2.3- Dihydro-1,4-dithiin, conformations

In 2,3-dihydro-1,4-dioxin, -1,4-oxathiin, and -1,4-dithiin derivatives and their benzo analogues, the values of the vicinal proton coupling constant ratios and ring dihedral angles were determined and found to be consistent with half -chair conformation <820MR(18)92>. 

Whereas 1,4-dioxin is planar, 1,4-dithiin 173 and 1,4-oxathiin are significantly distorted from planarity [98JST11]. For 173 a planar nonequilibrium structure was computed to be destabilized by about 2.5 kcal/mol with respect to the boat con-former. The half-chair and boat conformations of 3,4-dihydro-l,2-dithiin, 3,6-dihydro-1,2-dithiin, 4//-l,3-dithiin, and 2,3-dihydro-1,4-dithiin 174 were investigated at the MP2/6-31G level [98JCC1064] (see the structures of the corresponding dioxines in Scheme 114). The half-chair conformers are stabilized with respect to the corresponding boat conformers, and the most stable structure was found to be the half-chair conformer of 174. 

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