The reaction of the acetates of alpha-chloro methyl hemiacetals with nucleophiles. Synthesis of 2-hydroxy-1,4-dithianes

The acetates of the methyl hemiacetals of alpha-halo aldehydes, 1, easily undergo nucleophilic attack by lithium thiolates or ethane dithiolate affording alpha-thioalkyl aldehydes 2 or 2-hydroxy-1,4-dithianes 3, respectively. 3-monosubstituted dithianes 3 smoothly dehydrate to 2,3-dihydro-1,4-dithiines 4, whereas 3,3-disubstituted derivatives give dimers 6. The single-crystal X-ray analysis of 6b shows that it crystallises in a racemic form, with the dithiane ring in a chair conformation and the exocyclic S(3) atom axially bonded to the ring. The crystal packing appears to be determined mainly by a weak C-H ... O hydrogen bonding interaction.