Hydroboration fundamentals

It would be difficult to overestimate the role that the polyether antibiotics have played in the development of organic synthesis, particularly in the area of acyclic stereocontrol. These molecules have inspired many spectacular achievements in organic synthesis, achievements that have dramatically expanded the power and scope of the science. In fact, it would not be inaccurate to attribute much of our understanding about the factors controlling acyclic stereoselectivity for such fundamental processes as hydroboration,5 epox-... 

Asymmetric hydroboration followed by oxidation is used to give optically active alcohols. For example, addition of (+)-IpcBH2 to 1-phenylcyclopentene followed by oxidation gives S,2R)-trans-2-phenylcyclopentanol in 100% e.e. (Equation B2.9). The structure of the product alcohol reveals that the homochiral hydroborating reagent encounters fewer unfavourable steric interactions with alkene substituents if it approaches the lower face of the alkene as drawn in Equation B2.9. This preference determines the absolute stereochemistry of the product. (The regiochemistry and relative stereochemistry of the product are determined by fundamental hydroboration characteristics.)... 

The examples we have used to show the fundamentals of hydroboration-oxidation have been, necessarily, simple ones. In practice, synthesis generally involves more complicated molecules, but the principles remain the same. For example ... 

Thus orientation of addition in hydroboration is controlled in fundamentally the same way as in two-step electrophilic addition. Hydrogen becomes attached... 

Fundamental studies directed toward the elucidation of the mechanism of olefin i.e.f isobutylene, polymerizations yielded a new method for the synthesis of novel linear and tri-arm star telechelic polymers and oligomers [1,2]. The synthesis involves the use of bi- or tri-functional initiator/transfer agents, so called inifers (binifers and trinifers), in conjunction with BCI3 coinitiator and isobutylene, and gives rise to polyisobutylenes carrying exactly two or three terminal -CH2-C(CH3)2Cl groups. These liquid telechelic polyisobutylene chlorides can be readily and quantitatively converted to telechelic polyisobutylene di- or tri-olefins [2,3] which in turn can quantitatively yield by hydroboration/oxidation telechelic polyisobutylene di- and triols [4,5]. 

In an amazing transformation, optically pure erythronolide template 927 is generated from 926 by a double dioxanone-dihydropyran [3,3] sigmatropic rearrangement [248] (Scheme 124). The template contains the fundamental 13-carbon framework of erythronolide A or B (600) with seven of its asymmetric centers established. The synthesis of 927 requires 18 steps to complete, and the genesis of all the stereochemistry can be traced to isobutyl D-lactate (913). It is speculated that the remaining six stereocenters of erythronolide B can be introduced by hydroboration of the three olefinic segments of the bis(dihydropyran) 926. 

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