Download (direct link):
e.e. > 96 %
A number of functionalized dithioesters have been utilized for carbophilic and/or thiophilic additions . We shall mention only the potentialities offered by the readily available enolizable p-oxodithioesters (for a review, see ): R'COCH(R2)CS2Me.
A mole of an organometallic first reacts with the enol proton and a second mole (not necessarily the same reagent) adds exclusively to the carbonyl function, giving (3-hydroxydithioesters in good yields. After protection of the OH group the dithioester function can be used for formation of another carbon-carbon bond, and the starting (}-oxodithioesters can be viewed as a3d’ or aV synthetic equivalents .
lA^SMe -RH RlA^ASMe
ÎÍ î OH s
& ê IT
The reaction of organocuprates with dithioesters has been reported . High yields of tertiary thiols were obtained via a novel sequence of a “double-barrelled” carbophilic addition, such as with lithiocuprates:
SMe ii) H2s/NH4C1
The same results were obtained with magnesiocuprates (or a catalytic amount of Cu(i)). The simultaneous formation of two carbon-carbon bonds in a simple efficient step is in itself valuable.
The addition of organometallic reagents to the sulfines derived from dithioesters has also been investigated and shown to be promising [351, 352].
22.214.171.124 Thionoesters, thioketones, thioaldehydes, and thioamides
Only a few reports deal with the reaction of thionoesters with organometallic reagents, and carbophilic addition was observed as the usual mode. The reaction of O-ethyl thioesters with alkyllithiums afforded, after methylation, good yields of monothioacetals, which can be hydrolysed to ketones .
S i) RzLi
Ai) R Li I chloramine-T
_ --------- R1—C-OEt ---------------------- ri^r2
OEt It) Mel I R R
Nicolaou and his group [139, 354,355] used this nucleophilic addition to thionolactones as a route to cyclic ethers, via an efficient desulfurization step with tin hydrides (Ph3SnH or n-Bu3SnH). (±)-Lauthisan and a brevetoxin A fragment were prepared according to this scheme.
To a stirred solution of (±)-8-hexyl-2-oxocanethione (prepared from the reaction of the lactone and Lawesson’s reagent as in Section 2.8.3) (126 mg, 0.55 mmol) in anhydrous THF (6ml) at -78°C was added, under
Lrgon, ethyllithium (0.59 ml of a 1.4 ì solution in THF, 0.83 mmol). The Enaction mixture was stirred at -78°C for 5 min before quenching with ¦Mel (0.07 ml, 1.1 mmol). Dilution with ether (50 ml) followed by washing Lwith water (2 X 10 ml), drying (MgS04), concentration, and flash column Ichromatography (3% ether in petroleum ether) gave the title compound
I (125 mg, 85%).
[ From  with permission.
f (±)-cis-2-Ethyl-8-hexyloxocane (lauthisan)
The preceding compound (125 mg, 0.46 mmol) was stirred together with triphenyltin hydride (0.29 mmol, 1.15 mmol) and AIBN (5 mg) in toluene (8 ml) at reflux temperature for 20 min. Concentration and silica gel chromatography (2% ether in petroleum ether) gave the title ether (95.6 mg, 92%).
From  with permission.
One case of thiophilic addition has been reported for a bisthionester :
This fits with the general tendency for thiophilic addition to be favoured by a strong withdrawing group a to the thiocarbonyl function .
As mentioned before, it was with thioketones that the first cases of thiophilic addition were evidenced, but many other modes of reaction have been observed concurrently: carbophilic addition, reduction, coupling, double addition, and enethiolization . We will not discuss these reactions here, which have not been used much for synthetic purposes.
With thioamides few results are available. Only carbophilic additions have been reported [329, 358]. This mode of addition of organolithiums ] was used for novel syntheses of unsymmetrical ketones and a-alkylated amines. Although enethiolization of aliphatic thioamides by alkyllithiums limits the scope of these synthetic routes the yields are good and reactions simple to run .
O ,11 tfu j? « V
R1 "R5 in í,î® R' NR! Ù UAIH4 R' nr|
4.2.3 Reactions of enethiolates
Again for synthetic applications, thio- and dithioesters are particularly useful. Their enethiolates are easily generated . 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.