Stereoselective hydroxyalkylations of ( S )‐2‐azetidinecarboxylic acid

The enolate generated from S ‐phenyl ( S )‐1‐benzyl‐2‐azetidinecarbothioate is converted into racemic products 2 by treatment with electrophiles. The bicyclic oxazolidinone derivative 12 has been prepared from trimethylsilyl ( S )‐1‐(trimethylsilyl)‐2‐azetidinecarboxylate ( 11 ) and pivalaldehyde without racemization. Addition of 12 to aldehydes and hydrolysis gives the products 13 a‐h . The structure of the adduct 13 h from 4‐pyridinecarbaldehyde was determined by X‐ray crystal structure analysis, proving that the trigonal centers of the bicyclic enolate and of the aldehyde have combined with relative topicity unlike . Thus, the reaction resembles that of the homologous proline derivative described previously, and the novel products are formally D ‐ allo ‐threonines with an ethylene bridge between the nitrogen and the adjacent carbon atoms (Scheme 4) and with absolute configuration R , R . The mechanism of the C C bond‐forming reaction is discussed.