Barton-type deoxygenation

The same polymer-supported organotin reagent was also used in the Barton-type deoxygenation of steroid derivatives, sugar derivatives and diols160. Furthermore, the radical deamination of secondary and tertiary amines via the corresponding isocyanides was also reported160,169. 

SCHEME 40. Barton-type deoxygenation using a polymer-supported organotin hydride... 

The hydrolysis of 97 in alkaline conditions, followed by acid treatment, allowed the intramolecular lactonization, affording the hydroxylactone 98. Barton-type deoxygenation of 98 yielded the expected (+)-dihydronepetalactone 99. 

Barton-type deoxygenation of 100, with the same procedure previously described in scheme 20, gave the acetate 101 which is a known intermediate in the synthesis of iridomyrmecin 102. 

An interesting neophyl-type radical rearrangement process has been established for the synthesis of azabicycles, which are not readily accessible by other means. Barton McCombie deoxygenation of xanthate 70 under slow addition of (TMS)3SiH and AIBN in refluxing toluene furnished the 2-azabenzonorbor-nane derivative in good yield (Reaction 7.72) [82]. 

The other type of radical chemistry of importance in the carbohydrate field is one-electron reductions. A handful of these reactions (such as the metallic Zn reduction of acetobromoglucose to triacetylglucal) have been used in synthesis for decades, but, starting with the Barton-McCombie deoxygenation of sugars in the mid-1970s there has been an explosion of interest, as increasingly sophisticated cascades of elementary radical steps have been devised. Such reactions are driven by the homolysis of weak bonds such as Sn-H or N-O under conditions of photolysis or mild thermolysis. Nature uses a similar basic principle in Type II ribonucleotide reductases, where the weak bond in question is the cobalt-carbon a bond in the corrin cofactor. ... 

Several other types of polymer-bound stannanes have since been reported, including a polymer-bound distannane used in radical-mediated cyclizations and radical addition to acetylenes [87], and stannanes with longer tethers to the polymer [88, 89] applied in the Barton-McCombie deoxygenation of alcohols [90, 91] and... 

The toxicity of organotin derivatives and the difficulty in removing tin residues has spurred considerable eflforts to devise catalytic systems or, preferably, completely tin-free processes for conducting radical reactions. The use of poly(methylhydrosiloxane) in conjunction with a small amount of hexabutylditin oxide, a combination of reagents initially proposed by Grady and Kuivila [21a], has recently been applied to Barton-McCombie type deoxygenations [21b]. Several silanes have been examined... 

Dldeoxylnosine has been prepared by Barton deoxygenation of the 5 -0-benzoyl-2 -deoxy compound. Liquid-liquid and solid-liquid phase transfer methods have been used to prepare 2 -deoxyribofuranosides of pyrrolol2,3-dlpyrimidlnes (7-deazapurines). which have been converted to 2, 3 -dideo3Qrcompounds by Barton-type procedures amongst the... 

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. 

The products were obtained in excellent yields after simple hexane extraction. The removal of bromine and iodine proceeded smoothly. The removal of PhSe afforded methylcyclohexanone, indicating that the decarbonylation of acyl radicals takes place. The efficiency of deoxygenation of alcohols (Barton-McCombie reaction) is independent of the type of thiocarbonyl derivative (i.e. 0-arylthio-carbonate, O-thioxocarbamate, thiocarbonylimidazole or xanthate), as previously reported for (TMS)3SiH in organic solvents.25... 

Use of Tin Hydrides. From the mechanistic point of view, the reductive elimination of methyl xanthate derivatives of (3-hydroxy sulfones entails an interesting variation of the Julia reaction since an initial fragmentation of the C-0 bond through a Barton-McCombie-type radical deoxygenation takes place. Final aryl sulfonyl radical elimination affords the corresponding alkene (Eq. 52).98,99... 

TABLE 7. Alkanes through Barton-McCombie-type radical deoxygenations... 

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