Dithioesters carbophilic addition

From a synthetic point of view, both thiophilic and carbophilic additions on dithioesters afford pathways for carbon-carbon bond formation, and as the dithioacetals formed in these reactions are easily converted into carbonyl or methylene groups the dithioesters are equivalent to the four synthons... 

The possibility of conversion of the dithioacctal into the corresponding ketone without reconjugation of the double bond is a noteworthy j feature of this synthetic scheme. The syntheses of iso-ar-turmerone and isoartemisia ketone mentioned above proceeded similarly via a carbophilic pathway. In the latter case a p-ethylenic dithioester was not isomerized during the following overall sequence carbophilic addition-methylation-dithioacetalization [343]. ... 

Reformatsky reagents were shown to undergo carbophilic addition with a variety of thiocarbonyl compounds (thioketones, dithioesters, trithiocarbonates and xanthates). Elimination of sulfur (or of an alkylthio group) led to the formation of carbon-carbon double (or single) bonds [448]. Typically ... 

The reaction of organocuprates with dithioesters has been reported [350]. High yields of tertiary thiols were obtained via a novel sequence of a double-barrelled carbophilic addition, such as with lithiocuprates ... 

Exceptions to the carbophilic addition of heteronucleophiles to thiocarbonyl compounds were demonstrated with thiols and thioaldehydes [160] or dithioesters [36] bearing an a-electron withdrawing group, which undergo selective thiophilic addition. 

Reactions of organomagnesium compounds with a,(3-unsaturated dithioesters are somewhat unpredictable, 1,2-, 1,4-carbophilic and thiophilic addition having been observed [6], With a,(3-unsaturated thioamides [7] and imidothioesters [6], however, 1,4-carbophilic addition appears to predominate. 

The product is a new organomagnesium compound, which may be used in situ for further reactions reactions with dithioesters are particularly useful, giving functionalized dithioketals [2], It is interesting to contrast the following example with the reaction involving carbophilic addition described in the same paper (see p. 148) [3]. 

The thiophilic addition D is preferential when the temperature is less than — 17°C and when R is an ethyl or an isopropyl group similar results are observed with other dithioesters [33]. With CHaMgBr or CH3MgI, carbophilic addition is preferential if the reaction is performed at 20°C or at reflux, leading to the thiol E. 

Carbophilic addition of prenylmagnesium bromide and other allylic Grignard reagents to the thiocarbonyl group of dithioesters occurs exclusively with rearrangement of the allylic... 

A number of functionalized dithioesters have been utilized for carbophilic and/or thiophilic additions [120]. We shall mention only the potentialities offered by the readily available enolizable p-oxodithioesters (for a review, see [349]) R COCH(R2)CS2Me. 

---