β-elimination of protected aldono-1,4-lactones as a general approach to the synthesis of 2-keto-3-deoxyaldonic acids containing four to six carbon atoms

The well-known β-elimination of protected aldonolactones is used for the synthesis of the short-chain 2-keto-3-deoxyaldonic acids 1–4 containing four to six carbon atoms. The key step is the facile β-elimination step which generates the desired 2-keto-3-deoxy acids as protected enol 1,4-lactones in excellent yields. Smooth deprotection then leads to the 2-keto-3-deoxyaldonic acids. In the case of the protected d-galactono-1,4-lactone 6 an epimerisation is observed during the elimination process. This enables the synthesis of both 2-keto-3-deoxy-d-hexonic acids with either d-erythro (1) or d-threo (2) configuration in good to excellent yields in only three steps starting with commercially available d-galactono-1,4-lactone (5).