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DOI: 10.1055/s-0030-1258557
Additive-Controlled Stereoselective Glycosylations of Oxazolidinone-Protected Glucosamine and Galactosamine Thioglycoside Donors Based on Preactivation Protocol
Publikationsverlauf
Publikationsdatum:
03. September 2010 (online)
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Abstract
Based on a pre-activation protocol, the stereoselectivity of oxazolidinone-protected amino sugar thioglycoside donors towards glycosylations can be controlled by additives. Either α- or β-selectivity could be obtained by changing additives. 2,4,6-Tri-tert-butylpyrimidine (TTBP) was the best β-directing additive, while thiophene worked as the best α-directing additive. The bifunctional additives such as tetrabutyl ammonium iodide (TBAI) afforded either α- or β-selectivity depending on the amount added. Poor α-selectivity of some glycosylations without any additives was greatly improved by adding TBAI or thiophene.
Key words
glycosylation - stereoselectivity - preactivation - additive - carbohydrate
- Supporting Information for this article is available online:
- Supporting Information
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References and Notes
General Procedures
for Glycosylations of Donors 1 or 2 with Acceptors 3-8
Tf2O
(8.7 µL, 0.052 mmol, 1.3 equiv) was added to a stirred mixture
of donors 1 or 2 (21.2
mg, 0.048 mmol, 1.2 equiv), BSM (11.1 mg, 0.052 mmol, 1.3 equiv),
and activated 4 Å MS (300 mg, powder) in CH2Cl2 (3
mL) at -73 ˚C under nitrogen atmosphere.
The reaction mixture was stirred for 5 min, after loss of the donor
detected by TLC, the additive (0.1-2.0 equiv) was added
to the mixture. After stirring for 15 min, a solution of the acceptor 3 (15.0 mg, 0.040 mmol, 1.0 equiv) or other
acceptors in CH2Cl2 (0.2 mL) was added dropwise
to the reaction mixture. The mixture was stirred and warmed up to
r.t. slowly, quenched by Et3N (0.1 mL). The precipitate
was filtered off, and the filtrate was concentrated. The residue
was purified by column chromatography on silica gel to give the
products.
Representative
Procedures for Detecting Intermediates after Activation of Donor
1 by Variable Temperature NMR Spectroscopy
To a solution
of donor 1 (8.7 mg, 0.02 mmol), BSM (5.1
mg, 0.024 mmol) in CD2Cl2 (0.5 mL) in a NMR
tube at -60 ˚C, under an argon atmosphere,
was added 1.2 equiv of Tf2O (0.024 mmol, 4.1 µL).
The NMR tube was immediately transferred to the pre-cooled NMR probe
(-60 ˚C), and ¹H NMR
was recorded. Subsequently the temperature of the probe was raised
in 10 ˚C steps with monitoring by ¹H NMR.
Product 9 was purified by column chromatography on silica gel (PE-EtOAc, 3:1); R f = 0.3 (PE-EtOAc, 1.5:1). ¹H NMR (400 MHz, CDCl3): δ = 7.59-7.61 (m, 2 H), 7.33-7.40 (m, 8 H), 6.25 (d, 1 H, J = 2.8 Hz, H-1′), 5.55 (s, 1 H), 5.30 (s, 1 H), 4.70 (d, 1 H, J = 3.6 Hz, H-1), 4.64 (d, 1 H, J = 11.6 Hz), 4.53 (d, 1 H, J = 11.6 Hz), 4.11-4.34 (m, 6 H), 3.77-3.84 (m, 2 H), 3.72 (t, 1 H, J = 10.0 Hz), 3.51-3.56 (m, 2 H), 3.37 (s, 3 H), 2.40 (s, 3 H), 2.08 (s, 3 H), 2.04 (s, 3 H). ¹³C NMR (100 MHz, CDCl3): δ = 170.38 (2 C), 169.35, 152.90, 137.54, 137.11, 128.80, 128.62, 128.54, 128.47, 128.05, 126.29, 126.20, 101.14, 97.94, 95.01, 82.39, 78.38, 72.98, 72.07, 72.00, 68.87, 68.03, 65.80, 61.77, 61.54, 56.06, 55.17, 23.80, 20.58, 20.54. ESI-MS: m/z = 686 [M + H]+, 703 [M + NH4]+, 708 [M + Na]+. Anal. Calcd for C34H39NO14: C, 59.56; H, 5.73; N, 2.04. Found: C, 59.30; H, 5.69; N, 1.97.
20Product 10 was
purified by column chromatography on silica gel (PE-EtOAc,
1.5:1); R
f
= 0.1
(PE-EtOAc, 1.5:1). ¹H NMR (400 MHz,
CDCl3): δ = 7.48-7.50 (m,
2 H), 7.31-7.38 (m, 8 H), 5.59 (s, 1 H), 5.56 (s, 1 H),
5.08 (d, 1 H, J = 7.6
Hz, H-1′), 4.64 (d, 1 H, J = 11.8
Hz), 4.54 (d, 1 H, J = 3.6
Hz, H-1), 4.52 (d, 1 H, J = 12.0
Hz), 4.32 (dd, 1 H, J = 7.6,
12.0 Hz), 4.15-4.24 (m, 3 H), 3.95-4.08 (m, 3
H), 3.63-3.79 (m, 4 H), 3.30 (s, 3 H), 2.38 (s, 3 H), 2.10
(s, 3 H), 1.98 (s, 3 H). ¹³C NMR (100
MHz, CDCl3): δ = 171.25, 170.28, 169.33,
153.54, 137.91, 137.35, 128.86, 128.56, 128.18, 128.03, 127.45,
126.04, 103.01, 100.85, 98.25, 80.63, 78.79, 77.23, 76.33, 72.98,
72.23, 68.84, 64.06, 62.53, 61.33, 57.55, 55.22, 25.08, 20.59, 20.56.
ESI-MS:
m/z = 686 [M + H]+,
703 [M + NH4]+,
708 [M + Na]+, 724 [M + K]+.
Anal. Calcd for C34H39NO14: C,
59.56; H, 5.73; N, 2.04. Found: C, 59.34; H, 5.67; N, 1.96.
The α-anomers and β-anomers were identified by their ¹H NMR coupling constants for anomeric protons. For α-anomers, J 1,2 = 2.4-2.8 Hz; for β-anomers, J 1,2 = 7.2-7.6 Hz.