Thioaldehydes 5-oxides

The geometry of the silyl sulfines 30 was determined by LIS measurements. During chromatographic purification on silica gel, a partial protiodesilylation to the corresponding thioaldehydes 5-oxides was observed (vide infra). 

Protiodesilylation of thioacylsilane 5-oxides 30 (silyl sulfines) offers a unique mild synthetic path to thioaldehyde 5-oxides 40 (monosubstituted sulfines), which... 

A detailed study14 of the stereochemistry of fluoro-desilylation (Table 5) showed that the removal of silicon is a stereospecific process occurring with retention of configuration. Accordingly, from the E-isomers of thioacylsilane 5-oxides 30, the Z-thioaldehyde 5-oxides 40 were obtained. The myrtanyl derivative (entry 9) is the first enantiomerically pure thioaldehyde 5-oxide ever prepared. In contrast, loss of stereochemical integrity was observed during the desilylation of tert-butyl trimethylsilyl sulfine (entries 4 and 5) and mesityl-trimethylsilylsulfine (entries 10, 11, and 12). It was demonstrated that the loss of stereospecificity results from a fluoride-induced equilibration of thioaldehyde 5-oxides after the desilylation. 

Thioaldehyde 5-oxides were subjected to 1,4-cycloaddition with a variety of... 

The behavior of monosubstituted sulfmes is in contrast to that of unsymmetrically disubstituted analogues in the cycloaddition reaction, which occurred with retention of the stereochemistry.44 The deviant result was attributed to a Z to E isomerization of sulfmes during the cycloaddition and to a surprisingly high reactivity of the -thioaldehyde 5-oxides compared with the Z-isomers.43... 

Thioaldehyde oxides are reported to be one of the lachrymatory agents in onions and are speculated to dimerize via an initial [4+2] cycloaddition to a five-membered ring that rearranges to a 1,2-dithietane dioxide. The latter has also been isolated from extracts of onions (80JA2490) and is reported to be the only characterized derivative of a 1,2-dithietane (Scheme 121). 

The cycloaddition reactions of the reactive a-oxo sulfines 45 with dienes are well investigated and [4+2] cycloadducts 46 are obtained. Benzoyl substituted thioaldehyde -oxides already react with 2,3-dimethyl-l,3-butadiene at —78 °C. 

Thioaldehydes and thioketones oxidized at sulfur are additively named as thioaldehyde oxides, dioxides and thioketone oxides, dioxides, respectively. Besides, the traditional names sulfine and sulfene are still used for these species. 

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

Thiocarbonyl compounds can be generated thermally in the gas-phase from a variety of precursors. Bock et al. [77JCS(CC)287 82CB492] have shown that pyrolysis of 1,2,4-trithiolane derivatives (81) is especially advantageous for this purpose. The fragmentation reactions were monitored by PE spectroscopy. By this method thioaldehydes including thioformaldehyde and thioketones that polymerize readily were obtained. 1,2,4-Trithiolane 4-oxide (82) yields a mixture of thioformaldehyde and thioformaldehyde oxide. 

The corresponding 1,3-dithiolane S -oxides were prepared by oxidation of the thioacetals with m-chloroperbenzoic acid (mCPBA) and, notably, this approach has proven successful in the obtention of both thioaldehydes and thioketones. 

In the sulfur literature only a few examples of thioaldehyde S-oxides 99 (mono-substituted sulfines) have been reported due to the low stability of these compounds, and they were not prepared by oxidative methods since the starting thioaldehydes are also very unstable. These problems were circumvented by using silyl thioketones249,250 as precursors which, by controlled oxidation with mCPBA, afforded the corresponding thioacylsilane S-oxides 98. These sulfines were subsequently desilylated with BU4NF (equation 104) and the stereochemistry of this process has been studied in detail408. This indirect route has been applied to the preparation of aromatic and aliphatic, not enethi-olizable, thioaldehyde S-oxides. 

A novel synthesis of a-unsaturated sulfines has been introduced by Bra-verman et al. [99]. Et3N or DABCO treatment of allylic and benzylic tri-chloromethyl sulfoxides triggered the elimination of chloroform and formation of the sulfines. It must be stressed that these sulfines are thermally relatively stable, and this stands in high contrast to the corresponding thio-carbonyl compounds unsaturated thioaldehydes cannot be monitored under the same experimental conditions and have to be used at very low temperature or trapped in situ. The first synthesis of thioacrolein S-oxide was achieved by flash vacuum thermolysis of an anthracene allyl sulfoxide [100], and both isomers in a (Z E) ratio of 78 22 were characterised by NMR spectroscopy at -60 °C. 

The reaction of alkoxides with thioaldehyde S-oxides, generated from sul-finates, was carefully examined by Baudin et al. [163]. A number of products were isolated, formation of which was explained by carbophilic addition of the alkoxide anions. 

Thioaldehyde S-oxides behave as dienophiles with 1,3-dienes [241]. Cycloaddition involves the carbon-sulfur double bond of the heterocumulene to furnish dihydro-2H-thiapyran S-oxides (Table 6, entry 1). The cis trans product ratio was found to depend upon the initial diene/sulfine ratio. A large excess of diene selectively provided the cis cycloadducts, as a result of a stereospecific reaction of the (Z) sulfine. With a produced proportion of diene, mixtures of cis trans isomers are obtained, with a preference for the cis compound. The Italian authors have demonstrated that the slower reaction allows a (Z) to (E) isomerisation of the sulfines prior to addition. 

Cycloaddition reactions of thioaldehydes and sulfines are most probably encountered in plants, as elegantly and soundly shown by the group of Eric Block during their investigation of sulfur products occurring in the Allium species (for a review see [91]). They were able [92, 93] to isolate bicyclic dithioacetal oxides, called zwiebelanes, and also to synthesise them from a thioxosulfine, already described in this review (Sect. 2.6, Scheme 18). An extremely rich stereochemical and analytical study has resulted. 

The desilylated products 31 and 32 (Scheme 20) were obtained by the protiodesilylation of a number of thioacylsilane adducts and the corresponding sulfones obtained by oxidation of the cycloadducts with oxone (potassium hydrogen persulfate). Compounds 31 are formally derived from unstable thioaldehydes and the cyclic sulfones 32 from thioaldehyde 5,5-dioxide (sulfenes) (Scheme 20). It should be noted that sulfenes produced by dehydrochlorination... 

Thiophene 1-oxides have been shown to undergo syn-n-face see 110 selective Diels-Alder reactions. The oxide was shown to preferentially undergo Diels-Alder reaction with a variety of thioaldehydes, thioketones <03TL5159> and dienophiles <03JA8255>, through the w-n-face to give 111-117. 

Nakayama and co-workers have shown that 3, 4-di-/er/-butylthiophene 1-oxide can serve as trapping agents for unstable thioaldehydes and thioketones. Benzothiophenium salts 118 have been shown to undergo [4+2] cycloaddition with a number of dienes to give compounds of type 119<03JOC731>. 

Sulfines, which are thiocaibonyl 5-oxides, have been used many times as heterodienophiles. As with thioaldehydes, the development of this field has been limited more by the availability of these often unstable species rather than by problems with the Diels-Alder chemistry. It should be noted that sulfines can exist as configurationally stable ( )- or (Z)-isomers and this stereochemistry can play a role in the outcome of the cycloaddition reactions. 

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