The primordial influence of the ring size in the Lewis acid-promoted reaction of sterically unhindered isocyclic allylsilanes with 3-butyn-2-one is reported. In all the cases, the classical Sakurai-Hosomi reaction is, for the most part or fully, suppressed. With cyclohexanic frameworks, allyltrimethylsilanes like 1a-c afford mainly or solely H-ene components. This chemoselectivity is quite different from the one of cyclopentanic core compounds 1d-f which act primarily as [2 + 2] cycloaddition agents at the expense of the H-ene products procedure. Furthermore, the regioselectivity of the H-ene products formation is opposite for the two skeletons. Using Me 2 AlCl, the [2 + 2] cyloaddition of five carbon atoms isocyclic allylsilanes proceeds with complete chemoselectivity and provides an efficient synthesis of functionalized bicyclo [3.2.0] heptenes.