Pyrroles from oxazoles

Abstract

The review is devoted to the transformation of oxazoles (including those with cationoid and mesoionic structures) into compounds of the pyrrole series.The search for new methods for the synthesis of heterocyclic systems is always actual, and the development of new methods for the construction of the pyrrole ring is particularly significant: the pyrrole ring is the basis of life on Earth, as it is the major subunit of the vitally important biomolecules hemoglobin and chlorophyll. Like any other heterocycle, the pyrrole ring can either be assembled from acyclic precursors (one or several synthons) or another heterocycle can be used as its precursor, i.e., the pyrrole can be obtained by recyclization. Transformation of the oxazole ring can provide an interesting and unusual method for the production of pyrrole. Oxazoles are widely used as starting compounds in synthesis (e.g., of pyridines, imidazoles, and other heterocycles), but the diversity of the methods used for the transformation of oxazoles into pyrroles has not yet been duly reflected in the review literature.Preparation of this review was prompted by the discovery by author and his group of an entirely new family of oxazole transformations to pyrroles in a series of condensed heterocycles containing a bridging nitrogen atom. This transformation, which has already been included in the latest edition of the classic textbook [1] and in a recent monograph on the chemistry of oxazoles [2], deserves comparison with all other examples (both known and exotic) of the oxazole conversion to pyrroles. Such a comparison, with the emphasis on the structural type of the transformation and on comparison of monocycles with condensed systems, is the theme of the present review, which is of interest both to synthesis specialists and to a wide range of teachers and high school students.It is not necessary to duplicate the known examples of the pyrrole synthesis from oxazoles (presented in the books [2,3] in the form of detailed tables), and they will only be used in the general schemes in order to complete the picture. In view of classification it is convenient to divide the oxazoles from which pyrroles can be obtained into simple subfamilies -neutral oxazoles (mesoionic and "normal") and oxazolium salts. PYRROLES FROM "NORMAL" OXAZOLES WITH NEUTRAL STRUCTUREDuring Vilsmeier formylation of the oxazole 1 the pyrrole 2 was isolated [4].

Synthesis and Toxicity Evaluation of New Pyrroles Obtained by the Reaction of Activated Alkynes with 1-Methyl-3-(cyanomethyl)benzimidazolium Bromide

“…From the marine alkaloid group with the tri-and tetrasubstituted pyrrole ring, lamellarins O, Q, R for their anticancer activity [5] and storniamids A-D for antibacterial activity [3] stand out (Figure 2). Additionally, pyrroles are useful building blocks [18][19][20][21] in the design and synthesis of naturally occurring compounds, functionalized materials, and new bioactive condensed heterocycles, and as a consequence, the interesting synthetic aspects and applications of the compounds possessing the pyrrole core have been reviewed [9][10][11][12][13]18,[22][23][24][25][26][27][28][29]. The 1,4-dihydropyridine (DHP) class is frequently the first choice of drug used in the treatment of hypertension [30] and angina [31].…”