Brominations of Cyclic Acetals from α‐Amino Acids and α‐ or β‐Hydroxy Acids with N‐Bromosucinimide

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/CCl4/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 2H, 4H‐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.