1.3‐Dipolare Cycloadditionen, LVI. Eine bequeme Synthese von N‐substituierten Pyrrolen aus mesoionischen Oxazolonen und Alkinen

TLDR: In this paper, a convenient synthesis of N-substituted Pyrroles from Mesoionic Oxazolones and Alkynes has been presented, which can be used for cycloadditions if one treats them with acetic anhydride in the presence of suitable acetylenic dipolarophiles.

Abstract: 3-Methyl-2.4-diphenyl-oxazolium-5-oxid (2) vereinigt sich bei 0−130° mit Alkinen (Acetylen und Alkylderivate, phenylierte und acylierte Acetylene, Acetylen-carbonsaureester) unter Kohlendioxid-Abgabe zu den Pyrrolen 3, 5–13. Zahlreiche N-Acyl-sek.-aminosauren, bei denen die Isolierung des Oxazolium-5-oxids nicht gelingt, lassen sich in situ an Alkine zu den Pyrrol-Abkommlingen (20–28, 33–36, 38, 39) cycloaddieren, wenn man mit Acetanhydrid in Gegenwart des Dipolarophils behandelt. Die Additionsrichtung wird weniger von den Substituenten als von der Natur des Oxazolium-5-oxid-Rings bestimmt. 1,3-Dipolar Cycloadditions, LVI. A Convenient Synthesis of N-Substituted Pyrroles from Mesoionic Oxazolones and Alkynes 3-Methyl-2,4-diphenyl-oxazolium 5-oxide (2) combines at 0−130° with alkynes (acetylene and 1-alkynes, phenylated and acylated acetylenes, acetylenecarboxylic esters) to give initially adducts which then eliminate carbon dioxide to form the pyrroles 3, 5–13. Numerous N-acyl sec-amino acids which do not allow the isolation of oxazolium 5-oxides on anhydrization, can be utilized for cycloadditions if one treats them with acetic anhydride in the presence of suitable acetylenic dipolarophiles. The pyrroles 20–28, 33–36, 38, 39 have been prepared by this method. The direction of cycloaddition is determined mainly by the nature of the oxazolium 5-oxide ring rather than by the substituents in the 2- and 4-position.