Short synthesis of the common trisaccharide core of kankanose and kankanoside isolated from Cistanche tubulosa

Article abstract of DOI:10.3762/bjoc.9.80

A short synthetic approach was developed for the synthesis of a common trisaccharide core found in kankanose, kankanoside F, H₁, H ₂, and I isolated from the medicinally active plant Cistanche tubulosa. All glycosylations were carried out under nonmetallic reaction conditions. Yields were very good in all intermediate steps.

Full text of DOI:10.3762/bjoc.9.80

Short synthesis of the common trisaccharide core of
kankanose and kankanoside isolated
from Cistanche tubulosa
Goutam Guchhait and Anup Kumar Misra*§
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 705–709.
Bose Institute, Division of Molecular Medicine, P-1/12, C.I.T. Scheme
VII M, Kolkata 700054, India
doi:10.3762/bjoc.9.80
Received: 09 January 2013
Accepted: 20 March 2013
Published: 11 April 2013
Email:
Anup Kumar Misra* - akmisra69@gmail.com
* Corresponding author
§ Fax: 91-33-2355 3886
Associate Editor: S. Bräse
© 2013 Guchhait and Misra; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
Cistanche tubulosa; glycosylation; kankanoside; synthesis;
trisaccharide
Abstract
A short synthetic approach was developed for the synthesis of a common trisaccharide core found in kankanose, kankanoside F, H1,
H2, and I isolated from the medicinally active plant Cistanche tubulosa. All glycosylations were carried out under nonmetallic reac-
tion conditions. Yields were very good in all intermediate steps.
Introduction
Cistanche tubulosa (C. tubulosa), an Orobanchaceae parasitic C. tubulosa has been used in the folk medicine for several
plant found in Africa, Asia and Arabia, has been traditionally years, it is beneficial to find out the biological activities of the
used as folk medicine and tonic for the treatment of blood- individual compounds present in the C. tubulosa extracts. In
circulation-related disorders, impotence, sterility and order to establish the detailed medicinal potential of individual
body weakness [1-3]. A significant number of bioactive com- components, it is essential to have higher quantities of the com-
pounds have been isolated from C. tubulosa and have shown pounds, which are difficult to isolate from the plant source.
promising medicinal activity such as hepatoprotective and Therefore, development of concise chemical synthetic strate-
vasorelaxant activities [4-6]. Most of the compounds isolated gies would be the best option to gain access to these com-
from C. tubulosa and related species are phenylethyl oligo- pounds on a large scale. A few reports are available in the
saccharides, iridoids, terpenes and lignans [4-7]. Recently, literature for the synthesis of phenylethyl oligosaccharides
Yoshikawa et al. isolated and characterized a significant [8,9]. In this context, we developed a synthetic strategy for the
number of phenylethyl oligosaccharides, which include synthesis of the common trisaccharide core of kankanose,
kankanose, kankanoside F, H1, H2, I, etc. [4,5]. Since kankanoside F, H1, H2 and I isolated from C. tubulosa thereby
705

Beilstein J. Org. Chem. 2013, 9, 705–709.
exploiting newly developed regio- and stereoselective glycosyl- selective 6-O-glycosylation of compound 7 with D-glucose-
ation conditions (Figure 1). This straightforward synthetic derived trichloroacetimidate derivative 4 [12] furnished tri-
strategy employs a minimum number of steps.
saccharide derivative 8 in 71% yield, which was confirmed by
the spectral analysis [signals at δ 4.75 (br s, H-1B), 4.54 (d,
J = 8.0 Hz, H-1C), 4.22 (d, J = 8.0 Hz, H-1A) in the 1H NMR
Results and Discussion
The target trisaccharide 1 in the form of its 2-phenylethyl glyco- and at δ 100.8 (C-1A), 100.5 (C-1C), 98.8 (C-1B) in the
side was synthesized from the suitably functionalized monosac- 13C NMR spectra]. Saponification of compound 8 by using
charide derivatives 2 [10], 3 [11], and 4 [12], which were 0.1 M sodium methoxide in methanol furnished compound 1 in
prepared from the commercially available reducing sugars 94% yield. Spectral analysis of compound 1 unambiguously
(Figure 1). The key features of this synthetic strategy are (a) the confirmed its formation [signals at δ 5.16 (br s, H-1B), 4.37 (d,
application of two regioselective glycosylations by using J = 7.5 Hz, H-1A), 4.32 (d, J = 8.0 Hz, H-1C) in the 1H NMR
glycosyl acceptors with two hydroxy groups; (b) application of and at δ 103.5 (C-1A), 102.9 (C-1C), 101.3 (C-1B) in the
molecular iodine to the functional group transformations [13]; 13C NMR spectra] (Scheme 1).
and (c) activation of the glycosyl trichloroacetimidate deriva-
tive by using nitrosyl tetrafluoroborate (NOBF4) [14].
