Alkyl 2-Chloro-2-cyclopropylideneacetates—Remarkably Versatile Building Blocks for Organic Synthesis

Strain in small rings has evolved as one of the principles used to control reactivity and chemoselectivity in transformations of organic compounds. The combination of small rings with multiple bonds and functional groups establishes composite functionalities which demonstrate unique multiple reactivity and thereby potentially high synthetic utility. This survey concentrates on a family of compounds which combines the chemistry of methylenecyclopropanes and that of electron-acceptor-activated alkenes, namely alkyl 2-chloro-2-cyclopropylideneacetates of types 1–3. This special feature makes the compounds 1–3 multifunctional as well as highly reactive, and thus extremely versatile building blocks for organic synthesis. Not only is the general synthetic access to methylenecyclopropanes 1–3 presented here, but particularly their rich chemistry as highly reactive Michael acceptors, dienophiles, dipolarophiles and general cyclophiles which leads to a wide range of different types of functionally substituted cyclopropane derivatives, spirocyclopropane-annelated hetero- and carbocycles, mono- and oligocondensed cycles, natural and unnatural amino acids and peptidomimetics, and more. Finally, the first results obtained with polymer-bound substrates of types 1–3 in a combinatorial approach to libraries of potentially biologically active compounds are presented.