Thionocarbonates reaction

By application of the Corey-Winter reaction,vicinal diols 1 can be converted into olefins 3. The key step is the cleavage of cyclic thionocarbonates 2 (1,3-dioxolanyl-2-thiones) upon treatment with trivalent phosphorus compounds. The required cyclic thionocarbonate 2 can be prepared from a 1,2-diol 1 and thio-phosgene 4 in the presence of 4-dimethylaminopyridine (DMtVP) ... 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

The precise reaction conditions for optimal yields depend upon the specific reagents and both thermal290 and photochemical291 conditions have been developed. Phenyl thionocarbonates are easily prepared and are useful in radical generating reactions.292 A variety of other thiono esters, including xanthates and imidazolyl thiocarbonates also can be used.293... 

It is claimed that this reaction proceeds via a 2, 3 -thionocarbonate or a thiocarbamate derivative. If the 5 -OH group of uridine was not protected, the yield of the 2,2 -anhydro-l-(j -D-arabinofuranosyl)uracil was low. 

Iodotrimethylsilane formed in situ from the reaction of chlorotrimethylsilane and sodium iodide, also effects the conversion of 2-ene-l,4-diols to 1,3-dienes (equation 16)46. Allylic thionocarbonates on heating with triphenylphosphite undergo deoxygenation (Corey-Winter reaction) to generate olefins47. This procedure has been used for making hexatrienes (equation 17)47b. 

Cyclic thionocarbonates are formed under soliddiquid phase-transfer catalysed conditions from the reaction of diols with carbon disulphide [63]. The reaction has been specifically described for the reaction of carbohydrates, but should be generally applicable to all diols. 

Silylated 1,4-cyclohexadienes were introduced as reducing agents in radical chain reactions such as dehalogenation, deoxygenation via thionocarbonate ester and deselenization [120]. Two examples are given in Reactions (4.75)... 

Preparation of polycyclic ortho esters by the reaction of 1,2-hydroxy-benzene thionocarbonate with trimethyl phosphite [184, 185]. 

The absolute configuration of 1 was set by conjugate addition to the oxazoline 3 followed by trapping of the product anion, following the precedent of Meyers. Reduction of the oxazoline to the alcohol, followed by thionocarbonate formation then set the stage for the key aryl transfer reaction. 

The tributyltin hydride reduction usually proceeds without complications. The most common byproduct is starting alcohol, which is postulated to be derived from a mixed thioacetal. Use of the phenyl thionocarbonate ester has been reported to minimize this side reaction in cases where it is a problem.6... 

Cyclic thionocarbonates (41) can be cleaved to olefins (the Corey-Winter reaction)274 by heating with trimethyl phosphite275 or other trivalent phosphorus compounds276 or by treatment with bis(l,5-cyclooctadiene)nickeI.277 The thionocarbonates can be prepared by treatment of 1,2-diols with thiophosgene and 4-dimethyIaminopyridine (DMAP) 278... 

Decarbonylation of acyl halides 7-20 Cleavage of Michael adducts 7-21 Deoxygenation of vic-diols 7-22 Cleavage of cyclic thionocarbonates 7-23 Deoxidation of epoxides 7-24 Desulfurization of episulfides 7-25 Reaction of a-halo sulfones with bases (Ramberg-Backlund)... 

Since its introduction by Barton and McCombie, the deoxygenation of thionocarbonyl derivatives of alcohols has become an important synthetic reaction and a valuable method for the generation of carbon-centered radicals.3-4 Xanthates, thionobenzotes, thionocarbonyl imidazolides, aryloxy thionocarbonate, N-phenylthionocarbamates and oxalate esters are conveniently deoxygenated with tin or silicon hydrides in boiling benzene or toluene.4-5... 

The solution of l,2 5,6-di-0-isopropylidene-ct-D-glucofuranose-3-0-(4-fluorophenyl)-thionocarbonate [Eq. (12)] 32 (83 mg, 0.2 mmol), hypophosphorous acid (0.1 mL, 1 mmol), and triethylamine (0.154 mL, 1.1 mmol) in dioxane (1.5 mL) under argon was treated with AIBN (6.5 mg) twice at 20 min intervals and under reflux. After completion, the reaction was washed with water and dried over anhydrous MgS04. After evaporation of the solvent, the residue was analyzed by H NMR to give 100% of the deoxy product 30. 

A solution of the starting 2,4-bis-thionocarbonate [Eq. (15)] 37 (174 mg, 0.40 mmol), chlorotrimethylsilane (0.31 mL, 2.4 mmol), and triethylamine (0.5 mL, 3.6 mmol) in benzene (3 mL) was stirred for 1 h at room temperature. After filtration, the solvent was evaporated. The residue was dissolved in toluene (1 mL) and diphenylsilane (294 p,L, 1.6 mmol) was added. The reaction mixture was heated to reflux and treated under argon with 150 mL portions of an AIBN solution (262 mg of AIBN in 3 mL of dioxane) five times (at 20 min intervals) under reflux. After evaporation of the solvent under reduced pressure, the residue was analyzed by H NMR to give 85% of the 2,4-dideoxy-3-trimethylsilyl-l,6-anhydro-D-glucose product 38. 

