RESUMEN
(4 + 2) Cycloaddition plays an important role in the synthesis of versatile carbocyclic/heterocyclic compounds with its high atom- and step-economy. Additionally, with mild conditions and indispensable functional group compatibility, the radical reaction has been recognized as a useful tool in organic chemistry. Given the enormous impact of radical-mediated (4 + 2) cycloaddition processes and their promising applications, we summarize and highlight the recent works in this attractive area. On the basis of the types of radicals that initiate different (4 + 2) cycloaddition processes, we classify them into processes involving alkenyl cations or alkenyl radicals, aryl radicals, acyl radicals, alkyl radicals, and heteroatom radicals, and this review places special emphasis on the reaction design and mechanisms, which will stimulate future developments in radical-mediated intermolecular (4 + 2) cycloaddition.
RESUMEN
Chemodivergent tandem radical cyclization offers exciting possibilities for the synthesis of structurally diverse cyclic compounds. Herein, we revealed a chemodivergent tandem cyclization of alkene-substituted quinazolinones under metal- and base-free conditions, this transformation is initiated by alkyl radicals produced from oxidant-induced α-C(sp3 )-H functionalization of alkyl nitriles or esters. The reaction resulted in the selective synthesis of a series of mono- and di-alkylated ring-fused quinazolinones by modulating the loading of oxidant, reaction temperature, and reaction time. Mechanistic investigations show that the mono-alkylated ring-fused quinazolinones is constructed by the key process of 1,2-hydrogen shift, whereas the di-alkylated ring-fused quinazolinones is mainly achieved through crucial steps of resonance and proton transfer. This protocol is the first example of remote second alkylation on the aromatic ring via α-C(sp3 )-H functionalization and difunctionalization achieved by association of two unsaturated bonds in radical cyclization.
RESUMEN
A convenient and straightforward approach for the radical cascade cyclization/hydrolysis of CN-containing 1,6-enynes with simple ethers under metal- and base-free conditions is described. This strategy provides a variety of valuable ethers-substituted polyheterocycles via the construction of three C-C bonds, one C=O bond, and two new six-membered rings within a single procedure. The resulting products can smoothly undergo follow-up conversions to various useful scaffolds. The methodology shows excellent functional group tolerance, high step- and atom- economy, and mild reaction conditions, which can be further scaled up to gram quantity in a satisfactory yield.
