Conformational studies (1,2) of the cyclic sulfites of 2-substituted-propane-1,3-dials have indicated that these esters occupy preferentially a chair form with axial S=O bond. X-ray analyses (3,4) of two compounds have shown that the sulfite group holds the remainder of the ring in au almost ideal staggered conformation, all dihedral angles in the ring being close to 60'.
We now report that cis-trans isomeric sulfites can be equilibrated by acid catalysts. Although isomerisation takes place rather slowly with RW3-etherate, due to partial decomposition of the catalyst, the equilibrium position may be reached from either side and is independent of the catalyst concentration.
Ten sulfites, all occurring both as ois and trans isomers, have been investigated. From the final cis-trans ratio's, measured in various solvents by R?dR or GLC techniques, equilibrium constants Ki were evaluated. The results in terms of the free-energy differences -&Go = RT ln Ki are given in Table 1.
The experimental evidence for the structural and conformational assignments impliedly given in the last column has been (1,2) or will be published (5) elsewhere, together with details about synthetic procedures.
It has previously been observed (2), that the conformational homogeneity of the isomers may be affected seriously in polar eolvents, due to the strong interaction between a highly polar conformation and the surrounding dielectric.
l Part II: H.F. van Woerden, Tetrahedron Letters 24C7 (1966).