Diasterofacial selectivity

The D-fructose-derived, chiral, nonracemic l,3-oxazin-2-one derivative 260 exerted smooth stereocontrol, resulting in high levels of asymmetric induction and good chemical yields in various synthetic transformations. The chiral fragments 256 and 261 formed in the aldol or a-bromination reactions of the A -propionyl derivative 257 could easily be removed from the parent auxiliary by mild hydrolysis (Scheme 48). The Diels-Alder cyloadditions of the A -acryloyl and A -cinnamoyl derivatives of 260 were also characterized by excellent diasterofacial selectivity <1998T9765>. 

Aldol reactions of chiral dioxolanones (113) and (114) are summarized in Scheme 6 and Table 9. ° With both (113) and (114), essentially perfect diasterofacial selectivity is observed. The simple dia-stereoselection is modest to good, and is dependent on the enolate counterion. For the lithium and magnesium enolates, the sense of simple diastereoselection is the same as is observed with the achiral dioxolanone (107) and the chiral dioxolanone (110). Use of the zirconium enolate generally reverses the sense of simple diastereoselection, although the isomer ratios are not very high in some cases. 

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