Conclusion
In summary, a straightforward synthetic strategy was devel-
The treatment of D-glucose pentaacetate with 2-phenylethanol oped for the prompt synthesis of a common trisaccharide core
in the presence of borontrifluoride diethyl etherate furnished of the kankanose, kankanoside F, H1, H2 and I isolated from the
2-phenylethyl 2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (2) in extract of C. tubulosa. All the steps are high yielding, and
84% yield [10]. Saponification of compound 2 by using 0.1 M glycosylations were highly regio- and stereoselective. Because
sodium methoxide in methanol followed by benzylidene acetal of the simplicity of the synthetic strategy, it can be applied in a
formation by using benzaldehyde dimethylacetal in the pres- scaled-up preparation.
ence of molecular iodine [13] furnished compound 5 in 86%
Experimental
yield. Regioselective 3-O-glycosylation of compound 5 with
L-rhamnose derived trichloroacetimidate derivative 3 [11] in General methods
the presence of NOBF4 [14] followed by acetylation in the same All reactions were monitored by thin-layer chromatography
pot furnished disaccharide derivative 6 in 76% yield. In this with silica-gel-coated TLC plates. The spots on TLC were visu-
case, NOBF4 acts as a promoter for the activation of the alized by warming ceric sulfate (2% Ce(SO4)2 in 2 N H2SO4)
glycosyl trichloroacetimidate derivative as well as the acetyla- sprayed plates on a hot plate. Silica gel 230–400 mesh was used
tion of the sugar derivative with acetic anhydride. The forma- for column chromatography. 1H and 13C NMR spectra were
tion of compound 6 was confirmed by its spectral analysis recorded on a Bruker Avance 500 MHz by using CDCl3 as the
[signals at δ 5.44 (s, PhCH), 4.79 (br s, H-1B), 4.37 (d, solvent and TMS as the internal reference unless stated other-
J = 7.5 Hz, H-1A in the 1H NMR and at δ 101.9 (PhCH), 101.3 wise. Chemical shifts δ are expressed in parts per million
(C-1A), 97.4 (C-1B) in the 13C NMR spectra]. Removal of the (ppm). ESIMS were recorded on a Micromass mass spectrom-
benzylidene acetal group under neutral conditions by using eter. Optical rotations were recorded in a Jasco P-2000 spec-
triethylsilane and Pd/C [15] resulted in the formation of disac- trometer. Commercially available grades of organic solvents of
charide diol 7 in 80% yield. NOBF4 catalyzed regio- and stereo- adequate purity were used in all reactions.
Figure 1: Structure of the synthesized trisaccharide core found in kankanose, kankanoside F, H1, H2 and I isolated from Cistanche tubulosa.
706

Beilstein J. Org. Chem. 2013, 9, 705–709.
Scheme 1: Reagents: (a) 0.1 M CH3ONa, CH3OH, room temperature, 3 h, 98% for compound 5, 94% for compound 1; (b) PhCH(OCH3)2, I2, CH3CN,
room temperature, 1.5 h, 86%; (c) NOBF4, CH2Cl2, −20 °C, 20 min; (d) acetic anhydride, room temperature, 1 h, 76% in two steps; (e) Et3SiH,
10% Pd/C, CH3OH/CH2Cl2 (1:1, v/v), room temperature, 30 min, 80%; (f) NOBF4, CH2Cl2, −35 °C, 30 min, 71%.
