2- Oxazolin-5-ones stereochemistry

A ferrocenyloxazoline with only one adjacent position available for deprotonation will lithiate at that position irrespective of stereochemistry. This means that the same oxazoline can be used to form ferrocenes with either sense of planar chirality. The synthesis of the diastereoisomeric ligands 311 and 313 illustrates the strategy (Scheme 143), which is now commonly used with other substrates to control planar chirality by lithiation (see below). Ferrocene 311 is available by lithiation of 305 directly, but diastereoselective silylation followed by a second lithiation (best carried out in situ in a single pot) gives the diastereoisomeric phosphine 313 after deprotection by protodesilylation ". ... 

Now one of the two OH groups must be replaced by NH2 with retention of configuration. This can be achieved by the ortho-ester method like that summarised in chapter 23 for propylene oxide. The intermediates 188 and 189 can be isolated but the process works better if 189 is hydrogenated to give the oxazoline 184 in 89% yield over the three steps. Note that the stereochemistry of 189, and hence 184, comes from a double inversion at the position a to the ester group. 

All of the three mentioned mechanisms involve oxocarbenium ion-like transition states and a pair of carboxylic acids at the active site however, they differ in several aspects. The inverting mechanism is a one-step reaction that results in the formation of a product with inverted stereochemistry at the anomeric center. The other two alternatives are retaining in stereochemistry at the anomeric center and differ from each other primarily in the nature of the intermediate in the second mechanism, this species is a covalent enzyme adduct, whereas in the third case it is believed to be a bicyclic oxazoline or oxazolinium ion. ... 

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