Pyrroles (1H-pyrroles) is a class of hetero-aromatic organic compound with empirical formula C4H5N consisting of a five-membered ring with a nitrogen atom (Fig.1).It is a colorless volatile liquid while turns dark readily upon exposure to air. Substituted derivatives are also called pyrroles, such N-methylpyrrole, C4H4NCH3. Pyrroles are important synthons in the synthesis of natural products. They exhibit remarkable biological properties such as antitumour activities, anti-inflammatory, antimicrobial, hypolipidemic. Phosphino-substituted N-aryl pyrroles, a novel class of sterically demanding and electron rich biaryl phosphine ligands, exhibit high turnover rates and low catalyst loadings.
Fig. 1 The Full structural formula, Skeletal formula with numbers, Ball-and-stick model and Space-filling model of Pyrroles (cited from Wikipedia)
Biologically, pyrroles tend to construct the key structure of porphyrin rings, which act as an active moiety in chlorophyll, heme, vitamin B12 or bile pigments. This skeleton is also found in wide range of pharmacologically active compounds (Fig.2), being incorporated either as a substituent or with various substitutions on the ring itself. Some of the drugs containing pyrrole moiety are already available in market while rest are under clinical trials. Various diarylpyrrole derivatives were evaluated for the anticoccidial activity by both in vitro and in vivo assays. N-substituted derivatives of 2,3-diarylpyrrole were also evaluated against commercially important strains of Eimeria in chickens. Moreover, Tolmetin and Zomepirac are two well-known pyrrole acetic acid derivatives utilized for the treatment of rheumatoid arthritis and pain. Carson and Wong reported that methylation of acetic acid chain in the Zomiperac markedly increases the anti-inflammatory potency which was measured by performing the rat paw kaolin edema assay.
Fig.2 Biological activities of pyrroles and its derivatives.
Over the past two decades, calixpyrroles, have emerged as important members of the supramolecular chemistry pantheon due to their ability to act as receptors for anions, cations, and ion pairs. A large number of calixpyrrole-based anion- and ion pair receptors were developed by functionalizing the backbone of parent calixpyrrole at its meso positions or at one of more b-pyrrolic sites. Over the past ten years strapped calixpyrroles have garnered considerable attention and have led to advances that have yet to be recapitulated in the case of simple calixpyrroles. For instance, as compared to calixpyrroles, strapped calixpyrroles often display enhanced affinities and greater selectivity towards anionic or ion pair guests.
Fig. 3 (a) Chemical structures of calixpyrrole 1 and (b) generalized representation of a strapped calixpyrrole 2 bearing at least one “closed” strap.
Pyrroles-based building blocks have been applied to the production of a wide array of conjugated materials. These have included not only traditional-type materials, but also n-type and ambipolor materials, with resultant band gaps ranging from those typical of polythiophene (~2.0 eV), to values as low as 0.56 eV. In addition, these materials have exhibited enhanced fluorescence in comparison to typical thiophene-based materials.
Ryder group have sought routes to conducting pyrrol-based polymer systems based on thiophene and/or pyrrole that both are soluble and exhibit ordered crystalline, or liquid crystal phases. Crystallinity or liquid crystallinity in these materials can not only provide an insight into the electronic conductivity, but is also expected to lead to new applications that exploit anisotropic behaviour associated with the discrete, orthogonal conductivity mechanisms along the conjugated strands of the polymer material and between adjacent strands.Consequently, such materials are considered to be an important potential new feedstock for the plastic electronics industry.