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Synlett 2010(9): 1315-1318  
DOI: 10.1055/s-0029-1219919
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

On the Hydrogenation of Glycosyl Oxazolines

Daniel J. Coxa, Thomas B. Parsonsa, Antony J. Fairbanks*b
a Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA, UK
b Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
Fax: +64(3)3642110; e-Mail: antony.fairbanks@canterbury.ac.nz;
Further Information

Publication History

Received 19 March 2010
Publication Date:
06 May 2010 (online)

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Abstract

An investigation is undertaken into the propensity of glycosyl oxazolines to undergo reductive cleavage by catalytic hydrogenation. Results indicate that the protecting groups on carbohydrate hydroxyl groups modulate the rate of glycosyl oxazoline reduction; electron-withdrawing ester groups curtail reaction so that reductive cleavage of benzyl ethers and esters, or reduction of azide elsewhere in the molecule may be readily achieved.

Key words

carbohydrates - oxazolines - hydrogenation - protecting groups - reactivity tuning

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http://www.cazy.org/fam/GH85.html.

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Typical Procedure for Catalytic Hydrogenation
Glycosyl oxazoline (ca. 20 mg), NaHCO3 (2 equiv), and Et3N (20 equiv) were dissolved in a mixture of t-BuOH (1.2 mL) and H2O (0.2 mL). The mixture was degassed and put under an atmosphere of nitrogen. Palladium black was added, and the mixture was then stirred at r.t. under an atmosphere of hydrogen until TLC (EtOAc) indicated complete consumption of the starting material (R f typically ca. 0.35), and the formation of a polar product (R f = ca. 0.0). Mass spectrometric analysis indicated the presence of a single product. The reaction mixture was filtered through Celite,® the Celite® washed twice with H2O (2 × 0.5 mL) and the combined aqueous fractions lyophilized.

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Selected Data for Compound 4 IR (KBr disc): νmax = 3425 (br, OH), 1646 (s, C=N) cm-¹. ¹H NMR (500 MHz, D2O): δ = 2.01 (3 H, s, CH3), 3.26 (1 H, at, J = 10.4 Hz, H-1ax), 3.39 (1 H, dt, J = 3.1, 9.3 Hz, H-5), 3.54 (1 H, at, J = 9.3 Hz, H-3), 3.60 (1 H, at, J = 9.3 Hz, H-4), 3.86-3.95 (2 H, m, H-1eq, H-2), 3.97-3.98 (2 H, m, H-6, H-6′). ¹³C NMR (125.8 MHz, D2O): δ = 21.9 (q, CH3), 51.3 (d, C-2), 62.9 (dt, C-6), 67.2 (t, C-1), 69.6 (d, C-4), 74.3 (d, C-3), 80.1 (dd, C-5), 174.6 (1 × s, C=O). ³¹P NMR (202.6 MHz, D2O): δ = 4.6. MS (ES-): m/z (%) = 284 (100) [M - H]-. HRMS (ES-): m/z calcd for C8H15NO8P [M - H]-: 284.0541; found: 284.0547.

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Selected Data for Compound 15
IR (KBr disc): νmax = 3320 (br, NH stretch), 1750 (s, C=O), 1671 (s, C=N), 1663 (s, C=O) cm-¹. ¹H NMR (400 MHz, C6D6): δ = 1.61 [3 H, d, J 2a,CH3 = 2.7 Hz, N=C(CH3)], 1.68, 1.72, 1.77 (9 H, 3 × s, 3 × CH3), 3.49 (1 H, ddd, J 4,5 = 9.4 Hz, J 5,6 = 6.8 Hz, J 5,6 ′ = 2.9 Hz, H-5), 3.74 (1 H, m, H-4), 4.37 (1 H, dd, J 6,6 ′ = 12.3 Hz, J 5,6 = 6.8 Hz, H-6), 4.53-4.63 (2 H, m, H-2, H-6′), 5.40 (1 H, m, H-3), 5.52 (1 H, d, J 1,2 = 7.2 Hz, H-1), 6.66 (1 H, br d, J = 9.2 Hz, NH). ¹³C NMR (100 MHz, C6D6): δ = 13.4 [q, C=N(CH3)], 20.3, 20.4, 22.5 (3 × q, 3 × CH3), 47.4 (d, C-2), 64.6 (d, C-4), 64.8 (t, C-6), 71.6 (d, C-5), 71.8 (d, C-3), 100.2 (d, C-1), 168.7 (s, C=N), 169.2, 170.1, 170.2 (3 × s, 3 × C=O). MS (ES+): m/z (%) = 451 (100) [MNa+]. HRMS (ES+): m/z calcd for C14H20N2NaO7 [MNa+]: 451.1163; found: 451.1163.

