Carbon-oxygen bond reductive cleavage

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Use has been made of the bond cleavage processes initiated by an adjacent carbonyl function for the modification of steroidial ketols such as 18 [97], Reduction in ethanol eliminates the hydroxyl function and in the same reaction, the carbonyl function is reduced to a secondary alcohol. In compound 19 where there are several groups to act as electrophores, carbon-oxygen bond cleavage is initiated from the most easily reduced dienone function [98], Cleavage of the carbon-oxygen bond in an a-acetoxycarbonyl function is achievable in good yields from multifunctional compounds such as the sesquiterpene taxol [99]. 

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

The reductive lithiation of cyclic benzofused ethers, for example, 510, with 4,4 -di-/< rt-butylbiphenyl (DTBB) and lithium gives intermediate organolithiums, for example, 511 and 512, that can be quenched with a variety of electrophiles to give general products 513 and 514 (Scheme 92). The process is not synthetically useful for 4H-chromene as carbon-oxygen bond cleavage occurs in both directions <2002TL4907>. 

Treatment of substituted phthalans 1172 with lithium metal in the presence of catalytic quantities of naphthalene leads to reductive cleavage of the arylmethyl carbon-oxygen bond to form a stable dilithium compound 1173, which upon trapping with carbon dioxide furnishes isochroman-3-ones 1174 (Scheme 289) <1996JOC4913>. 

The sodium borohydride reduction of benzotriazoles 138 and 139 gave, in good yield, benzotriazole 140. The cleavage of the carbon-nitrogen bond by the complex hydride thus occurred in preference to reduction of the ring or carbon-oxygen bond cleavage. 

A cleavage of carbon-oxygen bonds caused by amalgam reduction is found, for example, in the reduction of dihydroxyacetoxime to isopropylamine [121] and 2-hydroxy-4-phenyl-3-butenoic acid to 4-phenyl-3-butenoic acid [122]. These reactions are analogous to those discussed in Chapter 23. 

Cyclic alkyl aryl ethers lead also to functionalized organolithium compounds by reductive carbon-oxygen bond cleavage in arene-catalyzed lithiation process. Thus, the treatment of 2,3-dihydrobenzofuran (47) with an excess of lithium in the presence of a catalytic amount of DTBB in THF at 0°C gives the dianion (48) which after reaction with different carbonyl compounds and final hydrolysis with water leads to... 

The cleavage of carbon-oxygen bonds from alkenyl or aryl phosphates can be accomplished under reductive conditions with a low valent metal. As vinyl phosphates can be formed readily from ketones, this procedure provides a method to convert a ketone to an alkene. For example, the alkenyl phosphate 74 was prepared by trapping the enolate formed on reduction of the enone 73 and was converted into the alkene 75 (7.55). The chemistry therefore provides a method to prepare structurally specihc alkenes. Low-valent titanium (prepared for example by reduction of titanium(III) chloride with potassium metal) is a convenient alternative to lithium or sodium in liquid ammonia or an amine for the reductive cleavage of alkenyl or aryl phosphates. 

Both carbon-oxygen bonds of the acetal functionality in the procyanidin A-1 (91) and A-2 (92) derivatives are thus susceptible to reductive cleavage under acidic conditions. This process is presumably triggered by random protonation of the acetal oxygens and concomitant... 

The reductive cleavage of carbon-oxygen bonds occurs readily when the carbon atom is substituted by a phenyl ring ... 

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