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

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.

thioglycoside - glycosylation - disarmed donor - Lewis acid - NIS

References and Notes

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29

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.

30

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.