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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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From [ 151 ] with permission.
With allylic Grignard reagents, the carbophilic addition on the thiocarbonyl of phenylisothiocyanate occurred as expected exclusively with inversion of the allylic chain. From crotyl and prenyl magnesium halides the compounds (4f) and (4g) were obtained in yields of 77 and 72%, respectively.
(4) (R2=Me)
s
(4a) R‘ - Et 68%
(4b) R'-i-Bu 67%
(4c) R1 = t-Bu 59%
(4d) R1 - allyl 41%
(4e) R1 - MethaUyl 78%
S
(4f)
(4g)
A similar procedure was used with organolithium reagents [152], and when the reaction was run in ether HMPA was added before the alkylation step.
Conjugation of the double bond with the thiocarbonyl was readily obtained by treatment with a catalytic amount of triethylamine for the P-unsaturated dithioesters (4d), (4e) and (4f). At room temperature only dimers (resulting from a Diels-Alder reaction) were isolated [153] for the a-ethylenic dithioesters (4h) and (4i), derived from (4d) and (4f).
(4h) (4i)
give dimers at rt
S
dimer of (4h)
stable at rt
The synthesis of thionoselenoestcrs has been reported [154—156]. Foi instance, treatment of thionoesters by dimethylmethylselenoaluminium smoothly yielded the methyl derivatives [155].
S S
Me2AlSeMe ö
r! À÷ or2 ** R1 A. SpMe
2.8.4 Thioamides
More stable than many other thiocarbonyl compounds, thioamides have been known and used for a long time. Reliable methods of synthesis [119] were introduced as early as 1815 by Gay-Lussac for the reaction of nitriles with hydrogen sulfide and 1878 by Hoffman for the thionation of amides by means of tetraphosphorus decasulfide.
RCN + H2S
RCSNH2
R'-CO-NR2!*3 ----—------> R*-CS-NR2R3
Ä, solvent
The preparation of thiopyrrolidone by this reagent is described in Section 4.3.5. Many modifications have been proposed. Thus the addition of various bases led to new useful species for the carbonyl to thiocarbonyl conversion in appropriate solvents. Let us mention some of them:
• sodium hydrogen carbonate or sodium sulfide solubilized P4Si0 [157] (Scheeren’s reagents) in a polar solvent;
• triethylamine-solubilized P4S,0 [158] (in dichloromethane or acetonitrile at room temperature);
• organolithium/P,Sl0 reagent (4:1) in THF [159].
Another efficient method, widely used, was developed by Lawesson with the reagent that bears his name [137, 160]. The Lawesson, Heimgartner and Davy reagents (cf. Section 2.8.2) are also excellent and widely used. Although Davy’s reagents were reported to allow thionation of amides and ketones under milder conditions [161], the convenient Lawesson’s reagent is the most popular. Examples appear regularly in the literature, such as the following preparation of 7V,/V-diethylthioacetamide [162] or the syntheses of 4-thioisoflavones [163].
N ,N-Diethylthioacetamide
A mixture of N,N-diethylacetamide (0.6 g) and Lawesson’s reagent (1.25 g) in 1,2-dimethoxyethane (30 ml) was stirred at room temperature for 40 min. The solvent was removed under reduced pressure and then the residue was chromatographed on a silica gel column with benzene/ethylacetate (4:1) as the eluent to yield TV, .V-diethylthioacetamide (615 mg, 90%), b.p. 45°C/3 torr.
From [162] with permission.
The procedure has been applied to the synthesis of thiopeptides [137] and, particularly, for the thionation of cyclosporine [164].
A reagent prepared in THF from P4S)0/Na2S (1:1) converts nitriles into thioamides at ambient temperature [165].
0 P4SI0 + Na2S (1:1) THF. 20°C
The most commonly used thiation reagents (see [166] for a discussion about thiation reagents) and particularly P4Sl0 and its surrogates are not suitable for acid-labile compounds. Some mild methods have been devised, for instance the conversion of nucleosides to thionucleosides [167], an example of which is given here. The nucleoside (protected on the sugar
Ë
W N- Î PhCOSH N
(/W'
RQ ° POCI3. Et3N '4N MeCOSH. MeOH^ ^ N
triazole îã
NO PhCOSH ^ °
RO
R=ffluMe2Si
moiety) yielded quantitatively a triazolo derivative which reacted with a thioacid to give the thionucleoside at room temperature and in excellent yield: the scope of this interesting procedure has not been investigated.
The synthesis of /V-substituted thioamides by /V-alkylation is an unusual route, as, with most electrophiles, S-alkylation is the rule. However, a two-step procedure based on the remarkable properties of a benzotriazolyl substituent has been proposed [168].
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