Allylic hydroperoxides natural product synthesis

In natural product synthesis it is sometimes important to eliminate selenoxide oxidatively to form the corresponding olefin. Alumina in combination with tertiary-butyl hydroperoxide in THF offers much improved yields in comparison to conventional methods [77]. Lebar et al. report the application of this reagent in the synthesis of primary allylic alcohols (e.g. equation 4.13) [78]. 

Singlet Oxygen.—Singlet oxygen reacts with dienes to give e do-peroxides and with alkenes to give allylic hydroperoxides or dioxetans. " The mechanism of the latter reaction is still under investigation. The use of these reactions in natural products synthesis has been comprehensively reviewed, "— " and attention will be concentrated on recent work. 

The allylic oxidation of alkenes by O2 involves an ene reaction, and proceeds with rearrangement s as in Scheme 4. The intermediate allylic hydroperoxide (5) can be reduced to yield an allylic alcohol (6), or be treated with base to give an unsaturated carbonyl conqiound (7). The reaction works best on tri- or tetra-substituted alkenes, and the relative preference for attack is Me - CH2 CH. The O2 allylic oxidation has been used in the synthesis of a large number of natural products, including some naturally occurring allylic hydroperoxides. It is possible that O2 reactions of this type are involved in biosynAetic processes. 

Sharpless asymmetric epoxidation ° is an enantioselective epoxidation of an allylic alcohol with ferf-butyl hydroperoxide (f-BuOOH), titanium tetraisopropoxide [Ti(0-fPr)4] and (-b)- or (—)-diethyl tartrate [(-b)- or (—)-DET] to produce optically active epoxide from achiral allylic alcohol. The reaction is diastereoselective for a-substituted allylic alcohols. Formation of chiral epoxides is an important step in the synthesis of natural products because epoxides can be easily converted into diols and ethers. 

Two carbonyl fragments could also be formed on acid-catalyzed hydrolysis of the aUyhc hydroperoxides that mostly proceeds through Hock-type fragmentation. AUyHc alcohols obtained by reduction of the corresponding allylic hydroperoxides are versatile building blocks with diverse synthetic utility they could also be either regioselectively dehydrated to l,3-dienes or oxidized to a,P-unsaturated ketones. The ene-reaction was also utilized in the synthesis of some natural products and diastereoselective allene derivatives. ... 

---