The radical is generated on the sugar template using mostly the tin hydride method and halo derivatives [72,87,89], thionocarbonate [90-91], or selenium derivatives [91,94, 95]. Again, in these reactions, the kinetically controlled 5-exo cyclization is always... 

Of all the radical reactions, the exo-l,5-cycIization of a hex-5-enyl radical to cyclopen-tylmethyl radical and its subsequent trapping by various reagents have attracted the most attention from synthetic chemists (Scheme 1) [4-7]. Starting materials that are most often used for the tin method (initiation of the chain by trialkyl tin radical) are halides, sulfides, selenides, or thionocarbonates. The generation and cyclization of the radical proceeds under exceptionally mild neutral conditions, and these conditions are compatible with a wide variety of common functional groups. A prototypical example of an application in carbohydrate chemistry is shown in Scheme 2 [8]. Readily available 2,3-di-O-isopropyl-ideneribonolactone 1 was converted into the bromoacrylate 2 in three steps. Radical... 

Thiocarbonyl compounds for which the C=S group is linked to two heteroatoms, as in thionocarbonates, thionocarbamates, thioureas and trithiocarbonates, have been in general less directly involved as intermediates in synthesis, and we have considered here only xanthates which intervene in the Chugaev reaction (see Section 3.1) and Barton radical reactions (see Section 3.2). They are traditionally prepared by reaction of an alkoxide anion, generated from the alcohol and a strong... 

Di-2-pyridyl thionocarbonate, 127 Formaldehyde, 136 Iodomethyltrimethylsilane, 315 Palladium(II) acetate, 232 Tributyl(iodomethyl)tin, 314 Ene reaction Ethylaluminum dichloride, 5 Oxygen, singlet, 228 8-Phenylmenthol, 243 Zinc bromide, 349 Enol acylation... 

The utilization of the azido group as radical acceptor in radical reactions shown in equation 56, has been tested by Kim and coworkers84,85 using (TMS SiH, which is relatively inert towards azides when compared to BusSnH. Therefore, alkyl bromides and thionocarbonates can be used as precursors of radicals, generated by (TMS SiH which... 

A mixture of cyclic thionocarbonate (0.36 mmol), H3P02 (50% aqueous solution, 0.13 ml, 1.26 mmol), and AIBN (12 mg) in dioxane (3 ml) was heated at 80 °C under an argon atmosphere for 30 min. After the reaction, dichloromethane was added to the reaction mixture. The obtained mixture was then washed with sat. aq. NaHC03 solution. The organic layer was dried over MgS04. After removal of the solvent, the residue was chromatographed on silica gel (eluent hexane/ethyl acetate = 8/2) to give the reduction product in 70% yield [38]. 

Thionolactones.a The reaction of thionocarbonic acid derivatives of 4-phenyl-3-butenol with Bu3SnH (AIBN) can provide thionolactones as the major product via a radical cyclization. 

Azine approach. 1 -Pyridinimines undergo 1,3-dipolar cycloaddition reactions with thiones. In the reaction between 2-isoquinolinimine and carbon disulfide the mesoionic thiadiazole (716) is formed the formation of (716) involves a secondary dehydrogenation of the initial adduct. With diphenyl thionocarbonate, phenoxy group expulsion is succeeded by cyclization leading to the adduct (717) (62TL387). 

Some reactions follow paths different from the single bond per electrophile sequence. Dibutylstannylene acetals derived from all-tra/is-diols react with phenoxythiocarbonyl chloride at room temperature in 1,4-dioxane to give noncyclic phenylthionocarbonates, but when ds-diols are present, cyclic thionocarbonates spanning the oxygen atoms of the cw-diol are obtained both for pyranosides70 and for furanosides.71... 

Table 2. Barton-McCombie deoxygenation of phenyl thionocarbonate esters comparison of the catalytic with the stoichiometric reaction [25].

The action of thiophosgene on carbohydrates in the presence of base and the reaction of a sodium-carbohydrate with a suitable 0-[(alkyl-thio)thiocarbonyl] derivative both give rise to intermolecular thionocarbonates. 

In their study of the reactions of methyl 3,4-0-isopropylidene-2-0-(sodium thiolthiocarbonyl)-/3-D-arabinopyranoside, Foster and Wolfrom showed that when bis(methyl 3,4-0-isopropylidene-/3-D-arabinopyranoside) 2,2 -thionocarbonate (VI) was treated with mercuric oxide, bis(methyl 3,4-0-isopropylidene-j8-D-arabinopyranoside) 2,2 -carbonate was produced. 

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