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This review aims to present the most recent contributions in the chemistry of nitrosoalkenes and azoalkenes, highlighting the chemical behavior that makes them important and versatile building blocks in organic synthesis. These are heterodienes used in the assembly of a variety of heterocyclic systems, spanning from five- to seven-membered heterocycles, as well as for the functionalization of heterocycles.

Corroles and hexaphyrins are porphyrinoids with great potential for diverse applications. Like porphyrins, many of their applications are based on their unique capability to interact with light, i.e., based on their photophysical properties. Corroles have intense absorptions in the low-energy region of the uv-vis, while hexaphyrins have the capability to absorb light in the near-infrared (NIR) region, presenting photophysical features which are complementary to those of porphyrins. Despite the increasing interest in corroles and hexaphyrins in recent years, the full potential of both classes of compounds, regarding biological applications, has been hampered by their challenging synthesis. Herein, recent developments in the synthesis of corroles and hexaphyrins are reviewed, highlighting their potential application in photodynamic therapy.

The reactivity of nitrosoalkenes toward dipyrromethanes, pyrrole, and 2,5-dimethylpyrrole is described. 1-(p-Bromophenyl)nitrosoethylene shows a different chemical behavior with these heterocycles than the previously reported reactions of ethyl nitrosoacrylate, which proceeds via a Diels-Alder reaction. 1-(p-Bromophenyl)nitrosoethylene reacts with dipyrromethanes and pyrrole to afford two isomeric oximes via conjugate addition followed by rearomatization of the pyrrole unit. On the other hand, this nitrosoalkene reacts with 2,5-dimethylpyrrole through an initial conjugate addition followed by intramolecular O- and N-nucleophilic addition with the formation of the corresponding bicyclic oxazine and five-membered cyclic nitrone, respectively. Quantum chemical calculations, at the DFT level of theory, indicate that the barriers associated with the Diels-Alder reactions of ethyl nitrosoacrylate are over 30 kJ/mol lower than those that would be required for the cycloadditions of 1-(p-bromophenyl)nitrosoethylene. Thus, calculations predict that the Diels-Alder reaction is privileged in the case of ethyl nitrosoacrylate and point to a different reaction pathway for 1-(p-bromophenyl)nitrosoethylene, corroborating the experimental findings.

The synthesis of new 3‐tetrazolyl‐1,4,5,6‐tetrahydropyridazines by Diels–Alder reactions of ethyl 3‐(tetrazol‐5‐yl)‐1,2‐diaza‐1,3‐butadiene‐1‐carboxylates is reported. These 1,2‐diaza‐1,3‐dienes reacted with electron‐rich heterocycles, nucleophilic olefins and cumulenes, as well as with electron‐deficient dienophiles, to give the target compounds regioselectively. Computational studies corroborated the rationalization of the observed reactivity and selectivity.

Allelopathy, the release of chemicals by plants that inhibit the germination and growth of competing species, can be an important trait for invasive success. However, little is known about potential biogeographical differences in allelopathy due to divergent regional eco-evolutionary histories. To test this, we examined the allelochemical potential of the highly invasive species Centaurea solstitialis from six world regions including native (Spain, Turkey) and non-native ranges (Argentina, Chile, California and Australia). Seeds from several populations in each region were collected and grown under common garden conditions. Allelopathic potential and chemical composition of three leaf extract concentrations of C. solstitialis from each region: 0.25%, 0.5% 0.75% (w/v−1) were assessed on the phytometer Lactuca sativa. The main allelochemicals present in the leaf-surface extract were sesquiterpene lactones that varied in major constitutive compounds across regions. These leaf extracts had strong inhibitory effects on L. sativa seed germination and net growth. Summed across regions, the 0.25% concentration suppressed germination by 72% and radicle elongation by 66%, relative to the controls. At the 0.5% concentration, no seeds germinated when exposed to extracts from the non-native ranges of Argentina and Chile, whereas germination and radicle growth were reduced by 98% and 89%, respectively, in the remaining regions, relative to controls. Germination and seedling growth were completely inhibited at the 0.75% concentration extract for all regions. Some non-native regions were characterized by relatively lower concentrations of allelochemicals, suggesting that there is biogeographical variation in allelopathic expression. These findings imply that rapid selection on the biochemical signatures of an exotic invasive plant species can be highly region-specific across the world.

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The introduction of side chains at the 1‐ and 9‐positions of dipyrromethanes was explored by using the hetero‐Diels–Alder reaction of azoalkenes. New 5,5′‐diethyldipyrromethanes and 5‐phenyldipyrromethanes that were functionalized with open‐chain hydrazone moieties, including derivatives with tetrazolyl groups, were prepared. Furthermore, the synthesis of new calix[4]pyrroles and bilanes was achieved by employing the bis(hetero‐Diels–Alder) reaction of azoalkenes with 5,5′‐diethyldipyrromethane.