Effect of conformational structure on the structural directivity of the opening of acetal rings by organomagnesium compounds

2,4-Dimethyl-1,3-dioxane having a conformation of a “chair” with a cis-equatorial arrangement of substituents, under-goes cleavage by ethylmagnesium bromide at the C2-O1, and C2-O3 bonds with the same probability, whereas cis- and transisomers of 2,4-dimethyl-1,3-dioxolane with conformations close to planar, react under the same reaction conditions with strict specificity at the C2-O1 bond, thus indicating strong spatial screening of the O3 atom in 4-methyl substituted 1,3-dioxolanes.