Sulphur extrusion reactions

Reactions in which a sulphur atom that bridges or interconnects two carbon groups is extruded with formation of a carbon—carbon bond between the two carbon groups is termed a sulphur extrusion reaction (equation 90). These types of reactions have proven to be of synthetic utility and are treated in this section. 

Sulphuranes - see also Dialkoxysulphuranes, Oxysulphuranes as intermediates 406 Sulphur bonding 484-493 Sulphur-containing groups, stereoelectronic effects of 584-594 Sulphur dioxide addition to 215-217 extrusion of 137, 140, 141, 158, 163, 397-402, 801, 805, 962, 1098 Sulphur monoxide, extrusion of 397-402 Sulphur trioxide, reactions of 217, 218 Sultenes, as intermediates 743 Sultines 679, 943 as photolytic products 881, 882... 

The reaction with benzothiophens provides a route to substituted naphthalenes by thermal ring-opening and sulphur extrusion of the photoadducts (equation 80) >... 

Sulphur extrusion and biaryl formation is reported for (129) which gives (130) on irradiation in the presence of trimethyl phosphite however, the reaction fails for (131), which is recovered... 

Cyclophanes. Many reports have appeared relating to these compounds. Most of them describe their synthesis, either by the sulphur-extrusion process reported several times in earlier sections or by some form of cycloaddition reaction. 

The Diels-Alder adduct of sulpholene and cyclopentadiene is a useful starting material for substituted diene synthesis121. The diene moiety is unmasked by retro-Diels-Alder reaction and sulphur dioxide extrusion under flash vacuum pyrolysis conditions (equations 74 and 75)122,123. 

Formation of naphthalene from phthalic anhydride and thiophene implies the extrusion of sulphur. Products arising from the reaction of this sulphur, possibly monatomic and hence highly reactive (Sidhu et al., 1966) with thiophene might be expected. Indeed, the mass spectrum revealed a product of molecular weight 116 and an isotopic distribution establishing the elemental composition as C4H4S2, doubtless thio-phenethiol. The relative amounts of the products were ... 

Furans give 2,5-cycloadducts on irradiation with alkynes (equation 75) . Thiophens probably behave similarly , but the product isolated is a substituted benzene which arises by extrusion of sulphur from the adduct (equation 76). The photochemical reaction with thiophen involves a triplet excited state of the thiophen, but both furan and thiophen cycloadditions can also be brought about thermally , (compare the pyrrole reaction in equation 74). 

Extrusion of sulphur dioxide from cyclic systems leading to dienes has proved to be a synthetically useful reaction Thermolysis of cis- and irons-2,5-disubstituted sulp-holenes, which can be readily obtained through regio- and stereoselective alkylation, proceeds in a stereospecific manner affording 1,3-dienes of high stereochemical purity, as predicted by symmetry rules (equations 68 and 69). On the other hand, a photochemical process is not completely stereospecific (equation 68). 2,5-DiaIkylative cyclization... 

We can apply this technique to another cheletropic reaction—the extrusion of sulphur dioxide from the sulphones (138 and 140).130 In this case, the reaction... 

The reaction of (148) with glycol and hydrogen peroxide gave (149). Compound (19b) has been reduced with liAlH4 to the corresponding alcohol, which was converted into the aldehyde and bromomethyl and aminomethyl derivatives that are of pharmaceutical interest. A mild method for the hydrolysis and decarboxylation of various amides of methyl 3-aminothiophen-2-carboxylates has been developed. Thermal decomposition of (150) led to cleavage of the thiophen ring, with extrusion of sulphur and formation of the isothiazole (151). ... 

Finally, thermal sulphur dioxide extrusion from c/j,-2,5-disubstituted 2,5-dihydrothiophene 1,1-dioxides, generated by a retro-Diels-Alder reaction, has been used for the synthesis of pellitorine (1) and pipercide (31) [64] (Scheme 19). 

A novel extrusion of S02 is reported for the dithiine dioxides 104 to afford low yields of the thiophenes 105 and 106. The exact step at which the S02 loss occurs is not known but the proposed intermediates 107 and/or 108 have support from the formation of pyrrole derivatives when the reaction is carried out in the presence of n-butylamine103. Sunlight-induced extrusion of sulphur dioxide from the pyranone dioxide 109 is similar to the above and results in the formation of the reactive cyclopentadienone 110 which can be trapped by dienophiles or in their absence forms a trimer104. The platinum complex 111 is photochemically labile and decomposes into benzyne on irradiation105. 

Thiophen Dioxides and their Dihydro and Tetrahvdro Derivatives. - Flash vacuum pyrolysis of cobalt complexes (194) of thiophen 1 1-dioxides led to extrusion of sulphur dioxide and the formation of cyclobutadiene complexes (195). Stereochemical labelling experiments were employed in order to determine the mechanism of this transformation.114 A detailed publication on the reaction of... 

Cycloaddition of the tetrazine (317) to cyanamides R2NCN, followed by extrusion of nitrogen, gives the 1,2,4-triazines (318) in good yield. A similar reaction with iV-(thionitroso)dimethylamine (McaN—N=S) gives the triazole (319), possibly by extrusion of sulphur from an initially formed thiatriazine. " ... 

The thermal extrusion of sulphur dioxide from sulphones and sulpholene adducts is a well known procedure for producing 1,3- or 1,4-dienes and skipped polyenes. Lithium aluminium hydride has now been shown to effect this extrusion in good yield under mild conditions (refluxing ether)." The technique is similar to, but apparently mechanistically different from, the LiAlH4-promoted retro-Diels-Alder reaction of triazolinedione adducts reported recently by Barton. 

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