Thiophene endoperoxides

The thiophene endoperoxides ate considerably more labile than the furan endo-peroxides 26), but diimide is sufficiently reactive even at —30 to —50 °C to trap them in the form of thiaozonides 13. 

The thiophene endoperoxide (18) is a powerful episulfidation reagent. The stereospecific transformation of cis- or trani-cyclooctene suggests a concerted process. First-order consumption kinetics have now shown that (18) is not itself the active -transfer reagent two such intermediates are proposed, possibly oxathiiranes such as (19) or (20), based on similarities in the trends for epoxidations of the same substrate by DMDO. ... 

The reactions of a number of ozonides and alkenes in the presence of BF3 Et20 resulted in the formation of E- and Z-l,2-dioxolanes in 12-70% yields (Scheme 30) <83JA6279>. 2,5-Dimethyl-thiophene endoperoxide can act as a sulfur- or oxygen-atom transfer agent in analogous reactions to those shown by furan endoperoxides <86JA2775>. 

The thiirene 6 was proposed as an intermediate in the formation of a cycloadduct 23 obtained from the reaction of a thiophene endoperoxide 36 <1996CC177> and cyclooctyne under thermal conditions (Scheme 6). The structure of 23 has been verified by X-ray crystallography <2002JA8316>. 

Another interesting synthesis of fused-ring thiiranes was reported by Weinkotz and co-workers in a sulfur-transfer reaction from 3,3,7,8-tetramethyl-9,10-dioxa-ll-thiatricyclo[6.2.1.0 ]undec-6-en-5-one (a derivative of thiophene endoperoxide) 37 to cyclic olefins under thermolysis conditions, or at low temperature using tetraphenylporphine cobalt(ll) complex as a catalyst (Scheme 56) <1996CC177>. The endoperoxide was prepared by photo-oxygenation... 

Several cycloaddition reactions of vinylthiophene and vinylbenzo[6]thiophene are known. 2-Vinylthiophene easily adds 4-phenyI-l,2,4-triazoline-3,5-dione to give (308) (74JA5591). 2-Vinyl-thiophenes also undergo stereospecific cycloaddition with singlet oxygen to produce the corresponding endoperoxides (Scheme 87) (75TL4471). 

It is known that thiophenes react with singlet oxygen forming endoperoxides, which are thiaozonides, and whose thermal reactions have been investigated. A novel reaction of acylthiophene endoperoxides 213 (photosensitized oxidation with tetraphenylporphyrine at —30 °C) involves their treatment with triphenylphosphine. The nucleophilic attack finally leads to a furan derivative 214 by elimination of O and S atoms bound to phosphorus (Scheme 71) <1995TL7431, 1998JA8914>. 

Another example of the extmsion of sulfur from a thiaozonide intermediate is given in Section 3.10.2.2. In that sequence, the endoperoxide radical cation resulted from the addition of triplet oxygen to the radical cation of a tetrasubstituted thiophene. 

Using this method, precursors of thiophene containing tricyclic prostaglandin endoperoxide analogs are available by addition of thienyl organomercurials to norbornadiene in the presence of palladium(II) chloride/lithium chloride and subsequent conversion of the nortricyclenyl palladium product 32 with carbanion reagents. ... 

Singlet oxygen reacted stereospecifically with 2-vinyl-thiophens in a 1,4-cycloaddition to give thermally stable 1,4-endoperoxides (147). The previously discovered [2 + 2] photoaddition of benzophenone to 2,5-dimethylthiophen to give oxetans has been extended to other carbonyl derivatives. Mechanisms explaining the formation of pyrroles, obtained by u.v. irradiation of a number of thiophens in the presence of propylamine, have been discussed. Irradiation of (148) led to the Dewar thiophen (149). The structures of dimerization products and Diels-Alder adducts with furans were elucidated. ... 

Unlike furan, thiophene was found to be unreactive under photooxygenation conditions. The alkylated derivative 2,5-dimethylthiophene 33, however, is reactive and the structure of the endoperoxide 34 was unambiguously proven by low-temperature NMR studies, as well as by diimide reduction to the saturated thermally persistent endoperoxide 35. ... 

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