The pyrrole molecular framework is found in a large number of natural and synthetic compounds of great importance. Since functionalized pyrroles are essential for the progress of many branches of science, its synthesis by simple, efficient and eco-friendly routes are particularly attractive in modern organic and bio-organic chemistry. To this end, a number of synthetic methods have been developed, in which the Paal–Knorr pyrrole synthesis stands out to be the easiest route to synthesize pyrroles. In spite of the efficiency, Paal–Knorr synthesis of pyrroles is considered limited by harsh reaction conditions, such as prolonged heating in acid, which may degrade sensitive functionalities in many potential precursors. Through this route almost all dicarbonyls can be converted to their corresponding heterocycles and therefore it is a synthetically valued process. To address the adverse issues this reaction route has undergone numerous modifications recently and today it can be said that this reaction route is a prominent green route for the synthesis of pyrroles. This review is a tour from the evolution and application of this harsh synthetic route to the eco-friendly greener route developed for the synthesis of pyrroles.

中文翻译：

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Paal–Knorr吡咯合成：从常规合成到绿色合成

在大量非常重要的天然和合成化合物中发现了吡咯分子骨架。由于功能化的吡咯对于许多科学分支的发展必不可少，因此通过简单，高效和环保的途径合成吡咯在现代有机和生物有机化学中特别有吸引力。为此，已经开发了许多合成方法，其中Paal–Knorr吡咯合成方法是合成吡咯的最简单方法。尽管效率高，但仍然认为吡咯的Paal-Knorr合成受到苛刻的反应条件的限制，例如长时间在酸中加热，这可能会降低许多潜在前体的敏感功能。通过这种途径，几乎所有的二羰基化合物都可以转化为其相应的杂环，因此这是一个具有综合价值的过程。为了解决不利的问题，该反应路线最近进行了许多修改，今天可以说，该反应路线是合成吡咯的重要绿色路线。这篇评论是从这种苛刻的合成路线的演变和应用到为吡咯的合成开发的环保绿色路线的旅程。