1.2- Dithiolane-3,5-diones

Fig. 54 Pyridine catalyzed ring-expansion polymerization of dithiolane-2,4-dione...

Cyclohexane-1,2-dione reacts with ethylene glycol (TsOH, benzene, 6 h) to form the diprotected compound. Monoprotected 1,3-oxathiolanes and 1,3-dithiolanes are isolated on reaction under similar conditions with 2-mercaptoethanol and eth-anedithiol, respectively. ... 

Ueno and Okawara (184) were the first to explicitly formulate a conjugated thiocarbonyl ylide as an intermediate in the reaction of l,3-dithiolane-2-thione with 4-bromophenacyl bromide. The initially formed thiocarbonyl ylide undergoes deprotonation with sodium hydride to give 2-(4-bromophenyl)-l-oxa-4,6,9-trithias-piro[4.4]non-2-ene. 1,3-Diacylated thiocarbonyl ylides of type 149 (Scheme 5.45) have also been proposed as intermediates in the reaction of 1,3-diphenylpropane-1,3-dione with thionyl chloride. This reaction leads to 2,2,4-tribenzoyl-5-phenyl-... 

As another important mechanistic type, fragmentation into at least three products that are formed more or less simultaneously has to be mentioned. Such [5 - 2 + 2 + 1] reactions were encountered in 2-pyrrolidinones (65, 67), certain pyrazolidine derivatives (68, 72, 73), l,2,4-triazolidine-3-ones (69) and l,2,4-triazolidine-3,5-diones (70), 2//-pyrrol-2-ones (71), 1,3-dithiolane 1-oxides (78), 1,2,3,4-thiatriazoles (74), 1,2,4-oxadiazolines (75), and in several rings containing sulfur in an oxidized state (76-79,82). Most of these molecules have an exocyclic double bond. 

The 7-(5-amino-l,3-dithiolan-2-ylidene)-3-phenyl-5,6,7,8-tetrahydro[l,2,4]triazolo[3,4-A][l,3,4]thiadiazepin-6,8-dione 105 was obtained through the intermediate M by treating 98 with CS2 and chloroacetonitrile in a one-pot reaction under PTC (DMF/K2CO3/TBAB) (Scheme 10). 

The 1,3-dipolar cycloaddition of the carbonyl ylide (31) to the aldimine (32) produces the adduct (33), which has been used to synthesize the taxol C(13) side-chain (34), which is known to be required for the antitumour activity of taxol (Scheme 9).35 The dirhodium tetraacetate-catalysed decomposition of l-diazo-5-phenylpentane-2,5-dione (35) yields the carbonyl ylide (36), which cycloadds to methylenecyclopropanes (37) to produce spirocyclopropanated 8-oxabicyclo[3.2.1]octan-2-ones [(38)-(40)] in 6-75% yields (Scheme 10).36 The 1,3-dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides with thiobenzophenone produce both regioisomeric 1,3-dithiolanes as expected. However, in the case of highly sterically hindered thiocarbonyl ylides, methylene transfer leads to the formation of 4,4,5,5-tetraphenyl-l,3-dithiolane.37,38... 

Among the parent compounds, oxazoles, isoxazoles, imidazoles and benzothiazoles are important for synthetic transformations. The partially or completely hydrogenated systems, especially 1,3-dioxolanes, 1,3-dithiolanes, 4,5-dihydrooxazoles, oxazol-5(477)ones, 4,5-dihydroisoxazoles, imida-zolidine-2,4-diones and 2,4-dihydro-3i/-pyrazol-3-ones are of wider application. 

The first is fluorination of cyclopenta[c]thiophene-4,6-dione 106 by treatment with A-fluoro-6-(trifluoromethyl)pyridinium-2-sulfonate (MEC-04B) in ethyl acetate to give l,3-dibromo-5,5-difluorocyclopenta[c]thiophene-4,6-dione 107 in 84 % yield. Then, conversion of the two carbonyl groups to difluoromethylene groups was accomplished via formation of the bis-l,3-dithiolane derivative 108 followed by desulfurative fluorination with hydrofluoric acid-pyridine complex and dibromatin (l,3-dibromo-5,5-dimethylhydantoin) in dichloromethane to afford 1,3-dibromohe xafluorocyclopenta[c]thiophene 109 in a two-step yield of 73 % [Id, 67]. 

The selective ketalization of a typical Of-diketone, cyclohexane-1,2-dione, has been studied in some detail [259, 260]. It appeared that the monoketal, once formed, ketalizes more rapidly than the enolic starting material, thus accounting for the low yield of the mono-dioxolan (15-18%). The mono-oxathiolan and -dithiolan were formed in somewhat h her yield. Various nucleophilic reactions could be performed on the ee ketone function in these compounds. 

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