Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides
(Chemical Equation Presented) 3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-α-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in...
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| Autores principales: | , , |
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| Formato: | JOUR |
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| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_00223263_v73_n18_p7224_Manzano |
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| Sumario: | (Chemical Equation Presented) 3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-α-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in the facial selectivity of the epoxidation reaction. Thus, the free HO-2 in 3 (the 6-O-acetyl derivative of 5) directs the attack of m-chloroperbenzoic acid from the more hindered α face of the molecule to give 2-propyl 6-O-acetyl-3,4-anhydro-α-D-allopyranoside (7) accompanied by the β epoxide 6 as a very minor product. Reverse diastereoselectivity has been obtained when the HO-2 in 3 was substituted by a bulky tert-butyldimethylsilyl (TBS) group. In this case, the major isomer was the 2-O-TBS derivative of 6 (α-D-galacto configuration). The ring-opening of sugar epoxides by nucleophilic per-O-acetyl-1-thio-β-D-glucopyranose (11) was employed as a convenient approach to the synthesis of (1→3)-and (1→4)- thiodisaccharides. For example, ring-opening of the oxirane 7 by 11 led to the expected regioisomeric per-O-acetyl thiodisaccharides β-D-Glc-S-(1→3)- 4-thio-α-D-Glc-O-iPr (12) and β-D-Glc-S-(1→4)4-thio-α-D- Gul-O-iPr (13). Regioselectivity in the construction of the (1→4)- thioglycosidic linkage could be achieved by hindering C-3 of the 3,4-anhydro sugar with a bulky silyloxy group at the vicinal C-2. For instance, coupling of the 2-O-TBS derivative of 7 with 11 led regioselectively to the protected thiodisaccharide β-D-Glc-S-(1→4)-4-thio-α-D-Glc-O-iPr (27). The utility of the approach was demonstrated through the synthesis of sulfur-linked analogues of naturally occurring (laminarabiose and cellobiose) and non-natural disaccharides (i.e., β-D-Glc-(1→4)-α-D-Gul). © 2008 American Chemical Society. |
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