Diazocinones: synthesis and conformational analysis

Synthesis of dibenzosuberenone-based novel polycyclic π-conjugated dihydropyridazines, pyridazines and pyrroles

The synthesis of novel polycyclic π-conjugated dihydropyridazines, pyridazines, and pyrroles was studied. Dihydropyridazine dyes were synthesized by inverse electron-demand Diels–Alder cycloaddition reactions between a dibenzosuberenone and tetrazines that bear various substituents. The pyridazines were synthesized in high yields by oxidation of dihydropyridazine-appended dibenzosuberenones with PIFA or NO. p-Quinone derivatives of pyridazines were also obtained by H-shift isomerization following the inverse electron-demand Diels–Alder reaction of tetrazines with p-quinone dibenzosuberenone. Then these pyridazines were converted to the corresponding pyrroles by reductive treatment with zinc. It was observed that all the dihydropyridazines obtained gave absorbance and emission at long wavelengths.

Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction

Reaction of 3,6-disubstituted-1,2,4,5-tetrazines with water and PEt₃ forms the corresponding 1,4-dihydrotetrazine and OPEt₃ . Thus PEt₃ , as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron-deficient tetrazine and electron-rich PR₃ , including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO₂ CN=NCO₂ Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O₂ , something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine-like intermediate is undetectable, but its protonated form is characterized, including by X-ray structure and NMR spectroscopy.

3,6-Substituted-1,2,4,5-tetrazines: tuning reaction rates for staged labeling applications

Cycloaddition reactions involving tetrazines have proven to be powerful bioorthogonal tools for various applications. Conceivably, sequential and selective labeling using tetrazine-based reactions can be achieved by tuning the reaction rate. By varying the substituents on tetrazines, cycloaddition rate variations of over 200 fold have been achieved with the same dienophile. Upon coupling with different dienophiles, such as norbornene, the reaction rate difference can be over 14,000 fold. These substituted tetrazines can be very useful for selective labeling under different conditions.

Novel 3,6-unsymmetrically disubstituted-1,2,4,5-tetrazines: S-induced one-pot synthesis, properties and theoretical study

18 unprecedented 3,6-unsymmetrically disubstituted-1,2,4,5-tetrazines were synthesized, and their spectral and electrochemical properties are studied. A systematic theoretical investigation based on DFT calculations was carried out.

Copper-Free Click for PET: Rapid 1,3-Dipolar Cycloadditions with a Fluorine-18 Cyclooctyne

The strain-promoted click 1,3-dipolar cycloaddition reactions involving azides and cyclooctynes for the synthesis of triazoles offer the advantage of being able to be performed in biological settings via copper-free chemistries. While strained reagents conjugated to optical dyes and radiometal conjugates have been reported, cyclooctyne reagents labeled with fluorine-18 ((18)F) and radiochemically evaluated in a copper-free click reaction have yet to be explored. This report describes the conversion of a bifunctional azadibenzocyclooctyne (ADIBO) amine to the (18)F-labeled cyclooctyne 4, the subsequent fast copper-free 1,3-dipolar cycloaddition reaction with alkyl azides at 37 °C (>70% radiochemical conversion in 30 min), and biological evaluations (serum stability of >95% at 2 h). These findings demonstrate the excellent reactivity of the (18)F-labeled cyclooctyne 4 with readily available azides that will allow future work focusing on rapid copper-free in vitro and in vivo click chemistries for PET imaging using (18)F-labeled cyclooctyne derivatives of ADIBO.

Cyclopenta[b]pyrroles from triazines: synthetic and mechanistic studies

The pyrrolidine-mediated reactions of 3,5-disubstituted 1,2,4-triazines with cyclobutanone lead to cyclopenta[b]pyrroles, which can be derivatized into hydrazones and oximes. The cyclopenta[b]pyrrole ring system likely arises through a tandem [4 + 2] cycloaddition/cycloreversion/ring rearrangement reaction. In contrast, 3,6-disubstituted 1,2,4-triazines undergo a simple nucleophilic 1,4-addition with cyclobutanone to give 1:1 adducts.

Mitsunobu Coupling of Nucleobases and Alcohols: An Efficient, Practical Synthesis for Novel Nonsugar Carbon Nucleosides

A simple facile synthesis of substituted purine derivatives has been developed by using Mitsunobu conditions for an alcohol and a respective nucleobase. A wide range of alcohols produces good to excellent yield (>90%). The resulting purine analogues show good regioselectivity with N-9 substitution as the dominant products in most of the cases. Application of diastereospecific alcohols reveals a complete inversion of the carbon stereogenic center giving a single diastereomer. More than two dozen novel nucleobase derivatives have been prepared in high yield.

Transannular Anti-Michael Addition: Formation of 4H-Pyrazolo[5,1-c]thiazines

The reaction of 2-(diphenylmethylene)thietan-3-one (2) with 1,2,4,5-tetrazines (3a-c) in KOH/MeOH/THF gives 4H-pyrazolo[5,1-c]thiazines (7a-c). This no vel condensation reaction proceeds via the intermediacy of an 8-(diphenylmethylene)-2H-1,4,5-thiadiazocin-7(8H)-one (5), which undergoes a multi-step rearrangement including a rare anti-Michael addition.