2-Phenylethyl 4,6-O-benzylidene-β-D-glucopyranoside (5): A tion of the starting material (TLC; hexane/EtOAc 4:1), acetic
solution of compound 2 (5.0 g, 11.05 mmol) in 0.1 M CH3ONa anhydride (3 mL) was added to the reaction mixture, and the
in CH3OH (70 mL) was stirred at room temperature for 3 h and mixture was stirred at room temperature for 1 h. The reaction
was neutralized with Amberlite IR-120 (H+) resin. The reaction mixture was diluted with CH2Cl2 (100 mL) and the organic
mixture was filtered and concentrated under reduced pressure. layer was washed with satd. NaHCO3 and water, dried
To a solution of the deacetylated product (3.1 g) in CH3CN (Na2SO4) and concentrated to a crude product, which was puri-
(10 mL) was added benzaldehyde dimethylacetal (2.5 mL, fied over SiO2 by using hexane/EtOAc (3:1) as an eluant to give
16.65 mmol) followed by molecular iodine (0.3 g, 1.18 mmol), the pure product 6 (4.2 g, 76%). Yellow oil; [α]D25 −60 (c 1.2,
and the reaction mixture was stirred at room temperature for CHCl3); 1H NMR (500 MHz, CDCl3) δ 7.39–7.11 (m, 10H,
1.5 h. The reaction mixture was evaporated and co-evaporated Ar-H), 5.44 (s, 1H, PhCH), 5.23 (dd, J = 10.0, 3.5 Hz, 1H,
with toluene (3 × 20 mL) under reduced pressure to give the H-3B), 4.97 (t, J = 8.0 Hz, 1H, H-2A), 4.89 (br s, 1H, H-2B),
crude product, which was purified over SiO2 by using hexane/ 4.85 (t, J = 10.0 Hz, 1H, H-4B), 4.79 (br s, 1H, H-1B), 4.37
EtOAc (2:1) as an eluant to give pure compound 5 (3.5 g, 86%). (d, J = 7.5 Hz, 1H, H-1A), 4.27 (dd, J = 10.5, 5.0 Hz, 1H,
White solid; mp 158–160 °C (EtOH); [α]D25 −16 (c 1.2, H-6aA), 4.04–3.97 (m, 2H, H-5A, CH2), 3.78 (t, J = 9.5 Hz, 1H,
CHCl3); 1H NMR (500 MHz, CDCl3) δ 7.48–7.21 (m, 10H, H-3A), 3.69 (t, J = 10.0 Hz, 1H, H-6bA), 3.60–3.54 (m, 2H,
Ar-H), 5.50 (s, 1H, PhCH), 4.35 (d, J = 7.7 Hz, 1H, H-1), 4.32 H-4A, CH2), 3.39–3.36 (m, 1H, H-5B), 2.81–2.78 (m, 2H, CH2),
(dd, J = 10.5, 4.9 Hz, 1H, H-6a), 4.17–4.12 (m, 1H, CH2), 2.01, 1.91, 1.89, 1.88 (4 s, 12H, 4 COCH3), 0.59 (d, J = 6.0 Hz,
3.78–3.72 (m, 3H, H-3, H-6b, CH2), 3.52 (t, J = 9.3 Hz, 1H, 3H, CCH3); 13C NMR (125 MHz, CDCl3) δ 170.0, 169.9,
H-4), 3.47–3.39 (m, 2H, H-2, H-5), 2.99–2.93 (m, 2H, CH2), 169.8, 169.4 (4 COCH3), 138.5–126.3 (Ar-C), 101.9 (PhCH),
2.90 (br s, 1H, OH), 2.54 (br s, 1H, OH); 13C NMR (125 MHz, 101.3 (C-1A), 97.4 (C-1B), 79.0 (C-4A), 76.8 (C-3A), 73.3
CDCl3) δ 138.1–126.2 (Ar-C), 103.3 (C-1), 101.9 (PhCH), 80.5 (C-2A), 71.3 (C-4B), 70.6 (C-2B), 70.5 (CH2), 68.7 (C-6A), 68.4
(C-5), 74.5 (C-3), 72.9 (C-4), 71.1 (C-2), 68.6 (CH2), 66.4 (C-3B), 66.6 (C-5B), 66.2 (C-5A), 36.0 (CH2), 20.9, 20.8, 20.7,
(C-6), 36.1 (CH2); ESIMS: 395.1 [M + Na]+; Anal. calcd for 20.6 (4 COCH3), 16.5 (CH3); ESIMS: 709.2 [M + Na]+; Anal.
C21H24O6: C, 67.73; H, 6.50; found: C, 67.57; H, 6.65.
calcd for C35H42O14: C, 61.22; H, 6.16; found: C, 61.05; H,
6.35.
