R/ireo-Selectivity

The azide 53 was prepared by r/ireo-selective addition of Tms azide to the a,P-unsaturated ketone the thiazole could be unmasked to give a carboxy group, thus leading to the regioisotner 54 of NeuNAc.52... 

Several studies have described the successful enhancement of the ( )-selectivity in alkene formation by increasing the steric requirements of the phosphonate. This selectivity may be derived by increasing the selective formation of the r/ireo- 3-hydroxyphosphonate. 

Allylation. Active barium is generated by reduction of Bah with 2 equivalents of Li biphenylide in THF at room temperature. Allylic chlorides are converted to the allylbarium reagents, which can be used to effect allylation with excellent regio- and stereoselectivities. r/ireo-Homoallylic alcohols from a y-selective reaction with aldehydes are obtained. ... 

Under kinetic conditions, E-enolates produce predominantly r/ireo-products in aldol condensations, making erythro-oldols difficult to prepare from cyclic ketones. However, readily isolable titanium enolates show pronounced erythro-selectivity in their reactions with aldehydes, irrespective of enolate geometry, the diastereoselection being high for reactions of cyclic ketones [equation (66)]. Triphenyltin enolates react in an analogous manner. ... 

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