Thietan 1-Oxides

Whereas the transformations thietane - thietane oxide- thietane dioxide are easy to perform192, as is the reverse transformation thietane dioxides ->thietanes188, no method of reducing the sulfonyl group to a sulfoxy group is available as yet. 

The proton spectra analysis of thietane, thietane oxide and thietane dioxide at 100 and 300 MHz in the temperature range — 140 to 190 °C confirmed the puckered structure for the oxide (5a) with the sulfinyl oxygen in the equatorial orientation, as inferred from chemical-shift considerations180. It appears that the repulsive-type 1,3-interactions between the oxygen and the 3-substituents184 are operating between oxygen and the axial proton on C-3 in the unsubstituted thietane oxide (5a). For the thietane dioxide (5b ... 

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

Based on NMR chemical shift assignments and the use of recorded spin-spin coupling constants it was determined193 that in both 2,4-diphenyl-substituted thietane oxides (186a,b) the dominant conformers are those in which the S—O bond is equatorial and, therefore, in the trans-2,4-isomer186 one phenyl group (i.e. R1) is syn-axial to the S— O bond, whereas in the cis-2,4-diphenyl isomer 186b both phenyls are anti-equatorial to the S—O bond. 

It might well be that, compared with other thietane oxide systems, the larger pucker angle here is due to the two bulky 2,4-phenyl substituents that tend toward equatorial conformation. 

The structures of four-membered rings are of considerable interest, owing in part to the low-frequency ring puckering vibration203. The comparison of the structures and conformational preferences of thietane oxides and dioxides discussed above with those of dithietane oxides and dioxides is therefore appropriate and will follow. 

The crystal structures of several thietane oxides have been determined. Bond lengths and angles are given in Table 6. 

TABLE 6. Bond lengths" and bond angles in selected thietane oxides R1 5(5 186) and 1,3-dithietane tetroxide... 

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

FIGURE 1. Sulfur lone-pair and %<, ionization patterns in 1,3-dithietane, thietane oxide and 1,3-dithietane oxide. 

The proton spectra of thietane oxide (5a) and thietane dioxide (5b) have been studied in order to evaluate whether the oxidation at the sulfur atom changes the established 35° puckering of the ring218, and whether a correlation is possible between structure and... 

The previously discussed conformational study of 3-substituted thietane oxides using lanthanide shift reagents185 corroborates the conclusions derived from other NMR studies and suggests that all rrans-3-substituted thietane oxides prefer an equatorial oxygen conformation when the thietane oxides are bound to shift reagents. 

Carbon-13 chemical shifts of the a- and (8-carbon atoms of various unsubstituted and 3-substituted thietane oxides and dioxides have been recorded and correlated by the equations S = ay + bx and Sf = ax + by where a and b are parameters characteristic of the sulfoxide or sulfone (y) and the substituent (x)216. The values of the substituent parameters were found to parallel those which determine the effect on the 13C chemical shifts when hydrogen is replaced by a substituent224. 

TABLE 8. Chemical shifts (ppm)" and coupling constants for selected thietanes, thietane oxides and X... 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

It is noteworthy that in spite of the demonstrated acidity of the a-hydrogens in thietane oxides and dioxides, attempted mono- or dialkylations of these systems have been unsuccessful thus far. 

The method of choice for preparing thietane oxides is the oxidation of thietanes. This can be conducted using hydrogen peroxide, sodium hypochloride194, sodium metaperiodate66, NaI0474c and m-chloroperbenzoic acid185. 

Oxidation of the thietanes provides thietane oxides (equation 81). 

The addition of sulfenic acids to olefins207 has been successfully applied in the synthesis of thietanoprostanoids, the thietane analogues of prostaglandin245. The general synthetic scheme is presented in equation 83207. The key step is the thermolysis of either erythro- or (7ireo-2-f-butylsulphinyl-3-vinyl-l-ol (209) to give the corresponding alkenesulfenic acids 210, which cyclize spontaneously to a mixture of stereoisomeric thietane oxides. 

Given any thietane, oxidation of the sulfur to a sulfone with peracids202,203 or H20 274c is straightforward and in most cases neither intervenes chemically with other sites nor alters the structural features or stereochemistry of the thietane ring. 

Stepwise decomposition of thietane oxides should be influenced by the relative stabilities of the developing radical centers, whereas the subsequent selection between... 

Dithietane oxides, Thietane oxides, Thiete oxides... 

The preparation and investigation of the thietane oxide system (5a) is largely associated with stereochemical and conformational studies . The investigation of the thietane dioxides (5b) is substantially related to the chemistry of sulfenes , the [2 -I- 2] cycloaddition of which with enamines is probably the method of choice for the synthesis of 5b . The study of the thiete dioxide system (6) evolved, at least in part, from the recognition that the unstable thiete system 183 can be uniquely stabilized when the sulfur in the system is transformed into the corresponding sulfone , and that the thiete dioxide system is very useful in cycloadditions and thermolytic reactions. The main interest in the dithietane oxides and dioxides (7) appears to lie in the synthetic challenge associated with their preparation, as well as in their unique structural features and chemical behavior under thermolytic conditions . ... 

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