Dithioester enethiolates

The thiophilic addition of aliphatic Grignard reagents to dithioesters deserves some comment. Although they are less reactive than organolithium compounds, their reactions are selective. With enethiolizable dithioesters, enethiolization cannot be avoided with the organolithium species, and at low temperatures this can become exclusive [332]. Enethiolization does not interfere in the case of the less... 

The addition reaction of enethiolates towards Michael acceptors has been investigated (for reviews, see [120, 362]). It was established that lithium dithioester enethiolates undergo regioselective 1,4-addition with a wiik ... 

The stereochemical aspects of the Michael addition of lithium dithioester enethiolates in the challenging acyclic series have been examined [362]. A stereospecific addition of a c/s-enethiolate to an ( )-enone furnishes the anti isomer (6). 

The aldol reaction of dithioester enethiolates has been used for the synthesis of dithiolactones [123], chiral substrates for the Claisen rearrangement [124, 125], and oxathianes [126]. Thioamide enethiolates may be employed in this reaction as well as shown with /Tthiolactams [26], or with a precursor of the antitumour agent vinblastine [127]. 

The reactive species in these additions has not been fully elucidated. It is likely that considerations similar to those involved in the addition of dithioester enethiolates to enones are involved (vide supra). 

Again for synthetic applications, thio- and dithioesters are particularly useful. Their enethiolates are easily generated [120]. Potassium and lithium amides and LDA can be conveniently used, and deprotonate the dithioesters quantitatively in THF at -78°C. Methyllithium under the same conditions also acts as a base and not as a nucleophile, and it has the advantage that its conjugate acid is the totally inert methane. 

When R is an alkyl group, the 3-oxodithioesters available through sulfhydrolysis of the acylation products could not be prepared either by direct condensation of carbon disulfide on a ketone or via oxidation of the P-hydroxy dithioesters formed in the aldolization reaction of enethiolates (see Section 4.2.4). 

Applied to dialkenyl disulfides, the dihetero sigmatropic shift gives rise to 1,4-dithiocarbonyl derivatives. The precursors are easily obtained by oxidation of the enethiolates, and the reaction has been observed with thioketones [500]. thioamides [501], dithioesters [356, 502] and thionesters [503],... 

Metzner reports that lithium dithioenolates, which are softer nucleophiles than the corresponding carbonyl enolates, add kinetically in a 1,4-mode exclusively to ot, 3-enones and the diastereoselectivity preferences are similar to that for ester enolate additions.l43a d Typically, kinetic deprotonation of dithioesters affords predominantly the (Z)-enethiolate (170) which is opposite to that for esters 139 thus, with ( )-enones, anti adducts (172) are obtained. In contrast, the addition of methyl dithioacetate to ot,P-disubstituted enones affords predominantly syn adducts (174 and 175) which is a consequence of intramolecular protonation of the resultant enolate (Scheme 68).143e... 

Amination of enethiolates has only been recently tackled by Beslin and Marion [136] and Hartke et al. [137]. Silylketene dithioacetals or lithium enethiolates react with azodicarboxylates to yield a-hydrazino-dithioesters, thionoesters or thioamides. Homologous dithioesters, bearing an amino moiety on the f5 position, have been prepared from the reaction of silylketene dithioacetals with an imine in the presence of a Lewis acid [138]. 

The noteworthy cis selectivity of the deprotonation of thiocarbonyl compounds was used to prepare (Z) ketenedithioacetals by starting from alkyl dithioesters and allylating the enethiolates. Conversely, (E) ketenedithioace-... 

Oxidation of enethiolates derived from dithioesters 123 with A -sulfonyloxaziridine 117 gave the corresponding vinyl sulfenates 124. Subsequent alkylation with alkyl halides led to ketene dithioacetal A-oxides 125 in good to excellent yields (Equation 5) <2004JOC6916>. 

A second significant difference deals with the stereochemistry of the deprotonation [41]. The reaction of thiocarbonyl compounds leads to cis enethiolates (Sli and R groups are located on the same side of the C=C bond) predominantly with dithioesters [48] or exclusively with thioamides [47, 49] and thioketones [50]. Under similar (kinetic) conditions, analogous esters or ketones lead to the trans enolates. 

In analogy to the Ireland version of the oxygen Claisen reaction, direct transposition of the enethiolate generated by deprotonation of allyl dithioesters [58]. 

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