Synthesis 2015; 47(18): 2805-2818
DOI: 10.1055/s-0034-1380223
special topic
© Georg Thieme Verlag Stuttgart · New York

Cycloaddition Reactions of Propyne Iminium Salts and Enol Ethers

Philipp Kratzer
Institute of Organic Chemistry I, University of Ulm, 89081 Ulm, Germany   eMail: gerhard.maas@uni-ulm.de
,
Holger Gerster
Institute of Organic Chemistry I, University of Ulm, 89081 Ulm, Germany   eMail: gerhard.maas@uni-ulm.de
,
Gerhard Maas*
Institute of Organic Chemistry I, University of Ulm, 89081 Ulm, Germany   eMail: gerhard.maas@uni-ulm.de
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Publikationsverlauf

Received: 18. Mai 2015

Accepted after revision: 20. Juni 2015

Publikationsdatum:
04. August 2015 (online)


Dedicated to Professor Manfred Regitz on the occasion of his 80th birthday

Abstract

Acetylenic iminium salts (propyne iminium salts) react with cyclic enol ethers in a thermal [2+2] cycloaddition reaction to afford condensed cyclobutene iminium salts. With acyclic enol ethers, a cycloaddition/electrocyclic ring-opening sequence occurs to give (1-alkoxybuta-1,3-dien-2-yl)methane iminium salts as mixtures of diastereoisomers. An analogous scenario is observed with 5-phenyl-2-vinylfuran as the nucleophilic olefinic component. With 1-methoxy-3-trimethylsilyloxy-1,3-butadiene, propyne iminium salts undergo a [4+2] cycloaddition. For all reactions, traces of acid must be quenched with solid K2CO3 or CaO to avoid immediate polymerization of the enol ether.