Rerf-Butyl hydroperoxide epoxidation

The asymmetric epoxidation of allylic alcohols with cumene hydroperoxide or rerf-butyl hydroperoxide (TBHP) was first examined by using chiral amino alcohol-Mo complexes 45) and V complexes with chiral hydroxamic acid ligands (Scheme 20) 46). The highest optical yields were 33% with geraniol and 50% with 2-phenylcinnamyl alcohol. Combined use of VO(acac)2 and a hydroxamic acid derived from proline led to 80% optical yield with 2-phenylcinnamyl alcohol 47). 

The first practical method for asymmetric epoxidation of primary and secondary ailylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (-J-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and rerf-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are sufficient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the ailylic alcohol. 

Less regularly used reagents are rerf-butyl hydroperoxide/terr-butyliithium,600,601 ozone.602 dioxiranes,603,604 fluorine/ water/acetonitrile,605 or A, A -diethylhydroxylaminc.606 Alkenes carrying a donor substituent can also be epoxidized with peracids.607-609 Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclectivity.610... 

The only derivative of 1,4-cineole (554) we feel constrained to mention is not even a natural product. As a herbicide, however, Cynmethylin (622) will doubtless soon be present in the environment. It was synthesized by epoxidation of l-menthen-4-ol (547), using vanadium-catalyzed rerf-butyl hydroperoxide. 

One of the most important asymmetric syntheses is the Sharpless epoxidation. In this reaction, an allylic alcohol is transformed, by reaction with rerf.-butyl hydroperoxide (TBHP) in the presence of titanium tetra-wo-propoxide (Ti(/-PrO)4) and diethyl tartrate (DET), to the corresponding epoxy alcohol, with high enantiomeric purity. By the application of either (+)- or (-)-DET, the reaction product with the desired stereochemistry can be obtained [1,2] (Scheme 1). "O" (-)-DET... 

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