We also hypothesised that the allylic alcohol in ketene acetals 6 might be activated by the adjacent electron-rich alkene making further selective dehydration at C-3 possible. Treatment of the glucose-derived alkene 6e with the Lewis acidic reagent B(OCH2CF3)3 led to dehydration and cyclisation to form a diastereomeric mixture of THFs 14 (Scheme 7) with essentially complete conversion, although chromatographic purification of the THFs led to much lower isolated yields. The arabinose derived thioalkene 6a cannot undergo a similar dehydration as 6e to form a THF at C-3, but treatment of 6a with In(OTf)3 led instead to cyclisation at C-1, presumably via a similar stabilised allylic cation. This leads via hydrolysis to the ,-unsaturated lactone which subsequently reacts with the liberated thiophenol to yield a diastereomeric mixture of known lactones 15 in 38% yield (unoptimized). These interesting heterocyclic compounds are potentially useful building blocks for asymmetric synthesis, with THFs 14 structurally similar to catechol-O-methyl transferase (COMT) bisubstrate inhibitor 16 and the anti-tumour natural product (+)-varitriol 17. 27 Scheme 7. Synthesis of chiral heterocycles from selected ketene dithioacetals via i) Lewis acid mediated reaction with electrophilic dimethyl acetal to give lactone 13 ii) borate ester B(OCH2CF3)3 mediated dehydration to give THF 14, with structural similarity to the natural product (+)-varitriol, iii) Lewis acid mediated dehydration to give lactone 15. Catechol-O-methyltransferase (COMT) bisubstrate inhibitor 29 , (+)-varitriol and Branimycin intermediate show with structurally similar glucose derived THF and arabinose-derived lactone scaffolds highlighted in red.