, M.; Konobe, M.; Goto, T. Carbohydr. Res. 1987,
71, 193–199. doi:10.1016/S0008-6215(00)90886-3
5.Dawe, R. D.; Fraser-Reid, B. J. Chem. Soc., Chem. Commun. 1981,
180–1181. doi:10.1039/C39810001180
doi:10.1002/(SICI)1521-3773(19991115)38:22<3384::AID-ANIE3384>3
.0.CO;2-7
3
53.Cipolla, L.; La Ferla, B.; Lay, L.; Peri, F.; Nicotra, F.
Tetrahedron: Asymmetry 2000, 11, 295–303.
1
doi:10.1016/S0957-4166(99)00480-2
1
54.Cipolla, L.; Lay, L.; Peri, F.; Nicotra, F. J. Org. Chem. 1997, 62,
6678–6681. doi:10.1021/jo970127f
6.Gemmell, N.; Meo, P.; Osborn, H. M. I. Org. Lett. 2003, 5, 1649–1652.
doi:10.1021/ol030023t
55.Petrušová, M.; BeMiller, J. N.; Petruš, L. Tetrahedron Lett. 1996, 37,
2341–2344. doi:10.1016/0040-4039(96)00280-8
56.SanMartin, R.; Tavassoli, B.; Walsh, K. E.; Walter, D. S.; Gallagher, T.
Org. Lett. 2000, 2, 4051–4054. doi:10.1021/ol006682c
57.Urban, D.; Skrydstrup, T.; Beau, J.-M. Chem. Commun. 1998,
955–956. doi:10.1039/a801196f
7.Steinhuebel, D. P.; Fleming, J. J.; Du Bois, J. Org. Lett. 2002, 4,
2
93–295. doi:10.1021/ol010273e
8.Hanessian, S.; Pernet, A. G. Adv. Carbohydr. Chem. Biochem. 1976,
3, 111–188. doi:10.1016/S0065-2318(08)60281-4
3
9.Daves, G. D., Jr.; Cheng, C. C. Prog. Med. Chem. 1976, 13, 303–349.
doi:10.1016/S0079-6468(08)70141-3
58.Urban, D.; Skrydstrup, T.; Beau, J.-M. J. Org. Chem. 1998, 63,
2507–2516. doi:10.1021/jo971727h
0.Poonian, M. S.; Nowoswiat, E. F. J. Org. Chem. 1980, 45, 203–208.
doi:10.1021/jo01290a002
59.Bouvet, V. R.; Ben, R. N. J. Org. Chem. 2006, 71, 3619–3622.
doi:10.1021/jo051938j
1.Gupta, A.; Vankar, Y. D. Tetrahedron 2000, 56, 8525–8531.
doi:10.1016/S0040-4020(00)00775-4
And references therein.
2.Pachamuthu, K.; Vankar, Y. D. J. Org. Chem. 2001, 66, 7511–7513.
doi:10.1021/jo0103322
60.Tsuji, J. Palladium Reagents and Catalysts; Wiley: Chichester, 2004;
pp 29–35.
3.Agarwal, A.; Rani, S.; Vankar, Y. D. J. Org. Chem. 2004, 69,
61.Overman, L. E. J. Am. Chem. Soc. 1976, 98, 2901–2910.
doi:10.1021/ja00426a038
6137–6140. doi:10.1021/jo049415j
4.Babu, J. L.; Khare, A.; Vankar, Y. D. Molecules 2005, 10, 884–892.
doi:10.3390/10080884
62.Mercer, G. J.; Yang, J.; McKay, M. J.; Nguyen, H. M.
J. Am. Chem. Soc. 2008, 130, 11210–11218. doi:10.1021/ja803378k
63.Gupta, P.; Vankar, Y. D. Eur. J. Org. Chem. 2009, 1925–1933.
doi:10.1002/ejoc.200801301
5.Gupta, P.; Kumari, N.; Agarwal, A.; Vankar, Y. D. Org. Biomol. Chem.
2
008, 6, 3948–3956. doi:10.1039/b810654a
6.Jayakanthan, K.; Vankar, Y. D. Carbohydr. Res. 2005, 340,
688–2692. doi:10.1016/j.carres.2005.07.024
64.VanRheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett. 1976, 17,
1973–1976. doi:10.1016/S0040-4039(00)78093-2
2
7.Sridharan, V.; Menéndez, J. C. Chem. Rev. 2010, 110, 3805–3849.
doi:10.1021/cr100004p
8.Nair, V.; Balagopal, L.; Rajan, R.; Mathew, J. Acc. Chem. Res. 2004,
License and Terms
3
7, 21–30. doi:10.1021/ar030002z
9.Agarwal, A.; Vankar, Y. D. Proc. Indian Natl. Sci. Acad., Part A 2005,
1, 309–326.
0.Linker, T.; Sommermann, T.; Kahlenberg, F. J. Am. Chem. Soc. 1997,
19, 9377–9384. doi:10.1021/ja971084n
1.Yin, J.; Sommermann, T.; Linker, T. Chem.–Eur. J. 2007, 13,
0152–10167. doi:10.1002/chem.200701151
This is an Open Access article under the terms of the
Creative Commons Attribution License
7
(http://creativecommons.org/licenses/by/2.0), which
1
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
1
2.Yadav, J. S.; Reddy, B. V. S. Synthesis 2002, 511–514.
doi:10.1055/s-2002-20968
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
3.Ghosh, R.; Chakraborty, A.; Maiti, S. B. ARKIVOC 2004, xiv, 1–9.
doi:10.3998/ark.5550190.0005.e01
(http://www.beilstein-journals.org/bjoc)
4.Lin, H.-C.; Du, W.-P.; Chang, C.-C.; Lin, C.-H. Tetrahedron Lett. 2005,
4
6, 5071–5076. doi:10.1016/j.tetlet.2005.05.061
5.Hosokawa, S.; Kirschbaum, B.; Isobe, M. Tetrahedron Lett. 1998, 39,
917–1920. doi:10.1016/S0040-4039(98)00047-1
The definitive version of this article is the electronic one
which can be found at:
1
6.Brawn, R. A.; Panek, J. S. Org. Lett. 2010, 12, 4624–4627.
doi:10.3762/bjoc.10.27
doi:10.1021/ol1019629
306