α-Alkoxyallylation of Activated Carbonyl Compounds. A Novel Variant of the Michael Reaction

Article abstract of DOI:10.1021/jo00175a030

Enolic or readly enolizable carbonyl compounds undergo α-alkoxyallylation upon reaction with acetals of α,β-enals or ethoxyallene at temperatures ranging from 200 deg C to ambient.Whereas reactions of the highly enolic or acidic carbonyl compounds (endocyclic β-diketones, α-cyano ketones, α-nitro carbonyl compounds, and α-hydroxy-methylene derivatives) occurred simply upon heating, alkylation of the less acidic exocyclic β-diketones and β-keto esters was best carried out in the presence of 1 mol percent of Ni(acac)2 as a catalyst.Pyridinium p-toluenesulfonate was employed as a catalyst for alkylations with acrolein ethylene acetal.Although ethoxyallylation of acylic substrates (e.g., ethyl acetoacetate, diethyl malonate, and ethyl cyanoacetate) with acrolein diethyl acetal proved to be slow, these and related alkylations could be conviently accomplished by use of the corresponding α-hydroxymethylene derivatives.Unsaturated acetals bearing a methyl or phenyl substituent at C-2 can be employed for alkoxyallylation, but the reaction appears to be incompatible with a methyl group at C-3.The mechanism of these reactions probably involves either direct C-allylation of the carbonyl compound on the γ-position of an alkoxyallyl carbocation intermediate or an indirect pathway via O-allylation at the α-position of the carbocation followed by Claisen rearrangement.