Relative Reactivities of Acetals and Orthoesters in Lewis Acid Catalyzed Reactions with Vinyl Ethers. A Systematic Investigation of the Synthetic Potential of Acetals and Orthoesters in Electrophilic Alkoxyalkylations of Enol Ethers

Article abstract of DOI:10.1021/jo00248a006

The relative reactivities of acetals and orthoesters in BF3<*>OEt2-catalyzed reactions with methyl vinyl ether (-78 deg C, CH2Cl2) have been determined by competition experiments.A reactivity increase by 5 orders of magnitude was found in the series: saturated acetals < methyl orthoformate < benzaldehyde acetals < α,β-unsaturated acetals; formaldehyde acetals as well as orthoacetates and orthobenzoates did not react under these conditions.The k_rel values of the para-substituted benzaldehyde acetals follow a Hammett ? correlation (ρ = -4.6).Whereas the k_rel values of the aldehyde acetals are correlated with the corresponding rate constants of acid-catalyzed hydrolyses, ketals and orthoesters devaite from this correlation.It is concluded that the k_rel listing in Scheme II can be used to predict the results of Lewis acid catalyzed additions of acetals and orthoesters toward vinyl ethers: The formation of 1:1 addition products may only be expected, if the relevant functional group of the reactants is listed below the functional group of the potential 1:1 products.