Aldol Reactions of Chiral Imides and Ester Enolates

2 Aldol Reactions of Chiral Imides and Ester Enolates 

Enolates of chiral carboximides linked to Evans or Oppolzer s auxiharies (+)- or ( )-Xcbo. -Xc -po and -Xcbs react with aldehydes, giving rise to syn a-branched /3-hydroxy carboxyhc acid derivatives (iyra-aldols) with excellent stereoselectivities, generally exceeding 96% and in chemical yields above 70% . Both boron enolates and chlorotitanium enolates have been used in isotopic synthesis. 

Both procedures normally give (Z)-enolates in greater than 99% selectivity. These react with aldehydes via a cychc six-membered transition state of the Zimmerman-Traxler type, with one face of the imide enolates hindered by the auxiliary, forcing attack of the aldehyde at the opposite face. As with diastereoselective a-alkylations (previous section), the stereodirecting influence of the auxiliaries generally overrides that imposed by ancillary stereogenic centers in the aldehyde substrates . 

The normal yn-selectivity of (Z)-boron and chlorotitanium enolates can be overridden by diverting the cyclic transition state of the aldolization processes to an open one . This can be accomplished either by using (Z)-0-silyl-iV,0-ketene acetals as starting materials or by treating the boron enolate solution with an aldehyde pre-complexed with a second Lewis acid 406,112,113 latter case, enantiopure 2-oxazolidinone- 

An atypical anti product (134) was also produced in the reaction of Schiff base 133 with (45)-4-phenyl-3-[(55)-5-(4-fluorophenyl)-5-hydroxypentanoyl]-l,3-oxazolidin-2-one (131) in the presence of -Pr2EtN and TMSCl followed by addition of TiCl4. The resultant compound was obtained in 29% radiochemical yield after crystallization. This result may be explained by intermediate formation of an (9-silyl-A,(9-ketene acetal. Adduct 134 was 

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Aldol reactions of chiral imide and ester enolates ... 

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