Thiane oxides reactions

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids299 (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism246,300 and thus facilitates the desired stereospecific synthesis301. The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene301. 

The Pummerer reaction of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position . The overall mechanism is illustrated in equation 129. 

Thiane oxides have been shown to be reduced cleanly back to the thiane with phosphorus pentasulfide under conditions to which sulfones, sulfinates, ketones, esters and amides are inert (78CJC1423) the potential of this reaction, though not yet applied, is obviously considerable, especially when coupled with the old-established Raney nickel desulfurization technique. Thiane itself is desulfurized to pentane (with traces of cyclopentane) but the opportunity to construct alkyl chains of great complexity regio- and stereo-specifically is there. At the very least, the reduction to tetrahydrothiopyrans presents a very useful entree into a wide range of 2-substituted thianes (Scheme 2). 

The enantioselective reaction of a-sulfinyl carbanions involves both desym-metrization of the prochiral alkyl groups and diastereoselection. Simpkins and coworkers have reported the asymmetric induction of a traws-thiane oxide with a camphor-derived chiral lithiiun amide giving products with complete diaster-eoselectivity and with good enantioselectivity (Table 6) [59,60]. 

Electrocatalytic formation of Me SO from DMSO was effected using cw-[Ru(0) (py)(bpy)2] VPt electrode/water-DMSO cf. mech. Ch. 1 [113], Oxidation of thian-threne-5-oxide by RuO /aq. Na(IO )/CCyO°C gave a mixture of the sulfone and thianthrene-5,5-dioxide with minor amounts of the disulfoxide and also thian-threne-5,5,10-trioxide comparisons were made between the behaviour of RuO, CrOjClj and [MnO ] for these reactions cf. mech. Ch. 1) [114]. 

In this synthesis the geometry of the acyclic double bonds is controlled through their formation as part of the thiane ring. Thiacyclohexanone (711) was converted to 4-thia-l-methylcyclohexene by reaction with methylmagnesium iodide and subsequent dehydration. Metallation of (712) with s-butyllithium and alkylation of the anion with the epoxide (713) gave a tertiary alcohol which was dehydrated to yield (714). A second alkylation of (714) with trails-4-chloro-3-methyl-2-butene 1-oxide (715) completed the carbon skeleton of the Cis juvenile hormone. Reduction of (716) with lithium in ethylamine and then desulfurization with Raney nickel led to trienol (717), a product converted previously to (718). 

Tetrabutylammonium periodate in the refluxing mixed solvent toluene/methanol (5/1 by volume) cleanly oxidizes the sulfides to sulfoxides. When three equivalents of tetrabutylammonium periodate were used, the test sulfides 2-dodecyl thiolane and 2-dodecyl thiane were >99% converted after a 20-minute reflux to the corresponding sulfoxides (98%) and sulfones (2%). After 60 minutes the yield of sulfone had increased to 10%. The reaction is highly dependent on the exact conditions employed. Small deviations from the recommended conditions may produce unacceptable results (24). Sulfones of saturated sulfides are difficult to reduce (22) and thus their generation should be minimized for maximum recovery of the sulfides. 

Reactions.— The stereoselectivity of the abstraction of diastereotopic protons a to trico-ordinated sulphur and the stereochemistry of electrophilic reactions at the carbanion centres of cFlithio-thian 1-oxides still excite considerable interest. The rate factors for base-catalysed H-D exchange of a-protons in several conforma-tionally biassed or rigid thianium cations have been measured with increasing rigidity, the reactivity of decreases, while that of remains about constant. It... 

Oxidation of cw-2,6-diphenylthian with Br2-H20 gave mainly the all-equatorial isomer, while its reaction with Bu OCl furnished its epimer. In contrast, oxidation of the 4-hydroxy-thian (59) with Bu OCl afforded (60), having the SO... 

Detailed studies of the a-chlorination of substituted thian S-oxides have been published. This reaction also showed remarkable stereoselectivity, and the determination of the stereochemistry of the products has given valuable mechanistic information. Mcmochlorination of either cis- or trans-4-t-butylthian S-oxide gave the same products the major one, (44), formed in 95% yield, had the oxygen atom equatorial and the chlorine axial. The minor chlorination product (45) also had the chlorine atom and sulphur lone-pair trans . ... 

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