Cyclic thionocarbonate

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

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) ... 

Cyclic thionocarbonates (41) can be cleaved to alkenes (the Corey-Winter reac-... 

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. 

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)... 

B. P. Roberts and T. M. Smits, Regioselectivity in the ring opening of 2-phenyl-l,3-dioxan-2-yl radicals derived from cyclic benzylidene acetals and comparison with deoxygenation of a carbohydrate diol via its cyclic thionocarbonate, Tetrahedron Lett., 42 (2001) 3663-3666. 

During the time frame covered by this chapter, stannanes have been reported to have been involved in radical chemistry not involving the more traditional functionalities. For example, Marzi and coworkers reported that the reduction of cyclic thionocarbonates (e.g. 82) with BusSnH under standard radical conditions affords cyclic acetals that can then be further transformed into 1,2-diols (equation 57)219. This transformation represents a new approach to the protection of these diols. Zehl and Cech described the use of Bu3SnH in the reduction of azide (83) to the corresponding amine (equation 58)305, while Hanessian and his associates reported the Ph3SnH-mediated free-radical reduction of the tertiary oxalate (84) (equation 59)309. This transformation represents a departure from the more typical reduction of a pyridinethioneoxycarbonyl (PTOC) oxalate ester321. 

Treatment of cyclic thionocarbonate (36) derived from 1,3-diol, with the Bu3 SnH/AIBN system generates the corresponding monodeoxygenated product (37), as... 

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]. 

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... 

Corey and Winter converted 1,2-diol into a cyclic thionocarbonate 4.23 on heating with thiocarbonyldiimidazole (4.22) in toluene or xylene. Thionocarbonates 4.23 can also be prepared by the reaction of diol with n-butyllithium, followed by the reaction with carbon disulfide and methyl iodide. Desulfurization-decarboxylation is carried out by heating the cyclic thionocarbonate 4.23 with trimethylphosphite, and alkene is produced (Scheme 4.17). 

Cyclic thionocarbonates (78) by heating with trimethyl phosphite yield (Z)-alkenes (84) (the Corey-Winter reaction). The starting thionocarbonates (78) may be prepared by condensation of thiophosgene with v/c-glycols (82) (X = 0) or v/c-thiols (82) (X = S) (Scheme 43). The Corey-Winter reaction may proceed via the carbenoid intermediate (83), which subsequently collapses to yield the (Z)-alkene... 

Deoxygenation. The centrally located functionality of 2,3-dihydroxycarbonyl compounds is selectively destroyed via their cyclic thionocarbonates, by means of BujSnH. Ring expansion of certain fused cyclopropylmethyl xanthates occurs, the extent of which has some dependence on the concentration of BujSnH. 

Olefin synthesis. Corey and Winter" devised a novel olefin synthesis from 1,2-diols utilizing in the first step the reaction with thiocarbonyldiimidazole (2), a reagent prepared from 2 moles of imidazole and 1 mole of thiophosgene. Corey postulated that treatment of the cyclic thionocarbonate (3) with a reagent which would abstract sulfur to form the curbenc (4) would effect cis elimination to the olefin and found that trimethyl phosphite effects this reaction cleanly and in high yield. [Pg.1350]

A series of thioanhydroaldoses and thioanhydropentitols, for example 50, has been prepared via the electrophilic bis-cyclic thionocarbonate 51, which was in turn obtained by treatment of the monobenzyl pentitol 52 with diimidazolyl thione (Im2CS) <04TL4365>. 

Multiple Chugaev eliminations of several xanthates within the same molecule are possible. For example, triol 28 was smoothly converted to the corresponding methyl xanthates and subsequently heated to yield ellacene (29), a precursor to the polyquinenes.21 The Chugaev elimination of xanthates derived from 1,2-diols, however, often yields other products such as cyclic thionocarbonates.2... 

The Corey-Winter olefin synthesis is a two-step transformation of a diol 1 to an olefin 3.1"3 A cyclic thionocarbonate 2 is prepared from the diol 1, and subsequent heating of 2 with phosphite affords olefin 3. 

In 1963, Corey and Winter disclosed their discovery of a new method for the stereospecific synthesis of olefins from 1,2-diols. Treatment of mejo-hydrobenzoin 4 with carbonyldiimidazole in toluene at reflux gave the corresponding cyclic thionocarbonate 5 with quantitative conversion. Exposure of 5 to trimethylphosphite then afforded cis-stilbene 6 in 92% yield. fraHJ-Stilbene could be obtained by starting with the corresponding tram-diol. This two-step reaction sequence could also be used for the synthesis of cyclic olefins. Treatment of rraiw-cyclodecanediol 7 with... 

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