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Synlett 2010(19): 2853-2856  
DOI: 10.1055/s-0030-1259016
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

La(OTf)3: An Efficient Promoter for Thioglycoside Activation in Conjunction with N-Iodosuccinimide

Somnath Mukherjee, Balaram Mukhopadhyay*
Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, Mohanpur Campus, BCKV Main Campus, Mohanpur, Nadia 741 252, West Bengal, India
Fax: +91(33)25873031; e-Mail: sugarnet73@hotmail.com;
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Publication History

Received 12 July 2010
Publication Date:
03 November 2010 (online)

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Abstract

Use of La(OTf)3 as a Lewis acid promoter for N-iodosuccinimide-mediated activation of thioglycosides is reported. The glycosylation reactions proceeded smoothly with good to excellent yields and stereoselectivity.

Key words

thioglycoside - glycosylation - disarmed donor - Lewis acid - NIS

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p -Methoxyphenyl 2,3-Di- O -benzoyl-4,6- O -benzylidene-β- D-glucopyranosyl-(1→3)-4- O -acetyl-2,6-di- O -benzyl-β- D-glucopyranoside (26): ¹H NMR (CDCl3, 500 MHz):
δ = 7.87, 7.83 (2 × d, 4 H, ArH), 7.38-7.13 (m, 21 H, ArH), 6.87, 6.68 (2 × d, 4 H, ArH), 5.67 (t, J = 9.5 Hz, 1 H, H-3′), 5.47 (s, 1 H, CHPh), 5.42 (d, J = 3.0 Hz, 1 H, H-4), 5.38 (dd, J = 8.0, 9.5 Hz, 1 H, H-2′), 5.17 (d, J = 8.0 Hz, 1 H, H-1′), 4.76 (d, J = 8.0 Hz, 1 H, H-1), 4.68 (d, J = 10.5 Hz, 1 H, CH2Ph), 4.44, 4.40 (2 × d, J = 10.5 Hz, 2 H, CH2Ph), 4.30 (d, J = 10.5 Hz, 1 H, CH2Ph), 4.29 (m, 1 H, H-6a′), 3.92 (dd, J = 4.0, 9.5 Hz, 1 H, H-3), 3.89 (t, J = 9.5 Hz, 1 H, H-4′), 3.78 (m, 2 H, H-2, H-6b′), 3.68 (m, 2 H, H-4′, H-6a), 3.67 (s, 3 H, ArCH3), 3.52 (m, 3 H, H-5, H-5′, H-6b), 2.11 (s, 3 H, COCH3). ¹³C NMR (CDCl3, 500 MHz): δ = 170.4 (COCH3), 165.6, 165.1 (2 × COPh), 155.4, 151.3, 137.8, 136.8, 133.3, 133.2, 129.8, 129.7, 129.4, 129.3 (2), 129.0, 128.5 (2), 128.4 (2), 128.3 (2), 128.2 (2), 128.0(2), 127.9 (2), 127.8 (2), 127.7 (2), 126.1 (2), 118.3 (2), 114.6 (2) (ArC), 102.7 (CHPh), 101.4 (C-1′), 101.1 (C-1), 79.4, 78.6, 76.2, 75.1, 73.8, 73.0, 71.9, 69.7, 68.7, 68.6, 66.5, 55.6 (ArCH3), 20.8 (COCH3). HRMS: m/z calcd for C56H54O15Na [M + Na]+: 989.3360; found: 989.3354.

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General procedure for glycosylation reactions: A mixture of acceptor (1.0 mmol), thioglycoside donor (1.2 mmol) and 4 Å MS in anhydrous CH2Cl2 (10 mL) was stirred under nitrogen for 30 min. La(OTf)3 (0.3 mmol) was added and the mixture was stirred at 0 ˚C until TLC (n-hexane-EtOAc, 2:1) showed complete consumption of the starting material. The mixture was filtered through a pad of Celite and the filtrate was washed successively with aq Na2S2O7 (2 × 20 mL), aq sat. NaHCO3 (2 × 20 mL), and brine (20 mL). The organic layer was separated, dried (Na2SO4), filtered and evaporated in vacuo. The residue was purified by flash chromatography using a suitable mixture of
n-hexane-EtOAc as eluent to afford pure glycosylated products.