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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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(hexane/ether 4:1) afforded (6) (5.9 g, 96% from (5)) as crystals, m.p. 52-53C, [a]D20=91.8"C (c 1.34 in CHC13).
From [248] with permission.
The use of chiral sulfoxides to transfer the chirality from sulfur to the a carbon has beer investigated, and the high asymmetric induction observed in chiral acyclic sulfoxides using a silicon-induced Pummerer-typc reaction is noteworthy [249].
An interesting oxidative decarboxylation which bears some analogy to the Pummerer reaction has been reported [250].
'vcoh LDA|2> ____NCS->
R2-'] 2 11) MeSSMe L, NaHC03, EtOH @SMe
),0 SMe Tp
R* xMe e!' SMe R1.____
^ XOE,
A plethora of other functional modifications are more or less directly associated with sulfur compounds. A few examples are discussed below.
An efficient synthesis of ethers involves hydrozirconation of thioketones with hydrochlorobis(cyclopentadienyl)zirconium [251].
R r_c . r 7r/H CeHfi r- RoCS-ZrCpo -----------------??-----*. R,CH-OMe
-S + Cp^Sr rtlh | Br2/MeOH
Appropriate cleavages of the zirconium sulfide (1) has led to sulfonyl halides, thioesters, and other derivatives.
Conversion of thiols to carbonyl compounds was realized via a
photochemical fragmentation of the corresponding phenacyl sulfides to thiocarbonyl compounds trapped in situ with t-butyldimethylsilyl nitronate ester and cleavage of the adduct with n-Bu4N+F~ [252].
The overall sequence occurs under nearly neutral conditions at room temperature and was shown to be an efficient tool for an oxidative desulfurization step required in sulfur-mediated synthesis of complex molecules such as methynolide and cytochalasin [253].
The thiolester group was used for deracemization of terpenic esters [254]. Racemic 5-phenyl thiocyclogeranate was deprotonated by n-BuLi, and the resulting enolate was protonated by a chiral aminoalcohol, (-)-jV-isopropylephedrine. The thiolester was obtained with the highest enantioselectivity (99% e.e.) reported for such a process (carried out on a 40g scale). With a sulfur group a selective (Z)-enolate was formed and protonation was slower than for esters.
99% e.e.
The thiolester was readily converted to target molecules, (5)-a-cyclocitral and (5)-a-damascone.
Deracemization of a terpenic thiolester
A solution of (/?,S)-S-phenyl cvclothiogeranate (40.0g, 153.8mmol) in THF (450ml) was cooled at -100 to -110C (ether, liquid nitrogen) and treated with 1.92 M n-butyllithium in hexane (120.2 ml, 230.8 mmol) (40 min, T *?100C). The solution was stirred between -105 and -100C for 2h. A solution of (-)-Ar-isopropylephedrine (63.7g, 307.7 mmol) in THF (100ml) was then added over 50min at 100C. The reaction mixture was cooled between -102 and -100C for 1 h and then allowed to warm up to 10C over 25 min. The mixture was then poured into vigorously stirred 5% aqueous NaOH and extracted with ether. The organic phase was washed successively with water, 5% aqueous HCl, saturated NaHC03, and saturated NaCl solution, dried over Na2S04, and concentrated. Distillation (110C/0.02 torr) afforded (S)-S-phenyl cyclothiogeranate (34.92 g, 87%), 99% e.e. [a]D20= -421 (c 0.04 in CHC13). The combined acidic aqueous phases were basified (10% KOH), extracted (ether), and distilled (90C/2 torr) to afford recovered (- )-7V-isopropylephedrine (62.4g, 98%).
From [2*>4] with permission.
Desulfurization-fluorination reactions of organosulfur compounds were used to synthesize organofluorine derivatives under mild conditions [255]. Hiyama et at. reported such processes for xanthates [256], dithiocarbamates [257] and arenedithiocarboxylates [258]. The procedure given here is representative.
TBA H2F3 . tetra-n-butylammonium dihydrogentri fluoride
Fluorination of methyl arenedithiocarboxylates
To a dichloromethane (1.5 ml) solution of ArCS2Me (0.5 mmol) and tetra-n-butylammonium dihydrogentrifluoride (1.5 mmol) was added DBH (2 mmol) in one portion at 0C, and the resulting mixture was stirred for 1 h at room temperature, then poured into an aqueous solution of NaHCO, and NaHS03, and extracted with diethyl ether. The ethereal layer was dried (Na2S04), filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to afford the trifluoromethyl-substituted aromatic
compounds in good yields (Table 3.2).
Oxidative desulfurization-fluorination of methyl arenedithiocarboxylates
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