1//-Thiete, calculations

Cycloadditions of nitrones, nitrile oxides or diazo compounds to thiete dioxides do not show the high stereoselectivity observed with acyclic vinyl sulfones, and mixtures of the two possible adducts are formed . The charge-transfer stabilization energy calculated according to the Klopman-Salem perturbational approach is able to account for the experimental trends of the isomer ratio in terms of the major stereochemical structural differences between the acyclic vinyl sulfones and the four-membered ring sulfones (see Section IV.B.3). 

The literature describing theoretical studies on monocyclic thietanes and thietes until 1995 has been fully covered in CHEC-II(1996). However, the described papers were confined to semi-empirical or molecular orbital(MO) calculations. During the last decade, computational abilities have increased dramatically, and several papers have appeared dealing with ab initio calculations of thietane structures. Extensive ab initio calculations have been carried out in order to establish the isotropic shielding constant a and chemical shifts 8 of 14 variously substituted thietanes 1 <2000MRC468>. These values have been computed using the HF/6-31++G and geometries for this purpose... 

Theoretical and experimental studies on the reaction of 3,4-bis(methoxycarbonyl)-1,2-dithiete with alkenes and alkynes yielding cycloadducts have been carried out. The activation energy of the interconversion of the 1,2-dithiete to 1,2-dithione was estimated by MO calculations. These calculations (MP2/6-31G(d)) show that the dithiete 19f is 5.8 kcal mol-1 more stable than the ethane-1,2-dithione cis-20f, and the tautomerization energy is 28.5 kcal mol-1 from the thiete 19f and this value of the activation energy supports the possibility of the tautomerization between the 1,2-dithiete 19f and ethane-1,2-dithione cis-20f, at least at high temperature <1996IJQ859>. 

Figure 7. General formulas, bond angles in the hetero ring, and other calculated parameters of naphtho[l,8-i/c]thiete and its V-oxides...

The calculated r and p values (01JST287) predict that all three representatives of this heterocyclic system shown in Figure 7 [parent naphtho[l,8-Z)c]thiete (34) and its two S-oxides (35 and 36)] should be sufficiently stable to exist in a free state. Experimental data are consistent with theory. All the three compounds have been prepared by several teams. 

The X-ray diffraction analysis of naphtho[l, 8-ic]thiete X,X-dioxide (36) (76JA6643) revealed the coplanar position of the four-membered hetero ring and the naphthalene skeleton, and the r value [the ratio of bond angles C4-C10-C5/ C1-C9-C8 (132.5/106°)] equaled 1.25 in good agreement with the calculated data as were also many other parameters. 

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