Electrophilic Substitutions of Five-Membered Rings

Iridium Acylnitrenoid-Initiated Biomimetic Cascade Cyclizations: Stereodefined Access to Polycyclic δ-Lactams

Ring-fused azacyclic compounds are important building units in the synthesis of biorelevant natural products, pharmaceutical agents, and molecular materials. Herein, we present a new approach to these condensed azacycles by a biomimetic cascade cyclization of arylalkenyl dioxazolones. This cascade reaction was found to proceed with excellent stereoselectivity and a high functional group tolerance. The substrate scope of arylalkenyl dioxazolones turned out to be highly flexible and extendable to additional terminating subunits, such as heteroaryl and alkynyl moieties. This biomimetic cyclization was elucidated to be initiated by an intramolecular transfer of the in situ generated electrophilic Ir-acylnitrenoid to the tethered olefinic double bond, leading to a key N-acylaziridine intermediate, which is in turn reacted with pendant (hetero)arenes or alkynes in a highly regio- and stereoselective manner to produce ring-fused azacyclic compounds.

EFFECT OF LEWIS ACID CATALYSTS ON THE POSITIONAL SELECTIVITY OF THE ELECTROPHILIC AROMATIC SUBSTITUTION ON α-SUBSTITUTED THIOPHENES: A CONCEPTUAL DFT INVESTIGATION

The α′/β regioselectivity of the electrophilic aromatic substitution of some thiophene α-substituted derivatives ( R = CHO, COMe or CO2Me ), catalyzed and not catalyzed by the Lewis acid, AlCl3 , has been investigated by means of the local nucleophilicity index, recently proposed by Pérez et al. [J Mol Struct: Theochem895: 86, 2009]. The quantum chemistry calculations, carried out at the B3LYP/6-311G(d,p) level of theory, show that the α′-substitution is preferred in absence of the catalyst, while the β-substitution is more favored in the presence of the catalyst. The theoretical results, predicted using DFT-based reactivity indices, are in good agreement with the experimental outcomes.

A mild and facile one-pot synthesis of N-methyl-acyl-pyrroles

N-Methyl-3-acylpyrroles were synthesized via a multicomponent reaction of dimethylacetylene dicarboxylate (DMAD), N-methylhydroxylamine and acylchlorides in the presence of KHCO₃. For comparison both conventional and microwave protocols were examined in this procedure. The reaction is clean and gives the products in good to excellent yields under conventional heating conditions at 40 ºC in anhydrous dichloromethane.

Origin of depressed dipole moments in five-membered, unsaturated heterocycles

The smaller molecular dipole moments of cyclopentadiene, pyrrole, furan, and thiophene, relative to their saturated analogs, are investigated using the HF/6-311++G**//HF/6-311++G** level of theory. Investigation of the atomic origins of the molecular dipole moments demonstrates that the decreased dipole moments of the unsaturated heterocycles result from simple perturbations of the butadiene fragment, rather than a delocalization of hetero-atom lone pairs into the π system. Comparison with the origins of the molecular dipole moments of the saturated analogs shows that a special delocalization mechanism is not necessary. Key words: ab initio, atoms-in-molecule, electronic effects, delocalization, polarization.