Thiocarbonyls/sulfur containing

In comparison with other 1,3-dipoles that have been extensively explored in organic synthesis (7), sulfur-centered 1,3-dipoles (1-4) are rather uncommon species. However, within the last two decades, remarkable progress has been made regarding both methods of generation and synthetic applications. In particular, thiocarbonyl ylides (1) were established as key intermediates useful for the preparation of sulfur-containing heterocyclic compounds. General methods for the preparation of thiocarbonyl ylides and their chemical reactivity have been reviewed (8-11). 

Two types of sulfur-containing five-membered heterocycles are convenient precursors of thiocarbonyl ylides, namely, 2,5-dihydro-1,3,4-thiadiazoles (20) and l,3-oxathiolan-5-ones (21) (Scheme 5.5). The precursors 20 are accessible by two different methods. 

Since in many instances we shall try to compare the properties of thiocarbonyl compounds with their carbonyl analogs, we recall that, in general, the theoretical treatment of sulfur containing systems is more demanding, essentially because of the larger number of electrons in the sulfur atom. This has restricted the ab initio calculations to relatively small compounds or to the use of small basis sets. Until quite recently, high level ab initio calculations were only reported for a few, small thiocarbonyl systems. 

In summary, it seems well established that supplementary d functions are indispensable in molecular orbital calculations of sulfur containing compounds. This is particularly true for thiocarbonyl compounds, wherein the anisotropy of the electrondensity distribution around sulfur is significantly greater34 than that around oxygen in carbonyl systems. 

The sulfur-containing alkaloid zapotidine has been found to have the structure (545) (61JA2022). A synthesis of (545) has been achieved from (544b) by lithium aluminum hydride reduction to iV -methylhistamine followed by ring closure to (545) with thiocarbonyl-diimidazole (67T239). 

A synthesis of 4,5-dithio-L-xylose, a thio sugar in which the competition between sulfur-containing furanose and pyranose rings could be studied, has not yet been successful. Periodate scission of 3,4-0-isopropylidene-l,2-dithio-l,2-(thiocarbonyl)-D-glucitol led to an aldehyde which could not be isolated in pure form. ... 

So far we have considered sulfur-containing carbanions obtained by metallation with appropriate bases of a to sulfur atom(s) acidic C-H bonds. For synthetic purposes this method has been by far the most used. Among the variety of other routes to such carbanions [203] we shall consider two methods which seem to be general and promising the reductive lithiation of phenyl thioethers and the thiophilie addition of organometallics to thiocarbonyl compounds (vide infra Section 4.1.2). 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

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