Deoxygenation with Bu3SnH

These esters can be deoxygenated with Bu3SnH or, as in the preceding example, with LiAlH4. ... 

The driving force for the fragmentation is formation of the C=0 double bond. If R reacts with Bu3SnH, a tributyltin radical is produced which continues the chain. Carried out in this manner this reaction is called the Barton deoxygenation of alcohols, since alcohols are precursors for the thiono esters. 

Radical deoxygenation of alcohols is important, and the reduction of xanthates prepared from alcohols, with Bu3SnH in the presence of AIBN is called the Barton-McCombie reaction (eq. 2.13) [37-51]. The driving force for the reaction is the formation of a strong C=0 bond from the C=S bond, approximately 10 kcal/mol stronger. This reaction can be used for various types of substrates such as nucleosides and sugars. Though methyl xanthates, prepared from alcohols with carbon disulfide and methyl iodide under basic conditions are very frequently used, other thiocarbonates, as shown in eq. 2.14, can also be employed. 

Eq. 7.2 shows a typical deoxygenative reaction of hederagenin (3) via methyl xanthate (4) with Bu3SnH in the presence of AIBN under benzene refluxing conditions. 

Deoxygenation of alcohols.1 Dithiocarbonates are converted to the corresponding alkane on treatment with Bu3SnH (1 equiv.) and (QHS),B (1 equiv.) in benzene. 

Deoxygenation of sec-alcohols.1 2 -Deoxynucleosides (1) can be deoxygen-ated efficiently to 2 3 -dideoxynucleosides (3) by reaction with N,N -thiocarbonyl-diimidazole in DMF (80°) to form imidazolides, which on reaction with methanol are converted into methylthionocarbonates (2). These crystalline derivatives are reduced by Bu3SnH to 3. 

A solution of Bu3SnH (200 mg, 0.69 mmol) inp-cymene (3 ml) was added dropwise over 2 h to a solution of methyl xanthate of hederagenin (50 mg, 0.087 mmol) in p-cymene (3 ml) at 150 °C. After the addition, the mixture was stirred at the same temperature for 10 h. After the reaction, CC14 was added and the mixture was refluxed for 3 h. After removal of the solvent, molecular iodine in ether was added until the iodine color remains. Then, the solution was washed with 10% aq. KF solution (5 ml), and the organic layer was dried over Na2S04. After removal of the solvent, the residue was recrystallized to give the deoxygenated product [2]. 

In the conjugate reduction of enals and enones, tin hydride is used catalytically by the combination with PhSiH3.58 Barton-McCombie deoxygenation reaction is performed by using Bu3SnH catalyst and PMHS as a stoichiometric reductant (Equation (16)).59... 

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