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Selected Data for Compound 17
IR (KBr disc): νmax = 3427 (br, OH stretch), 1672 (s, C=O), 1639 (br, C=N) cm-¹. ¹H NMR (500 MHz, D2O): δ = 1.93 [3 H, s, N=C(CH3)], 3.39-3.41 (1 H, m, H-5), 3.65 (1 H, dd, J 3,4 = 5.0 Hz, J 4,5 = 9.0 Hz, H-4), 3.76 (1 H, app t, J = 4.7 Hz, H-3), 3.81-3.87 (2 H, m, H-6, H-6′), 3.95 (1 H, m, H-2), 5.98 (1 H, d, J 1,2 = 7.4 Hz, H-1). ¹³C NMR (125.8 MHz, D2O):
δ = 13.1 [q, N=C(CH3)], 63.2 (dt, C-6), 66.4 (d, C-2), 68.2 (d, C-4), 72.7 (dd, C-5), 80.0 (d, C-3), 101.2 (d, C-1), 165.7 (s, C=N). ³¹P NMR (203 MHz, D2O): δ = 5.54. MS: (ES-): m/z (%) = 282 (100) [M - H]-. HRMS (ES-): m/z calcd for C8H13NO8P [M - H]-: 282.0384; found: 282.0389.

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Selected Data for 19
IR (KBr disc): νmax = 3429 (br, OH stretch), 1641 (br, C=N) cm-¹. ¹H NMR (500 MHz, D2O): δ = 2.07 [3 H, br s, N=C(CH3)], 3.42-3.45 (2 H, m, H-5a, H-5b), 3.63-3.68 (2 H, m, H-4b, H-6a), 3.72-3.83 (5 H, m, H-3b, H-4a, H-5c, H-6b, H-6′a), 3.86-3.96 (4 H, m, H-3c, H-4c, H-6c, H-6′b), 4.00-4.08 (2 H, m, H-2c, H-6′c), 4.11 (1 H, d, J = 2.7 Hz, H-2b), 4.18 (1 H, br d, J = 6.7 Hz, H-2a), 4.38 (1 H, s, H-3a), 4.76 (1 H, s, H-1b), 5.11 (1 H, s, H-1c), 6.08 (1 H, d, J 1,2 = 7.3 Hz, H-1a). ¹³C NMR (125.8 MHz, D2O): δ = 13.0 [q, C=N(CH3)], 61.0 (t, C-6b), 61.5 (t, C-6a), 62.6 (dt, C-6c), 65.2 (d, C-2a), 66.0 (d, C-4c), 66.2 (d, C-4b), 69.3 (d, C-3a), 70.0 (d, C-3c), 70.1 (m, C-2b, C-2c), 70.9 (d, C-5a), 72.8 (dd, C-5c), 76.1 (d, C-5b), 77.6 (d, C-4a), 80.3 (d, C-3b), 99.9 (d, C-1a), 101.1 (d, C-1b), 102.5 (d, C-1c), 168.6 (s, C=N). ³¹P NMR (162 MHz, D2O): δ = 5.71. MS (ES-):
m/z (%) = 606 (100) [M - 2Na + H]-. HRMS (ES-): m/z calcd for C20H33NO18P [M - 2Na + H]-: 606.1441; found: 606.1436.