| Abstract
| - 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).
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