2-Phenylethyl (2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl)-
(1→3)-2-O-acetyl-4,6-O-benzylidene-β-D-glucopyranoside 2-Phenylethyl (2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl)-
(6): A solution of compound 5 (3.0 g, 8.06 mmol) and com- (1→3)-2-O-acetyl-β-D-glucopyranoside (7): To a stirred solu-
pound 3 (3.6 g, 8.28 mmol) in anhydrous CH2Cl2 (15 mL) was tion of compound 6 (4.0 g, 5.82 mmol) and 10% Pd/C (0.5 g) in
cooled to −20 °C under argon. To the cooled reaction mixture CH3OH/CH2Cl2 (15 mL, 1:1, v/v) was added Et3SiH (2.8 mL,
was added NOBF4 (1.0 g, 8.56 mmol), and the reaction mixture 17.53 mmol) dropwise, and the reaction mixture was stirred for
was stirred at the same temperature for 20 min. After consump- 30 min at room temperature. The reaction mixture was filtered
707

Beilstein J. Org. Chem. 2013, 9, 705–709.
through a Celite® bed, and the filtering bed was washed with 68.5 (C-4C), 68.2 (C-4B), 67.5 (C-3B), 61.7 (C-6C), 35.9 (CH2),
CH2Cl2 (50 mL). The combined filtrate was concentrated under 20.9, 20.8, 20.7 (3 C), 20.6 (3 C) (8 COCH3), 17.3 (CH3);
reduced pressure to give the crude product, which was purified ESIMS: 951.3 [M + Na]+; Anal. calcd for C42H56O23: C, 54.31;
over SiO2 by using hexane/EtOAc (1:1) as an eluant to give H, 6.08; found: C, 54.18; H, 6.25.
pure compound 7 (2.8 g, 80%). Yellow oil; [α]D25 −30 (c 1.2,
CHCl3); 1H NMR (500 MHz, CDCl3) δ 7.28–7.18 (m, 5H, 2-Phenylethyl (α-L-rhamnopyranosyl)-(1→3)-[β-D-glucopy-
Ar-H), 5.23 (dd, J = 10.0, 3.5 Hz, 1H, H-3B), 5.10–5.09 (m, 1H, ranosyl)-(1→6)]-β-D-glucopyranoside (1): A solution of com-
H-2B), 5.05 (t, J = 10.0 Hz, 1H, H-4B), 4.94 (t, J = 8.0 Hz, 1H, pound 8 (2.0 g, 2.15 mmol) in 0.1 M CH3ONa in CH3OH
H-2A), 4.87 (d, J = 1.8 Hz, 1H, H-1B), 4.41 (d, J = 8.0 Hz, 1H, (50 mL) was stirred at room temperature for 3 h and neutral-
H-1A), 4.16–4.06 (m, 2H, CH2), 3.92–3.86 (m, 1H, H-6aA), ized with Amberlite IR-120 (H+) resin. The reaction mixture
3.82–3.78 (m, 1H, H-6bA), 3.69–3.61 (m, 2H, H-3A, H-5B), was filtered and concentrated to give the crude product, which
3.58 (t, J = 9.0 Hz, 1H, H-4A), 3.54–3.31 (m, 1H, H-5A), was purified over Sephadex® LH-20 gel by using CH3OH/H2O
2.91–2.83 (m, 2H, CH2), 2.13, 2.04, 2.01, 1.98 (4 s, 12H, (10:1) as an eluant to give pure compound 1 (1.2 g, 94%).
