Cationic Iron(III) Salt as an Initiator for Radical Cation‐induced [4+2] Cycloaddition

Abstract

We developed a cationic iron(III) salt initiator for radical cation‐induced [4+2] cycloaddition. Fe(OTf)3 [or Fe(ClO4)3] behaves as a highly active oxidant for both electron‐deficient and electron‐rich dienophiles. The scope of substrates is much broader than with FeCl3 reported previously. With highly active Fe(OTf)3, we demonstrate a formal total synthesis of Heitziamide A.

Transition‐Metal Catalyzed [4+2]‐Cycloaddition Reactions: A Sexennial Update

“…[188] In order to reinvestigate iron (III) initiators for the [4 + 2] cycloaddition of electron-deficient dienophiles, Horibe et al in the year 2020 reported a cationic iron (III) salt initiator for radical cation-induced [4 + 2] cycloaddition of electron-rich dienophile 213 and electron-rich diene 214 for the synthesis of [4 + 2] cycloadduct 215 (Scheme 60). [189] Highly active Fe(OTf There are multiple literature reports of Povarov reactions [190] based on the tertiary anilines, but there is no literature report for [4 + 2] cycloaddition using a secondary aniline as the substrate. Exploiting secondary/tertiary anilines 217, Ding et al in 2021 outlined the synthesis of quinolone-spiroquinzolinones 219 by an iron-catalyzed cross-dehydrogenative coupling [4 + 2] annulation of 217 and quinazolinones 218 (Scheme 61).…”

Recent advances in radical-mediated intermolecular (4 + 2) cycloaddition

“…Delightfully, in the following year, they explored the reaction with Fe(OTf ) 3 as a catalyst and tested substrates substituted with electron-withdrawing groups, again obtaining high yields, which successfully expanded the scope of substrates (Scheme 11b). 21 Thiophene with a conjugated diene structure has likewise been used in (4 + 2) cycloaddition reactions. In 2019, Chiang and Lei's group achieved the first visible-light-induced (4 + 2) cycloaddition reaction of thiophene and alkynes (Scheme 12).…”

“…Delightfully, in the following year, they explored the reaction with Fe(OTf) 3 as a catalyst and tested substrates substituted with electron-withdrawing groups, again obtaining high yields, which successfully expanded the scope of substrates (Scheme 11b). 21…”

Regioselectivity Umpolung in Asymmetric Diels–Alder Reaction of ortho-Formyl-Substituted Cinnamates and Dienals via Double Aminocatalysis

“…Moreover, we designed the activated cinnamates possessing an additional formyl group, which would further lower the LUMO energy via vinylogous activation by potentially forming iminium ions I , as proposed in Scheme c. Therefore, such a double catalytic mode with a single amine, through simultaneous LUMO-activation of cinnamate derivatives and HOMO-activation of 2,4-dienals (via trienamines II) , would not only enhance the reactivity but also switch the regioselectivity of traditional cinnamate dienophiles in the DA reaction . Moreover, the resulting multifunctional cycloadducts III might further render a cascade intramolecular aldol reaction under aminocatalytic conditions, straightforwardly affording the polyhydrophenanthrene frameworks, a type of fused tricyclic system ambiguous in natural products and receiving wide interest in organic synthesis …”