The reaction of 1,2-disubstituted 2-imidazolines with electron-deficient alkynes proceeds as a pseudo-threecomponent process and forms imidazolidines with an N-vinylpropargylamine fragment. Heating the resulting imidazolidines in xylene on air leads to an effective formation of polysubstituted pyrroles through a domino sequence of aza-Claisen rearrangement/ transannular nucleophilic addition/oxidative ring opening reactions. The direct one-pot transformation of 2-imidazolines to pyrroles has been also realized.
A novel isocyanide-based multicomponent synthesis of alkyl aryl(indol-3-yl)acetimidates has been established. Starting from aryl(indol-3-yl)methylium tetrafluoroborates, aromatic isocyanides and alcohols, the imidates were obtained in moderate to very good yields. Consecutive four-component synthesis of the above mentioned imidates from N-alkylindoles, aromatic aldehydes, aromatic isocyanides and alcohols was also proposed. In addition, it was shown that in the presence of water, aryl(indol-3-yl)methylium tetrafluoroborates reacted with isocyanides to furnish aryl(indol-3-yl)acetamides.
The
transformation of 2-imidazolines into 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines has been realized. A pseudo-three-component
reaction of 2-imidazolines with terminal electron-deficient alkynes
(2 equiv) first generates imidazolidines, containing an N-vinylpropargylamine fragment. The latter can then undergo a base-catalyzed
domino aza-Claisen rearrangement/cyclization reaction sequence, simultaneously
constructing pyrrole and pyrazine rings. The process works in a broad
substrate scope, delivering pyrrolo[1,2-a]pyrazines
in good to excellent yields (45–90%). This two-step approach
can be carried out in a one-pot fashion without a noticeable decrease
in yield. Remarkably, a three-component protocol for the introduction
of two different alkynes has been also developed.
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