Yamamoto epoxidation vanadium catalysts

The asymmetric synthesis of florfenicol (104) from the oxidation of 4-methylthiobenzaldehyde by TBHP in the presence of Yamamoto s vanadium catalyst resulted in 37% overall yield intermediate formation of aUyUc alcohol (105) and its epoxidation to (25, 35)-epoxide (106) in 75% yield and 90% ee was involved. TBHP oxidation of jS-dicarbonyls in the presence of vanadyl acetylacetonate in benzene involved the activated methylene group in formation of intermediate trioxo derivatives and is accompanied by decomposition of the carbon skeleton... 

Homoallylic alcohols can be asymmetrically epoxidized using a chiral vanadium catalyst equipped with the hydroxaraic acid ligand 45, as exemplified in Yamamoto s concise synthesis... 

Yamamoto has studied a variety of readily available, chiral hydroxamic-acid based ligands, which have proven effective in vanadium-catalyzed enantioselective epoxidations of allylic (Equation 14) [84, 85] and homoallylic alcohols (Equation 15) [86], as well as in molybdenumunfunctionalized olefins [87], For example, treatment of 74 and 77 with tert-BuOOH and 1 mol % vanadium catalyst 75 furnished epoxides 76 and 78 in 97 and 91 % ee, respectively [85]. 

Other metal-based epoxidation catalysts have been explored to overcome some of the hmitations of the Sharpless procedure. One drawback with the Sharpless asymmetric epoxidation is the slightly lower ees often obtained when using cis-olefin substrates. The group of Yamamoto have achieved highly enantioselective epoxidations of ds-alkenes using vanadium(V) oxytriisopropoxide in the presence of C2-symmetric bishydroxamic acid ligands such as (4.23). In contrast to the Sharpless procedure this process is not hampered by the presence of air or... 

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