Chemo- and regioselectivity in the Lewis acid-induced reaction of sterically unhindered isocyclic allylsilanes with 3-butyn-2-one.

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-e 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 bicycle [3.2.0] heptenes. Copyright (C) 1996 Elsevier Science Ltd