A new method is described for regioselective partial deprotection of carbohydrates protected as benzylidene acetals. This deprotection was accomplished for each of the six methyl pyranosides (4, 5, and 18-21) studied by irradiation of the protected sugar and .¡V-bromosuccinimide (NBS) in the presence of water. Under these conditions the benzylidene (1,3-dioxolane) ring in each compound opened to give a methyl pyranoside with an axial benzoyloxy group and an equatorial hydroxy group. For example, irradiation of methyl 3,4-O-benzylidene {R or S)-6-deoxy-2-0-(2,2-dimethylpropanoyl)-a-L-galactopyranoside (18 or 19) with NBS, barium carbonate, and water resulted in the formation of methyl 4-0-benzoyl-6-deoxy-2-0-(2,2-dimethylpropanoyl)-a-L-galactopyranoside ( 22) in 72% yield. In a similar manner compounds 4 and 5 gave 10 and compounds 20 and 21 produced 23. The advantages of this deprotection process are described.
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