Contrasting conformational behavior of 5-methylsulfonyl-1,3-dioxane and -1,3-dithiane in the minimization of steric and electrostatic repulsive interactions

The preparation of novel 1,3-dithiane derivatives, cis- and trans-2-tert-butyl-5-methylsulfonyl-1,3-dithiane, was achieved by acid-catalyzed condensation of 2-methylsulfonyl-1,3-propanedithiol with pivalaldehyde. The former dithiol was obtained from allylmethylsulfide via the Knochel–Normant bromination–rearrangement protocol. Configurational and conformational assignment of cis- and trans-5 was based on 1H NMR analysis, and revealed that the trans isomer adopts a normal chair conformation. By contrast, cis-5 adopts a twist-boat conformation in order to minimize the steric and electrostatic repulsive interactions that an axial methylsulfonyl group engenders. Chemical equilibration cis-5⇌trans-5 shows the latter to be more stable by 1.50 kcal/mol.