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With reactive Michael partners the reaction had to be run at lower temperatures (-98°C for methyl vinyl ketone for a 30% yield), and with less reactive ones the temperature had to be raised for the addition to occur (~10°C for 3-methylcyclohexenone for a 52% yield).
Some potentialities of the 8-ketodithioesters are illustrated by the easy conversions of the dithioester group in ester and amide functions . The overall sequences are indirect 1,4-addition on 2-cyclohexenone of, respectively, an ester enolate for (4) or an amide enolate for (5).
0 LDA/THF, -78°C. 5 min Î
Î R4 S
MeO OMe CuO, CuClj Î H3o*
Ë î ÍçÎ® Î
s m-CPBA Î
The stereochemical aspects of the Michael addition of lithium dithioester enethiolates in the challenging acyclic series have been examined . A stereospecific addition of a m-enethiolate to an (?)-enone furnishes the anti isomer (6).
With a,p-disubstituted enones an unexpected mode of stereocontrol was evidenced [365, 366]. Syn products (7) were obtained by addition of the lithium enethiolate of methyl dithioacetate to an a,p-disubstituted enone via diastereoselective auto-protonation of the intermediate enolate generated in the 1,4 addition.
Î R3 SLi
The lithium enolates of thionoesters [134, 367] and thioketones  exhibited a soft nucleophilic character.
Thioamides are strongly electrophilic and easily alkylated  at sulfur, yielding iminium salts in a quantitative yield. This property has often been used, notably for the Eschenmoser reaction (see Section 4.3.5).
4.2.7 Thioesters and imidothioesters in C-C bond formation
Evans  and Masamune [370,371] have pioneered the use of thioesters for stereocontrol of the aldol reaction. The accompanying scheme summarizes the reactions of boron and silicon enolates of t-butyl thiopropanoate with aldehydes . Both reactions are stereoconvergent. (Z) and (?)-enolates afford the same diastereoisomer: syn with the boron
4.2.6 Alkylation of thioamides
derivatives, anti for the BF3:OEt2-mediated condensations of the compounds. The diastereoselectivities were excellent. For R1 = Ph anti/syn ratios were, respectively, 1:25 and 21:1 for 72 and 95% yields.
Other examples of the uses of enolate derivatives of thioesters | highly diastereoselective aldol reactions were reported by Gennari [3 and Hanaoka et al. [374, 375]. The latter reported the reaction between chromium-complexed benzaldehyde (1) and the titanium enolate of thioester (2) as the first step of sequences directed towards the synthesis (+)-goniofufurone  and the taxol C13 side-chain . They also ur cobalt-complexed acetylenic aldehydes for the selective formation of sy aldols .
<^-™s * "âëÄøè 1
Cr(CO)3 (1) (2)
Thioesters have been used for carbon-carbon bond formation in transition metal-catalysed Grignard reactions. In the accompanying; scheme two sequential cross-coupling reactions with S-phenyl carbonochloridothioate and Grignard reagents led to a general and efficient ketone synthesis [377,378].
° R‘MgX/Ni(ll) ° R2MgX/Fe(n)
PhS'^Cl THF, rt PhS'^R1 THF, o°C
As an example, the synthesis of (Z)-jasmone via a 1,4-diketone was realized in an overall yield of 56% based on S-phenylcarbono-chloridothioate .
Imidothioesters are easily obtained through S-alkylation of thioamides, and we have noted (see Section 2.8.3) a general preparation of such compounds from isothiocyanates via addition of an organometallic and S-alkylation.
They can be used for C-C bond formation by nucleophilic displacement of the alkylthio group, as in the example given here, which led to an imine
BOND-FORMING REACTIONS nediate in the synthesis of an ant venom alkaloid .
1 V n-BuMgCl ^ \
MeS'^N^'n-ˆ7Hi5 Et2o, reflux*^ nC4H9^N n'C?Hi5
..Imidothioesters offer a number of other possibilities. Thus, it was ^ported that N-phenyl thioimidoesters can be metallated at low iperature by LDA or n-butyllithum. Although the resulting enaminates awn in the scheme) were alkylated on nitrogen when R' was an alkyl up, more interesting alkylation occurred regioselectively at carbon when
1 was a vinyl or aryl group  and also with R1 =H or Me3Si .