Abstract
A catalyst-free [3 + 2] cycloaddition reaction of electron-deficient alkynes and o-hydroxyaryl azomethine ylides in water was developed, affording pyrroline derivatives in moderate to high yields (up to 90%).
Highlights
Nitrogen-containing heterocycles are widely present in various drugs, natural products, organic catalysts, and synthetic intermediates.[1]
Krause’s group discovered in 2004 that when an amino group or a substituted amino group was present in the α-position of allene, under the catalysis of gold, intramolecular self-cyclization can occur to form an N-substituted pyrroline derivative (Scheme 1, eq 1).[6]
The Fan group, Soriano group, and Harmata group successively discovered that the substrate can undergo intramolecular cyclization under the catalysis of different transition metals such as copper, platinum, and silver.[7]
Summary
Nitrogen-containing heterocycles are widely present in various drugs, natural products, organic catalysts, and synthetic intermediates.[1]. In addition to the methods described above, the use of azomethine ylides and electron-deficient alkenes was an effective approach to obtain 3-pyrroline derivatives since Grigg first reported it in 1978. They used a very active dimethyl acetylenedicarboxylate and N-phenylmaleimide refluxing in toluene to obtain the corresponding pyrroline derivative.[12] It is significant to emphasize that Deng’s group successfully catalyzed the cyclization of azomethine ylides with electrondeficient alkynes using silver and copper (eq 4) and achieved asymmetric synthesis of pyrroline derivatives.[13,14] under their catalytic system, only the terminal electrondeficient alkynes can react with azomethine ylides. Compared to traditional transition-metal catalysis[17] or phosphine-catalyzed reaction of azomethine ylide with electron-deficient alkynes, the highlight of our work is that no catalyst was used and no organic solvents were used. Last but not the least, the raw materials we used are easy to prepare
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