Selective C-acylation of CH-active dicarbonyl compounds with ketenylidenetriphenylphosphorane: syntheses and structures of 3-phosphoranylideneacyltetronic acids, 3-phosphoranylideneacyl-4-oxocoumarins, and 4-phosphoranylideneacylpyrazol-5-ones

Abstract

Selective C- over O-/N-acylation of acyclic, carbocyclic and heterocyclic carbonyl compounds bearing active α-CH groups is described. β-Ketoesters give the corresponding α-acylylidic derivatives in good yields. Likewise, tetronic acids and 4-hydroxycoumarins furnish the respective 3-acylylidic derivatives, and pyrazol-5-ones exclusively yield the corresponding 4-acylylidic compounds despite the possibility of tautomerism. X-Ray single crystal structure and NMR analyses of the product tricarbonyl ylides are presented and structure–reactivity interdependencies are discussed. As a rule, compounds with wide-spread conjugation of the π-system like 9/13 do not readily undergo Wittig olefination. Acylylides 11 with separated β-keto moieties react normally.