ORGN 269 |
| alpha-Haloalkylmagnesium chlorides constitute a remarkable class of main-group carbenoids which possess unusually high configurational stability. We have discovered that these organometallic reagents will insert into the carbon-boron bond of simple boronic esters with excellent stereochemical fidelity. Successful iteration of this stereospecific homologation process, if achievable, will allow for a fully programmable asymmetric synthetic technique well suited to the construction of polyketides. Proof of concept studies will be presented in which the carbenoids of interest were generated in situ by established sulfoxide ligand exchange methodology. In one example, addition of (R)-1-chloro-2-phenylethylmagnesium chloride (2), with an enantiomeric excess of at most 84%, to B-(phenethyl) catechol borane at –78 deg C followed by warming and subsequent oxidation, delivered (S)-1,4-diphenylbutan-2-ol (3) with an enantiomeric excess of 80% (Figure 1). The involvement of a stereoretentive SE2 reaction between carbenoid and boronate is implicated by the stereochemical outcome of this reaction.
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Asymmetric Reactions and Syntheses
8:00 AM-12:00 PM, Monday, 14 March 2005 Convention Center -- Room 11A, Oral
Division of Organic Chemistry |
