Efficient acetylation of carbohydrates promoted by imidazole

Article abstract of DOI:10.1002/ejoc.200500555

An efficient per-O-acetylation of carbohydrate derivatives and unprotected reducing sugars promoted by imidazole is reported. The reaction conditions have been successfully employed to acetylate carbohydrate derivatives containing acid-susceptible functional groups. In most of the cases the yields obtained were excellent. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejoc.200500555

SHORT COMMUNICATION
Efficient Acetylation of Carbohydrates Promoted by Imidazole[‡]
Pallavi Tiwari,[a] Rishi Kumar,[b] Prakas R. Maulik,[b] and Anup Kumar Misra*[a]
Dedicated to Professor B. P. Chatterjee[‡‡]
Keywords: Carbohydrate / Acylation / Synthetic methods / Protecting groups / Imidazole
An efficient per-O-acetylation of carbohydrate derivatives
and unprotected reducing sugars promoted by imidazole is
susceptible functional groups. In most of the cases the yields
obtained were excellent.
reported. The reaction conditions have been successfully em- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ployed to acetylate carbohydrate derivatives containing acid-
Germany, 2005)
carbohydrates using a stoichiometric quantity of acetic an-
hydride in the presence of HClO4/SiO2. Although, above-
mentioned reagents catalyze the acetylation efficiently, most
of them are incompatible for the acetylation of carbo-
hydrate derivatives containing acid-sensitive functional
groups. In most of the instances, acetic anhydride is being
used in an excess quantity, which requires extra efforts for
its neutralization in large-scale preparations of acetylated
carbohydrate derivatives. Besides this, acetylation of unpro-
tected reducing sugars resulted an isomerized product mix-
tures of pyranose and furanose in many occasions. There-
fore, a search for a new mild, less toxic and efficient catalyst
for acetylation of carbohydrates minimizing isomerization
and loss of sensitive functional groups is still continuing.
Prompted by a recent report[24] on the synthesis of esters
of simple alcohols catalyzed by imidazole under microwave
conditions, we have explored the catalytic potential of imi-
dazole in the acetylation of carbohydrates particularly with
those having acid-sensitive functionalities. In this report we
disclose an efficient economical method for the acetylation
of carbohydrate derivatives using acetic anhydride pro-
moted by imidazole.
Introduction
One of the most commonly used techniques for the pro-
tection of hydroxy groups in the synthesis of oligosaccha-
rides is acetylation. In carbohydrate chemistry, per-O-acety-
lated sugars are inexpensive and useful intermediates for the
synthesis of several natural products containing glycosides,
oligosaccharides and other glycoconjugates.[1] The most
often used protocol for the acetylation of sugar alcohols
employs a large excess of acetic anhydride and pyridine as
solvent and activator despite its toxicity and unpleasant
odor.[2,3] In some cases, pyridine derivatives, such as, 4-(di-
methylamino)pyridine and 4-(pyrrolidino)pyridine have
been added to the reaction as co-catalyst to speed up the
acetylation reaction.[4,5] Besides some classical reaction pro-
tocols for the acetylation of carbohydrates,[6–8] a variety of
reagents have been developed for the acetylation of carbo-
hydrate derivatives avoiding the use of pyridine which in-
cludes several Lewis acid catalysts[9–14] and a number of
heterogeneous catalysts.[15–18] ZnCl2/sodium acetate combi-
[20]
nation[19] or InCl3
with acetic anhydride under micro-
wave conditions have also been reported recently for the
acetylation of carbohydrates. Few reports have also ap-
peared on the acetylation of carbohydrates using ionic li-
quids as solvents and catalysts.[21,22] Considering the fact
that solid supported catalysts have extra advantages over
homogeneous catalysts in terms of purification of the pro-
ducts, we have recently reported[23] per-O-acetylation of
Results and Discussion
Imidazole is well known for its use in several organic
reactions.[25–31] It has a relatively lower toxicity than some
[‡] CDRI communication no. 6809.
[a] Medicinal and Process Chemistry Division, Central Drug Re-
search Institute,
Chattar Manzil Palace, Lucknow 226001, UP, India
[b] Molecular and Structural Biology Division, Central Drug Re-
search Institute,
Chattar Manzil Palace, Lucknow 226001, UP, India
E-mail: akmisra69@rediffmail.com
[‡‡]Indian Association for the Cultivation of Science, Jadavpur,
Kolkata, India.
