Diastereoselective Claisen Rearrangements Using Chiral Auxiliaries

3 Diastereoselective Claisen Rearrangements Using Chiral Auxiliaries 

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A review which covers sigmatropic rearrangements in addition to Diels-Alder reactions, 1,3-dipolar cycloadditions, electrocyclic reactions, and ene reactions has appeared. The stereochemistry of [3,3]-sigmatropic reactions of chiral carbon compounds has been reviewed, as have diastereoselective Claisen rearrangements of substrates bearing chiral auxiliary and enantioselective variants of achiral substrates. Examples of [3,3]-sigmatropic rearrangements used in the synthesis of various types of 3-chromene derivatives have been reviewed. ... 

The chiral auxiliary mediated aza-Claisen rearrangement of /V-allylketcnc. V.O-acetals also allows the diastereoselective construction of quaternary carbon centers642. Butyllithium proved to be an unsuitable base for the neutralization step in this case because the increased steric hindrance at C-l causes C-2 nucleophilic addition to become competitive with C-l deprotonation. However, this problem can be overcome by the use of lithium tov-butoxide or lithium isopropoxide. This is shown for the achiral. V-allylketene A. O-aceta] precursor 8. 

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