Hypervalent iodine system for debenzylation of sugars

Article abstract of DOI:10.1080/00397910701465065

A simple and mild system using 4,4′-bis-(dichloroiodo)-biphenyl in combination with tetraethylammonium bromide (TEAB) at room temperature has been developed for selective debenzylation of sugars. Acetates, benzoate, and sensitive glycosidic linkages are unaffected. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701465065

Synthetic Communicationsw, 37: 2647–2650, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701465065
Hypervalent Iodine System for
Debenzylation of Sugars
Vikas N. Telvekar
Department of Pharmaceutical Sciences and Technology, University
Institute of Chemical Technology, Matunga, Mumbai, India
Abstract: A simple and mild system using 4,40-bis-(dichloroiodo)-biphenyl in combi-
nation with tetraethylammonium bromide (TEAB) at room temperature has been
developed for selective debenzylation of sugars. Acetates, benzoate, and sensitive
glycosidic linkages are unaffected.
Keywords: 4,40-bis-(dichloroiodo)-biphenyl, debenzylation, sugars, tetraethylammo-
nium bromide
INTRODUCTION
Benzyl ethers are widely used as protecting groups for carbohydrate
compounds because of their stability toward a wide variety of reaction con-
ditions. Although numerous methods are available for their removal,[1–5]
problems associated with reagent incompatibility, slow debenzylation rate,
low yield, and selectivity in the presence of multiple functionalities. Our
group has been working extensively on development of novel methodologies
under mild reaction conditions using hypervalent iodine reagents.[6] It was
observed that 4,40-bis-(dichloroiodo)-biphenyl is very good reagent for the
iodochlorination of alkenes.[7] We observed that the 4,40-bis-(dichloroiodo)-
biphenyl in the presence of tetraethylammonium bromide (TEAB) was a
highly efficient system for selective debenzylation at room temperature. In
the absence of TEAB, debenzylation did not take place.
Received January 8, 2007
Address correspondence to Vikas N. Telvekar, Department of Pharmaceutical
Sciences and Technology, University Institute of Chemical Technology, Matunga,
Mumbai 400019, India. E-mail: vikastelvekar@rediffmail.com
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V. N. Telvekar
For the initial study, benzyl 4-O-benzoyl 2,3-O-isopropylidene-a-L-ara-
binopyranoside was chosen as a substrate, and the reaction with 4,40-bis-
(dichloroiodo)-biphenyl in the presence of TEAB was performed. It was
observed that the starting material was consumed within 25 min as
indicated by thin-layer chromatography (TLC) analysis, and the reaction
mixture became clear. After usual workup, the debenzylated derivative
(entry 1) and 4,40-diiodobiphenyl were recovered after silica-gel
chromatography.
Encouraged by this result, varieties of protected carbohydrates were
subjected to a similar reaction, the results of which are presented in
Table 1. Notably acetates, benzoate, and sensitive glycosidic linkages are
unaffected by the reaction conditions.
Table 1. Debenzylation using 4,40-bis-(dichloroiodo)-biphenyl and TEAB
Time Yieldb
(min) (%)
Entry
1
Compound
Producta
35
90
2
30
90
3
4
25
30
89
91
5
35
90
6
7
30
35
87
90
aAll products were characterized by 13C NMR, 1H NMR, IR, and optical rotation and
compared with reported data.
bIsolated yields after purification.

Hypervalent Iodine System for Debenzylation of Sugars
GENERAL EXPERIMENTAL PROCEDURE
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Benzyl 4-O-benzoyl 2,3-O-isopropylidene-a-L-arabinopyranoside (3.40 g,
10 mmol) was added to a stirred suspension of 4,40-bis-(dichloroiodo)-
biphenyl (1.7 g, 30 mmol) and TEAB (2.5 g, 12 mmol) in dry chloroform
(10 ml) at room temperature. The reaction was monitored by TLC. After
25 min, the reaction mixture was quenched with water (25 ml); the organic
layer was separated and washed with water (2 ꢀ 25 ml), 10% sodium
bicarbonate (2 ꢀ 15 ml), 10% aqueous sodium bisulfite solution
(2 ꢀ 15 ml), and finally water (2 ꢀ 10 ml). The chloroform layer was
separated, dried over sodium sulfate, and concentrated, and the resultant
residue was purified by silica-gel column chromatography (10% EtOAc–
hexane) to afforded an ab mixture (1:1), that is, 4-O-benzoyl 2,3-O-isopropy-
lidene-ab-L-arabinopyranose (2.40 g, 90%). Mp ¼ 588C; [a]D20 ¼ 232
(c ¼ 1, CH2Cl2); IR (KBr, cm21) ¼ 1725 (C55O, ester), 3460 (OH,
hydroxyl); 1H NMR (d ppm, 500 MHz, CDCl3, ab mixture 1:1): 7.712
(S, 10H), 5.58 (dt, J ¼ 9.7 & 5.8 Hz, 1H, H-4), 5.54–5.52 (m,1H, H-4),
5.50 (t, J ¼ 9.6 Hz, 1H, H-3), 5.37 (br, d, J ¼ 3.2 Hz, 1H, H-1), 5.23
(t, J ¼ 9.6 Hz, 1H, H-3), 4.74 (d, J ¼ 8.1 Hz, 1H, H-1), 4.05 (dd, J ¼ 9.56
& 7.93 Hz, 1H, H-2), 4.09–4.11 (m, 1H, H-2), 3.97–3.71 (m, 4H, H-5),
1.47 (s, 6H) & 1.42 (s, 6H); 13C NMR (d ppm): 165.786 (C55O), 133.184,
136.740, 129.711, 129.629, 128.062, 127.825 (aromatic), 111.023
(isopropyl carbon), 102.46 (C1), 70.85 (C2), 70.38 (C3), 69.90 (C5), 69.14
(C4), 26.587 & 26.309 (2 ꢀ CH3); Found: C, 61.0; H, 6.1; O, 32.5 %; calc.
for C15H18O6: C, 61.2; H, 6.16; O, 32.61%.
ACKNOWLEDGMENTS
The author thanks the University Grant Commission (UGC) India for the
financial support under the Major Research Scheme and T. F. R for
recording NMR (500 MHz).
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