Base-catalyzed Dehydrobromination of Several α-Bromoacetals

Base-catalyzed dehydrohalogenation with potassium t-butoxide in t-butyl alcohol of the acetals obtained from homologues of α-bromoacetaldehyde and ethylene glycol[2-(α-bromoalkyl)-1,3-dioxo-lanes] or 1,3-propanediol[2-(α-bromoalkyl)-1,3-dioxane] provides the corresponding ketene acetal, in some cases exclusively and in others as the major product along with a smaller proportion of the α,β-unsaturated acetal. In contrast, similar dehydrohalogenation conditions convert the acetals obtained from homologues of α-bromoacetaldehyde and monohydroxy alcohols, to the α,β-unsaturated acetals, in some cases exclusively, and in others as the major product accompanied by a smaller proportion of the corresponding ketene acetal.The preference for the ketene acetal formation from the 2-(α-bromoalkyl)-1,3-dioxolanes (the ethyleneacetals) is believed due to greater ease of approach by base to the methine proton as a result of the restricted shape of the 1,3-dioxolane ring. Approach by base to the methine proton of the α-bromoalkyl dialkylacetals is more hindered by the two alkoxy groups, which cause preferred attack by base at the β proton to provide the α,β-unsaturated acetal.The proportion of α,β-unsaturated acetal obtained from base-catalyzed dehydrohalogenation of 2-(α-bromoalkyl)-1,3-dioxolanes can be greatly increased if the reaction is carried out in dimethyl sulfoxide. This marked change in proportion of products is thought to be due to a change in mechanism occasioned by the dimethyl sulfoxide.