The Stereoselective Synthesis of 3-Mannosides

The same group used a ketal tether as an alternative connecting group in the synthesis of the 1,4-linked C-disaccharide 236 [85 b]. Tebbe methylenation of acetate 237 provided the corresponding enol ether 238, which upon treatment with alcohol 235 in the presence of CSA at -40°C in acetonitrile, furnished linked disaccharide 239 in 81% yield. Subsequent radical cyclization, acidic hydrolysis of the tether and peracetylation provided the D-mannose-containing C-disaccharide 236 as the major product in 35% yield from 239 (Scheme 10-75). Cyclization was not completely stereoselective and a small amount of the )8-C-manno isomer was also isolated (a/)3 10 1). This result is in contrast to similar studies on tether-directed /J-C-mannoside syntheses (vide infra) where a much shorter tether attached to the axial 2-hydroxyl group forces obtention of the desired P-configuration. 

Furstner and Konetzki have successfully adopted this approach for the synthesis of the fungal metabolite 1 [15]. Treatment of 2-oxo bromide donor 17 with alcohol 27 in the presence of aluminosilicate solely afforded (3-glycoside 28, which was reduced without further purification with sodium borohydride. Mannoside 29 was obtained in a 78% yield over two steps with excellent stereoselectivity (Scheme 8.7). 

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