Scheme 1.
Eur. J. Org. Chem. 2005, 4265–4270
DOI: 10.1002/ejoc.200500555
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4265

P. Tiwari, R. Kumar, P. R. Maulik, A. K. Misra
Table 1. Acetylation of carbohydrate derivatives using Ac2O (1.2 equiv. per OH) and imidazole (0.6 equiv.) at room temperature.
SHORT COMMUNICATION
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Efficient Acetylation of Carbohydrates Promoted by Imidazole
SHORT COMMUNICATION
Table 1. (continued)
[a] 6.0 equiv. of Ac2O used. [b] 20 equiv. of Ac2O used. [c] 15 equiv. of Ac2O used.
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P. Tiwari, R. Kumar, P. R. Maulik, A. K. Misra
SHORT COMMUNICATION
widely used acetylation catalysts, such as pyridine, 4-(di- ported in the cited references. In the case of reducing
methylamino)pyridine (DMAP), etc. Earlier, N-acylimida- sugars, per-O-acetylation gave a mixture of α- and β-ace-
zole derivatives have been used for the acylation of simple tates, the ratio of which was determined by NMR spec-
alcohols and carbohydrates.[32–34] In this endeavor, imida- troscopy.
zole has been successfully applied as a catalyst for the acety-
To further establish the efficacy of the present protocol,
lation of carbohydrates (Scheme 1). The findings of the a comparison study has been carried out to the previously
acetylation of a variety of carbohydrate derivatives contain- reported methods for acetylation of carbohydrates, which
ing both acid-labile and acid-stable functional groups and are presented in Table 2. Most of the previously reported
unprotected reducing sugars are listed in Table 1.
protocols take either longer reaction times for completion
In order to ascertain the catalytic potential of imidazole or require prior preparation of catalysts or use of hazardous
for this transformation, a series of experiments have been chemicals as activators or cannot be used with carbohydrate
carried out at room temperature by varying the quantity of derivatives containing acid-labile functional groups. From
acetic anhydride (2.0–1.0 equiv. per OH) and imidazole the comparison in Table 2, it is clear that the present proto-
(1.0–0.2 equiv.). Under optimized conditions acetic anhy- col is in many aspects more effective than the previously
dride (1.2 equiv. per OH) and imidazole (0.6 equiv.) in ace- reported methods. Although in some instances, the yield is
tonitrile (4.0 mL per mmol of substrate) at room tempera- comparable to the earlier reports, the most notable advan-
ture can successfully acetylate carbohydrate derivatives in tage of the present protocol is that it can acetylate carbo-
almost quantitative yield. Use of other commonly used sol- hydrate derivatives containing acid-labile functional groups
vents e.g. CH2Cl2, CHCl3, THF, DMF, nitromethane could very efficiently without any side reactions.
not produce result similar as with acetonitrile. Acid-suscep-
tible functional groups (benzylidene, isopropylidene,
TBDMS, TBDPS, dithioacetal, etc.) present in the carbo-
hydrate derivatives remained intact under the reaction con-
dition, which is the most notable advantage of this reaction
methodology. In order to extend the scope of this catalyst
further, acetylation of unprotected reducing sugars were
also investigated under these reaction conditions. In most
of the cases clean conversion has been observed and per-O-
acetylated products were isolated in almost quantitative
yield either by two-phase aqueous workup or by simple
evaporation of the solvent followed by column chromatog-
raphy. It is pertinent to note that no formation of per-O-
acetylated glycofuranose derivatives has been observed,
which was reported earlier in heterogeneous catalyst pro-
moted acetylations of unprotected reducing sugars.[15–18] In
the case of hydrated sugars a slight excess of acetic anhy-
dride was required with a longer reaction time because of
the partial consumption of water present in the starting ma-
terials. Products of all known compounds gave acceptable
Conclusions
A mild less toxic and efficient protocol for the acetylation
of carbohydrates has been devised using imidazole as acety-
lation activator. This methodology has been further ex-
tended to acetylate unprotected reducing sugars to form
per-O-acetylated glycopyranoses. In most of the cases quan-
titative yields were obtained. Being operationally so simple,
this methodology does not need any aqueous workup and
thereby reduces the efforts in purification of the products.
