Boronic Ester Intermediates without Functional Substituents

Boronic Ester Intermediates without Functional Substituents 

The reaction of boronic esters with (dihalomethyl)lithium is particularly well suited to the building of asymmetric structures in which there are no functional substituents. At the end of the synthesis, the boronic ester group can be replaced by reaction with hydrogen peroxide to form the corresponding asymmetric secondary alcohol [11,12, 29]. 

The first test of pinanediol (a-chloroalkyl)boronic ester chemistry included syntheses of (2S,3S)- and (2J ,3S)-3-phenyl-2-butanol, each in 94-96% diastereopurity and, assuming independent stereoselection for installation of each of the two adjacent stereocenters, 99% enantiopurity [11]. These targets were chosen because their absolute configurations had been estabhshed by Cram s classic work [30]. Zinc chloride catalysis had not been discovered, and other features of the syntheses are also ob- 

Nucleophilic displacement of a-halogen substituents is facilitated by the boronic ester group, and elimination reactions do not normally occur [9], This early observation has been substantiated by the large body of chemistry that has accumulated since. 

In view of the instability of (p-bromoalkyl)boronic esters in the presence of base, it is not expected that carbon insertion into the C-B bond of an (a-chloroalkyl)boronic ester would yield anything but elimination products. However, pinanediol (o-chlorovinyl)boronate proved resistant enough to P-elimination to allow insertion of a chloromethyl group into the C-B bond [39]. 

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Boronic Ester Intermediates without Functional Substituents... 

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