Carbon-oxygen double bonds stereoselective synthesis

These reactions contain stereoselective and stereospecilic components. The starting materials 20 and 22 are chiral and racemic and the faces of the double bonds (in both cases) are diastereotopic. Epoxidation is stereospecilic since there is only one extra atom in the epoxide it must react with the two carbon atoms on the same side if the reaction is concerted. But, with allylic alcohols the reactions are also stereoselective since the OH will form a hydrogen bond with the peracid and hence deliver the oxygen syn to the alcohol. It could in principal react to give the anti product but it is not possible to make (as the major product anyway) the anti isomers 24 and 25 by this method. Another example of a reaction that has both stereoselective and stereospecilic components appears in the pumiliotoxin synthesis in Chapter 21. 

Acetals are unique as chiral auxiliaries since the prochiral carbon atom is tetrahedral, not trigonal, as in all other examples.t l In the presence of a Lewis acid the acetal function becomes a powerful electrophile, capable of reacting with electron-rich double bonds. The origin of the selectivity is believed to be the preferential complexation of the Lewis acid with the less-hindered oxygen as in (102).[4i] Reaction takes place by Sn2 displacement with inversion at the electrophilic carbon to give (103). Of interest in this synthesis of the lactone portion of mevinolin (105)[42] is the use of 3-elimi-nation to remove the chiral auxiliary and internal chelation to reduce the ketone (104) stereoselectively, as already described in section... 

Irradiation of 2-acyl-2,3-dihydro-4i/-pyrans [e.g. (90)] in the presence of 1-methylnaphthalene selectively excites the singlet state of the carbonyl group while quenching the triplet state. The oxygen of the carbonyl group becomes attached to C-6 of the double bond with concomitant stereoselective transfer of a hydrogen atom from C-4 to the carbon atom of the carbonyl group [e.g. (91)]. This process has been applied to the synthesis of exo-brevicomin (92) (Scheme 30), the sex attractant of the western pine beetle. ... 

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