Enantiomerically enriched cyclopropene derivatives, the smallest possible unsaturated carbocycles, are of great synthetic interest since they serve as versatile reactive building blocks. Their reactivity results from the relief of the ring strain in the small molecule. They can be transformed into a wide variety of complex chiral structures and a special emphasis will be directed towards the preparation of enantiomerically enriched methylene- and alkylidenecyclopropane derivatives. The ready availability of a wide range of these chiral entities now provides an excellent opportunity to discover new and unique transformations that can further enrich mainstream synthetic methodology.

The copper-catalyzed carbomagnesiation (or hydrometalation) reaction of chiral cyclopropenylcarbinol derivatives, obtained by means of a kinetic resolution of secondary allylic alcohols, leads to an easy and straightforward preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn-carbometalation followed by a syn-elimination reaction. To gain further insight into the reaction mechanism of the carbometalation, the diastereoselective formation of cyclopropylcarbinol was also achieved and was found to be very sensitive to the nature of the organometallic species used for the addition reaction. Cyclopropylcarbinol could also be prepared through a diastereoselective reduction of cyclopropenylcarbinol derivatives. Finally, functionalization of enantiomerically enriched cyclopropenylcarbinols into the corresponding acetate or phosphinite derivatives leads, under mild conditions, to various enantiomerically pure heterosubstituted alkylidenecyclopropanes.

The straightforward approach: The copper‐catalyzed carbomagnesiation reaction of chiral cyclopropenylcarbinol derivatives, obtained through kinetic resolution of secondary allylic alcohols, leads to the preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn carbometalation followed by a syn elimination reaction.

Open sesame! A selective metal‐catalyzed ring opening of alkylidenecyclopropane derivatives leads to various functionalized acyclic derivatives possessing challenging quaternary stereocenters. The key feature of this transformation is that the ring opening is faster than the reductive elimination and is highly regioselective; a single ring‐opened product was always observed and the stereointegrity of the quaternary stereogenic center remains unaffected in the process.

Enantiomerenangereicherte Cyclopropenderivate, die kleinstmöglichen ungesättigten Carbocyclen, sind von großem Interesse für die Synthese, da sie als vielseitige reaktive Bausteine eingesetzt werden können. Die Reaktivität rührt von der Ringspannung her, die durch Umsetzungen gemildert wird. Die Cyclopropenderivate können zu komplexeren chiralen Strukturen umgesetzt werden, wobei in diesem Aufsatz besonders die Herstellung von enantiomerenangereicherten Methylen‐ und Alkylidencyclopropanderivaten herausgehoben wird. Die einfache Verfügbarkeit chiraler Bausteine dieser Art bildet eine ausgezeichnete Grundlage für die Entdeckung neuartiger Reaktionen, die das Repertoire der organischen Synthese bereichern können.