Pyrone als Substrate für Pd‐katalysierte [4 + 3]‐Cycloadditionen

Abstract

Ein neuartiges Reaktionsverhalten von Pyronen wurde bei deren Umsetzung mit dem Palladiumkomplex 1, einem 4π‐Elektronenreagens, beobachtet. Während Pyrone normalerweise mit 4π‐Elektronensystemen als 2π‐Elektronensubstrate reagieren, gehen sie mit 1 [4 + 3]‐Cycloadditionen zu Siebenringverbindungen wie 2 und 3 ein. Als Erklärung für dieses abweichende Verhalten wird ein zweistufiger Verlauf der Cycloaddition vorgeschlagen.

Organocatalytic Enantioselective Cross‐Vinylogous Rauhut–Currier Reaction of Methyl Coumalate with Enals

“…As methyl coumalate typically serves as an electrondeficient partner to react with electron-rich species, [7] and the known reactions of this heterocycle with electron-deficient partners usually require either harsh or photoreaction conditions, [6c,10] under our mild reaction conditions,w eb elieved that methyl coumalate was unlikely to undergo Friedel-Crafts alkylation with enals (Scheme 3, pathway B). This assumption was further supported by the failed reactions using the [b] ee [%] [c] …”

“…While using methyl coumalate as ap artner for conjugate addition with electron-deficient Michael acceptors has not been reported, the underlying mechanism of av ariety of known transformations of this heterocycle indicates that C6 is the most electrophilic site and can be attacked by nucleophilic reagents. [7] Thus,w es urmized, as depicted in Scheme 1, that the C6 of methyl coumalate could be attacked by an ucleophilic reagent (Nu) to generate adelocalized enolate at either C5 or C3 (A), which could trigger the subsequent intermolecular Michael addition to the a,b-unsaturated iminium B (generated from the enal 1 and ac hiral amine catalyst). The resulting adduct C could then undergo rapid aromatizationdriven elimination to deliver the final RC product 2.T he overall process would represent an asymmetric cross-vinylogous RC reaction enabled by an ucleophilic reagent (Nu) and achiral amine organocatalyst.…”