Tetrahedron Letters
4
77% yield. Formation of compound 2 was confirmed from its
In conclusion, we have developed one-pot, two step iterative
glycosylation and [2+3] block glycosylation strategy for the
synthesis of the O-antigen of Escherichia coli O181 as its p-
methoxyphenyl glycoside. An iterative [1+1+1] one-pot
glycosylation protocol was applied to accelerate the assembly of
the trisaccharide backbone. Several elegant concepts such as
H2SO4-silica as a promoter, ‘armed-disarmed’ glycosylation, the
use of orthogonal anomeric protecting groups, selective
spectral analysis [signals at ), 5.66 (d, J = 8.1 Hz, 1 H, H-1D),
5.48 (s, 1 H, PhCH), 5.33 (d, J = 3.1 Hz, 1 H, H-1A), 4.91 (brs, 1
H, H-1B), 4.61 (brs, 1 H, H-1E) in the 1H NMR and 101.9
(PhCH), 101.5 (C-1D), 99.8 (C-1B), 98.4, (C-1C), 97.2 (C-1E),
96.2 (C-1A) in the 13C NMR spectra] (Scheme 4).
activation of one glycosyl donor in the presence of other, and so
on, have been successfully incorporated in the synthetic strategy.
Finally, the global deprotection of the protected
pentasaccharide 2 began with the series of reactions involving (a)
removal of phthalimido group under hydrazinolysis,26 N-
acetylation, (b) finally removal of benzylidene acetal, benzyl
ethers, and reduction of azido group using a combination of
triethylsilane and 20% Pd(OH)2-C (Pearlman’s catalyst)23
followed by N-acetylation using acetic anhydride in methanol
furnished the target pentasaccharide 1 as its p-methoxyphenyl
(PMP) glycoside in 56% overall yield (Scheme 4).
Acknowledgments
H. K. thanks CSIR, New Delhi for providing Senior Research
Fellowship. The author gratefully acknowledges financial
support by DST-SERB (Scheme No.EMR/2017/001791), New
Delhi and SAIF Division of CSIR-CDRI for providing the
spectroscopic and analytical data. CDRI communication no.
510/2018/PKM.
References and notes
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Scheme 4: Reagents and conditions: (a) NIS, H2SO4-silica,
CH2Cl2, MS 4Å, 20 C, 30 min, 77%. (b) (i) NH2NH2.H2O,
C2H5OH, 80°C, 6 h; (ii) acetic anhydride, pyridine, rt, 2 h; (c)
Et3SiH, 20% Pd(OH)2-C, CH3OH, room temperature, 4 h; (d)
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Conclusion