Epoxidation hydroxy group directing effects

Interestingly, MCPBA epoxidation of cis alcohol 16 affords a mixture of diastereomeric epoxides (55 45 mixture). Furthermore, protection of the allylic alcohol as TBS ether (17) and subsequent epoxidation results as well in hardly any stereochemical selectivity (53 47 mixture). With regard to these results it is suggested that the trans-allylic hydroxy group is effectively involved in directing the MCPBA epoxidation event. 

Hydroxy groups exert a directive effect on epoxidation and favor approach from the side of the double bond closest to the hydroxy group.78 Hydrogen bonding between the hydroxy group and the reagent evidently stabilizes the TS. 

In the peracid epoxidation of E or Z medium-size ring allylic alcohols, often in both diastereomeric transition states a directing effect by the hydroxy group is feasible. The strong preference for trans epoxidation by peracids [99.8 0.2 for (Z)-2-cyclooctenol, see Houben-Weyl, Vol. E13/2, pp 1189,1190] is mainly a consequence of the transannular ring strain in the conformer with pseudoaxial hydroxy group. 

Since epoxidation at the vinyl double bond is unproductive, it is desirable to direct reaction on the al-lene moiety. This can be accomplished by taking advantage of the hydroxy-directed epoxidation of allylic alcohols using the t-butyl hydroperoxide/vanadium(V) system.The directing effects of both allylic and homoallylic type hydroxy groups have been examined at both positions of the vinylallene unit. " At the 1-position (64), primary, secondary and tertiary allylic tdcohols are effective, while only primary homoallylic alcohols have bran examined (equation 35). Presumably the directing effect of the hydroxy groups favors formation of the intermediate allene oxide (65). A sample of the compounds prepared by this route is shown in Scheme 32. ... 

Directing effects of various functional groups on the stereoselectivities of peroxy acid epoxidations have been studied in detail. In the case of allylic alcohols, the weak directing effect of the hydroxy... 

The influence of the conformational factors, which play a decisive role in directing oxide fission in the above cases is no longer operative in the case of 3-keto-5a,6a-epoxides and their 3-ethylene ketals. With these substrates the —I effect of the BFs-complexed 3-keto or 3-ketal grouping predominates leading to the fluorohydrins. Thus, treatment of both 5a,6a-oxidopregnane-3,20-dione (35) and its 3,20-bisethylene ketal with BFg-etherate in benzene-ether affords in 45% yield the 6jff-fluoro-5a-hydroxy-derivative (36) and its 3-ethylene ketal, respectively. which are converted into the 6a-fluoro-A -CH3... 

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