4 COCH3), 1.21 (d, J = 6.0 Hz, 3H, CH3); 13C NMR White powder; [α]D25 −11 (c 1.2, CH3OH); 1H NMR
(125 MHZ, CDCl3) δ 170.0, 169.9, 169.7, 169.5 (4 COCH3), (500 MHz, CD3OD) δ 7.26–7.16 (m, 5H, Ar-H), 5.16 (br s, 1H,
138.5–126.3 (Ar-C), 100.7 (C-1A), 98.8 (C-1B), 84.8 (C-4A), H-1B), 4.37 (d, J = 7.5 Hz, 1H, H-1A), 4.32 (d, J = 8.0 Hz, 1H,
75.2 (C-3A), 71.4 (C-2A), 70.7 (C-4B), 70.4 (C-2B), 69.9 (2C, H-1C), 4.15–4.13 (m, 1H, H-6aC), 4.10–4.04 (m, 1H, CH2),
C-5B, CH2), 68.6 (C-3B), 67.7 (C-5A), 62.2 (C-6A), 36.0 (CH2), 4.03–3.98 (m, 1H, H-5B), 3.95–3.92 (m, 1H, H-2B), 3.88–3.82
20.9, 20.8, 20.7, 20.6 (4 COCH3), 17.4 (CH3); ESIMS: 621.2 (m, 1H, H-6aA), 3.81–3.73 (m, 2H, H-6bC, CH2), 3.70 (dd,
[M + Na]+; Anal. calcd for C28H38O14: C, 56.18; H, 6.40; J = 10.0, 3.5 Hz, 1H, H-3B), 3.68–3.64 (m, 1H, H-6bA), 3.50 (t,
found: C, 56.05; H, 6.55.
J = 10.0 Hz, 1H, H-4A), 3.48–3.44 (m, 2H, H-4B, H-5A), 3.39
(t, J = 10.0 Hz, 1H, H-4C), 3.30–3.35 (m, 4H, H-2A, H-3A,
2-Phenylethyl (2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl)- H-3C, H-5C), 3.21 (t, J = 9.0 Hz, 1H, H-2C), 2.94–2.90 (m, 2H,
(1→3)-[2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)-(1→6)]- CH2), 1.25 (d, J = 6.0 Hz, 3H, CH3); 13C NMR (125 MHz,
2-O-acetyl-β-D-glucopyranoside (8): A solution of compound CD3OD) δ 138.6–125.8 (Ar-C), 103.5 (C-1A), 102.9 (C-1C),
4 (1.7 g, 3.45 mmol) and compound 7 (2.0 g, 3.34 mmol) in an- 101.3 (C-1B), 82.6 (C-4A), 76.6 (2C, C-3A, C-3C), 75.6 (C-5A),
hydrous CH2Cl2 (20 mL) was cooled to −35 °C under argon. To 74.3 (C-5C), 73.7 (C-2C), 72.6 (C-4C), 71.0 (C-4B), 70.9
the cooled reaction mixture was added NOBF4 (410 mg, (C-2A), 70.6 (CH2), 70.2 (C-2B), 68.6 (C-3B), 68.5 (C-5B), 68.3
3.51 mmol), and the reaction mixture was stirred at the same (C-6C), 61.3 (C-6A), 35.8 (CH2), 16.5 (CH3); ESIMS: 615.2 [M
temperature for 30 min. The reaction mixture was diluted with + Na]+; Anal. calcd for C26H40O15: C, 52.70; H, 6.80; found: C,
CH2Cl2 (100 mL), and the organic layer was washed with satd. 52.56; H, 7.0.
NaHCO3 and water, dried (Na2SO4), and concentrated to a
crude product, which was purified over SiO2 by using hexane/
Supporting Information
EtOAc (3:1) as an eluant to give the pure product 8 (2.2 g,
71%). Yellow oil; [α]D25 −32 (c 1.2, CHCl3); 1H NMR
Supporting Information File 1
(500 MHz, CDCl3) δ 7.17–7.07 (m, 5H, Ar-H), 5.12 (dd,
J = 10.0, 3.5 Hz, 1H, H-3B), 5.07 (t, J = 10.0 Hz, 1H, H-3C),
4.99–4.93 (m, 3H, H-2B, H-4B, H-4C), 4.89 (t, J = 10.0 Hz, 1H,
H-2C), 4.82 (t, J = 10.0 Hz, 1H, H-2A), 4.75 (br s, 1H, H-1B),
4.54 (d, J = 8.0 Hz, 1H, H-1C), 4.22 (d, J = 8.0 Hz, 1H, H-1A),
4.16 (dd, J = 12.0, 5.0 Hz, 1H, H-6aC), 4.07–3.96 (m, H-6aA,
H-6bC, CH2), 3.69–3.65 (m, 1H, H-6bA), 3.60–3.56 (m, 1H,
H-5A), 3.55–3.48 (m, 1H, CH2), 3.43 (t, J = 10.0 Hz, 1H,
H-3A), 3.42–3.35 (m, 1H, H-5C), 3.33–3.31 (m, 1H, H-5B),
2.77–2.74 (m, 2H, CH2), 2.08, 2.07, 2.01, 2.00, 1.99, 1.96, 1.94,
1.90 (8 s, 24H, 8 COCH3), 1.10 (d, J = 6.0 Hz, 3H, CH3);
13C NMR (125 MHz, CDCl3) δ 170.7, 170.2, 170.0, 169.9,
169.6, 169.4, 169.3, 169.2 (8 COCH3), 138.6–126.2 (Ar-C),
100.8 (C-1A), 100.5 (C-1C), 98.8 (C-1B), 84.3 (C-5C), 74.8
(C-5B), 72.7 (C-3C), 71.9 (C-4A), 71.5 (C-2B), 71.1 (C-5A),
70.8 (C-2C), 70.3 (CH2), 70.2 (C-2A), 70.0 (C-3A), 68.7 (C-6A),
1H NMR and 13C NMR spectra of compounds 1, 2, 5, 6, 7
and 8.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-80-S1.pdf]
Acknowledgements
G. G. thanks CSIR, New Delhi for providing a Senior Research
Fellowship. This work was supported by CSIR, New Delhi
[Project No. 02 (0038)/11/EMR-II].
