Using L-arabinose, which is available from waste sugar-beet pulp, as a test substrate, the corresponding ethyl-and phenyl thioacetals were prepared via reported procedures. Treatment of the ethyl thioacetal with K2CO3/dimethyl carbonate (DMC) led to the formation of a complex mixture of products. However, reaction of the readily formed phenyl thioacetal 5a under similar conditions led to the formation of the diphenyl ketene thioacetal 6a as a single product. In addition, purification of the phenylthioacetal derivatives could be achieved via recrystallisation, avoiding the need for column chromatography. Interestingly, unlike the reactions of the corresponding hydrazones, the THF was not formed, and a selective dehydration took place exclusively at the C-2 position to give alkene 6a in near quantitative yield (15 mmol scale, Scheme 2). Scheme 2. Thioacetal protection of L-arabinose followed by selective dehydration under mild conditions. The PhS groups in 5a make the C-1 proton fairly acidic, and hence, it is clear that an elimination reaction can take place readily when the C-2 hydroxyl group is activated by DMC. The formation of similar ketene dithioacetals has previously been reported as a problematic side reaction under harsh conditions involving reactions of protected derivatives with strong bases (e.g. sodium methylsulfinylmethylide or n-BuLi). Given that our reaction conditions are very mild, and that the reaction is selective and high yielding, this potentially offers a readily scalable method for the selective C-2 deoxygenation of sugars without the need for hydroxyl protecting groups. The scope of this approach more generally was then explored (Scheme 3). Selective dehydration was carried out with an array of sugar dithioacetals, derived from aldose sugars, in moderate to excellent yields (48-99%) for several pentose and hexose sugars (6a, 6b, 6e, 6f). However, some thioacetals, such as those derived from D-ribose (5c), L-rhamnose (5d) and D-mannose (5g), gave little to no conversion to the alkene. A common feature of the unsuccessful substrates is anti-stereochemistry at the C-2 and C-3 positions. This potentially provides a useful insight into the mechanism of the reaction, which is likely to occur via (reversible) formation of a cyclic carbonate at C-2/C-3, through reaction of the polyol with dimethyl carbonate. This then subsequently undergoes elimination by removal of the acidic C-1 proton (Scheme 4).