Retention oxazoline ring

In this aforementioned Heine reaction the initial ring opening takes place by iodide ions. Subsequent ring closure by S 2 displacement of iodide by reaction with the negative oxygen center then leads to the products. This process proceeds with double inversion at the same carbon atom, thus with net retention. Hydrolysis of these oxazolines gives j9-hydroxy-a-amino acids (Scheme 31) [1,38]. The stereochemical course of ring expansion is the same as that observed in Scheme 29. 

Hori and co-workers studied the BF3 OEt2 catalyzed isomerization of a chiral Al-acylaziridine 213 to the oxazoline 214. It was established that the ring expansion proceeds with retention of configuration. The authors have proposed a Sivri mechanism for this transformation through the transition state 213a. Ab initio molecular orbital calculations agree well with this hypothesis (Scheme 8.59). 

Hydroxy-3-phenylthiopropylamine stirred ca. 20 hrs. with cyanogen bromide and Na-acetate as acid acceptor in methanol 2-amino-5-phenylthiomethyl-2-oxazoline. Y 88%. F. e. s. E. R. Freiter, A. H. Abdallah, and S. J. Strydcer, J. Med. Chem. 16, 510 (1973) with CICN, without acid acceptor, stereospecific ring closure with retention or inversion of configuration, interconversion of stereo-isomeric 2-aminoalcohols, s. H. Wollweber and R. Hiltmann, Arch. Pharm. 306, 284 (1973). 

---