Efficient Wittig–Horner synthesis of acyclic α-enones with an asymmetric carbon at the γ-position using barium hydroxide

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Abstract
β-Monosubstituted acylic α-enones with an asymmetric carbon at the γ-position: (E)-(5RS)-5-phenylhex-3-en-2-one, (4a), (E)-(6RS)-2,2-dimethyl-6-phenylhept-4-en-3-one, (4b), (E)-(4RS)-1,4-diphenylpent-2-en-1-one, (4c), (E)-(5RS)-6,6-dimethyl-5-phenylhept-3-en-2-one, (4d), (E)-(6RS)-2,2,7,7-tetramethyl-6-phenyloct-4-en-3-one, (4e), and (E)-(4RS)-5,5-dimethyl-1,4-diphenylhex-2-en-1-one, (4f), have been prepared by Wittig–Horner reaction of aliphatic aldehydes and 2-oxoalkylphosphonates with different degrees of steric hindrance. The use of activated barium hydroxide C-200 as the base in 1,4-dioxane provided an efficient and stereoselective synthetic method, remarkably free of competing side-reactions. The reaction course was found to depend on the phosphonate acidity: triethyl phosphonoacetate reacts through an interfacial solid-liquid mechanism, while the more acidic 2-oxoalkylphosphonates dissolve the barium hydroxide and the reaction takes place in the homogeneous phase probably due to a phase-transfer-like mechanism. The anionic species formed from the 2-oxoalkylphosphonates by the action of barium hydroxide C-200 in 1,4-dioxane were detected by 31P n.m.r. spectroscopy. The α-enones (4) are useful model compounds for the study of diastereofacial selectivity in nucleophilic addition reactions.

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