Regioselective Ring Opening of Selected Benzylidene Acetals. A Photochemically Initiated Reaction for Partial Deprotection of Carbohydrates

Article abstract of DOI:10.1021/jo00180a008

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 N-bromsuccinimide (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-O-(2,2-dimethylpropanoyl)-α-L-galactopyranoside (18 or 19) with NBS, barium carbonate, and water resulted in the formation of methyl 4-O-benzoyl-6-deoxy-2-O-(2,2-dimethylpropanoyl)-α-L-galactopyranoside (22) in 72percent yield.In a similar manner compounds 4 and 5 gave 10 and compounds 20 and 21 produced 23.The advantages of the deprotection process are described.