Abstract
(R)‐5‐Bromo‐6‐(bromomethyl)‐2‐(tert‐butyl)‐2H,4H‐1,3‐dioxin‐4‐one (2) derived from (R)‐3‐hydroxybutanoic acid is used for substitutions and chain elongations at the side‐chain C‐atom in the 6‐position of the heterocycle (→3–6, 10–13). Subsequent simultaneous reductive debromination and double‐bond hydrogenation (Pd/C,H2)occurs with essentially complete diastereoselectivity (>98% ds), with H transfer from the face opposite to the t‐Bu group (→15–20, Table 1). Hydrolytic cleavages of the dioxanones then lead to enantiomerically pure β‐hydroxy‐acid derivatives (overall self‐reproduction of the stereogenic center of 3‐hydroxybutanoic acid or alkylation in the 4‐position of this acid with preservation of configuration).