Ethers, benzyl methyl reductive cleavage

Benzyl derivatives of aliphatic alcohols 73> are not reduced below the discharge potential of (C2H5)4N + in DMF [—2.9 V(SCE)]. But, they show a wave around —3.1 V(SCE) in solutions of (C4H9)4N+. Efficient reductive cleavage of benzyl methyl ether was achieved n) in THF-8% H20 with 0.25 M (C4H9)4NBF4. Under these conditions, toluene, which is the cleavage product may also be reduced and the... 

Selective reduaion of aromatic acids and their derivatives to aldehydes can be achieved electrochemi-cally. Reduction of the carbinolamines is observed, as in the formation of anilinomethylpyridine (Scheme 13) and 4-methylpyridine. Reductive cleavage of benzyl methyl ethers can be achieved leading to hydrocarbons when the ring substituents are electron attracting. 

Enders and Jegelka [88] have used l,3-dioxan-5-one 122, a protected dihydroxyacetone derivative, to construct enantiomerically pure C5- to C9-deoxycarbohydrates. For example, reaction of 122 with SAMP gives the hydrazone 123, which is deprotonated and alkylated with methyl iodide to yield 124. The monoalkylated hydrazone is then alkylated in the same manner with chloromethyl benzyl ether to form 125. Cleavage of the hydrazone with ozone furnishes the protected ulose 126 (>98% de, >98% ee), which is deprotected to (—)-5-deoxy-L-r/ir o-3-pentulose 127. Reduction of 126 with L-Selectride, followed by deprotection, provides 5-deoxy-D-arabinitol 128 (>95% de, >95% ee) (Scheme 13.46). 

Reductive C-0 bond cleavage. Benzylic methyl ethers give benzyl anions which can be alkylated. The controlled stepwise cleavage of dimethyl acetals makes the method synthetically useful. 

Cleavage of propargyl ethers. With the low-valent Ti species (from TiCla, Mg in THF) the reductive cleavage of propargyl ethers is selective, since allyl, benzyl, and methyl ethers remain intact under these conditions. 

Analogous to the preparation of [ C]methyl ethers is the reaction of alkali thiolates with [ CJmethyl iodide to give the labeled thioethers. The synthesis of (5)-[methyl- C]-methionine (201), the most important representative of this substance class, may serve as an example i . It is conveniently available in 60-80% radiochemical yield by reaction of [ CJmethyl iodide with deprotonated (5)-homocysteine, generated in situ by reductive cleavage of the 5-benzyl. It is widely used as a [ C]methyl donor in fermentative labeling processes (see Chapter 12). 

---