Thiiranes thiocyanate

Sodium or potassium hydrogen sulfite reacts with several thiiranes to give disulfides of /3-mercaptosulfonic acid salts (76EGP122086). Potassium thiocyanate in dimethylformamide or aqueous ethanol isomerizes thiiranes (Scheme 84) (72CJC3930). 1,2-Dithiols are obtained by treatment of thiiranes with NaBH2S3 obtained from sodium borohydride and sulfur (73TL1401). 

Treatment of cyclic carbonates of 1,2-diols with thiocyanate ion at temperatures of 100 °C or higher yields thiiranes (Scheme 145) (66CRV297, 75RCR138). Thiourea cannot replace thiocyanate satisfactorily, and yields decrease as the carbonate becomes more sterically hindered. The reaction mechanism is similar to the reaction of oxiranes with thiocyanate (Scheme 139). As Scheme 145 shows, chiral thiiranes can be derived from chiral 1,2-diols (77T999, 75MI50600). 

Experiments with cyclic thioethers (80JCS(P1)1693), thiourea, thiocyanate and ethyl xan-thate always led to destruction of oxaziridines (73AJC2159). Products of complicated consecutive reactions could be isolated but only with some difficulty, e.g. (92) from a reaction with carbon disulfide (74JOC957), and (93), obtained by trapping with butadiene a product of a reaction between an oxaziridine and a thiirane (80JOC1691). 

In general, thiocyanate salts are used for the epoxide-thiirane conversions. The reaction proceeds by nucleophilic attack on the epoxide by thiocyanate ion followed by cyclization as shown for (121) (125). The formation of a... 

Acetic anhydride has also been used as the acylating agent. The formation of thiiranes from thiocyanatohydrins having a tertiary hydroxy group is best achieved by p-toluenesulfonic acid-catalyzed acetylation.The analogous thiocyanatohydrins with a secondary hydroxyl and a tertiary thiocyanate function give a predominance of epoxide from thiocyanatohydrin acetates since the hydrolysis rate of the secondary acetate grouping becomes competitive with that of the tertiary thiocyanate. 

The use of thiocyanic acid for thiirane synthesis is usually compatible with many functional groups found in natural steroids. The method has found application in androstanes, pregnanes, cholestanes, cholanates and cardeno-lides. However, the presence of a,j5-unsaturated carbonyl groups may decrease the yield of the thiocyanatohydrin due to conjugate addition of thiocyanic acid. Indeed, the 1,4-addition of thiocyanic acid has been carried... 

The use of potassium hydrogen carbonate for the cyclization of thiocyan-atohydrin mesylates containing alkali-sensitive groups has been reported, but the selectivity for thiirane formation is reduced. ... 

Infrared, nuclear magnetic resonance, ultraviolet, optical rotary dispersion and circular dichroism measurements have been used for the spectral analysis of thiiranes. A few steroidal thiiranes have been reported to possess infrared absorption in the range from 580 to 700 cm The intermediate thiocyanate derivatives (RSCN) have a strong sharp peak at 2130-2160 cm the isomeric isothiocyanate (RNCS) shows a much stronger but broad band at 2040-2180 cm. ... 

A quite different use of a thiocyanate salt is the conversion of an epoxide (oxirane) to an episulfide (thiirane) this normally heterogeneous reaction is markedly accelerated by the use of silica gel coated with, or finely ground with, potassium thiocyanate, rather than with the thiocyanate salt alone.17... 

A series of epoxides has been efficiently converted to the corresponding thiiranes using a polymer-bound quaternary ammonium thiocyanate 38. The polymeric byproduct of this transformation is the supported cyanate.56... 

Tamami, B. Kiasat, A. R. Synthesis of Thiiranes From Oxiranes Under Mild and Nonaqueous Conditions Using Polymer Supported Thiocyanate, Synth. Comrnun. 1996, 26, 3953. 

A classic prototype of these reactions is the conversion of oxiranes into thiiranes by thiocyanate ion (Scheme 29 CHEC 5.06.4.3). Inversion at both ring carbons makes the reaction stereospecific with respect to the E Z relation of the substituents on the oxirane carbons. 

In general, thiocyanate salts are used for the epoxide-thiirane conversions. The reaction proceeds by nucleophilic attack on the epoxide by thiocyanate ion followed by cyclization as shown for (121) - (125). The formation of a fused cyclic intermediate (123)143 is only possible if the sulfur and oxygen substituents are equatorial and ring A assumes a boat conformation. These restrictions limit the application of this procedure to the synthesis of 2,3-thiiranes which are usually obtained in less than 15% yield.144... 

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