SELENOGLYCOSIDES .3. SYNTHESIS OF PHENYL 2-(N-ACETYLAMINO)-2-DEOXY-1-SELENO-ALPHA-D-GLYCOPYRANOSIDE AND 2-AZIDO-2-DEOXY-1-SELENO-ALPHA-D-GLYCOPYRANOSIDE VIA AZIDO-PHENYLSELENYLATION OF DIVERSELY PROTECTED GLYCALS

Two methods are described for the preparation of diversely protected phenyl 2-azido-2-deoxy-alpha-D-selenoglycopyranosides from protected glycals. In the first one (method A), a peracetylated glycal is treated with sodium azide and diphenyl diselenide in the presence of (diacetoxyiodo)benzene in dichloromethane at rt. With 3,4,6-tri-O-acetyl-1,5-anhydro-2-deoxy-D-arabino-hex-1-enitol (tri-O-acetyl-D-glucal) an inseparable mixture of phenyl 2-azido-2-deoxy-alpha-gluco- and -alpha-manno-selenoglycosides is obtained (91% yield). With 3,4,6-tri-O-acetyl-1,5-anhydro-2-deoxy-D-lyxo-hex-1-enitol (tri-O-acetyl-D-galactal) only the alpha-galacto isomer is obtained (92%). Method A is not compatible with benzyl groups. In method B, a perbenzylated glycal is reacted with trimethylsilyl azide and tetra-n-butylammonium fluoride in the presence of N-phenylselenophthalimide. From protected D-glucal a gluco/manno mixture is obtained, whereas only the galacto isomer is formed from protected D-galactal (75% yield). The compatibility of method B with a variety of protecting groups is exemplified with 6-O-acetyl, 6-O-benzyl, and 6-O-(tert-butyldimethylsilyl)-3,4-O-isopropylidene-1,5-anhydro-2-deoxy-D-lyxo-hex-1-enitol. The same diastereocontrol is observed, and the alpha-D-galacto isomer is obtained (60-70% yield). Reduction of the azido group of these selenoglycosides with 1,3-propanedithiol in the presence of triethylamine and acetylation affords the corresponding phenyl 2-(N-acetylamino)-2-deoxy-alpha-D-selenoglycopyranosides in goed yield.