1.3- Oxathiole

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

Oxathiol-4-ones (4 nhydro-4-hydroxy-l,3-oxathiolium Hydroxides) (498)... 

The reaction of a thiocarbonyl and a-oxodiazo compound that leads to 1,3-oxathioles has been rationalized by a 1,5-dipolar electrocyclization reaction (178). It was suggested that an intermediate thiocarbonyl yhde bearing a C=0 function at the a-position (extended dipole) was first formed. Due to the low reactivity of a-oxodiazo compounds, these reactions were carried out at elevated temperatures or in the presence of rhodium acetate as the catalyst. In some cases, catalysis by LiC104 was also reported (77-80). 

Acyl-substituted thiocarbonyl ylides have been suggested to undergo 1,5-dipolar cyclization to give 1,3-oxathioles (55,179-183). Convincing evidence for the intermediacy of thiocarbonyl ylide 146 involves the formation of structure 147 starting from either two different pairs of reagents (179,180) (Scheme 5.44). 

When thiocarbonyl and ot-diazocarbonyl compounds are combined, acyl-substituted thiocarbonyl ylides 158 are generated from a nonisolable 3-acyl-1,2,4-thiadiazoline 157 (Scheme 8.36). In addition to giving acylthiiranes 159 and 1,3-dithiolanes 160, dipoles 158 can also 1,5-cyclize to produce 1,3-oxathioles 161. Acyl-thiocarbonyl ylides derived from diazoketones [e.g., HC(0)C(N2)R, R = Ph, f-Bu (219,220) 2-diazocyclohexanone (221)] produce 1,3-oxathioles [e.g., 162 (220), Scheme 8.36], while those derived from diazoesters (218,222,223) lead to thiiranes by 1,3-cyclization. Ylides derived from a-diazocarboxamides form 1,3-oxathioles (e.g., 163) and thiiranes (e.g., 159, R = f-Bu, R = NMePh, R = R" = Ph), depending on the nature of the substituents (220). A related 1,5-cyclization of an aminomethyl-thiocarbonyl ylide formed from dimethyl 3-anilino-2-diazobutanedioate was also reported (224). 

The manner in which a carbene was generated was found to be critical to the product distribution in a synthesis of 4(5//)-thiooxazolones (Scheme 6.9). Treatment of diethyl bromomalonate with excess triethylamine in the presence of benzoyl isothiocyanate afforded a mixture of the 4(5//)-thiooxazolone 25 (44%) and the 1,3-oxathiole 26 (minor). However, if the carbene was generated via copper-catalyzed decomposition of diethyl diazomalonate, then 26 was isolated as the major product, albeit in low yield (22%). 

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