摘要：

The preparation of novel electrophilic building blocks for the synthesis of enantiomerically pure compounds (EPC) is described. Thus, the 2-( tert -butyl)dioxolanones, -oxazolidinones, -imidazolidinones, and -dioxanones obtained by acetalization of pivalaldehyde with 2-hydroxy-, 3-hydroxy-, or 2-amino-carboxylic acids are treated with N -bromosuccinimide under typical radical-chain reaction conditions (azoisobuytyronitril/CCl 4 /reflux). Products of bromination in the α-position of the carbonyl group of the five-membered-ring acetals are isolated or identified ( 2, 5 , and 8 ; Scheme 1 ). The dioxanones are converted to 2 H , 4 H -dioxinones under these conditions ( 12 , 14 , 15 , 21 , and 22 ; Schemes 2 and 3 ). The products can be converted to chiral derivatives of pyruvic acid (methylidene derivatives 3 and 6 ) or of 3-oxo-butanoic and -pentanoic acid ( 16 and 23 ). The mechanism of the brominations is interpreted. The conversion of serine to enactiomcrically pure dioxanones 26–28 ( Scheme 4 ) is also discussed.

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