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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
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Some potentialities of the 6-ketodithioesters are illustrated by the easy conversions of the dithioester group in ester and amide functions [332]. The overall sequences are indirect 1,4-addition on 2-cyclohexenone of, respectively, an ester enolate for (4) or an amide enolate for (5).

e>-eu.
MeO OMc CuO, CuClj
o H3o
X/N/^OMe thf/h2o
(4)

s
U^NMe.*
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 m-enethiolate to an (?)-enone furnishes the anti isomer (6).
SLI
^v^SR2
A^Me
SR2
With a,p-disubstituted enoiies 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,|3-disubstituted enone via diastereoselective auto-protonation of the intermediate enolate generated in the 1,4 addition.
SLi
^SMe
R1' Y SMe
R2
The lithium enolates of thionoesters [134, 367] and thioketones [134] also exhibited a soft nucleophilic character.
4.2.6 Alkylation of thioamides
Thioamides are strongly electrophilic and easily alkylated [368] 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).
S tftc
riX^NR*
SR3
,1-^
NR2,
X
4.2.7 Thioesters and imidothioesters in C-C bond formation
Evans [369] 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 [372]. Both reactions are stereoconvergent. (Z) and (E)-enolates afford the same diastereoisomer: syn with the boron
-
r'cho
OSiR2,
derivatives, anti for the BF3:OEt2-mediated condensations of the silyl compounds. The diastereoselectivities were excellent. For R1 = Ph the 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 for highly diastereoselective aldol reactions were reported by Gennari [373] and Hanaoka et al. [374. 375]. The latter reported the reaction between a chromium-complexed benzaldehyde (1) and the titanium enolate of a thioester (2) as the first step of sequences directed towards the synthesis of (+)-goniofufurone [374] and the taxol C13 side-chain [375]. They also used cobalt-complexed acetylenic aldehydes for the selective formation of syn-aldols [376].
CHO
<^s *
Cr(CO)3
(1) (2) (3)
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 5-phenvl carbonochloridothioate and Grignard reagents led to a general and efficient ketone synthesis [377,378].
R'MgX/Nidi) R2MgX/Fe(H)
PhS^'Cl THF, rt PhS'^'R1 THF, o'C R2^4'R1
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 [378].
Imidothioesters are easily obtained through 5-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 5-alkylation.
R^NR2
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
intermediate in the synthesis of an ant venom alkaloid [379].
MeS
n-C7H,5 Et20, reflux
n-BuMgCl
n-C7H15
n-C4H9-' n-C7H15
IN
I
Me
ant toxin
Imidothioesters offer a number of other possibilities. Thus, it was reported that A/-phenyl thioimidoesters can be metallated at low temperature by LDA or n-butyllithum. Although the resulting enaminates (shown in the scheme) were alkylated on nitrogen when R1 was an alkyl group, more interesting alkylation occurred regioselectively at carbon when R> was a vinyl or aryl group [152] and also with R1 =H or Me3Si [380].
(*) added at -78C for efficient trapping of (5)
The preceding scheme also shows how a controlled thermal decomposition of the metallated thioimidoesters (enaminates (4)) by in situ trapping by an organometallic of the ketenimines (5) formed by loss of MeSLi allows a very direct route to ketones [381, 382]. An example of this application is given over.
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