Diols, deoxygenation

Saturated O-heterocycles as intermediates in synthesis of olefins by vicinal diol deoxygenation 84OR(30)457. 

During these studies. Cook and Andrews published the first observation of catalytic rhenium-mediated diol deoxygenation (10) (Eq. 2). Their studies were specifically directed toward alditol manipulation. These results showed a turnover frequency for 1,2-phenylethanediol that was consistent with the stoichiometric rate of diolate cycloreversion, indicating that reduction of Cp Re03 and subsequent cyclocondensation of the diol on the rhenium were... 

It is of note that Arterburn and coworkers have developed a heterogenized rhenium catalyst that is also effective for diol deoxygenation (17). 

Diol Deoxygenation. The PhsP-lm-Ia combination can also be used to convert vic-diols into the corresponding alkenes, although 2,4,5-triiodoimidazole is more effective than imidazole itself. Alternative combinations are Triphenylphosphine-lodoform-Imidazole, which can deoxygenate cis-diols but which is better suited to frons-isomers, and Chlorodiphenylphosphine-l2-lm, which can be used to deoxygenate v(c-diols when both are secondary or when one is secondary and one is primary... 

B. Reactions in Which Neither Leaving Atom is Hydrogen 17-17 Deoxygenation of Vicinal Diols... 

As vicinal dibromides are usually made by bromination of alkenes, their utility for synthesis is limited, except for temporary masking of a double bond. Much more frequently it is desirable to convert a diol to an alkene, and several useful procedures have been developed. The reductive deoxygenation of diols via thiono carbonates was... 

Diols can also be deoxygenated via Mv-sulfonate esters using sodium naphthalenide.294 Cyclic sulfate esters are also cleanly reduced by lithium naphthalenide.295... 

N-Ethylpiperidi 11 i um hypophosphite has been used as a reductant in deoxygenation of nucleoside diol xanthates in aqueous solution.300... 

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. 

The reduction of thiocarbonyl derivatives by EtsSiH can be described as a chain process under forced conditions (Reaction 4.50) [89,90]. Indeed, in Reaction (4.51) for example, the reduction of phenyl thiocarbonate in EtsSiD as the solvent needed 1 equiv of dibenzoyl peroxide as initiator at 110 °C, and afforded the desired product in 91 % yield, where the deuterium incorporation was only 48% [90]. Nevertheless, there are some interesting applications for these less reactive silanes in radical chain reactions. For example, this method was used as an efficient deoxygenation step (Reaction 4.52) in the synthesis of 4,4-difluoroglutamine [91]. 1,2-Diols can also be transformed into olefins using the Barton-McCombie methodology. Reaction (4.53) shows the olefination procedure of a bis-xanthate using EtsSiH [89]. 

A new synthesis of olefins by the reductive deoxygenation of vicinal diols using tungsten(iv) compounds, e.g. K2[WCl6], has been described. The process appears to be the reverse of the reaction of OsO with olefins and to involve transition state (90) since, in corresponding reactions, M0CI4 only forms... 

To complete the synthesis of brevicomin, the side-chain had to be deoxygenated. This could be achieved by a reduction of the keto group, eUmination of the resulting diol to the olefin and reduction of the double bond. 

Both unsymmetrical diols and alkenes can be prepared by applying these methods to mixtures of two different carbonyl compounds. An excess of one component can be used to achieve a high conversion of the more valuable reactant. A mixed reductive deoxygenation using TiCl4/Zn has been used to prepare 4-hydroxytamoxifen, the active antiestrogenic metabolite of tamoxifen. 

The A ring is closed by a Ti-mediated reductive coupling (step H-5). The C(ll)-C(12) double bond is introduced from the diol by deoxygenation of the thiocarbonate [step I-... 

From the difference between the enthalpies of formation of 2,5-dimethyl-2,5-dihydroperoxyhexane and 2,5-dimethyl-2,5-dihydroperoxyhex-3-yne, the enthalpies of hydrogenation of the alkyne are derived as —298.5 kJmol (g) and —309.6 kJmol (s). The gas phase value is quite a bit larger than that for its deoxygenated counterpart, 3-hexyne, of —272.4 kJmol (g) - However, the calculated solid phase enthalpy of hydrogenation of 2,5-dimethylhex-3-yn-2,5-dior to 2,5-dimethylhexane-2,5-diol is -295.7 kJmor. ... 

---