Routes to (2S)-2-methyltetrahydropyran-4-one of high optical purity starting from readily available chiral pool precursors and suitable for large-scale manufacture are described. In one approach, the key step is cyclisation of (S)-5-hydroxyhex-1-en-3-one, derived either from an alkyl (S)-3-hydroxybutyrate or (S)-propylene oxide. Formation of the tetrahydropyran ring directly via an intramolecular oxy-Michael reaction under acid-catalysed conditions resulted in loss of optical purity, whereas proceeding through the intermediate (2S)-2-methyl-2,3-dihydropyran-4-one, via an oxidative Pd-catalysed ring closure, followed by hydrogenation of the alkenyl bond, preserved the optical purity. An alternative approach to (2S)-2-methyl-2,3-dihydropyran-4-one is also reported, again starting from an alkyl (S)-3-hydroxybutyrate by elaboration to a carbonyl-protected (6S)-6-methyl-5,6-dihydropyran-2,4-dione derivative, followed by partial reduction and dehydration. Alternatively, the carbonyl group can be reduced out completely in one step to furnish (2S)-2-methyltetrahydropyran-4-one directly after deprotection.

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