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Catalytic iron- and cobalt-mediated enediene carbocyclizations
Abstract
The field of organic synthesis continues to evolve toward the construction of highly functionalized, structurally complex targets. Discovering new methods to form carbon-carbon bonds is of the utmost interest. The use of transition metals to perform this task has been evolving very rapidly over the past several years. Transition metal-mediated syntheses often enable novel bond constructions, for which there exists no classical counterpart. It is also worthy to note that the use of catalysis to perform a reaction is a strategy relevant to environmentally friendly synthesis, i.e. green chemistry. Classical approaches employ stoichiometric reagents, the byproducts of which must be removed and disposed of after the reaction. The future of organic synthesis will inevitably lie in the development and application of catalytic methods to perform efficient bond constructions. The focus of this research has been the efficient construction of highly functionalized ring systems. Construction of tetrasubstituted five membered rings, the core fragments contained in PGI2 analogue Cicaprost TM, iridoid glucoside (–)-gibboside, isoprostane 15-F2t -IsoP have been accomplished using a catalytic iron-mediated cyclization. CicaprostTM and (–)-gibboside have a common core fragment, which was synthesized in an 80% yield (single isomer) using an iron-mediated cyclization. The tetrasubstituted cyclopentane core contained in 15-F 2t-IsoP was constructed in 50% yield and good selectivity (9:1), using an iron-mediated cyclization. A novel cobalt-mediated reductive cyclization catalyst has been used in the construction of trans five- and six-membered rings (50–86% yield) and good to excellent selectivity (7 to 50:1, trans:cis). This catalyst has also been used in cascade cyclizations, where multiple rings can be constructed from an acyclic precursor. The use of an iron-mediated cyclization in the construction of the core tetrasubstituted cyclopentanes contained in CicaprostTM, (–)-gibboside, 15-F2t-IsoP has been efficient. The cobalt-mediated reductive cyclization catalyst has demonstrated its utility in the formation of five and six-membered rings, and cascade cyclizations. Catalytic iron- and cobalt-mediated cyclizations appear to be applicable in the efficient construction of a variety of ring containing targets.
Subject Area
Organic chemistry
Recommended Citation
Mehrman, Steven James, "Catalytic iron- and cobalt-mediated enediene carbocyclizations" (1999). ETD collection for University of Nebraska-Lincoln. AAI9942179.
https://digitalcommons.unl.edu/dissertations/AAI9942179