Along with these features, this method may be considered
as an attractive alternative to the existing methodologies for
the acylation of carbohydrates, particularly those contain-
ing acid-susceptible functionalities.
Experimental Section
General Experimental Protocol for the Acetylation of Carbo-
1H NMR and 13C NMR spectra that matched the data re- hydrates: To a magnetically stirred solution of the carbohydrate
Table 2. Comparative study of acetylation using different reported catalysts at room temperature.
[a] Domestic microwave irradiation.
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Efficient Acetylation of Carbohydrates Promoted by Imidazole
SHORT COMMUNICATION
(1.0 mmol) and acetic anhydride (1.2 equiv. per OH) in acetonitrile
(2.0 mL) was added imidazole (0.6 mmol) at room temperature and
the reaction mixture was stirred at ambient temperature for the
appropriate time as mentioned in Table 1. After completion of the
reaction (TLC; hexane/EtOAc, 1:1), the reaction mixture was
poured in water and extracted with CH2Cl2. The organic layer was
washed with aq. NaHCO3 and water, dried (Na2SO4) and concen-
trated under reduced pressure. Column chromatography of the
crude product on SiO2 using hexane/EtOAc (4:1) as an eluent fur-
nished pure acetylated carbohydrate derivatives (Table 1). In some
cases, direct evaporation of the solvent under reduced pressure avo-
iding two-phase partition of the reaction mixture followed by col-
umn chromatography furnished pure acetylated products. Spectral
data for compounds, which were not reported earlier follow below.
20.9 ppm. IR (KBr): ν = 2934, 2829, 2367, 1715, 1595, 1366, 1228,
˜
1105, 1030, 966, 719 cm–1. ESI-MS: m/z = 554 [M + Na].
C29H25NO7S (531): C 65.52, H 4.74; found C 65.68, H 5.0.
Methyl 2,3,4-Tetra-O-acetyl-6-O-di-tert-butyldiphenylsilyl-α-D-gluc-
opyranoside: Syrup. [α]2D5 = +124 (c = 1.5, CHCl3). 1H NMR
(CDCl3): δ = 7.65–7.64 (m, 4 H, aromatic H), 7.42–7.32 (m, 6 H,
aromatic H), 5.38 (t, J = 9.6 and 9.9 Hz, 1 H, 2-H), 5.03 (t, J =
9.9 and 9.3 Hz, 1 H, 3-H), 4.92 (d, J = 3.0 Hz, 1 H, 1ϪH), 4.82
(dd, J = 10.2 and 2.4 Hz, 1 H, 4-H), 3.85–3.80 (m, 1 H, 5-H), 3.71–
3.63 (m, 2 H, 6-Hab), 3.39 (s, 3 H, OCH3), 2.07, 1.98, 1.86 (3 s, 9
H, 3 COCH3), 1.05 [s, 9 H, C(CH3)3] ppm. 13C NMR (CDCl3,
75 Hz): δ = 169.8, 169.7, 169.0, 135.7–127.7 (aromatic C), 96.5,
71.0, 70.6, 69.9, 68.9, 62.6, 55.0, 26.8 (3 C), 20.7 (2 C), 20.5,
19.3 ppm. IR (neat): ν = 2390, 1715, 1580, 1366, 719 cm–1. ESI-
˜
5,6,7,9-Tetra-O-acetyl-4,8-anhydro-1,3-dideoxy-D-glycero-L-
MS: m/z = 581 [M + Na]. C29H38O9Si (558): C 62.34, H 6.86; found
glucononulose [1-(2,3,4,6-Tetra-O-acetyl-β-D-galactopyranosyl)ace-
C 62.10, H 7.05.
