Doppelt und dreifach funktionalisierte, enantiomerenreine C₄‐Synthesebausteine aus β‐Hydroxybuttersäure, Äpfelsäure und Weinsäure

Enantiomerically Pure Synthetic Building Blocks with Four C‐Atoms and Two or Three Functional Groups from β‐Hydroxy‐butanoic, Malic, and Tartaric Acid The pool of chiral, non‐racemic electrophilic building blocks, which are available from simple natural products in both enantiomeric forms is enlarged by the epoxides 3, 5 , and 10 , by the tosylate 12a , and by the aldehydes 18 ( cf. symbols A‐D , 14 , and Scheme 1 ). Key steps of the conversions leading from hydroxyacids to the building blocks are: epoxide‐opening by triethylborohydride ( 1 → 2a ) and tosylate reduction ( 12a → 12b ); the Mitsunobu inversion ( 2a → 4a ); the reduction of ( R, R )‐tartaric ester to ( R )‐malic ester by NBS ( N ‐bromosuccinimide) opening of the benzaldehyde acetal 8 and tin hydride reduction ( 6c → 7c ); the enantiomer enrichment of optically active ethyl β‐hydroxy‐butanoate through the crystalline dinitrobenzoate 21b . Detailed procedures are given for large scale preparations of the key intermediates. The enantiomeric purities of the building blocks are secured by correlations.