Anti dihydroxylation

As mentioned in the introductory section of this chapter, dihydroxylation reactions are characterized by the addition of OH and OH across an alkene. As an example, consider the dihydroxylation of ethylene to produce ethylene glycol  

There are a number of reagents well suited to carry out this transformation. Some reagents provide for an anfi dihydroxylation, while others provide for a sjn dihydroxylation. In this section, we will explore a two-step procedure for achieving antt dihydroxylation  

The first step of the process involves conversion of the alkene into an epoxide, and the second step involves opening the epoxide to form a tram diol (Mechanism 9.6). An epoxide is a three-membered, cyclic ether. 

You might be wondering why a weak nucleophile can participate in an Sn2 process at a secondary substrate. In fact, this reaction can occur even if the center being attacked is tertiary. This will be explained in Section 14.10. 

In the first part of the process, a peroxy acid (RCO3H) reacts with the alkene to form an epoxide. Peroxy acids resemble carboxylic acids in structure, possessing just one additional oxygen atom. Two common peroxy acids are shown below  

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Whereas the yyn-dihydroxylation reaction, which is described above, is performed in the presence of metal oxides KMn04 or OSO4, the anti-dihydroxylation is carried out with... 

Anti-dihydroxylation Epoxides may be cleaved by an aqueous acid to give glycols. Proton transfer from the acid catalyst generates the conjugate acid of the epoxide, which is attacked by nucleophiles such as water. The result is anti-dihydroxylation of the double bond. 

Anti dihydroxylation is achieved in two steps—epoxidation followed by opening of the ring with OH or H2O. Cyclohexene, for example, is converted to a racemic mixture of two tram-1,2-cyclohexanediols by anti addition of two OH groups. 

Draw the products formed when both c/s- and frans-2-butene are treated with a peroxyacid followed by "OH (in H2O). Explain how these reactions illustrate that anti dihydroxylation is stereospecific. 

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

Anti dihydroxylation (Section 12.9 A) The addition of two hydroxy groups to opposite faces of a double bond. 

Wittig-Homer-Emmons olefination of serinal derivative l-50 (see Scheme 13.27) with carbonylphosphorane 176 generates enone 177. Subsequent conversion to (—)-nojirimycin l-58 proceeds via anti dihydroxylation and reduction with NaBH4 (Scheme 13.61), whereas reduction with Red-Al leads to (—)-mannonojirimycin [109]. A similar approach has been applied to generate galactostatine [110]. 

The syn dihydroxylation of olefins is usually performed in the presence of metal oxide, for example, KMn04 or OSO4, and with t-BuOOH or HP as terminal oxidants. The anti dihydroxylation can be achieved with peracetic acid and m-ClC6H4C03H in water. However, the atom efficiency of these oxidants is low, and they form equimolar amounts of deoxygenated compounds [36a, bj. 

Diols are made by two reactions from Chapter 8 and revisited in 9-41 (d) either syn-dihydroxylation with OSO4, or anti-dihydroxylation via an epoxide using a peroxyacid and water. As this problem says to use inorganic reagents, the solution shown here will use OSO4... 

Oxidation by peroxycarboxylic acids (RC03H) and hydrolysis of epoxides to give anti- dihydroxylation... 

RC03H, such as meta-chloroperoxybenzoic acid (m-CPBA). RCO3H and then ring-opening of the epoxide with H07H20 or H7H20 to give stereoselective anti- dihydroxylation. H2/Pd/C. 

Anti dihydroxylation An oxidation-hydrolysis sequence in which an ami diol forms from an alkenc. 

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

Detemnine whether syn dihydroxylation of trans-2-butene will yield the same products as anti dihydroxylation of c/s-2-butene. Draw the products in each case and compare them. 

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. 

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