Aldol Reactions of Chiral Haloacetates

Chiral haloacetates of the Evans and Oppolzer type are highly potent and versatile synthons for the preparation of a broad variety of e.p. intermediates. The boron enolates of Evans haloacetates and the titanium enolates of Oppolzer s derivatives undergo aldol additions with aliphatic and aromatic aldehydes to provide syn 2-halo-3-hydroxy carboxylic acid derivatives with excellent diastereoselectivities, generally exceeding syn/anti ratios 50 1 . In the Oppolzer series, only with acetaldehyde (diasteromeric ratio 

Both antipodes are obtained in 70% yield upon heating sodium [1- C]- and [1,2- 2]-acetate with a mixture of Br2 and PBra followed by vacuum transfer of the resulting bromo[ C ]acetyl bromide into a suspension of the lithiated auxiliary. The requisite 

NR3 BUjBOTf, iPrjEtN (Evans) TiCl4, 1-ethylpiperidine (Oppolzer) 

As elaborated in Sections 11.3.6 and 11.3.8, the spectrum of valuable labeled building blocks, such as a-amino acids, /3-hydroxy a-amino acids of the aWo-threonine type, chiral ( )-enoyl imides, is significantly broadened by replacing the halogen in carbon-13/14-labeled (25)/(21 )-BABS with various functionalities (e.g. -N=CPh2, -NCS, -PO(OEt)2). In addition, replacement of the bromo group subsequent to aldolization provides further options  

Reaction conditions la PBra, Br2, TFAA (trace) 100-120 °C, 2 h, b. n-BuLi, 

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