Unsaturated Four-Membered Rings: Efficient Strategies for the Construction of Cyclobutenes and Alkylidenecyclobutanes.

Our recent studies of the diastereo- and enantioselective formation of strained alkylidenecycloalkanes drove us to more-thoroughly investigate the formation of four-membered rings for which only few efficient methods are described. We first developed a strategy to diversify the saturated part of the four-membered ring and applied it to a highly diastereoselective synthesis of more-elaborate alkylidenecyclobutanes, which completed our precedent studies. In parallel, cyclobutene structures were built employing simple organometallic methods and further functionalized to give a diverse range of new substitution patterns, which consequently enriched the pool of cyclobutene-based building blocks.