Thiocarbonyl compounds rearrangement

Some applications of the thio-Claisen rearrangement to the syntheses of thiocarbonyl compounds, particularly unsaturated dithiocstcrs and thionesters, and of its stereocontrol aspects are reviewed in [120]. A particularly high stereoselectivity was observed with S-allyl derivatives of P-hydroxydithioesters syn products were obtained exclusively, according to the accompanying scheme [495,496]. 

Keywords Thiocarbonyl compounds, Thiophilic addition, Cycloaddition reactions, Enethioli-sation, Eschenmoser reaction, Sigmatropic rearrangement. 

Pyrolysis of certain 0-[(alkylthio)thiocarbonyl] compounds yields, by an intramolecular rearrangement, members of another group of dithiocarbon-ates, the [Pg.111]

Base-induced S-allylation of dithioesters [17,18], thionoselenoesters [19] as well as tertiary thioamides [20-23] with allylic halides and alcohols [24] have been commonly used to generate S-allyl ketene-S,W-acetals (W = S,Se, N), which are prone for a thio-Claisen rearrangement. In fact, the marked trend of the starting thiocarbonyl compounds to render the cis-... 

Thiocarbonyl compounds are also substrates of monooxygenases, forming S-monoxides (sulfmes, reaction 3-A) and then S-dioxides (sulfenes, reaction 3-C). Asa rule, these metabolites cannot be identified as such because of their reactivity. Thus, S-monoxides rearrange to the correspondingcarbonyl by expellinga sulfur atom (reaction 3-D). This reaction is known as oxidative desulfurationand occurs in thioam-ides and thioureas (e.g., thiopental). As for the... 

On the other hand, the rearrangement leading to thiocarbonyl compounds, such as dithioesters, thionesters or thioamides, instead of thioketones, has been shown to be driven to the right side. The only exception was observed when the initial unsaturated sulfide exhibits an aromatic nature [35]. 

The sulfur version of the Claisen rearrangement has emerged as a facile route towards the formation of C-C bonds and as an attractive synthesis of thiocarbonyl compounds. It was performed with successful control of both relative and absolute configurations and provides access to a variety of sulfur heterocycles. Relatively few applications to the synthesis of natural products have been reported so far, but they illustrate the advantages of the sulfur version. 

An attempted synthesis of biotin using thiocarbonyl ylide cycloaddition was carried out (131,133,134). The crucial step involves the formation of the tetrahydrothiophene ring by [3 + 2] cycloaddition of a properly substituted thiocarbonyl ylide with a maleic or fumaric acid derivative (Scheme 5.27). As precursors of the thiocarbonyl ylides, compounds 25a, 72, and 73 were used. Further conversion of cycloadducts 74 into biotin (75) required several additional steps including a Curtius rearrangement to replace the carboxylic groups at C(3) and C(4) by amino moieties. 

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