Yamamoto epoxidation homoallylic alcohol

In a recent paper, Zhang and Yamamoto have described a modified BHA ligand (235d) that is suitable for highly enantioselective vanadium-catalyzed epoxidation of homoallylic alcohols (Scheme 102). Both tram- and cA-substituted epoxides were achieved with nearly complete enantioselectivities and good yields. 

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... 

The retrosynthetic analysis proposed by Yamamoto et is based on the asymmetric epoxidation of a homoallylic alcohol 58 obtained from (5)-limonene (Scheme 34.16). The (5)-homoallyl alcohol 58 was epoxidized diastereoselectively using hydroxamic acid 59 as ligand to give the corresponding epoxy alcohol 60 with 84% yield and 90% de. 

Zhang W, Yamamoto H. Vanadium-catalyzed asymmetric epoxidation of homoallylic alcohols. J. Am. Chem. Soc. 2007 129 286-287. 

General experimental procedure for Yamamoto epoxidation of homoallylic alcohols To a mixture of hydroxamic acid 21 (0.02 mmoi) and toiuene (0.25 mL) at room temperature was added VO(0/-Pr)3 (0.01 mmol). The mixture was stirred at room temperature for 8hours. After that time, 88% cumene hydroperoxide (1.5 mmol) was added, followed by homoallyhc alcohol (l.Ommol). The reaction mixture was stirred for 24 hours and then quenched by the addition of trimethyiphos-phite (1.5 mmol). The mixture was extracted with ethyi acetate. The combined organic layers were dried over Na2S04, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gei to give the desired epoxide. 

MakitaN, Hoshino Y, Yamamoto H. Asymmetric epoxidation of homoallylic alcohols and application in a concise total synthesis of (-)-a-bisabolol and (-)-8-ep -a-bisabolol. Angew. Chem. Int. Ed. 2003 42(8) 941-943. 

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]. 

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