An improved synthesis of 3,6-anhydro-D-glucal and a study of its unusual chemical reactivity

6-O-Tosyl-d-glucal 1 upon treatment with excess LiAlH4 unexpectedly gave 3,6-anhydro-d-glucal 2 as a major product in good yield. A crystal structure was obtained. Reaction of the anhydride 2 with N-iodosuccinimide (NIS) in excess methanol resulted in the formation of diastereomeric 2-deoxy-2-iodoglycosides. Addition of ceric (IV) ammonium nitrate and thiophenol to a solution of 2 in acetonitrile gave a mixture of 2-deoxy and 2,3-unsaturated thioglycosides. Reaction of 1,2:3,4-di-O-isopropylidine-α-d-galactopyranose with the anhydro sugar 2 in the presence of N-iodosuccinimide did not give the expected iodoglycoside mixture, but instead gave an unusual 1,4:3,6-dianhydride 7 as the major product.