Dienamine‐Induced Divinylcyclopropane–Cycloheptadiene Rearrangements

Sigmatropic rearrangements constitute an important group of pericyclic reactions. In contrast to cycloaddition reactions, examples of catalytic variants of electrocyclic reactions and sigmatropic rearrangements are still scarce in the chemical literature. Herein, we report the first organocatalytic Cope rearrangement of in situ‐generated divinylcyclopropanes. The reactive motif was generated by condensation of 4‐(2‐vinylcyclopropyl)but‐2‐enal derivatives with a secondary amine catalyst to form a transient dienamine. The cycloheptadiene products could be obtained in high yield and excellent diastereoselectivity. Importantly, the reaction was demonstrated to be stereospecific, proceeding under mild conditions, while exhibiting broad functional group tolerance.     

Caged Phosphate and the Slips and Misses in Determination of Quantum Yields for Ultraviolet‐A‐Induced Photouncaging

The quantum yields for photouncaging reactions are mostly determined relative to other uncaging reactions, often using 1‐(2‐nitrophenyl)ethyl‐phosphate (“caged phosphate”). Herein, we demonstrate that the quantum yields acquired by using this method can be off by an order of magnitude at the typical irradiation wavelengths around 350 nm and describe an easy‐to‐use alternative procedure using inexpensive azobenzene.     

Diastereoselective Construction of Densely Functionalized 1‐Halocyclopentenes Using an Alkynyl Halo‐Prins/Halo‐Nazarov Cyclization Strategy

A diastereoselective two‐step strategy for the synthesis of densely functionalized 1‐halocyclopentenes with several chiral centers has been developed. In the first step, a multicomponent alkynyl halo‐Prins reaction joins an enyne, a carbonyl derivative, and either a chloride, bromide, or iodide to produce a cyclic ether intermediate. In the subsequent step, the intermediate is ionized to generate a halopentadienyl cation, which undergoes an interrupted halo‐Nazarov cyclization. The products contain three new contiguous stereogenic centers, generated with a high level of stereocontrol, as well as a vinyl halide allowing for additional functionalization. The strategy creates two new carbon–carbon bonds, one carbon–halide bond, and one carbon–oxygen bond.