Phosphazenomalonates as Catalysts and Reactants in (4+3) Annulation to Acrolein

Cyclohepta[b]indole Core Construction via the Michael Addition/Friedel-Crafts Condensation Sequence

An approach to cyclohepta[b]indoles via the formal (4 + 3) annulation of readily synthesized 2-indolyl-derived 1,3-dicarbonyl compounds to acrolein and enones was developed. The process is initiated by catalytic Michael addition, wherein the adduct was generated in situ. The subsequent addition of a Bro̷nsted acid catalyst triggers a cascade process that includes intramolecular Friedel-Crafts hydroxylalkylation followed by dehydration. The cascade reaction is relatively undemanding since even degrading chloroform is able to initiate it. The obtained tetrahydrocyclohepta[b]indoles were prone toward easy dimerization, underscoring the high reactivity of the double bond in the seven-membered ring.

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion