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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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3.3 OXIDATION OF ALCOHOLS

RN=C=NR
H @ MeaSO
RNH-C=NR
0-SMe2
RNH-C=NR
R=cyclohexyl
r1r2choh RNHv
/
.Me
Many other reagents have been used in place of DCC. The most common is oxalyl chloride (Swern).
Analogously, the conversion of amines into imines was realized and indolines were converted into indoles in good yields [239].
3.4 ASYMMETRIC REDUCTION
3.4.1 Sulfmyl-mduced asymmetric reduction
The sulfinyl chirality has shown [86] considerable utility in asymmetric synthesis, particularly for carbon-carbon bond formation (see Sections
2.6.2 and 4.2.8). Its use as the chiral source in a NADII mimic has been reported [240]. Reduction of methyl benzoyl formate by enantiopure
l-propyl-(S)-3-(/?)-tolyl)sulfinyl-l,4-dihydropyridine yielded the (/?)-mandelate with 96% e.e.

Ph COaMe
g...
|l ll VtoI n"
{ H OH
Pr
Ph^COaMe
(AO-mandelate
3.4.2 (3-Hydroxysulfoximine-catalysed enantioselective reduction of ketones
The asymmetric reduction of ketones with borane or with NaBH4/Me3SiCl, catalysed by p-hydroxysulfoximines, afforded secondary alcohols in high yields with good enantioselectivities [241,242].
HN
N Phi,. II ..Ph
X
'Ph
10 mol* NaBH ! Me3SlCl
HO H Ph^-CHjjCl
88% yield. 88% e.e.
3.5 DECARBOXYLATION
Another useful aspect of the radical chemistry associated with thiocarbonyl compounds is the decarboxylation of acids. Typically the acyl derivatives (2) of yV-hydroxypyridine-2-thione (1) are prepared from the appropriate acid [243], and treated with tributyltin hydride.
The Pummerer reaction of sulfinyl compounds involves the formation of an a-functionalized sulfide [244, 245] from a sulfoxide. Acetic anhydride is commonly used as the electrophile, which adds to the sulfoxide to yield a sulfonium salt, and the rearrangement occurs through successive formations of an ylide (rate-determining step) and an alkylidene sulfonium, trapped by a nucleophile, or stabilized by a proton loss with formation of a vinyl sulfide.
HC
(2) /BUgSn *
SnBu,
(1)
3.6 THE PUMMERER REARRANGEMENT
c?'

n? Ac0<
H
p, -AcOH _________..

Good precursors of carbonyl compounds are formed in these processes and many applications deal with such syntheses. A comprehensive review has appeared in a 1991 issue of Organic Reactions [246] (479 references). The synthesis of n-octanal from n-octyl phenyl sulfoxide is described [246].
Repeated use of the Pummerer reaction has been made in the systematic syntheses of all L-hcxoscs [247]. The accompanying scheme summarizes the steps yielding the protected erythro- and threo-aldehydes from 4-benzhydryloxy-(?)-2-butene-l-ol (Ph2CHOCH2-CH=CH-CH2OH) (1) via (2) and (3).
(i)
R = PhjCH
AcO^^SPh O-
X.
OR
X
OR
CHO

OR
i) (a) Sharpless asymmetric epoxidation with L-(+)-tartrate. (b) PhSH, OH-, equilibration of the secondary epoxide with a primary epoxide and regioselective opening of the latter with benzene thiol, (c) Conversion of the thioether diol into the acetonide (2).
ii) Pummerer reaction: (a) m-CPBA; (b) Ac20, AcONa.
iii) DIBAH.
iv) Basic hydrolysis with epimerization at C2.
New examples of application appear regularly. In the following one [248] phenylsulfenylation of nopinone (4) was followed by oxidation to the sulfoxide. Pummerer rearrangement of the latter led directly to the a-phenylthio substituted a-unsaturated ketone (6) (overall yield 94%), starting material for further elaboration of natural sesquiterpene synthesis via 4,4-disubstituted nopinones (7).
(lR,5R)-6,6-Dimethyl-3-(phenylthio)bicyclo[3.1.1]hept-3-en-2-one
To a stirred solution of (5) (6.00 g, 24.4 mmol) in CH2C12 (100 ml), obtained by phenylsulfenylation of (4) with S-phenyl benzene-thiosulfonate, was added dropwise at 0C a solution of m-CPBA (80% purity, 5.26 g, 24.4 mmol) in CHZC1Z (50 ml), and stirring was continued at 0C for lh and then at room temperature for 10 h. The precipitate was filtered and the filtrate was washed successively with aqueous Na2S203, aqueous NaHC03 and brine, and then dried and filtered. To the CH2C12 solution was added acetic anhydride (4.5 ml, 47.1 mmol) and methanesulfonic acid (1.0 ml, 15.7 mmol). The resulting solution was stirred at 0C for 1 li, allowed to warm to room temperature and stirred for an additional 15h. Water (30ml) was added, the mixture was stirred for 30 min, and the aqueous layer was separated and extracted with CH2C12. The combined extracts were washed successively with NaHCO, and brine and dried. Removal of the solvent followed by chromatography of an oily residue on silica gel
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