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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 non-cyclic thioacetals have been used sometimes, particularly those bearing phenylthio groups or some special feature such as an extra electron-withdrawing group. The corresponding carbanion can then be used for a 1,4 addition on an -unsaturated ketone as reported [282].
The reaction of lithiodithioacetals with organoboranes led to borate complexes, and the formation of a carbon-carbon bond occurred through migration of an alkyl group from boron to carbon with cleavage of the carbon-sulfur bond. 2-Alkyl-l,3-benzodithioles were found to be particularly useful for a general and efficient synthesis of ketones, according to the accompanying scheme [283].
R1
o


n-BuU
H202 / NaOH
Treatment with mercuric chloride or methyl fluorosulfonate before the oxidation step initiated the migration of a second alkyl group from boron to carbon, and a synthesis of tertiary alcohols was thus at hand [284].
Ketones and tertiary alcohols were also available in good yields under mild conditions through interaction of trialkylboranes with lithium tris(phenylthio)methanide followed by oxidation [285].
Cyclopentyl propyl ketone (via a borate complex of lithiobenzodithicle)
D n-Buii
rrv-
ill) HjOs, NaOH
n-Butyllithium (2 mmol) in pentane was added to a stirred solution of 2-n-propyl-l,3-benzodithiole (0.392g, 2mmol) in THF (5ml) at 30C under a static pressure of dry nitrogen, and the reaction mixture stirred for 1.5 h. Tricyclopentylborane (2 mmol) in THF (5 ml) was added to the stirred solution (held at -30 C) and the reaction allowed to proceed at that temperature for 1.5h and then at room temperature for 3h. The product was oxidized by the addition of sodium hydroxide (10 ml, 5 ) and then hydrogen peroxide (7 ml, 50% CARE) followed by stirring overnight. The organic product was taken into light petroleum (200 ml), washed with water (2 x 200 ml). dried (Na,S04), filtered and evaporated. The crude product was purified by chromatography (silica gel 120-160 mesh) to give the pure ketone (0.203 g, 73%).
From [283] with permission.
4.2.1.2 Carboxy anion equivalents
Carbanions derived from orthothioformates behave as carboxy anion equivalents through alkylation with electrophiles followed by unmasking of the orthothiocarboxylate group [43].
i) n-BuIJ
H-CtSR)3-----------> E-C(SR)3-------- E-COjjMe
ill E
Some efficient syntheses have been reported such as those of p-hydroxy esters from tris(methylthio)methyllithium and epoxides. The case of a malate derivative from a commercially available chiral epoxide is described [286].
ill
,^
(MeSjeCH ------------------> (MeS)gCLi
THF/HMEA, -70C 70 to -45
) 2
OH 9H
NBS/NaHC03
(S)-(+)-3-Hydroxy-4-(benzyloxy)tris(methylthio)buiane
To a solution of tris(methylthio)methane (770 mg, 5 mmol) in THF/HMPA (12ml, 5:1), cooled to -70C under an atmosphere of nitrogen was added a 1.6 M solution of n-BuLi (3.1 ml, 5 mmol) in hexane. This yellow solution was stirred for 15 min. (/?)-(-)-2-(Benzyloxymethyl)oxirane (760 ml, 5 mmol) was added via a syringe, and the reaction flask was transferred to a dry ice/acetonitrile bath (-45C). After 30 min the reaction was poured into H20 and extracted with ethyl acetate. The organic phase was washed with H20, dried (MgS04) filtered and concentrated under reduced pressure. Silica gel chromatography (CH2C12) as the eluent) afforded the orthothiocarboxylate as a colourless oil (1.46g, 4.59mmol, 92%).
From [286] with permission.
Methyl 3-hydroxy-4-benzyloxybutanoate
To a suspension of the compound obtained above (500 mg, 1.57 mmol) and NaHCOj (1.58 g, 18.84 mmol) in Me0H/H20 (26 ml, 12:1), cooled in an ice bath, was added NBS (1.96g, 11.00 mmol). The reaction mixture was stirred at 5C for 5 min, then at ambient temperature for 10 min. A solution of aqueous 10% NajS^, (5 ml) was added, then the mixture was concentrated under reduced pressure to remove most of the MeOH. The reaction mixture was partitioned between H20 and Et20/hexane (1:1), then washed with water, dried (MgS04), filtered and evaporated under reduced pressure. Silica gel chromatography (35% EtOAc/hexane) afforded the p-hydroxy ester as a colourless oil (310mg, 88%).
From [286] with permission.
4.2.1.3 Thioacetal monoxides
iff R-S-CH2-S-R R-S-CH-S-R
R2
Oxidation of a thioacetal at one of the sulfur atoms offers some useful features. After formation of the carbaniun and addition to an electrophilic partner, the hydrolysis is easier than for dithioacetals and can be made with dilute sulfuric or perchloric acid. Moreover, the addition of a lithiated carbanion derived from these species to enones occurs in a 1,4 rather than a
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