Heterocyclic rings are important structural scaffolds encountered in both natural and synthetic compounds, and their biological activity often depends on these motifs. They are predominantly ...accessible via cycloaddition reactions, realized by either thermal, photochemical, or catalytic means. Various starting materials are utilized for this purpose, and, among them, diazo compounds are often encountered, especially vinyldiazo compounds that give access to donor-acceptor cyclopropenes which engage in 2+n cycloaddition reactions. Herein, we describe the development of photochemical processes that produce diverse heterocyclic scaffolds from multisubstituted oximidovinyldiazo compounds. High chemoselectivity, good functional group tolerance, and excellent scalability characterize this methodology, thus predisposing it for broader applications. Experimental and computational studies reveal that under light irradiation these diazo reagents selectively transform into cyclopropenes which engage in cycloaddition reactions with various dipoles, while under thermal conditions the formation of pyrazole from vinyldiazo compounds is favored.
1,2,3-Triazine 1-oxides are formed by nitrosyl addition from tert-butyl nitrite to the vinylogous position of vinyl diazo compounds. This transformation, which is a formal intermolecular 5 + 1 ...cycloaddition, occurs under mild conditions, with high functional group tolerance and regioselectivity, and can be employed for late-stage functionalization. Upon heating at refluxing chlorobenzene temperature, these triazine-N-oxides undergo dinitrogen extrusion to form isoxazoles in very high yields.
Responsibility sharing was a central commitment in the 2016 New York Declaration for Refugees and Migrants (annex 1, para 1). It was also a key commitment in the preamble to the landmark 1951 Refugee ...Convention, in which countries of first asylum are promised that their providing refuge will be met by 'international cooperation', without specifying its content. Yet, just as the 1951 Refugee Convention failed to define what international cooperation meant, so, too, the New York Declaration was long on principles but short on specific commitments.
Extrusion of dinitrogen from enol diazoacetates with a RhII catalyst generates metal enol carbenes. Subsequent vinylogous addition of these to N‐acyliminopyridinium ylides results in an effective ...formal 3+3 cycloaddition to give highly substituted 1,2,3,6‐tetrahydropyridazines in up to 98 % ee and high yield.
An efficient two-step protocol for the asymmetric synthesis of 1H-pyrrol-3(2H)-one derivatives in 99% ee from conveniently accessed 2,3-diketoesters has been developed.
Many food safety-related studies require tracking of introduced foodborne pathogens to monitor their fate in complex environments. The green fluorescent protein (GFP) gene (gfp) provides an easily ...detectable phenotype so has been used to label many microorganisms for ecological studies. The objectives of this study were to label major foodborne pathogens and related bacteria, including Listeria monocytogenes, Listeria innocua, Salmonella, and Escherichia coli O157:H7 strains, with GFP and characterize the labeled strains for stability of the GFP plasmid and the plasmid's effect on bacterial growth. GFP plasmids were introduced into these strains by a CaCl(2) procedure, conjugation or electroporation. Stability of the label was determined through sequential propagation of labeled strains in the absence of selective pressure, and rates of plasmid-loss were calculated. Stability of the GFP plasmid varied among the labeled species and strains, with the most stable GFP label observed in E. coli O157:H7. When grown in nonselective media for two consecutive subcultures (ca. 20 generations), the rates of plasmid loss among labeled E. coli O157:H7, Salmonella and Listeria strains ranged from 0%-30%, 15.8%-99.9% and 8.1%-93.4%, respectively. Complete loss (>99.99%) of the plasmid occurred in some labeled strains after five consecutive subcultures in the absence of selective pressure, whereas it remained stable in others. The GFP plasmid had an insignificant effect on growth of most labeled strains. E. coli O157:H7, Salmonella and Listeria strains can be effectively labeled with the GFP plasmid which can be stable in some isolates for many generations without adversely affecting growth rates.
Catalyst-dependent 4 + 2-cycloaddition reactions of azoalkenes from α-halohydrazones with enol diazoacetates have been developed. A 4 + 2-cycloaddition of enol diazoacetates with in situ formed ...azoalkenes produces tetrahydropyridazinyl-substituted diazoacetates promoted by only Cs2CO3. In contrast, donor–acceptor cyclopropenes, which are formed in situ from enol diazoacetates by Rh2(OAc)4-catalyzed dinitrogen extrusion, undergo 4 + 2-cycloaddition with azoalkenes to yield bicyclo4.1.0tetrahydropyridazines. These stable cycloaddition products undergo subsequent one-step transformations to form 6-alkylidenetetrahydropyridazines and 4,5,6,7-tetrahydro-1,2-diazepine derivatives in good yields.
A highly regio- and diastereoselective synthesis of bicyclic pyrazolidinone derivatives by rhodium(II) acetate catalyzed 3 + 3-annulation with enoldiazoacetates and azomethine imines has been ...achieved in high yield. A vinylogous reaction of the metal enol carbene with the azomethine imine initiates 3 + 3-cycloaddition, whereas reaction at the carbene center effects N–N-cleavage of the azomethine imine.
Reactions with the unstable and highly reactive zwitterionic intermediates generated in processes catalysed by transition metals are providing new opportunities for molecular constructions. Insertion ...reactions involve the collapse of zwitterionic intermediates, but trapping them would allow structural elaborations that are not currently available. To synthesize complex molecules in this manner, reactive electrophiles can be used to trap the zwitterionic intermediates. Here, we describe the use of imines, activated by chiral organocatalysts, and a highly efficient integrated rhodium and chiral Brønsted acid co-catalysed process to trap zwitterionic intermediates that have been proposed previously to undergo a formal C-H insertion reaction, allowing us to obtain polyfunctionalized indole and oxindole derivatives in a single step with excellent diastereoselectivity and enantioselectivity.
![Intermolecular [5 + 1]-Cycl...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1523-7060/small "Intermolecular [5 + 1]-Cycloaddition between Vinyl Diazo Compounds and tert-Butyl Nitrite to 1,2,3-Triazine 1‑Oxides and Their Further Transformation to Isoxazoles")
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