Alkenes anti dihydroxylation

Anti dihydroxylation of an alkene with a peroxyacid, followed by ring opening with OH or H20(12.9A)... 

Hydrogen peroxide in presence of tungsten oxide (WO3) or selenium dioxide (SeO ) react with alkene to give anti-dihydroxylation products (Scheme 73). 

A two-step procedure for anti dihydroxylation involves conversion of an alkene to an epoxide, followed by acid-catalyzed ring opening. 

Alternatively, diols can be made via dihydroxylation of an alkene. In Chapter 9, we explored reagents for achieving either syn or anti dihydroxylation. 

Hydrolysis of oxacyclopropanes furnishes the products of anti dihydroxylation of an alkene... 

Treatment of oxacyclopropanes with water in the presence of catalytic acid or base leads to ring opening to the corresponding vicinal diols. These reactions follow the mechanisms described in Section 9-9 The nucleophile (water or hydroxide) attacks the side opposite the oxygen in the three-membered ring, so the net result of the oxidation-hydrolysis sequence constitutes an anti dihydroxylation of an alkene. In this way, tram-2-butene gives me50-2,3-butanediol, whereas ds-2-butene furnishes the racemic mixture of the 1R, >R and 2S,3S enantiomers. 

The stereochemical consequences of the vicinal syn dihydroxylation of alkenes are conplementary to those of vicinal anti dihydroxylation. Show the products (indicate stereochemistry) of the vicinal... 

In the synthesis of polyhydroxylated polyenes, osmium tetroxide induced dihydroxylation of an alkene adjacent to a coordinated iron atom has been used as the key synthetic step115. The reaction has been shown to be highly diastereoselective for E-alkenes and diastereospecific for Z-alkenes. With both types of alkene, the reaction occurs anti to the coordinated iron group (equation 18). 

Dihydroxylation of an alkene with 0s04 is a specifically cis reaction the two OH groups add to the same side of the alkene. So E-22 gives one diastereomer (syn as drawn) of the diol 23 while Z-22 gives, by syn addition, a diol that can be re-drawn after rotation of a bond, as anti -23. 

The pheromone 151 possesses an anti-relationship between the two hydroxyl functions built in its structure (Scheme 43). The estabUshment of such an antistereochemistry requires the AD of a (Z)-olefin, a process which usually proceeds with moderate enantiocontrol. To overcome this difficulty, Lohray [120] performed the asymmetric dihydroxylation of the isomeric ( )-alkene 149 and inverted selectively one of the chiral center via an intramolecular lactonization reaction on the derived cyclic sulfate 150. 

The AD reaction was central in the preparation of (+)-cw-sylvaticin 41,27 a natural product found to have potent anti-tumor activity. The ability of this compound to inhibit ATP production by blockade of the mitochondrial complex I was thought to be the origin of this biological outcome. The AD reaction, in this example, exploited the preference of this reaction for the oxidation of 1,2-frans-alkenes over monosubstituted alkenes. The E,E-isomer of tetradecatetraene 42 could be chemoselectively dihydroxylated at both internal alkenes, while the terminal alkenes remained untouched. Thus, 43 was generated in excellent chemical yield. 

Epoxidation and Dihydroxylation of Alkenes There are several ways to convert alkenes to diols. Some of these methods proceed by syn addition, but others lead to anti addition. An important example of syn addition is osmium tetroxide-catalyzed dihydroxylation. This reaction is best carried out using a catalytic amount of OSO4, under conditions where it is reoxidized by a stoichiometric oxidant. Currently, the most common oxidants are f-butyl hydroperoxide, potassium ferricyanide, or an amine oxide. The two oxygens are added from the same side of the double bond. The key step in the reaction mechanism is a [3 + 2] cycloaddition that ensures the syn addition. 

As summarized in Scheme 2.8, these reactions provide access to three different overall stereochemical outcomes for alkene dihydroxylation, syn addition, anti addition, or stereorandom addition, depending on the reaction mechanism. In Section 2.5.4 we will discuss enantioselective catalyst for alkene dihydroxytation. These reactions provide further means of controlling the stereochemistry of the reaction. 

Note that the dihydroxylation reaction occurs by a syn addition of the two hydroxy groups to the same face of the alkene, such that the Z-alkene 77 gives the anti products 78, 79. 

Because OSO4 adds two hydroxyl groups to an alkene in a syn fashion, the overall product depends on the geometry of the alkene starting material it is stereospecific. It is similar to bromination (p. 439) in that respect, although of course bromination is an anti addition. You can see how two different diastereoisomers are produced from different alkenes in these two examples both dihydroxylations are mechanistically syn, but redrawing the product from the Z alkene in its more extended form reveals anti stereochemistry. 

We can carry out anti 1,2-dihydroxylation of a double bond by converting the alkene to an epoxide and then carrying out an acid-catalyzed hydrolysis (Section 11.15). 

Anti 1,2-dihydroxylation (Section 11.15) The installation of hydroxyl groups at adjacent carbons and on opposite faces of an alkene, often accomplished by ring-opening of an epoxide. 

---