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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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1,2-dithiolan series.
S-S S-S S-S S-S
V V V ^
/N\ s4 co2H
nereistoxin charatoxin asparagusic acid lipoic acid
Interestingly some 1,2-dithiin compounds have been shown to possess antiviral activity, including anti-HIV activity, and their chemistry is an active area of research associated with that of diatomic sulfur Sj [30,31].
The S3 unit is also to be found in compounds of natural origin [28, 32, 33]. A very simple one, isolated from an edible mushroom, exhibits both S2 and S3 moieties [34, 35]. 5-Methylthio-l,2,3-trithiane [28] is another example close to charatoxin, and the presence of an allylic trisulfide, and its role as a triggering device for their antitumour activity, in enediyne antibiotics [36,37], is truly astonishing.
NHCOaMe
s
s—s
< > V
from shiitake mushroom from a green alga ( )-calicheamicinone
algycone part of alicheamicin 7,'
Some disulfides, particularly dimethyl disulfide (DMDS), and trisulfides are important industrial compounds [13]. Disulfides find a use in synthesis
as sulfenylating agents [38], and the most usual method to prepare them is through formation of the S-S bond, and particularly by oxidation for symmetrical structures [39].
[Ol
2 RSH----------- RSSR
For unsymmetrical disulfides, side reactions often cause randomization of the products, leading to mixtures of symmetrical and unsymmetrical products. To avoid secondary processes, mild procedures occurring under neutral conditions have been developed for the synthesis of these disulfides as well as for trisulfides.
A very convenient method has been devised [40] for the conversion of thiols to ethyldithio derivatives as a routine procedure for protection of thiol-substituted organic acids. It uses a DMAP-catalysed exchange of the ethylthio group between a thiol and ethyl 2-pyridyl disulfide, prepared from the commercially available 2,2'-dithiobispyridine. Filtration through a macroporous sulfonic acidic resin and evaporation of the solvent yielded the disulfide directly.
dmap/ch2ci2
Ethyl 2-pyridyl disulfide
To a solution of 2,2'-dithiobispyridine (4.4 g, 20mmol) and a catalytic amount of BF3:OEt2 (5 ml) in dichloromethane (25 ml) was added ethanethiol (1.22 g, 20 mmol). The resulting solution was stirred for 2h at ambient temperature and the solvent was removed under reduced pressure. The residue was suspended in a mixture uf hexanes and ethyl acetate (50 ml, 7:3), and the insoluble 2-mercaptopyridine was removed by filtration. The filtrate was concentrated under reduced pressure and the residue was purified by chromatography (silica, dichloromethane), affording ethyl 2-pyridyl disulfide (2.84 g, 83%) as a pale-yellow liquid.
From [40] with permission 2-(Ethyldithio)hexadecanoic acid
To a solution of ethyl 2-pyridyl disulfide (55 mg, 0.32 mmol) and DMAP (33 mg, 0.27mmol) in dichloromethane (2ml) was added 2-mercaptohexadecanoic acid (77 mg, 0.27 mmol). The progress of the reaction was monitored by TLC (silica, CHCl,/MeOH 9:1). Upon completion (1 h) the product mixture was passed through a column (0.5 X
5 cm) of AG MP-50 (0.5 g, 3.5 meq (ml dry resin)1, Bio-Rad Laboratories). Elution with 3 ml of dichloromethane, followed by concentration of the filtrate under reduced pressure, afforded 2-(ethyldithio)hexadecanoic acid (91 mg, 98%) as a colourless oil.
From [40] with permission.
The following routes to unsymmetrical disulfides and symmetrical trisulfides use silyl sulfides and thiosulfmates [41. 42]. Thiosulfonates react similarly.
R'-S-S-R1 + 2 R^S-SiM&j _______ 2 R'-S-S-R2 + (Me3Si)20
Î
^ J,Ă* — R-S-S-S-R.WeAO
ROy * /s-SR
R
Dimethyl trisulfide (R = Me)
S-Methyl methylthiosulfinate (lg, 9 mmol) was dissolved in anhydrous chloroform and treated with bis(trimethylsilyl) sulfide (1.62 g, 9 mmol). The reaction mixture was heated to 60CC with stirring for 14 h, diluted with dichloromethane and washed several times with water. After work-up the crude material obtained was distilled under vacuum to obtain the pure trisulfide (830 mg, 73%), b.p. 65-68°C/25 lorr.
From [41J with permission.
2.4 THIO AND DITHIOACETALS AND RELATED COMPOUNDS
The widespread use of compounds derived from the 1,3-dithiane carbanion or its homologues as acyl anion equivalents (for applications see Section 4.2.1.1) followed the pioneering works of Corey and Seebach on nucleophilic acylation (for reviews, see [43] and [44]).
©< ) == "CH=0 R—( > -- R-C=0
The term “Umpolung" is most often used to describe the reversal of the carbon polarity when going from a carbonyl group to the carbanion of a dithioacetal.
The syntheses of dithioacetals are generally straightforward [43]. Standard methods may be unselective for multifunctional molecules. Therefore, new procedures have been developed. It has thus been reported that 1,3-dithianes are readily synthesized by reaction of aldehydes, ketones or acetals with 2-stanna-l,3-dithianes under catalysis of organotin triflates [45]. These odourless reagents are prepared from dialkyldichlorotin and 1,3-propanedithiol.
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