Olefins bishydroxylation

We have already seen the bishydroxylation using 0s04 and NMO this time lead tetraacetate is used to cleave the diol and yield the aldehyde. The phosphorane (Me0)2P(0)CHN2 is named the Gilbert-Seyferth reagent. 8 It basically behaves like the phosphoranes in the Horner-Wadsworth-Emmons reaction described above, except that the olefin subsequently loses nitrogen, creating the desired triple bond (also see Chapter 10). 

These reagents were first used for the bishydroxylation of olefins.406"408 It was later found that many of these reactions proceed via epoxides that undergo subsequent hydrolysis under the acidic conditions employed. Under basic or neutral conditions, these reagents can be used for the epoxidation of olefins409"414 ... 

The synthesis began, as shown in Scheme 2.16, with the alkylation and subsequent hydrolysis of compounds 160 and 161 to afford aldol precursor 162. An enantioselective aldol condensation, using D-proline as a catalyst, gave a 70% yield of 163 and 164 in a 1 1 ratio, both diastereomers being formed in 36% ee. Isolation of the desired 164 followed by dehydration and recrystallization then gave optically pure (-I-) 165 in 10-12% yield from the racemates 160 and 161. A stereospecific sequence involving ketone reduction and protection as the MOM ether followed by bishydroxylation of the olefin and subsequent acetonide formation gave the key intermediate 166 (74%). 

Xc lacks the substituent at the a position (or the a- and /3-positions, if they are part of an olefinic system) required to define the class of compound to be created (i.e., a-amino acid, a-hydroxyacid, a-bromo-/3-hydroxyacid, etc.). It is the reagent R that brings this information. This concept is illustrated for an amino acid in Figure 11.36. In other words, the requisite substitution pattern at these positions is generated by an appropriate diastereoselective a- or a, -functionalization reaction (e.g. an a-alkylation, a-aldolizatiom, a-amination, a-hydroxylation, conjugate addition, Diels-Alder reaction, bishydroxylation etc.). 

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