A New Radical-Based Route to Calothrixin B

Abstract

[structure: see text] A high-yielding totally regioselective intramolecular homolytic acylation of a quinoline ring constitutes the key step in a new synthesis of the pentacyclic indolo[3,2-j]phenanthridine alkaloid calothrixin B.

Access to Aromatic Ring-Fused Benzimidazoles Using Photochemical Substitutions of the Benzimidazol-2-yl Radical

“…This typically requires excess amounts of azo-initiator, and/or oxygen to give the 'oxidized' aromatic substitution product in the presence of full equivalents of the 'reductant' Bu 3 SnH. [2][3][4][5][6][7][8][9][10][11][12][13] Other disadvantages of this inherently inefficient reaction include the requirement for slow syringe pump addition of initiators, and the separation, then disposal of toxic organotin waste. Bowman and co-workers reported intramolecular five and six-membered aryl radical displacements of phenylsulfanyl groups at the benzimidazole-2-position to give aryl ring-fused adducts (Scheme 1).…”

(Me3Si)3SiH: Twenty Years After Its Discovery as a Radical‐Based Reducing Agent

“…www.chemeurj.org 16 in good yield. [39] However, the whole process is not the result of a radical chain reaction. The radical derived from the decomposition of AIBN also participates in the oxidation or rearomatization of the intermediate radical upon completion of the chain.…”

Iridium(III) Photocatalysis: A Visible‐Light‐Induced Dearomatizative Tandem [4+2] Cyclization to Furnish Benzindolizidines

“…Alternatively, the catalysts’ excited states can first be reductively quenched by an electron donor to produce the mono‐reduced complexes, and in case of catalyst 3 (where E * ox is just −0.96 V), this leads to the Ir II species [Ir(dtbbpy)(ppy) 2 ] (dtbbpy=4,4′‐di‐ tert ‐butyl‐2,2′‐bipyridine; ppy=2‐phenylpyridine) as a suitable reductant for unactivated organic halides ( E ox =−1.51 V) 1a. Intra‐5 and intermolecular6 radical annulations of functionalized indole substrates have been utilized extensively in the preparation of polycyclic indole and indoline structures, some of which have proven suitable precursors to complex indole alkaloids in total synthesis 5a,b,f–h,k. 6a Complementary to thermal methods, photoredox‐initiated radical additions to indoles have also been developed, but the methods reported to date have encompassed only intramolecular aromatizative cyclizations1c, 3a and intermolecular oxidative functionalizations 7.…”