tone]: Yield: 96%; white solid; m.p. 91–92 °C. [α]2D5 = +5.6 (c = 1.5,
1
CHCl3). H NMR (CDCl3, 300 MHz): δ = 5.34 (br. s, 1 H, 7-H),
Acknowledgments
5.01–4.98 (m, 2 H, 5-H and 6-H), 4.05–3.98 (m, 2 H, 9-Hab), 3.96–
3.85 (m, 2 H, 4-H and 8-H), 2.74 (dd, J = 16.3 and 8.5 Hz, 1 H,
3-Ha), 2.45 (dd, J = 16.4 and 3.4 Hz, 1 H, 3-Hb), 2.16, 2.15, 2.02,
2.01, 1.96 (5 s, 15 H, 5 COCH3) ppm. 13C NMR (CDCl3, 75 Hz):
δ = 204.5, 169.9 (2 C), 169.7, 169.6, 74.4, 74.3, 71.9, 69.3, 67.9,
Instrumentation facilities from SAIF, CDRI is gratefully acknowl-
edged. P. T. and R. K. thank CSIR and DOD New Delhi for pro-
viding fellowships. This project was partly funded by the Depart-
ment of Science and Technology (DST), New Delhi (Project no.
SR/FTP/CSA-10/2002), India.
61.6, 45.6, 30.9, 20.7, 20.6 (2 C), 20.5 ppm. IR (KBr): ν = 2960,
˜
1741, 1713, 1598, 1439, 1386, 1220, 1030, 726 cm–1. ESI-MS: m/z
= 411 [M + Na]. C17H24O10 (388): calcd. C 52.57, H 6.23; found
C 52.30, H 6.50.
[1] P. J. Garegg, Acc. Chem. Res. 1992, 25, 575–580.
[2] G. Hofle, W. Steglich, H. Vorbruggen, Angew. Chem. Int. Ed.
Engl. 1978, 17, 569–583.
[3] E. F. V. Scriven, Chem. Soc., Rev. 1983, 12, 129–161.
[4] C. S. Hudson, J. K. Dale, J. Am. Chem. Soc. 1915, 37, 1264–
1270.
2,3-Di-O-acetyl-5-O-tert-butyldimethylsilyluridine: Yield: 92%;
white solid; m.p. 84 °C. [α]2D5 = –2.5 (c = 1.5, CHCl3). 1H NMR
(CDCl3, 300 MHz): δ = 10.18 (br. s, 1 H, NH), 7.72 (d, J = 9.0 Hz,
1 H), 6.15 (d, J = 6.0 Hz, 1 H), 5.62 (d, J = 9.0 Hz, 1 H), 5.20–
5.18 (m, 2 H), 4.09 (br. s, 1 H), 3.85–3.73 (m, 2 H), 2.04, 1.98 (2 s,
6 H, 2 COCH3), 0.85 [s, 9 H, C(CH3)3], 0.06 (s, 6 H, 2 CH3Si)
ppm. 13C NMR (CDCl3, 75 Hz): δ = 169.3, 169.0, 163.0, 150.6,
139.0, 103.0, 85.0, 83.4, 73.1, 71.4, 63.0, 25.7 (2 C), 25.4, 20.4, 20.1,
[5] B. Yu, J. Xie, S. Deng, Y. Hui, J. Am. Chem. Soc. 1999, 121,
12196–12197.
[6] M. L. Wolfrom, A. Thompson, Methods Carbohydr. Chem.
1963, 2, 211–215.
[7] J. A. Hyatt, G. W. Tindall, Heterocycles 1993, 35, 227–234.
[8] H. Binch, K. Stangier, J. Thiem, Carbohydr. Res. 1998, 306,
409–419.
18.1, –5.8 (2 C) ppm. IR (KBr): ν = 3202, 3072, 2929, 2858, 1749,
˜
1715, 1460, 1380, 1241, 1125, 1101, 1046, 834, 813, 778, 757 cm–1.
ESI-MS: m/z = 465 [M + Na]. C19H30N2O8Si (442): calcd. C 51.57,
H 6.83; found C 51.28, H 7.08.
[9] K. P. R. Kartha, R. A. Field, Tetrahedron 1997, 53, 11753–
11766.
[10] J.-C. Lee, C.-A. Tai, S.-C. Hung, Tetrahedron Lett. 2002, 43,
851–865.
Phenyl (2,6-Di-O-acetyl-3,4-O-isopropylidene-β-
D
-galactopyrano-
syl)-(1Ǟ4)-2,3,6-tri-O-acetyl-1-thio-β- -glucopyranoside:
D
Syrup.
[α]2D5 = +5.5 (c = 1.5, CHCl3). 1H NMR (CDCl3, 300 MHz): δ =
7.52–7.20 (m, 5 H, aromatic H), 5.23 (dd, J = 10.8 and 8.1 Hz, 1
H, 2-H), 4.92 (t, J = 9.5 Hz each, 1 H, 2Ј-H), 4.87–4.80 (m, 1 H),
4.67 (d, J = 9.8 Hz, 1 H, H-1), 4.47 (d, J = 11.2 Hz, 1 H, 1-H),
4.35–4.20 (m, 3 H), 4.19–4.02 (m, 3 H), 3.95–3.90 (m, 1 H), 3.80–
3.55 (m, 2 H), 2.08, 2.04 (2 s, 15 H, 5 COCH3), 1.52, 1.31 [2 s, 6
H, C(CH3)2] ppm. 13C NMR (CDCl3, 75 Hz): δ = 170.5, 170.4,
170.0, 169.4, 169.3, 133.2–128.4 (aromatic C), 111.0, 100.6 (C-1Ј),
85.7 (C-1), 78.1, 77.2, 76.2, 73.8, 73.4, 72.9, 71.2, 70.6, 63.4, 62.7,
[11] A.-A. Tai, S. S. Kulkarni, S.-C. Hung, J. Org. Chem. 2003, 68,
8719–8722.
[12] A. I. Vogel, Vogel’s Textbook of Practical Organic Chemistry,
5th ed., Wiley, New York, 1989, pp. 644–651.
[13] K.-C. Lu, S.-Y. Hsieh, L. N. Patkar, C.-T. Chen, C.-C. Lin,
Tetrahedron 2004, 60, 8967–8973.
[14] F. Dasgupta, P. P. Singh, H. C. Srivastava, Carbohydr. Res.
1980, 80, 346–349.
[15] P. M. Bhaskar, D. Loganathan, Tetrahedron Lett. 1998, 39,
2215–2218.
[16] P. M. Bhaskar, D. Loganathan, Synlett 1999, 129–131.
[17] R. Kumareswaran, K. Pachamuthu, Y. D. Vankar, Synlett
2000, 1652–1654.
[18] G. M. Christensen, J. Org. Chem. 1962, 27, 1442–1443.
[19] C. Limousin, J. Cleophax, A. Petit, A. Loupy, G. Lukacs, J.
Carbohydr. Chem. 1997, 16, 327–342.
[20] S. K. Das, K. A. Reddy, V. L. N. R. Krovvidi, K. Mukkanti,
Carbohydr. Res. 2005, 340, 1387–1392.
[21] S. Murugesan, N. Karst, T. Islam, J. M. Wiencek, R. J. Lin-
hardt, Synlett 2003, 1283–1286.
[22] S. A. Forsyth, D. R. MacFarlane, R. J. Thomson, M.
von Itzstein, Chem. Commun. 2002, 714–715.
[23] A. K. Misra, P. Tiwari, S. K. Madhusudan, Carbohydr. Res.
2005, 340, 325–329.
27.7, 26.5, 21.0 (3 C), 20.9 (2 C) ppm. IR (neat): ν = 2923, 2855,
˜
2363, 1730, 1461, 1218, 769 cm–1. ESI-MS: m/z = 707 [M + Na].
C31H40O15S (684): C 54.38, H 5.89; found C 54.75, H 6.12.
Phenyl 3-O-Acetyl-4,6-O-benzylidene-2-deoxy-2-phthalimido-1-thio-
β-D
-glucopyranoside: White solid; m.p. 115 °C. [α]2D5 = +18.3 (c =
1.5, CHCl3). 1H NMR (CDCl3, 300 MHz): δ = 7.85–7.71 (m, 4 H,
aromatic H), 7.42–7.25 (m, 10 H, aromatic H), 5.85 (t, J = 9.5 and
9.0 Hz, 1 H, 3-H), 5.80 (d, J = 10.6 Hz, 1 H, 1-H), 5.50 (s, 1 H,
PhCH), 4.41 (d, J = 5.9 Hz, 1 H, 4-H), 4.30 (t, J = 10.2 and 10.1
Hz, 1 H, 2-H), 3.82–3.70 (m, 3 H, 5-H and 6-Hab), 1.87 (s, 3 H,
COCH3) ppm. 13C NMR (CDCl3, 75 Hz): δ = 170.5, 168.2, 167.6,
137.3–124.1 (aromatic C), 102.1, 84.3, 79.4, 70.9, 69.0, 54.7,
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SHORT COMMUNICATION
[24] T. Hirose, B. G. Kopek, Z.-H. Wang, R. Yusa, B. W. Baldwin,
Tetrahedron Lett. 2003, 44, 1831–1833.
