Pyrrole-containing natural products

Fig. 1 Indole and pyrrole containing natural products and therapeutics...

Scheme 18.15 Structures of the pyrrole-containing natural products dioxapyrrolomycin and pyrrolonitrin.

Scheme 16.11 An early contribution for the synthesis of a pyrrole-containing natural product by using an intramolecular C-H arylation strategy.

A few instances of pyrrole-, indole-, carbazole-containing natural products are known from insects and higher animals, in addition to tryptophan-containing peptides. 

Reddy and colleagues devised a novel method for the divergent synthesis of 2,3-disubstituted indoles or 2,4,5-trisubstituted pyrroles from a-diazoketone 126. In the presence of copper(II) triflate, treatment of a-diazoketone 126 with enamine 129 gave pyrrole 130 if 126 was instead treated with aniline derivative 127, indole 128 resulted. The study culminated in the formal synthesis of indole-containing natural product homofascaplysin C (130L464). 

Pyrrolo[3,2-df pyridazines are a class of interesting and useful A -heterocycles [40—42]. However, synthetic methods for such heterocyclic compounds have been very much limited such as condensation of pyrrole-2,3-diones with hydrazine. There are no reports on one-pot multi-component synthesis of pyrrolo[3,2-(f py-ridazines [43]. Moreover, synthetic methods for pyrrole-2,3-diones are also very limited [43]. On the other hand, transition-metal-mediated reactions of azides are of great importance and versatility in organic synthesis, because azides could be readily transformed into a wide variety of valuable A -containing natural products and medicinal agents [44-50]. 

Fig. 2 General Structural Features of Pyrrole Containing Marine Natural Products...

Lycogalic acid (131) and halitulin (132) represent unique analogs (Fig. 8) to the 3,4-diaryl pyrrole scaffold of the natural products previously discussed. Lycogalic acid (131) contains indole rings attached at both C-3 and C-4 of... 

Often, in the synthesis of natural products containing the indolizidine substructure, it is necessary to modify a preformed indolizidine ring. This is the case in the synthesis of (+)-myrmicarin 217 191 where the key step is the closure of the third ring through an electrophilic substitution on the pyrrole nucleus (Scheme 45) <2000JOC2824>. 

Many natural products contain a pyrrole subunit as the basic core. Of particular interest is the remarkable diversity of biological activity associated to the... 

Further applications to preparations of nitrogen-containing heterocycles such as pyrroles (83,84) and indoles (85) by Feldman and co-workers [Eqs. (17), (18)] made these reactions more important and useful for natural product synthesis [66,67]. 

The number of new natural products containing pyrroles and their benzo derivatives has increased considerably during the past decade. Due to space restrictions, only the most representative examples are included, and these witness the variety of structures and biological functions of natural products containing pyrroles, indoles, carbazoles, and related skeletons. 

Dicarbonyl compounds are valuable intermediates in the preparation of natural products and related compounds containing pyrrole, thiophene, furan or cyclopentenone ring systems. 

The following section looks at how palladium-catalyzed C-H functionalization has been successfully applied in synthetic strategies enabling rapid and elegant routes to complex natural products containing the indole and pyrrole nucleus. There are a number of metal-mediated examples where stoichiometric quantities of transition metals are employed to affect the desired transformation however, there are very few cases of catalytic functionalization with in the context of complex molecule synthesis. 

The pyrrole ring, although not very common in nature, occurs in some very important natural products. A few antibiotics contain a pyrrole ring, one of the simplest is pyrrolnitrin 22 ... 

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