3-Indolylacyl Radical Cyclizations upon Pyridines and Tetrahydropyridines: Access to Ergoline-Related Indole [cd]-fused Isoquinolines

Oxidative Cyclization of Kynuramine and Ynones Enabling Collective Syntheses of Pyridoacridine Alkaloids

A cerium(III)-catalyzed oxidative cyclization of kynuramine and ynones has been reported as a key reaction in the total synthesis of marine pentacyclic pyridoacridine alkaloids featuring different ring connectivity patterns. The formation of tricyclic benzonaphthyridine rings was identified in the oxidative process. By combining with an intramolecular acylation and the chemoselective late-stage functionalization of pyridine rings, different approaches with 4-10 steps have been designed to accomplish the synthesis of alkaloids demethyldeoxyamphimedine (1), amphimedine (2), meridine (3), isocystodamine (4), N-methylisocystodamine (5), N-hydroxymethylisocystodamine (6), 9-hydroxyisoacididemin (7), neolabuanine A (8), and ecionine A (9).

General, Mild, and Metal-Free Synthesis of Phenyl Selenoesters from Anhydrides and Their Use in Peptide Synthesis

A mild, practical, and simple procedure for phenyl selenoesters synthesis from several anhydrides and diphenyl diselenide was developed. This transition-metal-free method provides a straightforward entry to storable Fmoc-amino acid selenoesters which are effective chemoselective acylating reagents. An application to oligopeptide synthesis was illustrated.

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

Herein is a pertinent review of recent photochemical homolytic aromatic substitution (HAS) literature. Issues with using the reductant Bu3SnH in an oxidative process where the net loss of a hydrogen atom occurs is discussed. Nowadays more efficient light-induced chain reactions are used resulting in HAS becoming a synthetic mechanism of choice rivaling organometallic, transition-metal and electrophilic aromatic substitution protocols. The review includes aromatic substitution as part of a tandem or cascade reaction, Pschorr reaction, as well as HAS facilitated by ipso-substitution, and Smiles rearrangement. Recently visible-light photoredox catalysis, which is carried out at room temperature has become one of the most important means of aromatic substitution. The main photoredox catalysts used are polypyridine complexes of Ru(ii) and Ir(iii), although eosin Y is an alternative allowing metal-free HAS. Other radical initiator-free aromatic substitutions have used 9-mesityl-10-methylacridinium ion and N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide) as the photoredox catalyst, UV-light, photoinduced electron-transfer, zwitterionic semiquinone radical anions, and Barton ester intermediates.

Copper promoted radical addition/cyclization of azobisisobutyronitrile with arylacrylamides: a convenient process to synthesize 3-(2′-cyano alkyl) oxindoles

A CuI promoted radical addition/cyclization of azobisisobutyronitrile with alkenes for the preparation of 3-(2′-cyano alkyl) oxindoles was developed.

Barton esters for initiator-free radical cyclisation with heteroaromatic substitution

S-(1-Oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium hexafluorophosphate (HOTT) facilitates the first examples of efficient radical cyclisation with (hetero)aromatic substitution via Barton ester intermediates. Cyclopropyl and alkyl radicals allow access to five, six and seven-membered alicyclic-ring fused heterocycles with and without an additional fused cyclopropane, including the skeleton of the anti-cancer agent, cyclopropamitosene, expanded, and diazole analogues. Radical initiators are not required for cyclisation from carboxylic acid precursors.