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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
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* [f^SH + PhSOaNH2
Allylic 14 thiols were prepared in 70-80% yield by this procedure. The reversibility under mild conditions of the ene reaction with N-sulfinylsulfonamides RS02N=S=0 had been exploited in allylic deuteration and tritiation of olefins [516].
4.5.4 Cycloaddition reactions
The role of sulfur functionalities in such processes has received much attention in the recent general literature [482-484] as well as in a number of reviews (inter alia [120, 485]). The bond of a thiocarbonyl group can be directly involved in (2 + 2), (3 + 2) and (4 + 2) cycloadditions (see [120] and references therein).
In view of their very high reactivity with 1,3-dipoles, thiocarbonyl compounds were designated as superdipolarophiles [517]. Adducts were obtained with a large variety of dipoles and led to useful synthetic transformations. A few examples are given here (see [120] for references).
Cycloreversion of adducts with nitrile oxides provides one of the most convenient routes from thiocarbonyl to carbonyl compounds.
Nitronate esters have been used for the same purpose of C=S to C=0 conversion, and the procedure was notably applied in a (+)-methynolide total synthesis [518].
The cleavage of adducts from thiocarbonyl compounds with azides or nitrile imines yielded imines.
Cycloaddition of diazoalkanes with thiocarbonyl compounds occurs usually in the regiochemistry derived from a thiophilic addition of the carbon end of the dipole to sulfur, yielding a 1,3,4-thiadiazoline (1).
These cycloadducts are readily cleaved, at ambient temperature, with loss of nitrogen, yielding thiocarbonyl ylides (2) as reactive intermediates, which were used in the syntheses of thiiranes and alkenes (see [120] for references).
Highly hindered alkenes were obtained in this way, and the example given here is typical [128].
(t)-Thiocamphor (see Section 2.8.2) (505 mg, 3 mmol) and 3-diazo-2,2,4,4-tetramethylpentane in THF (5 ml) were heated under N2 to reflux for 15h. The mixture was filtered and the solvent removed under reduced pressure. The crude episulfide was treated with n-Bu3P (3 ml, 12 mmol) and heated at 120C for 15 h. The mixture was treated with Mel (3 ml, 48.2 mmol) in petroleum ether (30 ml) (very exothermic), stirred for 2h, filtered and concentrated. Column chromatography on silica gel (elution with petroleum ether) afforded the olefin in a 64% yield.
From [128] with permission. Thiocarbonyl compounds in (4 + 2) cycloaddition
R3 -5 r
i) (t Bu) 2CN2/THF, reflux
ii) n-Bu3P
Most aspects of the reactivity of thiocarbonyl compounds in the Diels-Alder reaction, either as dienophiles or as heterodienes (see Section
2.8.3), are discussed in books [483, 519, 520] and reviews [120, 521]; we have previously noted (see Section 2.8.1) the trapping of thioaldehydes through their dihydrothiopyran adducts.
These adducts easily undergo cycloreversion; reactive unstable thioaldehydes can thus be conveniently stored and recovered, and their chemistry has been greatly developed over the last 10 years, mainly by Vedejs and Kirby [126, 253]. Vedejs elegantly used the thiopyranic heterocycles, via a [2,3] sigmatropic shift of a sulfur ylide [508], as precursors for medium sulfide rings, applied to the synthesis of complex natural products in the cytochalasin-zygosporin family [522, 523] for instance.
Although almost all kinds of thiocarbonyl compounds are good dienophiles, synthetic applications derived from the Diels-Alder adducts of thioketones, thionesters and dithioesters remain rather rare and focused on specific targets, as in the preparation of a fragment of the antibiotic erythronolide from the cycloadduct formed regioselectively from 2-methyl-l,3-pentadiene and an a-oxodithioester, a particularly efficient dienophile [524].
erythronolide fragment
4 5 4 2 Ancillary role of sulfur functionalities in cycloaddition reactions
The use of phenyl vinyl sulfoxide as an acetylenic synthon in Diels-Alder reactions was reported in 1978 [525]. By heating phenyl vinyl sulfoxide with anthracene in chlorobenzene at 130C for 120h, dibenzobarrelene was obtained in an 83% yield through elimination of phenylsulfinic acid (see Section 3.1) from the primary cycloadduct.
This early example is one of the numerous synthetic utilizations of the cycloadditions of olefins activated by sulfur atoms at various oxidation levels (vinyl sulfides, sulfoxides and sulfones: = -S(0)R, n = 0,1, 2). Most of the work carried out in this field has been pertinently reviewed and discussed in a 1988 Tetrahedron Report with 204 references [485]. Some specific aspects are underlined here and recent examples given.
(a) Vinyl sulfides
Vinyl sulfides are not good dienophiles towards electron-rich dienes. Their reactivity is much enhanced by the presence of a geminal electron-withdrawing group. The corresponding capto-dative olefins have been widely used, such as the following:
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