In previous papers, we have reported on the synthesis of 2,6-[1], 3,6-[2], and 4,6-thioanhydro sugars [3] that were shown to be important intermediates for the enantiospecific preparation of polyhydroxythianes and thiolanes, structurally related to the potent glycosidase inhibitors polyhydroxypiperidines and pyrrolidines [4]. Continuing with our efforts towards the synthesis of such thioanhydro sugars, we report the preparation of a 4,6-thioanhydroglucoside by base treatment of a 3-O-mesyl-galactopyranoside via a novel one-pot conversion involving four consecutive steps.
I. Results and discussion
An attempt to prepare the starting compounds, methyl 2-O-benzoyl-(1) and 3-O-benzoyl-4,6-O-benzylidene-a-D-galactopyranoside (2), in a regioselective manner by application of the method described by Munavu and Szmant [5] was unsuccessful, giving a mixture of both isomers in a similar ratio as that obtained by Overend and co-workers [6]. Therefore, the latter and shorter procedure was used for the synthesis of 1 and 2, requiring, however, a tedious separation of the mixture by column chromatography.
Conventional methanesulfonylation of 1 and 2 gave the corresponding 3-0-(3) and 2-O-methanesulfonyl derivatives (4). Due to close chemical shift and coupling constant Thioanhydro sugar derivatives, Part IV. For Part III, see ref. [2].