Phosphate esters reductive cleavage

Reductive removal of oxygen from aromatic rings can also be achieved by reductive cleavage of aryl diethyl phosphate esters. 

There are also examples in which phosphate esters of saturated alcohols are reductively deoxygenated.229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have indicated that the crucial C-O bond cleavage occurs after transfer of two electrons.230... 

Steenken S, Jagannadham V (1985) Reaction of 6-yl radicals of uracil, thymine, and cytosine and their nucleosides with nitrobenzenes via addition to give nitroxide radicals. OH catalyzed ni-troxide heterolysis. J Am Chem Soc 107 6818-6826 Steenken S, Jovanovic SV (1997) How easily oxidizableis DNA One-electron reduction potentials of adenosine and guanosine radicals in aqueous solution. J Am Chem Soc 119 617-618 Steenken S, Behrens G, Schulte-Frohlinde D (1974) Radiation chemistry of DNA model compounds, part IV. Phosphate ester cleavage in radicals derived from glycerol phosphates. Int J Radiat Biol 25 205-210... 

Appropriate quenching of a reductively formed lithium enolate with a carboxylic acid anhydride, chloride, methyl chloroformate or diethyl phosphorochloridate yields the corresponding enol esters, enol carbonates or enol phosphates. These derivatives may be transformed into specific alkenes via reductive cleavage of the vinyl oxygen function, as illustrated by the example in Scheme 8. 

The two crops (10.66 g) represent 94X recovery on recrystalllzatlon. The yield of recrystalHzed product Is 92X In the reductive cleavage of the phosphate ester and 73% overall. 

Although a radical may be formed on cleavage, it does not necessarily give rise to substitution product. Nucleophiles and electrons both compete for the radical. Reaction with an electron leads to an anion which then is protonated. The net result of the two electron capture sequence is removal of the side chain by reduction. Such a sequence can be used advantageously. Phenols may be dehydroxylated on a preparative scale by converting them to their phosphate esters prior to treatment with an alkali metal in ammonia.102 ... 

A synthesis of 10-epijunenol (417) cf. Vol. 8, p. 102) has been accomplished by photochemical cycloaddition of the cyclobutene ester (413) to piperitenone (414) followed by reductive cleavage of the phosphate ester (416) (Scheme 35). 

Replacement of the Phenolic OH Groins. Recendy the synthesis of completely dehydroxylated calix[4]arene 14d by reductive cleavage (K in liquid ammonia) of the tetrakis(diethyl phosphate) ester of p-tert-butylcalix[4]arene 9b has been reported [13]. Starting from the 1,3-di- and tetraesters of 9b we could isolate the partially dehydroxylated calix[4]arenes 14a-c [14]. 

Despite these modifications, Ireland s original method of both vinyl phosphate formation and reduction remains the most popular. In studies on the cationic rearrangements of [4.3.2]propellanes, Smith et converted a tricyclic ketone, via the diethyl vinylphosphate derivative, into the corresponding alkene (Scheme 26). In a similar manner, Kamata et al prepared A - and A -steroids (Scheme 27) with excellent control of regiochemistry. As shown in this example, esters are susceptible to cleavage under the standard reduction conditions (Li, NHa, Bu OH, -35 C), while acetals survive. 

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