References
1. Kobayashi, H.; Oguchi, H.; Takizawa, N.; Miyase, T.; Ueno, A.;
Usmanghani, K.; Ahmad, M. Chem. Pharm. Bull. 1987, 35, 3309–3314.
doi:10.1248/cpb.35.3309
708

Beilstein J. Org. Chem. 2013, 9, 705–709.
2. Xinjiang Institute of Traditional Chinese and Ethnologic Medicines., Ed.
Culture Techniques of Xinjiang Staple Medicinal Plants; Xinjiang
Science and Technology Press, 2004; pp 84–88.
3. Muraoka, O.; Morikawa, T.; Zhang, Y.; Ninomiya, K.; Nakamura, S.;
Matsuda, H.; Yoshikawa, M. Tetrahedron 2009, 65, 4142–4148.
doi:10.1016/j.tet.2009.03.040
4. Morikawa, T.; Pan, Y.; Ninomiya, K.; Imura, K.; Matsuda, H.;
Yoshikawa, M.; Yuan, D.; Muraoka, O. Bioorg. Med. Chem. 2010, 18,
1882–1890. doi:10.1016/j.bmc.2010.01.047
5. Yoshikawa, M.; Matsuda, H.; Morikawa, T.; Xie, H.; Nakamura, S.;
Muraoka, O. Bioorg. Med. Chem. 2006, 14, 7468–7475.
doi:10.1016/j.bmc.2006.07.018
6. Xie, H.; Morikawa, T.; Matsuda, H.; Nakamura, S.; Muraoka, O.;
Yoshikawa, M. Chem. Pharm. Bull. 2006, 54, 669–675.
doi:10.1248/cpb.54.669
7. Jiménez, C.; Riguera, R. Nat. Prod. Rep. 1994, 11, 591–606.
doi:10.1039/np9941100591
And references therein.
8. Das, S. K.; Reddy, K. A.; Mukkanti, K. Carbohydr. Res. 2007, 342,
2309–2315. doi:10.1016/j.carres.2007.06.022
9. Zhou, F.-Y.; She, J.; Wang, Y.-G. Carbohydr. Res. 2006, 341,
2469–2477. doi:10.1016/j.carres.2006.08.006
10.Sinha, B.; Pansare, V. S. Indian J. Chem., Sect. B 1980, 19B,
825–826.
11.van Steijn, A. M. P.; Kamerling, J. P.; Vliegenthart, J. F. G.
Carbohydr. Res. 1991, 211, 261–277.
doi:10.1016/0008-6215(91)80096-6
12.Schmidt, R. R.; Michel, J.; Roos, M. Liebigs Ann. Chem. 1984,
1343–1357. doi:10.1002/jlac.198419840710
13.Panchadhayee, R.; Misra, A. K. J. Carbohydr. Chem. 2008, 27,
148–155. doi:10.1080/07328300802030837
14.Sau, A.; Santra, A.; Misra, A. K. Synlett 2012, 2341–2348.
doi:10.1055/s-0032-1317135
15.Santra, A.; Ghosh, T.; Misra, A. K. Beilstein J. Org. Chem. 2013, 9,
74–78. doi:10.3762/bjoc.9.9
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.9.80
709