Magnesium bromide diastereofacial selectivity

The enantiomerically pure perhydro-l,3-oxazine-2,6-dione (72 R = menthyl Ar = 4-methoxyl-phenyl) reacts with benzaldehyde to give the benzylidene derivative (73). This compound shows high diastereofacial selectivity ( 98%) when used as either an oxadiene, or as a Michael acceptor. For example, with 2-methoxypropene, followed by thermolysis of the adduct in boiling 1,4-dioxane, it affords the ketoaldehyde (74) and with ethyl magnesium bromide it gives the adduct (75), acidic hydrolysis of which yields the chiral acid (76) (Scheme 15) <92CL485,92CPB1972). 

---