Skeletally Tunable Seven-Membered-Ring Fused Pyrroles

Chemical Chartographisis: a contemporary perspective in molecular design and synthesis

The use of flexible molecular systems in solution, without strictly controlling their behaviour, has frequently been productive. Their potential could increase by a more holistic view of the reaction(s) process(es) in which they are involved. In this perspective, we introduce a broader approach - "Chemical Chartographisis" - and discuss three projects in detail to illustrate its potential. The topics involve bimetallic 3d/4f species and coordination compounds built from benzotriazole-based and (a)symmetric salan ligands and focus on catalytic and, in less detail, biological-related examples.

Simple Synthetic Approach to N-(Pyridin-2-yl)imidates from Nitrostyrenes and 2-Aminopyridines via the N-(Pyridin-2-yl)iminonitriles as Intermediates

A facile, green, synthetic protocol of several substituted N-(pyridin-2-yl)imidates from nitrostyrenes and 2-aminopyridines via the corresponding N-(pyridin-2-yl)iminonitriles as intermediates is reported. The reaction process involved the in situ formation of the corresponding α-iminontriles under heterogeneous Lewis acid catalysis in the presence of Al₂O₃. Subsequently, α-iminonitriles were selectively transformed into the desired N-(pyridin-2-yl)imidates under ambient conditions and in the presence of Cs₂CO₃ in alcoholic media. Under these conditions, 1,2- and 1,3-propanediols also led to the corresponding mono-substituted imidates at room temperature. The present synthetic protocol was also developed on one mmol scale, providing access to this important scaffold. A preliminary synthetic application of the present N-(pyridin-2-yl)imidates was carried out for their facile conversion into the N-heterocycles 2-(4-chlorophenyl)-4,5-dihydro-1H-imidazole and 2-(4-chlorophenyl)-1,4,5,6-tetrahydropyrimidine in the presence of the corresponding ethylenediamine and 1,3-diaminopropane.

Dearomative (4 + 3) Cycloaddition Reactions of 3-Alkenylindoles and 3-Alkenylpyrroles to Afford Cyclohepta[b]indoles and Cyclohepta[b]pyrroles

The dearomative (4 + 3) cycloaddition reactions of 3-alkenylindoles with in situ-generated oxyallyl cations furnish cyclohepta[b]indoles, functionality-rich frameworks found in many bioactive compounds, including all pentacyclic ambiguine alkaloids. The analogous reactions between oxyallyl cations and 3-alkenylpyrroles afford cyclohepta[b]pyrroles. The cycloadducts are generally formed in good to high yields and diastereoselectivities and can be readily transformed into useful derivatives. Additionally, we report preliminary investigations into the enantioselective catalysis of the dearomative (4 + 3) cycloaddition using imidodiphosphorimidate catalysts.

Synthesis of Pyrrole via Formal Cycloaddition of Allyl Ketone and Amine under Metal-Free Conditions

A new metal-free synthesis of pyrrole from allyl ketone and amine has been established. The reaction proceeds via an thiolative activation of the C-C double bond with dimethyl(methylthio)sulfonium trifluoromethanesulfonate, followed by a nucleophilic ring-opening addition of primary amine to the generated episulfonium intermediate, and then an internal condensation and aromatization. This mild procedure provides a novel strategy to the construction of substituted pyrroles through a formal [4 + 1] cycloaddition reaction.

Polyoxometalate-Driven Ease Conversion of Valuable Furfural to trans-N,N-4,5-Diaminocyclopenten-2-ones

We investigated the catalytic efficacy of silicotungstic acid (H₄SiW₁₂O₄₀) polyoxometalate (POM) toward the reaction between furfural and amines that selectively yields trans-N,N-4,5-substituted-diaminocyclopenten-2-ones (trans-DACPs). H₄SiW₁₂O₄₀ facilitates the synthesis of a library of trans-DACPs with loadings as low as 0.05 mol %, in open air, without additives, in short times and good to high isolated yields. The protocol was applied to secondary amines as well as to aromatic primary amines with pK_b higher than ca. 9. The present catalytic synthetic protocol has an extended substrate scope with high yields and represents, to the best of our knowledge, the first polyoxometalate-driven paradigm as an efficient method to produce cyclopentanone frameworks under mild reaction conditions.