[25] H. A. Staab, H. Bauer, K. M. Schneider, Azolides in Organic
Synthesis and Biochemistry, Wiley-VCH, Weinheim, 2002, pp.
1–488.
[38]
[39]
[40]
[41]
S. K. Das, N. Roy, Carbohydr. Res. 1996, 296, 1079–1091.
V. Pozsgay, H. J. Jennings, J. Org. Chem. 1988, 53, 4042–4052.
M. Kiso, L. Anderson, Carbohydr. Res. 1985, 136, 309–324.
E. Fischer, M. Bergmann, A. Rabe, Ber. Dtsch. Chem. Ges.
1920, 53, 2362–2388.
P. L. Barili, G. Catelani, G. Fabrizi, D. Lamba, Carbohydr. Res.
1993, 243, 165–176.
H.-C. Tsui, L. A. Paquette, J. Org. Chem. 1998, 63, 9968–9977.
[26] T. Hirao, S. Santhitikul, H. Takeuchi, A. Ogawa, H. Sakurai,
Tetrahedron 2003, 59, 10147–10152.
[42]
[27] R. Gatri, M. M. El Gaied, Tetrahedron Lett. 2002, 43, 7835–
[43]
[44]
7836.
A. Roeen, J. I. Padron, J. T. Vazquez, J. Org. Chem. 2003, 68,
4615–4630.
[28] D. Mohajer, A. Rezaeifard, Tetrahedron Lett. 2002, 43, 1881–
1884.
[29] A. Khalafi-Nezhad, R. F. Alamdari, N. Zekri, Tetrahedron
2000, 56, 7503–7506.
[30] K. Ushijima, H. Gouzu, K. Hosono, M. Shirakawa, K. Kagos-
ima, K. Takai, H. Takaku, Biochim. Biophys. Acta 1998, 1379,
217–223.
[45] T. Halmos, R. Montserret, J. Filippi, K. Antonakis, Carbohydr.
Res. 1987, 170, 57–70.
[46] M. L. Wolfrom, A. Thompson, Methods Carbohydr. Chem.,
1963, vol. II, p. 427–430.
[47] R. U. Lemieux, T. Takeda, B. Y. Chung, Synthetic Methods for
Carbohydrates (Eds.: H. S. El Khadem), ACS Symposium
Series 1976, vol. 39, p. 90–115.
[31] D. Mohajer, S. Tangestaninejad, Tetrahedron Lett. 1994, 35,
945–948.
[32] D. Horton, W. Priebe, O. Varela, Carbohydr. Res. 1985, 144,
317–324.
[48] P. L. Durette, D. Horton, J. Org. Chem. 1971, 36, 2658–2669.
[49] R. S. Teague, Adv. Carbohydr. Chem. Biochem. 1954, 9, 185–
246.
[50] C.-T. Chen, J.-H. Kuo, C.-H. Li, N. B. Barhate, S.-W. Hon, T.-
W. Li, S.-D. Chao, C.-C. Liu, Y.-C. Li, I.-H. Chang, J.-S. Lin,
C.-J. Liu, Y.-C. Chou, Org. Lett. 2001, 3, 3729–3732.
[51] S. A. Forsyth, D. R. MacFarlane, R. J. Thompson, M.
von Itzstein, Chem. Commun. 2002, 714–715.
Received: July 22, 2005
[33] T. Kamijo, H. Harada, K. Lizuku, Chem. Pharm. Bull. 1984,
32, 5044–5047.
[34] Z. Szurmai, A. Liptak, Carbohydr. Res. 1982, 107, 33–41.
[35] D. Horton, J. H. Lauterbach, J. Org. Chem. 1969, 34, 86–92.
[36] G. Vic, J. J. Hasting, O. W. Howarth, D. H. G. Crout, Tetrahe-
dron: Asymmetry 1996, 7, 709–720.
[37] N. Khiar, M. Martin-Lomas, J. Org. Chem. 1995, 60, 7017–
7021.
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