Stereoselectivity in epoxidations

Allylic and cis-homoallylic alcohols are epoxidized readily, but frans-homoallylic and bishomoallylic alcohols react slowly, if at all. The stereoselectivity in the epoxidation of acyclic allylic alcohols is the same as and is comparable to that observed with r-BuOOH/VO(acac)2. The stereoselectivity in epoxidation of acyclic homoallylic alcohols is also the same but lower than that obtained with t-BuOOH/ VO(acac)2. Epoxidation of cyclic allylic alcohols proceeds more slowly and in lower yield than that of acyclic allylic alcohols. 

Curci and co-workers observed stereoselectivities in epoxidations with the caroate-acetone system opposite to that observed for Bu 02H/V(5+) epoxidations (Figure 6) <82JOC2670>. Whereas the vanadium system gives high cis selectivity with cyclic allylic alcohols, the caroate-acetone reagent gives predominantly trans selectivity ((115) is formed exclusively with vanadium). 

Methylcyclohexene shows little stereoselectivity in epoxidation, giving a 54 46 ratio favoring the trans-epoxide. ds-4,5-Dimethylcyclohexene, on the other hand, gives predominantly (87 13) trans,frans-4,5-dimethylcyclohexene oxide. Explain. 

---