4-π-Photocyclization of 1,2-Dihydropyridazines: An Approach to Bicyclic 1,2-Diazetidines with Rich Synthetic Potential

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

General access to cubanes as benzene bioisosteres

The replacement of benzene rings with sp3-hybridized bioisosteres in drug candidates generally improves pharmacokinetic properties while retaining biological activity1-5. Rigid, strained frameworks such as bicyclo[1.1.1]pentane and cubane are particularly well suited as the ring strain imparts high bond strength and thus metabolic stability on their C-H bonds. Cubane is the ideal bioisostere as it provides the closest geometric match to benzene6,7. At present, however, all cubanes in drug design, like almost all benzene bioisosteres, act solely as substitutes for mono- or para-substituted benzene rings1-7. This is owing to the difficulty of accessing 1,3- and 1,2-disubstituted cubane precursors. The adoption of cubane in drug design has been further hindered by the poor compatibility of cross-coupling reactions with the cubane scaffold, owing to a competing metal-catalysed valence isomerization8-11. Here we report expedient routes to 1,3- and 1,2-disubstituted cubane building blocks using a convenient cyclobutadiene precursor and a photolytic C-H carboxylation reaction, respectively. Moreover, we leverage the slow oxidative addition and rapid reductive elimination of copper to develop C-N, C-C(sp3), C-C(sp2) and C-CF3 cross-coupling protocols12,13. Our research enables facile elaboration of all cubane isomers into drug candidates, thus enabling ideal bioisosteric replacement of ortho-, meta- and para-substituted benzenes.

A Metal-Free Cyclobutadiene Reagent for Intermolecular [4 + 2] Cycloadditions

Cyclobutadiene is a highly reactive antiaromatic hydrocarbon that has fascinated chemists for over 60 years. However, its preparation and uses in chemical synthesis are sparing, in part due to its lengthy synthesis that generates hazardous byproducts including excess heavy metals. Herein, we report a scalable, metal-free cyclobutadiene reagent, diethyldiazabicyclohexene dicarboxylate, and explore its intermolecular [4 + 2] cycloaddition with various electron-deficient alkenes. We also demonstrate its utility in a three-step synthesis of dipiperamide G and a diverse array of product derivatizations including bromocyclobutadiene.

Expanding the Chemical Space of 1,2-Difunctionalized Cyclobutanes

An efficient approach to the synthesis of previously unavailable or hardly accessible 1,2-difunctionalized cyclobutanes (mostly with NH₂/NHBoc, OH, SH, or SO₂F groups attached to the carbocycle either directly or via a CH₂ unit) relying on the divergent strategy is described. This class of compounds provides sp³-enriched and conformationally restricted building blocks that are of special demand for medicinal chemistry. The target compounds were prepared not only as pure racemic (±)-cis- and (±)-trans-diastereomers but in some cases also as single enantiomers. The developed procedures are readily scaled up and allow obtaining the target compounds on an up to hundred-gram scale. On the basis of the results of 20 X-ray diffraction experiments, structural characterization of the 1,2-difunctionalized cyclobutane core was performed using the extended Cremer-Pople puckering parameters and exit vector (EVP) plots.

Construction of 3,12-Diazatetracyclododecane-dienes through Unexpected Visible-Light-Induced Radical Cascade Cyclization

A novel visible-light-induced cascade radical cyclization reaction of 3-cyano-4-aryl-1,4-dihydropyridines for the construction of 3,12-diazatetracyclododecane-diene derivatives is reported for the first time. In the presence of 410 nm blue LED lamp as the light source and ethanol as the solvent, the reactions proceed smoothly to afford photocyclization products in good yields. The process is carried out through the breaking of original C═C double bonds and the formation of three new single bonds in one pot and proved to be able to tolerate different substituents.

Synthesis of strained 1,2-diazetidines via [3 + 1] cycloaddition of C,N-cyclic azomethine imines with isocyanides and synthetic derivation

Strained diazetidines are assembled simply from 1,3-dipolar cycloaddition of isocyanides and C,N-cyclic azomethine imines, and their diversified transformations are presented.