The Game of Electrons: Organocatalytic Higher‐Order Cycloadditions Involving Fulvene‐ and Tropone‐Derived Systems

Abstract

The great progress that took place in the field of higher‐order cycloadditions involving fulvene‐ and tropone‐derived systems in the last few years is astonishing. By application of organocatalytic activation modes, new higher‐order reactivities have been identified and described in the literature. These approaches take advantage of the high reliability of organocatalysis, at the same time expanding its potential and paving new directions for its further evolution. In this Minireview, the progress in the field of organocatalytic higher‐order cycloadditions involving fulvene‐ and tropone‐derived systems is summarized and insights into mechanistic aspects of the developed reactivities are provided. Furthermore, the discussion on the nomenclatural issues related to cycloaddition reactions has been conducted and solutions to clarify the picture proposed.

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher‐Order Cycloaddition Reactions

“…Fulvenes have served as useful synthons in diverse types of cycloadditions, including higher‐order cycloadditions [15,122,123] . In view of the great potential of pentafulvenes, their application with FCCs has been explored.…”

“…Pentafulvenes have been employed in cycloadditions [122,123] with FCCs (as aforementioned) and a great variety of other compounds. The first description of a cycloaddition between a fulvene and an FCC was published in 2001.…”

“…Albrecht's group coined the terms higherene and higherenophile to describe the reactants in a higher‐order reaction [22] . The former refers to a larger polyene component able to contribute 6, 8 or 10π electrons, while a higherenophile is the smaller π component [22] . Other researchers have considered higher‐order cycloadditions as non‐pericyclic cycloadditions capable of constructing rings with more than six members [23] …”

“…Hoffmann recently offered an even more specific description of higher‐order cycloaddition reactions as those involving over 6π electrons in bonding changes, making it possible to synthesize medium‐sized rings [19–21] . Albrecht's group coined the terms higherene and higherenophile to describe the reactants in a higher‐order reaction [22] . The former refers to a larger polyene component able to contribute 6, 8 or 10π electrons, while a higherenophile is the smaller π component [22] .…”

Cephalodiones A–D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition

“…The higher-order cycloadditions defined as the reactions involving more than six p electrons have been attracting the wide interests of organic chemists owing to the fascinating reaction mechanism and/or the advantage in constructing the unique ring systems. [5] The cephalotane-type C 19 -norditerpenoids possessing attractive structures and significant antitumor potencies are the diagnostic metabolites of the Cephalotaxus genus, [6] which attracted tremendous interests from organic synthesis. [6a, 7] In our continuing study on this compound class, four C 19norditerpenoid dimers, named cephalodiones A-D (1-4) (Figure 1), were isolated and characterized from the seeds of C. fortunei var.…”