The influence of an allylic substituent on the chemistry of the carbon carbon π bond has been studied. Model compounds, with synclinal oxygen or carbon allylic substituents, have been sythesised. These include 5-methylene-2-phenyl-1,3-dioxane, 1-methylene-4-t-butylcyclohexane and E and Z 5,5'-di-t-butyl bis (1,3-dioxan-5-ylidene). Semi-empirical and ab initio computer calculations were carrried out for analogues of these compounds. The stereochemistry and regiochemistry exhibited by 5-methylene-1,3-dioxane, 1-methylene-4-t-butylcyclohexane and E and Z-5,5'-t-butyl bis (1,3-dioxanylidene), to electrophilic additions, was examined. Generally, methylene cyclohexane and methylene dioxane both exhibited endo (or axial) stereoselectivity with methylene dioxane showing greater specificity. Methylene cyclohexane exhibited Markownikoff regioselectivity whereas methylene dioxane surprisinly exhibited total anti-Markownikoff specificity. The calculations suggest that endo stereoselectivity is largely caused by hyperconjugation from the c_β/-x_γ/ bond with the HOMO. The greater endo selectivity exhibited by methylene dioxane is however, due to either exo bending of the double bond or, more likely, electrostatic considerations. It has been established that the observed regiochemistry in methylene dioxane is largely due to a hyperconjugative effect originating from the synclinal C-O bonds as the conformationally mobile model compound also studied, dimethoxy methylene propane, exhibits largely Markownikoff regiochemistry.